UNIVERSITY OF CINCINNATI

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1 UNIVERSITY OF CINCINNATI Date: 04/03/06 I, Abhay Vikas Joshi, hereby submit this work as part of the requirements for the degree of: Master of Science in: Materials Science & Engineering It is entitled: Development of a novel Si-modified Zn-Al eutectoid alloy for hot-dip batch galvanizing This work and its defense approved by: Chair: Dr. William J. van Ooij Dr. Relva Buchanan Dr. Ray Lin

2 Development of a novel Si modified Zn-Al eutectoid alloy for hot-dip batch galvanizing A dissertation submitted to the Division of Research and Advanced Studies of the University of Cincinnati in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in the Department of Chemical and Materials Engineering of the College of Engineering 2006 By Abhay Vikas Joshi B.E., Visvesvaraya National Institute of Technology, Nagpur, 2002 Committee Chair: Dr. William J van Ooij Committee members: Dr. Ray Lin Dr. Relva Buchanan ii

3 ABSTRACT The hot dip galvanizing process facilitates a combination of good strength and toughness of the substrate with excellent resistance to corrosion provided by the coating through a simple and economic technique. There are two broad categories to this process; the continuous hotdip galvanizing process and the batch hot-dip galvanizing process. The batch process is particularly important for preformed members or structures. Along with improvement in the life of galvanized steel, there is a growing environmental concern about leaching of zinc into the soil, and this project was an attempt to address this issue. Intensive research has resulted in successful development of new products like Galvanneal, Galfan and Galvalume in recent years, which are produced through the continuous galvanizing process. These products have varying degree of aluminum in the galvanizing bath. There has not been much development in the batch process, mainly due to the restrictions imposed by the process itself, and due to the incompatibility of the conventional zinc-ammonium-chloride flux with baths containing high aluminum content. A novel Cu-Fe and Cu-Sn chloride-free fluxes were therefore developed first, which work satisfactorily with baths containing Al as high as 50%. Prior research work on the Zn-22.3 wt% Al alloy showed that the galvanized coatings obtained from the binary Zn-Al eutectoid bath were an order thicker than the commercially acceptable norm of approximately 80 µm and the coating surface was rough and the coatings showed a lot of trapped porosities and outbursts. The ternary additions of Bi and RE in the Zn-Al eutectoid bath were made in order to control the thicknesses of the coatings, but with limited success. Ternary addition of wt% Si in the Zn-Al eutectoid bath had beneficial influence on the growth rate and morphology of the coatings. A coating as thin as µm iii

4 could be achieved. The porosities in the coatings were totally eliminated but the outbursts although, greatly reduced, were still a cause of concern. However, the Si-treated Zn-Al eutectoid bath emerged by far as the most promising alloy for hot-dip galvanizing of steel. Dr. W. van Ooij on behalf of the University of Cincinnati and Weert Groep, requested the assistance and galvanizing expertise of Teck Cominco Ltd. Product Technology Center (PTC) in completing the development of this alloy, understanding the underlying principles and scaling-up the development work from small laboratory furnaces to larger crucibles for dipping trials. Following the agreement between the three parties a joint project was started at PTC from 22 nd June, 2005 until 20 th September, The novel Cu-Sn and Cu-Fe fluxes were found to be ineffective during the initial stages of the work. A modified Zn Ammonium Chloride flux developed by PTC and Ferrotech Inc. was used and it was found to be working with this high Al bath. The thesis work was focused towards development and optimization of the experimental matrix to assess the effects of pretreatment chemistries, bath temperature, bath composition, immersion time and withdrawal rates on the coating quality. Evaluation and understanding of the coating properties was accomplished by doing in depth characterization of the coatings, produced at PTC, in the Advanced Materials Characterization Center at The University of Cincinnati. The thesis traverses through the study of the working mechanism of the Si modified Zn-Al alloy in an attempt to completely eliminate the outbursts observed on the coating surface and to produce a commercially acceptable coating quality. iv

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6 ACKNOWLEDGMENT I would like to express my gratitude to all those who provided their valuable help and support in completing this work. I would like to thank the Department of Chemical and Materials Engineering for giving me an opportunity to commence this thesis in the first instance, to do the necessary research work and to use the departmental facilities and also for the financial support in the form of University Graduate Scholarship. I am indebted to my research advisor Dr. Wim J van Ooij for his trust in my capability to manage the research of this nature, and for all his support, guidance and encouragement throughout the period of my graduate study. His tutelage, patience and the personal time he invested throughout my tenure as a graduate student are much appreciated. I gratefully acknowledge his keen insight in the science of corrosion, his energetic enthusiasm and generous support. I am thankful to Dr. Madhu Ranjan for his advice and his assistance. I am extremely thankful to Mr. Paul Kolisnyk, Mr. John Zervoudis and the entire management at the Teck Cominco Ltd. Product Technology Center, Canada, for being a part of this project work and for providing the research facilities. I am sincerely grateful to Mr. John Zervoudis, Mr. Graham Poag, Mr. Steven Murray, Mr. Pat Ginneti, Dr. Daniel Liu and the entire staff at PTC for their guidance, advice, and assistance at every stage of this work. Their inputs from the commercial standpoint went a long way in having a more viable and fruitful outcome of the work. I gratefully acknowledge the financial support provided by Weert Groep, The Netherlands, to carry out the research work at the University of Cincinnati. v

7 I am grateful to Dr. Ray Lin and Dr. Relva Buchanan for their critical comments, valuable suggestions, and time serving as members of my dissertation committee. My thanks are due to the faculty members and staff of the Department of Chemical and Materials Engineering, University of Cincinnati, who contributed to my education. Further, I would like to thank my friends Ashwini, Akshay, Deepti, Girish, Sumeet and many others for all their help, support, interest and valuable hints. Finally, I would like to thank my mother Meenal Joshi, to whom I dedicate my work, my sister and my entire family for their continued support and encouragement, without which, I would not have succeeded in my endeavor. vi

8 Table of Contents 1. INTRODUCTION OVERVIEW LITERATURE REVIEW Zinc-iron reaction Zinc aluminum reactions Zinc Aluminum Iron system a) Low-Al addition to galvanizing baths (<1%) b) Zn bath containing 5 wt% Al (Zn-Al eutectic alloy) c) Zn bath containing wt% Al Silicon in Zn-Al bath Pure Aluminum Fluxing Summary OBJECTIVE EXPERIMENTAL PROCEDURE Laboratory set up Furnace Crucible Thermocouple Sample Insertion Machine (SIM) Steel panels Sample cleaning Fluxing Bath preparation Analysis of the bath samples and the wet chemical analysis of the coating Bottom sample Top dross vii

9 2.9 Ingot analysis Galvanizing in the laboratory (30-kg crucible) Galvanizing in the shop (500-kg crucible) Coating thickness measurement Microstructure investigation: scanning electron microscopy Bend test Hardness determination Salt spray test Humidity test DC Polarization test RESULTS AND DISCUSSION Fluxes Working of the G flux Bath analysis Top dross Ingot analysis Bath Bath Depletion of Si in bath Galvanizing in the 500-kg crucible Dipping of various shapes and sizes of steels (Appendix 4) viii

10 3.10 Coating appearance of the Zn23Al-coated coupons dipped at various Si levels Coating Structure Characterization of the coatings Coatings dipped at % Si level Coatings dipped at 0.4% Si level Coatings dipped at 0.75% Si level Dipping of the special set of Si steels Coating thicknesses of the Zn23Al-coated coupons dipped at various Si levels in the bath Effect of immersion time and bath Si level on the coating thicknesses Effect of bath temperature Characterization of the top layer of the Zn23Al coating Wet chemical analysis of the Zn23Al coating dipped at 0.175% Si in the bath at 575 ± 5 C Bath metallographic analysis Spangle observed on the coupons Black spots on the coating surface Mechanical tests performed on the Zn23Al coated coupons The bend test on the coatings Hardness test Corrosion tests Salt spray test Humidity test DC polarization SUMMARY AND CONCLUSIONS FUTURE WORK REFERENCES ix

11 List of Tables Table 1 Fe-Zn phase characteristics 64 Table 2 Ternary solid phases in Fe-Al-Si system 64 Table 3 Composition of the special set of steels 65 Table 4 Parameters used for the in-house analysis of Si and Al 65 Table 5 Wet chemical analysis of the bath Si level (bath 1) 66 Table 6 Si level in bath and bottom samples 66 Table 7 EDS analysis of the intermetallics found in the top dross 67 Table 8 Wet chemical analysis of the ingots 67 Table 9 Wet chemical analysis of the bath 2 67 Table 10 Wet chemical analysis of the bath 3 68 Table 11 Bath analysis showing the depletion of the Si level 68 Table 12 Characterization of the coatings dipped at % Si bath (at%) 69 Table 13 Characterization of the coatings dipped at 0.4% Si bath (at%) for 1 min, carried out at Teck Cominco PTC 70 Table 14 Characterization of the coatings dipped at 0.4% Si bath (at%), carried out at UC AMCC 70 Table 15 Characterization of the coatings dipped at 0.75% Si bath (at%), carried out at UC 71 AMCC Table 16 Characterization of the special set of steels dipped at 0.4% Si bath 72 Table 17 Coating thicknesses obtained at various parameters 73 Table 18 Weight loss in the Galfan-coated and Zn23Al-coated coupons in the salt spray test 74 x

12 Table 19 Sample identification table for coupons used in the bend test 75 Table 20 Weight loss table for coupons used in the bend test 75 Table 21 Sample identification table for coupons used in the scribe test 76 Table 22 Weight loss table for coupons used in the scribe test 76 Table 23 Sample identification table for coupons used in the humidity test 77 Table 24 Weight loss table for coupons used in the humidity test 77 Table 25 EDS analysis of the black spots (wt%) 78 Table 26 Corrosion rate calculated with DC Potentiodynamic method 79 xi

13 List of Figures Figure 1 Iron-zinc equilibrium diagram...81 Figure 2 Microstructure of zinc coating formed after 5 mins in a 450 C zinc-bath...82 Figure 3 A schematic representation of Fe-Zn phase layer formation in zinc galvanizing bath, to corresponds to zero time and t1<t2<t3<t Figure 4 Al-Zn system...84 Figure 5 Primary deposition areas in the zinc corner...85 Figure 6 Zn-rich corner of the 460 C isothermal section of the Fe-Zn-Al phase diagram...86 Figure 7 Ternary Fe-Al-Zn phase diagram at 575 C...87 Figure 8 Growth Kinetics of Fe 2 Al 5 layer at 470 C for various bath Al content...88 Figure 9 Effects of bath Al content and alloying temperature on incubation period...88 Figure 10 Schematic diagram showing the Fe-Zn outburst growth behavior...89 Figure 11 Schematic of the Fe-Zn phase layer formation in 0.20 wt% Al-Zn galvanizing bath. to corresponds to zero time, and development occurs according to time such that to<t1<t2<t3<t Figure 12 Ternary Fe-Al-Si phase diagram at 600 C...91 Figure 13 Corrosion losses with time on atmospheric exposure for Al, Al-Zn and Zn hot dip coatings on steel...92 Figure 14 SEM image of the intermetallics found in top dross T Figure 15 Microstructure of the Zn23Al1Si alloy ingot showing black Si particles embedded in the Zn23Al matrix...93 Figure 16 Microstructure of the Zn23Al alloy ingot showing various phases of Zn and Al..93 Figure 17 Hot rolled steel dipped in 0.4% Si bath at 600 C for 5 min Mag 2000X...94 Figure 18 Hot rolled steel dipped in 0.4% Si bath at 575 C for 5 min Mag 2000X...94 Figure 19 Hot rolled steel dipped in 0.4% Si bath at 550 C for 5 min Mag 2000X...95 Figure 20 Hot rolled steel dipped in 0.75% Si bath at 575 C for 1 min Mag 2000X...95 Figure 21 Hot rolled steel dipped in 0.75% Si bath at 575 C for 5 min Mag 2000X...96 Figure 22 SEM image of the top surface of the Zn23Al coating dipped at 0.175% Si in the bath at 575±5 C (1000X)...96 xii

14 Figure 23 SEM images of cross sections of the coatings dipped at 0.175% Si in the bath with varying immersion time...97 Figure 24 SEM images of cross sections of the coatings dipped at 0.4% Si in the bath with varying bath temperature. Immersion time kept constant at 1 min Figure 25 Hot rolled steel dipped in 0.75% Si bath at 575 C for 1 min Mag 1000X...99 Figure 26 Hot rolled steel dipped in 0.75% Si bath at 575 C for 5 min Mag 1000X...99 Figure 27 Steel No. 2 dipped in 0.4% Si bath at 575 C for 5 min Mag 2000X Figure 28 Steel No. 3 dipped in 0.4% Si bath at 575 C for 5 min Mag 2000X Figure 29 Steel No. 4 dipped in 0.4% Si bath at 575 C for 5 min Mag 2000X Figure 30 Steel No. 7 dipped in 0.4% Si bath at 575 C for 5 min Mag 2000X Figure 31 Steel No. 7 dipped in 0.4% Si bath at 575 C for 5 min Mag 1000X Figure 32 Steel No.10 dipped in 0.4% Si bath at 575 C for 5 min Mag 1000X Figure 33 Steel No.10 dipped in 0.4% Si bath at 575 C for 5 min Mag 250X Figure 34 Steel No. 3 dipped in 0.4% Si bath at 575 C for 5 min Mag 1000X Figure 35 Steel No. 4 dipped in 0.4% Si bath at 575 C for 5 min Mag 250X Figure 36 SEM images of the samples from the bath surface (top) showing black oxides and from the middle portion of the bath (bottom) Figure 37 SEM images of the top surface of the coatings dipped at different temperature..106 Figure 38 The coupons showing black spots (burnt flux residues) Figure 39 SEM image of the top surface of the coating showing the black spot Figure 40 SEM images of the bent Zn23Al-coated coupons showing the edges in tension and compression Figure 41 SEM images of the bent Zn5Al-coated coupons showing the edges in tension and compression Figure 42 Figure showing the Zn5Al-coated (top) and Zn23Al-coated (bottom) coupons before the salt spray test Figure 43 Figure showing the Zn5Al-coated (left) and Zn23Al-coated (right) coupons after 1000 hrs in the salt spray test chamber xiii

15 Figure 44 Zn5Al-coated and Zn23Al-coated coupons after 1000 hrs in the salt spray chamber (left) and 3 weeks after they were removed from the salt spray chamber (right)..110 Figure 45 Zn23Al coated samples after 2000 Hrs in the salt spray chamber at Ecosil Technologies Inc Figure 46 Zn5Al and Zn23Al-coated coupons before (left) and after (right) weight loss study in the salt spray test Figure 47 Bent Zn5Al, galvanized and Zn23Al-coated coupons after the salt spray test Figure 48 Figure showing galvanized and Zn23Al-coated coupons before the salt spray test, immediately after the removal from the salt spray test and after cleaning the corrosion products Figure 49 Zn5Al and Zn23Al-coated samples after 48 days in the humidity test chamber..115 Figure 50 DC Polarization curves for a Zn23Al coated and a commercial HDG sample xiv

16 List of Appendices Appendix 1 Galvanizing laboratory (30 kg crucible) 117 Appendix 2 Galvanizing shop (500 kg crucible) 119 Appendix 3 Binary phase diagram of ZnCl 2 -NH 4 Cl 120 Appendix 4 Coating on an I-beam 121 xv

17 1. INTRODUCTION 1.1 OVERVIEW Steel is the most favored and widely used metal in our day-to-day life because of its excellent strength, formability and economics of production. However, the useful life of the steel components is greatly threatened because of its susceptibility to corrosion. For steel to be the building material of choice, a lot depends on prolonging its life by a protective coating. One of the most important commercially processing techniques used to enhance its life is by coating it with zinc or zinc alloys. This process is called galvanizing. Galvanizing is predominantly used to improve the atmospheric corrosion of steel by two methods, barrier protection and galvanic protection. The zinc coating, which provides a barrier between the steel and the corrosion environment, corrodes before corrosion can attack the steel. In galvanic protection, zinc is less noble or anodic to iron at ambient conditions, and therefore sacrificially corrodes to protect the substrate steel, even if some of the steel is exposed at cut edges or scratches in the coating. Galvanized zinc coatings applied by hot dip continuous or batch processes in molten zinc or zinc alloy baths have been used for many years to protect steel from atmospheric corrosion. Atmospheric CO 2 and moisture transform the non-protective zinc oxide or hydroxide into protective basic zinc carbonate 4ZnO.CO 2.4H 2 O [1], so that the zinc dissolution slows down. The reaction takes place in two stages, the zinc oxide forming first and later reacting with the dissolved CO 2 to form the basic salt. Hot-dipped coatings are very adherent, because a metallurgical bond forms between the steel and the coating at high temperatures. The anatomy of zinc-coated steel parts consists of (1) the overlay or coating alloy, (2) an interfacial layer between the overlay and the substrate steel containing a series of 1

18 intermetallic compounds, and (3) the substrate steel. These inter-metallic layers are brittle, and therefore detrimental to the formability of the sheets, however formability is not a major issue for pre-formed parts. The excess formation of the intermetallic layer is also undesirable as it affects the surface appearance of the coating. The growth of the inter-metallic layer is controlled with small addition of aluminum in the galvanizing bath, which has a higher affinity for iron. Fe and Al form their own inter-metallic compounds, which, if not controlled, gives rise to undesirable surface appearance [2]. Galvanizing is the generic term for any of several techniques for applying thin coatings of zinc (Zn) to iron (Fe) or steel stock or finished products to protect the ferrous base metal from corrosion. Galvanizing has immense techno-economic impact; a rough estimate puts corrosion losses at about 4 % of GDP of an industrialized nation. Enhancing life of galvanized products results in reduction in maintenance and replacement costs, thereby large savings in energy. There is continual effort to further enhance the useful life of galvanized products, and thus this project was aimed at development of a new composition for galvanizing. The hot-dip galvanizing is classified into (a) continuous process, where the steel strip moves uninterruptedly through the pretreatment, galvanizing, and the post treatment sections, and (b) batch process, where the preformed steel sections are treated a step at a time. The pretreatment, in either case, involves cleaning the steel surface of oil, grease, dirt and oxides so that the ferrous surface could react well with the molten bath. The continuous process has an edge over the batch process in pretreatment of the substrate for two principal reasons: (a) the reducing atmosphere in the furnace makes the steel very clean and protects it from environmental contaminations, and 2

19 (b) the preheating of the substrate to a temperature closer to that of the galvanizing bath, which makes the steel very reactive and yields excellent coatings in very short dipping times. On the other hand, the batch process involves frequent handling in natural oxidizing atmosphere, before as well as after galvanizing, and is thus vulnerable to environmental contamination. In order to protect the steel surface from environmental contamination, an additional step called fluxing is mandatory in the batch process. The process of fluxing and its composition is extremely important, for it should be compatible with the galvanizing bath without taking part in the reactions. The flux has been a major impediment in development of new bath chemistries. Lately, researchers have shown a lot of interest in the role of aluminum (Al) in a Zn galvanizing bath. The synergy of the excellent properties of Zn and Al has already been very well exploited by continuous galvanizers in commercial products like Galvalume (55 wt% Al) and Galfan (5 wt% Al), which exhibit improved corrosion properties. Even at low concentrations, Al is known to inhibit the development of Fe-Zn inter-metallic compounds due to the very rapid formation of an inhibition layer (Fe 2 Al 5 ) which acts as a diffusion barrier for Fe toward the bath and Zn towards the substrate. However, the efficiency of this inhibition is strongly dependent on the steel and bath compositions as well as galvanizing conditions. The nature of the inhibition layer, the deposition mechanisms and the Al level necessary for the inhibition are poorly understood, and is still a matter of debate. The sequence and kinetics of phase transformations at the steelliquid zinc interface needs to be appreciated in order to control the quality of galvanized coatings, and to develop new galvanizing alloys. In recent times there has been a growing environmental concern in Europe regarding leaching of Zn into the soil, which is likely to call for imposition of restrictions on the rate of leaching of Zn into the environment. The rate of leaching, which is a result of atmospheric corrosion and 3

20 weathering, can be controlled if the coating has greater resistance to corrosion. Research has shown that addition of Al in varying amounts not only reduces the rate of leaching of Zn by providing an excellent barrier protection but also suppresses the formation and growth of the brittle Fe-Zn inter-metallic compounds. There has not been any satisfactory development in the batch galvanizing process in spite of world-wide efforts. One of the biggest impediment being the incompatibility of the conventional zinc-ammonium-chloride flux with Al containing Znbaths. In order to address the environmental concern regarding Zn-leaching and to improve formability of galvanized steel, a research project for development of a new galvanizing alloy was initiated at the University of Cincinnati in 1997, funded by Weert Groep in The Netherlands. As a first step, a novel copper (Cu) based flux was developed which was found to be compatible with Zn- Al baths as well as with Zn-baths [47,48]. The basic alloy composition was arrived at by the following considerations: (a) Al as the alloying element for its well known corrosion resistance properties and for its ability to suppress the formation of brittle Fe-Zn inter-metallic compounds, (b) a eutectoid composition, which yields a structure composed of alternate harder and softer phases enhancing its toughness and formability. Thus, Zn-22.3 wt% Al eutectoid composition emerged as the candidate alloy. Ranjan researched on the Zn-22.3 wt % alloy [3] and found that the galvanized coatings obtained from the binary Zn-Al eutectoid bath were an order thicker than the commercially acceptable norm of approximately 80 µm. Besides being too thick, the coating surface was rough. He also found that the coatings showed a lot of trapped porosities and outbursting, which is caused by the discontinuities on the coating surface occurring due to the violent reactions between Fe and Zn. This can be attributed to the inadequacy or the breakdown of the inhibition effect of the Fe-Al 4

21 inter-metallic layer, known to form at the interface in Al-containing Zn-bath. The influences of ternary additions such as bismuth (Bi), rare-earth (RE) and silicon (Si) into a Zn-22.3 wt% Al eutectoid bath on coating quality with respect to thickness, structure and corrosion properties were investigated. The ternary additions of Bi and RE in the Zn-Al eutectoid bath were made in order to control the growth rate, but with limited success. The coating thickness remained an order thicker than desired. Burstings at the interface and porosities in the intermediate layer could still be noticed. Ternary addition of wt% Si in the Zn-Al eutectoid bath had beneficial influence on the growth rate and morphology of the coatings. A coating as thin as µm could be achieved, resulting from diffusion-controlled parabolic growth rate. The corrosion resistance of this alloy was atleast 8 times higher than the convention HDG alloys. The porosities in the coatings were totally eliminated but the outbursts, although greatly reduced, were still a cause of concern. However, the Si-containing Zn-Al eutectoid bath emerged by far as the most promising alloy for hot-dip galvanizing of steel. Ranjan also discussed a) characterization of the coatings from the alloy (Si containing bath), (c) growth and reaction mechanisms during galvanizing with Si containing bath, and (d) role of Si in controlling the reaction rate and also in providing an adherent coating. The motivation for this work was to further understand the mechanism of the forming of the alloy and to completely eliminate the outbursts observed on the coating surface. 5

22 1.2 LITERATURE REVIEW The following review is intended to present a preview of the structure and phases formed in the conventional zinc-galvanized coating, and the influence of aluminum addition in zinc galvanizing bath on changes in the structure and properties of the coating Zinc-iron reaction The binary Fe-Zn equilibrium diagram shows a series of intermetallics [4] towards the Zn-corner in equilibrium with liquid Zn and solid Fe, viz. ζ (zeta-fezn 13 ), δ (delta-fezn 7 ), Γ (gamma prime-fezn 4 ), and Γ (gamma-fe 3 Zn 10 ), with Fe content increasing from ζ through Γ phases (Figure 1). Table 1 summarizes the phases observed in the relevant portion of the binary Fe-Zn phase diagram. The Γ, δ and ζ phases are formed by peritectic reactions between liquid and solid phases, and the δ 1 phase is formed by the transformation of the δ phase. The three compounds which are very rich in iron appear to be formed without any particular difficulty; whereas the ζ phase is formed only very slowly with considerable undercooling. A typical morphology found for pure zinc hot-dipped coatings is shown in the Figure 2 [5]. The gamma (Γ + Γ) phases appear as a thin layer with a planar interface between the steel substrate and the delta phase layer. The delta phase has a columnar morphology as a result of a preferred growth perpendicular to the interface in a direction along the (0001) basal plane of the hexagonal structure [6]. As the immersion time increases, cracks form along this basal plane of the delta phase layer that can extend into the ζ phase layer above and the Γ phase layer below. The zeta phase has two layers depending upon the super-saturation of Fe in the melt. Adjacent to the delta phase layer, the zeta phase grows in a columnar morphology that is supersaturated in Fe. Continued growth of these crystals occurs rather than the formation of new zeta phase 6

23 crystals. However, if the zinc melt is supersaturated with iron, and there is sufficient nucleation of new crystals, numerous tiny zeta crystals can form in the melt, that are separated from each other by the solidified zinc ζ phase [7]. When a piece of iron is immersed into molten zinc the course of reactions between iron and zinc depends on the formation and growth of the alloy layers. At very short reaction time a finegrained ζ layer and a very thin δ 1 layer are formed. If the reaction proceeds further, the δ 1 layer grows very rapidly, and after a few minutes a thin Γ layer below the δ 1 layer is observed. This indicates that during the reaction, the ζ layer is formed first, then the δ 1 layer, and finally the Γ layer [8]. All these phases are formed in layers. A schematic representation as shown in Figure 3 of Fe-Zn phase layer formation with time in a zinc galvanizing bath has been suggested [5] Zinc aluminum reactions In the continuous galvanizing process, aluminum is added to the zinc bath to improve corrosion resistance by either allowing for the formation of a pure Zn overlay for galvanic protection by inhibiting the formation of Fe-Zn phases or by introducing multiphase microstructures in the overlay coating. In each case the resulting microstructures produce coatings that offer corrosion resistance by a combination of galvanic and barrier protection with corrosion products that in some cases are passive. The solubility of Zn in the Al solid solution (Figure 4) increases with temperature to 31.6 wt% at the eutectoid temperature (548 K) and 82.8 wt% at the eutectic temperature (655 K). The Zn terminal solid solution, ζ, is HCP with a maximum Al solubility of 1.1 wt% at the eutectic temperature, decreasing to 0.6 wt% Al at the eutectoid temperature, and 0.03 wt% Al at room temperature. The eutectoid reaction at the eutectoid point of 77.7 wt% Zn causes the ß phase decomposition at 548 K to produce an aluminum solid solution containing 31.5 wt % Zn, and a Zn-rich phase containing 0.6 wt% Al in solid solution [9]. The viscosity of 7

24 liquid alloys at constant superheating increases proportionally with zinc and it is 25-30% higher than that for Al at 30 wt% Zn. The plasticity of Al is decreased by Zn additions, but the alloys above 60 wt% Zn show super-plasticity [10-12]. The alloy with 77.7 wt% zinc, yielding eutectoid composition, is the focus of attention of this project Zinc Aluminum Iron system Of all the elements that are present in hot-dip galvanizing baths or are added to them with a definite purpose, aluminum has the greatest effect on the course of the reactions between iron and zinc. The course of reactions between iron and molten zinc containing aluminum has been investigated extensively, especially as galvanized coatings that are deformable can be produced only in this manner. As a result of stronger affinity of Fe for Al and the relatively high melting points of Fe and Fe-Al compounds, the regions in which these types of crystals separate out primarily from the melt are moved far into the Zn corner of this ternary system. This is shown in Figure 5, which reproduces the projection of these primary deposition areas on the concentration triangle in the zinc corner [13]. It is seen that the primary deposition region of the Γ phase is marked off by that of α-iron, and the primary deposition region of the δ 1 phase is marked off by that of the δ phase, so that at definite concentration regions, the primary deposition areas of iron and δ phase are in contact with those of the δ phase and the ζ phase. This means that phases, which are not in equilibrium with one another in the binary Fe-Zn systems, are brought into equilibrium by the addition of Al. It is researched that apart from the higher affinity of iron for aluminum, the equilibrium is determined by the fact that the α-iron mixed crystal can dissolve considerable quantities of Zn and Al, while the Al-Fe compounds Fe 2 Al 5 and FeAl 3 can dissolve large percentages of Zn in the 8

25 solid state [14]. At the same time, the solubility for Zn in the Fe-Al compound FeAl 2 and the solubility for Al in the Γ and ζ phases are low. Moreover, the δ phase which occurs in the binary Fe-Zn system only at higher temperatures, i.e. 530 C-685 C, is so strongly stabilized by Al that it is stable as Al-containing Fe-Zn compound even at the galvanizing temperatures of 450 C. Proceeding from the border system Fe-Zn, it is observed that there exists the two-phase equilibrium αfe-γ, Γ-δ 1, δ-ζ and ζ-melt between the homogenous phases αfe, Γ, δ 1, ζ which are stable at 500 C and the Fe-saturated Zn melt. By the addition of Al, beyond certain definite limits, they change over, via three-phase equilibrium, into the two-phase equilibrium αfe-γ, Γ- δ, δ- δ 1 and δ 1 -Zn melt and the ζ phase is eliminated. At still higher Al contents, after passing through other three-phase regions, the equilibrium αfeδ, Fe 2 Al 5 -δ and δ-zn melt are established. If the Al content is further increased, a two-phase equilibria between Fe 2 Al 5 and the melt is formed. With further increase in Al, this changes via three-phase equilibrium Fe 2 Al 5 FeAl 3 Zn melt, into the two-phase equilibrium FeAl 3 melt. At very high Al contents, there is a three-phase equilibrium between FeAl3, melt and a Zncontaining Al mixed crystal δ, which finally changes over in the Al corner into the two phase equilibrium FeAl 3 δ. Besides these equilibria, there exists, on the side of the Fe-Al alloys, twophase and three-phase equilibrium between: (a) α and phases α 1 and α 2, (b) α 1, α 2 and FeAl 2, (c) α, FeAl 2 and Fe 2 Al 5, (e) FeAl 2 and Fe 2 Al 5. From these equilibria, the two-phase equilibrium δ 1 melt and the neighboring three phase equilibrium vanish at 485 C, and a new two-phase equilibrium is formed between δ and ζ. At still lower temperature, there is also equilibrium between the individual Al-Zn phases and FeAl 3. 9

26 Most of the equilibrium lines proceed towards the zinc-rich corner of this ternary system. Therefore, in Zn-rich alloys, very slight Al additions are sufficient for the establishment of equilibrium between Fe-Al compounds and the Zn melt. This is particularly important for the formation and the structure of the alloy layers formed during the reaction between Fe and Zn melts containing Al. For Zn-Al melts, the following was established at 460 C for the Zn corner of the Zn-Fe-Al ternary equilibrium phase diagram [15, 16] (Figure 6): When the bath Al content is less than 0.10 wt%, the phase in equilibrium with the liquid is the ζ phase. When the bath Al content is between 0.10 and 0.14 wt%, the phase in equilibrium with the liquid is the δ phase. When the bath Al content is greater than 0.14 wt%, the phase in equilibrium with the liquid is the Fe 2-x Al 5 Zn x (η) phase. The isothermal section of the ternary Fe-Al-Zn system at 575 C (Figure 7) shows the presence of Fe-Zn intermetallics with limited solubility for Al towards the Zn-end and the presence of Fe-Al intermetallics with limited solubility of Zn towards the Al-end of the diagram [5]. The FeAl 3 phase can accommodate nearly 7 at% Zn, and the Fe2Al5 phase has maximum solubility close to 10 at% Zn. This means that excess of Zn in the coating produced during galvanizing would be rejected if a Fe-Al phase forms on the substrate a) Low-Al addition to galvanizing baths (<1%) Aluminum is the most important alloying element added to the hot-dip galvanizing bath, with different levels required in order to produce different properties in the bath. These additions are made to, (a) improve the luster of the coating, (b) reduce oxidation of the Zn-bath, and (c) to 10

27 obtain a ductile coating by suppressing the formation of brittle Fe-Zn phases. Very low levels of wt% are added to brighten the initial coating surface. The effect is related to the formation of continuous Al 2 O 3 layer on the coating surface that inhibits further oxidation by acting as a protective barrier layer. This effect is also responsible for reduced atmospheric oxidation of the zinc bath. In practice, wt% Al is added to the Zn galvanizing bath to suppress the brittle Fe-Zn inter-metallic compounds from forming at the steel coating interface by forming the Fe 2-x Al 5 Zn x inhibition layer. Inhibition is transitory, and results in an incubation that can increase (a) with increasing Al content in the bath, (2) using low bath temperature, (c) having low bath Fe content, (d) increasing agitation and (e) increasing the presence of Si in the steel [17]. Effect of Al on inhibition of the Fe-Zn reaction The formation of Fe-Zn compounds is inhibited by the formation of a continuous layer of the Fe- Al phase that is in equilibrium with the Al containing Zn-bath. Depending upon the Al content of the Zn bath, this phase can be a ζ, δ, or η (Fe 2 Al 5 ) layer [18]. The origin of the potential inhibition layer compound depends upon the Al concentration in the bath, and it can be seen from Figure 7 that the minimum Al content necessary for a full inhibition effect by Fe 2-x Al 5 (Zn x ) (η) is approximately 0.15 wt % Al at 450 C. The amount of Al in the Al- Fe alloy layer increases continuously with the Al bath content [19,20] (Figure 8). Thus, the incubation period (i.e., the time for Fe-Zn phases to form) increases with increasing Al content in the bath and it increases with decreasing bath temperature [21] (Figure 9). The formation of the inhibition layer is a two-stage process [22]. The first stage, associated with a high rate of Al uptake at the coating/substrate interface, is controlled by continuous nucleation of Fe2Al5, followed by a second stage diffusion-controlled growth process. During the initial 11

28 growth period (<1 s), the lower layer colonies grow to meet each other forming a compact layer. This very rapid interface-controlled reaction probably exhibits linear kinetics. The second growth period (>2s) corresponds to the formation of the upper, coarse crystal layer. The slower kinetics may reflect control by solid state diffusion of Fe through the already formed lower layer. Increasing the Al content or decreasing the bath temperature increases the stability of the Fe-Al inhibition layer. In baths containing more than 0.15 wt% effective Al, a continuous Fe-Al inhibition layer forms upon immersion. The Fe-Al layer was found to be essentially the orthorhombic (η) Fe 2-x Al 5 Zn x containing approximately 23 wt % Zn [23]. Breakdown of inhibition layer Al in small quantities (< 0.14 wt%) delays the Fe-Zn reaction rather than suppressing it completely, and eventually Fe-Zn outbursts form [24]. Even thick layers of Fe 2 Al 5 are destroyed after sufficiently long dipping in high Al-baths. Outbursts have been shown to nucleate at the substrate grain boundaries (Figure 10). One of the models proposed that inhibition layer breakdown occurs by Zn diffusion down short circuit paths in the Fe 2 Al 5 inhibition layer [23,25]. When Zn reaches the substrate, it reacts with Fe, nucleating Fe-Zn intermetallic phases at the Fe 2 Al 5 /subtrate interface that bursts the layer apart into the surrounding bath. Short-circuit paths through the Fe 2 Al 5 are expected to be the grain boundaries of the pancake structure layer or interfaces between the surface particles and the layer. It was proposed by M Guttmann, that Fe 2 Al 5 diffusion short circuits would coincide with emerging substrate grain boundaries, and hence, the effect of steel solute additions on ferrite grain texture and grain size can affect the rate of inhibition layer breakdown or outburst formation [25]. A finer grain size substrate will be more reactive, since more grain boundary area is available for reaction with the liquid Zn-bath on 12

29 a fine grain size surface resulting in more rapid Fe-Zn phase growth [26]. Increased grain size significantly retarded incubation time and promoted Fe 2 Al 5 inhibition layer stability. Reactivity at grain boundaries of the substrate depends upon the ability of the solute element to segregate to these sites. These alloying additions can be separated into elements that segregate to the grain boundary (e.g., C, P), and those that will form compounds (e.g., Ti, Nb) that will precipitate throughout the grain, leaving the grain boundary pure or clean. Clean grain boundaries will have no barrier to Fe-Zn compound formation, while segregated grain boundaries will reduce the thermodynamic activity at these sites, improving stability of the inhibition layer. The substrate grain boundary cleanliness is the dominant steel substrate structural feature that controls the kinetics of Fe-Zn alloy phase formation in Al-containing zincbaths. In carbon steels, as carbon segregates to the grain boundary [27], there is improvement in stability of the inhibition layer. Similarly P retards the growth of Fe-Zn phases, as P was found to segregate to ferrite grain boundaries in re-phosphorized low carbon steels, blocking the diffusion of Zn along the grain boundaries and lowering the thermodynamic activity [28, 29]. Interstitial free steels can contain additions of Ti and/or Nb at extremely low carbon levels. Ti-IF steels are more sensitive to outburst formation than Al-killed or even Nb-Ti steels [26], because these steels are more likely to form carbide, nitride, sulfide and phosphide precipitates in the grains, preventing segregation to the grain boundaries. The Fe-Zn phase formation for an IF steel in a Zn-bath containing 0.20 wt% Al follows the sequence outlined in the Figure 11 [30]. The δ phase forms first, followed by, the Γ phase. However, the ζ phase does not form with this bath composition. 13

30 1.2.3 b) Zn bath containing 5 wt% Al (Zn-Al eutectic alloy) A galvanizing bath of Zn-Al eutectic (5 wt% Al) with minor addition (0.05 wt%) of misch metal is used for the Galfan coating, with the objective of developing a zinc-rich coating without inter-metallics at the coating/substrate interface for improved formability [31]. For short dipping times at 450 C, no reaction was found to occur between Zn and Fe in baths containing 5-10 wt % Al. The inhibition layer was found to be rich in Al. The breakdown of the initial inhibiting layer by the formation of outbursts was first detected at 450 C for dipping times of 8 and 4 s in Zn3Al and Zn6Al baths, respectively [32]. It was suggested that the inter-metallic layer was Fe 2 Al 5 or a composite layer of Fe 2 Al 5 /FeZn 7 /FeZn 13 or two separate Fe-Al-Zn phases exist [33-34]. The initial formation of intermetallic phases after inhibition layer breakdown at 450 C is a characteristic outburst at the substrate/melt interface consisting mainly of the Fe 2-x Al 5 Zn x phase, with the outer part next to the eutectic alloy coating being Fe 1-x Al 3 Zn x [35]. Addition of misch metal is made to improve the wettability and fluidity of the molten bath without affecting the corrosion resistance of the coating. Misch metal ( wt%) delays the formation of a detectable inhibition layer at the interface from 4 s to approximately 64 s [36]. The rare earth elements segregate at the coating/substrate steel interface, thereby playing a major role in the stabilization of the interfacial layer. With increase in dipping temperature in the range of C the rates of intermetallic formation and growth were found to increase markedly in a bath with wt% misch metal [37], and growth was attributed to the formation of a Fe 3 Al porous layer [38] c) Zn bath containing wt% Al Aluminum at 55 wt% is added to the Zn-bath for Galvalume coatings on steel sheets [39]. The interfacial layers exhibit two morphologies. In the first morphology, the interfacial layers, Fe 2 Al 5 14

31 and FeAl 3, flake off from the iron panel and allow linear attack by the molten bath that is in direct contact with the iron. The reaction proceeds very rapidly, limited only by the diffusion of Al in the liquid Al-Zn bath to the substrate iron surface. In the second morphology the interfacial layers are adherent to the iron substrate. However, Al again diffuses to the iron substrate through liquid channels in the interfacial layers and the reaction proceeds rapidly. In both cases, the forming of Fe 2 Al 5 and FeAl 3 layers is exothermic in nature and consumes the panel in less that 2 min. Inhibition layer never has an opportunity to form. Silicon (1.6 wt%) is added to the bath to suppress the exothermic reaction by forming a solid interfacial layer that acts as a diffusion barrier for the reactive species, which is probably Al [39]. Therefore, in order for a reaction between the bath and the substrate to occur, the Al has to diffuse through the solid phase in the interfacial layer, which is considerably slower than liquid state diffusion. The composition of the interfacial layer of commercial Galvalume was reported to be in one study a single-phase layer containing 55.8 wt % Al, 33.7 wt % Fe, 6.7 wt % Si and 3.8 wt % Zn, and in another study a two phase layer containing 53.1 wt % Al, 25 wt % Fe, 8.9 wt % Si, 13 wt % Zn, and 56.6 wt % Al, 36.7 wt % Fe, 3 wt % Si, 3.7 wt % Zn [40]. As Al diffusion continues with longer dipping times, Fe 2 Al 5 and FeAl 3 phases containing 2-3 wt% Si form and continues to slowly consume the entire coating. Because of the rapid reaction in the bath, the process allows only a few seconds of dipping time, and therefore this material is produced through continuous galvanizing only Silicon in Zn-Al bath Silicon aids in the suppression of intermetallic growth at the steel/coating interface. For Al levels greater than about 18%, a thick intermetallic layer forms, unless Si is also present. Elimination of the thick intermetallic layer has the expected results of reduced thickness, improved adherence 15

32 and formability. It is believed that addition of Si removes Fe from the reacting system, delaying Fe-Zn phase formation such that a continuous, inhibiting Fe-Al phase layer had a chance to form [41]. An alternative explanation is that Si is itself part of the barrier layer and improves its effectiveness as a diffusion barrier by modifying its structure or the kinetics of formation. The ternary Fe-Al-Si system [42] shows the presence of several binary as well as ternary phases (Figure 12). The highlights of the Fe-Al-Si system are that (1) the binary Fe-Si phases would form only at very high concentrations of Si, (2) at the Al-end, the Fe-Al phases, especially FeAl 3 and Fe 2 Al 5 have a solubility of about 6 at% and 4 at% Si respectively, and (3) formation of ternary phases is promoted at Si levels of 12 at% and above. Table 2 summarizes the intermetallic phases formed by the ternary system [43] Pure Aluminum Weight losses by corrosion of hot-dipped pure aluminum coatings are lower than those of both pure zinc and aluminum-zinc coatings (Figure 13) [44]. However, aluminum often provides only barrier protection, especially in neutral solutions or in the atmosphere [45]. Rust spots at coating defects and cut edges appear fairly soon, despite the low overall corrosion losses of the coating. Thus thicker coatings are often required for equivalent protection against rusting. Further, the higher melting aluminum bath anneals the substrate steel and limits its strength Fluxing The fundamental purpose of flux used in batch hot-dip galvanizing includes preserving the reactive pickled steel surface until hot dipping and to enhance the reactivity of the steel during hot dipping. Conventionally zinc ammonium chloride has been in use as flux, and it meets the above requirements. However, it has a long list of shortcomings including fuming, development 16

33 of corrosive vapors, rather short holding time between fluxing and hot-dipping and above all it is incompatible with high aluminum concentrations in the zinc bath as aluminum reacts with the chlorides of the flux to form AlCl 3, which is volatile. The latter has impeded the development of batch process for a high aluminum alloy system. A novel fluxing method based on electroless plating, which would be compatible with the high Al content in the galvanizing bath was developed. Two patents (Fluxing process for galvanization of steel, Patent Number: 6,200,636 and high aluminum galvanized steel, Patent Number: 6,372,296) were applied for and granted to the University of Cincinnati, with the inventor being Dr. W. van Ooij and Prasanna Vijayan. The novel Cu-Sn flux was used in principle, but for comparison sake Cu-Fe flux was also experimented with [46-48]. The Cu/Sn thin metallic film is deposited on the pickled steel surface by a simple immersion technique. The flux bath is a mixture of the respective chloride salts of Cu, Sn and Fe, in certain ratios stabilized in an acid solution maintained at room temperature. This bath deposits a thin, uniform film of metallic copper on the steel. The film also contains about 1 wt% of tin. A stable thin metallic film of either Cu or Sn alone can be obtained and used as fluxes. However, a composite Cu/Sn flux is used to make avail of the synergistic effect that each have on the other during the thin metallic film deposition. When Cu alone is used, the pickled steel article tends to be coated with a thick flaky copper film. The deposition rate is extremely fast even at room temperatures and thick Cu films are obtained instantly. On galvanizing, these films yield poorly galvanized coatings. The coverage is complete with no bare spots, but the surface of the galvanized coating tends to be bristled. When Sn alone is used, the film deposition rate on pickled steel is slow and immersion of up to 5 minutes needs to be done. The presence of Sn in 17

34 the Cu/Sn flux controls the coating deposition rate and thin composite Cu/Sn metallic films are obtained. The amount of Sn in the Cu/Sn flux deposited is about 1 % of the total coating weight. The Cu-Sn film is dried prior to hot-dipping in the Zn-Al alloy bath. This ensures a chloride-free surface, whereby the formation of the volatile AlCl 3 is completely avoided. The Cu/Sn thin coating is very adherent, and prevents any oxidation of the base steel. The conventional triple salt flux melts at 350 C and chemically breaks down to form HCl vapor, which etches the steel, NH 3 and other corrosive and toxic gaseous products. Compared to this there is no fume evolution during hot dipping of steel fluxed with Cu/Sn or Cu/Fe flux. The Cu-Sn and Cu-Fe fluxes also have some disadvantages. Sn stimulates inter-granular corrosion in the steels whereas Cu attacks the steel of the kettle. Both these elements are undesirable in the bath metal and galvanizers have been apprehensive about their use. However, those were the only fluxes that appeared to be working with high Al containing baths Summary The reaction mechanism between solid Fe and Zn-bath containing Al is quite complex. As the solubility of Al in the Fe-Zn intermetallic is very low, a new set of Fe-Al-Zn intermetallics crystallize when the Al level in the bath exceeds 0.14 wt%. These Fe-Al or Fe-Al-Zn phases form a thin layer on the surface of the substrate steel and suppress the formation of Fe-Zn phases. The inhibition effect is transitory and depends on several factors such as bath chemistry, bath temperature and immersion time. Immersion of a steel panel in a Zn-bath containing low levels of Al causes dissolution of Fe and formation of Fe-Al binary phases or Fe-Al-Zn ternary phases, leading to rapid depletion of Al in the vicinity of the substrate surface. In addition to that, diffusion of Al and Zn towards the 18

35 substrate and Fe towards the melt at different rates, makes the bath dynamics extremely transient, therefore, the inhibition characteristics cannot be explained by thermodynamic equilibrium between Fe 2 Al 5 and the melt. The mechanism of formation of the inhibition layer and its subsequent breakdown has been a matter of debate, and are yet to be fully understood. Although, there is general agreement that the inhibition layer is Fe 2 Al 5 or its ternary derivative Fe 2-x Al 5 Zn x, there is difference in opinion on the mechanism of their formation. There are basically two schools of thought; (i) One advocates that a thin compact layer forms first on the substrate very quickly when in contact with melt, followed by a slow growth of the upper coarse layer, which is diffusion controlled. Both layers co-exist; (ii) the other group believes that a thin compact Fe 2 Al 5 layer forms first followed by precipitation of loosely adhering Fe-Al-Zn ternary layer on top of this layer. The ternary phase particles grow at the expense of the primary layer, which may get fully consumed. With time the ternary phase takes the form of FeZnAl 5. The inhibition layer, being transitory, finally gives way to the formation of Fe-Zn intermetallics. The end of inhibition does not mean dissolution of the Fe 2-x Al 5 Zn x phase, since this phase is in equilibrium with the melt. Of the various models proposed, the Zn diffusion mechanism sounds most logical. Zn diffuses from the melt through the inhibition layer enriching the substrate/fe 2- xal 5 Zn x interface with Zn, forming Fe-Zn phases between the substrate and the Fe 2-x Al 5 Zn x layer, which causes the outbursts. Even thick layers of Fe 2-x Al 5 Zn x are destroyed after sufficiently long immersion time in high-al-containing Zn-baths. The Zn diffusion takes place through short circuit paths like grain boundaries or liquid Zn channels. The coating layers in Galfan (5 wt% Al) and Galvalume (55 wt% Al) primarily consist of Fe 2- xal 5 Zn x and Fe 1-x Al 3 Zn x phases. The inhibition effect in these products is not very well understood. Since the presence of Al has been found inadequate in providing the desired 19

36 inhibition, its effectiveness is supplemented with additions of mischmetal and silicon, respectively in the two products. One of the major objectives of Al addition in the Zn galvanizing bath is to obtain improvement in formability of the coatings by suppressing the development of the brittle Fe-Zn intermetallics. As the inhibition layer formed in low Al baths is highly transitory, the influence of a few additives like misch-metal or silicon in Zn-Al bath on the formation of the inhibition layer can be investigated, taking cue from Galfan and Galvalume. Interesting compositions like Zn-Al eutectoid (22.3 wt % Al), are excellent candidates for research in this field. The corrosion properties and formability of coatings developed in a eutectoid composition holds excellent promise. 1.3 OBJECTIVE The objective of this work, with the inputs from the previous work done by Ranjan, was to develop a suitable galvanizing process to suppress outbursting of the galvanized coatings and to produce a commercially acceptable coating quality. Ranjan s work was concentrated on finding a suitable ternary addition to the Zn23Al alloy to improve the quality of the coatings. Ranjan s work revealed that Si when present in about 0.3 wt% in the Zn23Al alloy bath helps to control the thickness of the coatings. However, the coatings showed outbursts on its surface. Dr. W. van Ooij on behalf of the University of Cincinnati and Weert Groep, requested the assistance and sharing of the galvanizing expertise of Teck Cominco Ltd. Product Technology Center (PTC), Canada, in completing the development of this alloy. Following the agreement between the three parties a joint project was started at the PTC from 22 nd June, 2005 until 20 th September, The work tasks included, 20

37 a) baseline evaluation with dipping trials in a laboratory furnace with a general steel and then repeating the experiments with steels having varying levels of Si in it, b) develop and optimize the experimental matrix to assess the effects of pretreatment chemistries, bath temperature, bath composition, immersion time and withdrawal rates, c) scaling up of the developmental work from laboratory sized furnaces to large scale commercial kettles, d) evaluation and understanding of the coating properties. Evaluation and understanding of the coating properties was accomplished by doing in depth characterization of the coatings, produced at PTC, in the Advanced Materials Characterization Center of the University of Cincinnati. 21

38 2. EXPERIMENTAL PROCEDURE Experiments were carried out in two different capacity crucibles 1) 30 kg of zinc 2) 500 kg of zinc 2.1 Laboratory set up Furnace The galvanizing experiments were carried out in an electrically heated crucible furnace having a ceramic lid to cover the opening at the top. The furnace had the following characteristics: Volts = 240, Watts = 2.6 kw, Amp = 11, Phase = 1, Hertz = 50/60 Temperature range = 1200 C Crucible A silicon carbide crucible having the following dimension was used; Height: 180 mm; Top OD: 150 mm; Bilge: 150 mm; Bottom OD: 108 mm; Thermocouple Bath temperature is one of the most important parameters in galvanizing. In order to have a better monitoring and control of the bath temperature, a thermocouple with a ceramic sheath (300 x 6.35 mm) was installed in the furnace with the temperature indicator Sample Insertion Machine (SIM) Ideally, the insertion and retraction of the steel substrate into the bath should be steady and at a predefined rate to obtain a smooth and homogenous coating. In order to eliminate the jerky motion and unsteady speed associated with manual insertion and retraction of the panels, a 22

39 mechanized system was used in the galvanizing lab. The system was mounted on wheels and consisted of motorized insertion and retraction of the panels, with controls for speed of insertion and retraction. However, the 500-kg furnace had only two set of speeds, namely fast and slow. 2.2 Steel panels Cold-rolled steel coupons with the following chemistry were used for the initial trials with the fluxes and some dipping trials. C 0.041%, Si 0.02%, P 0.006%, Mn 0.25% Later, all the experiments were carried out with hot-rolled steel available at Teck Cominco Ltd. with the following chemistry C 0.19%, Si 0.24%, Mn 0.9%, P 0.021%, S 0.003% A set of Si special steels, available at Teck Cominco Ltd., namely Steel no. 2, 3, 4, 7 and 10 were also used for dipping trials. The C content was 0.09% in all the steels. The steel no. 10 had high phosphorus content. The composition of the steels is listed in Table Sample cleaning The steel panels were thoroughly cleaned in three stages to remove any dirt, grease or rust after the identification numbers were engraved. In the first stage, the samples were subjected to acetone cleaning. This was followed by treating them in NaOH (10 wt %) solution at 70 C for minutes. After rinsing with water, the samples were pickled in HCl (10 wt %) solution at room temperature for minutes. The samples were then scrubbed with a wire brush, rinsed with water and subjected to flux treatment. 23

40 2.4 Fluxing During the initial stage of the experimentation 3 different fluxes were tried. A Cu-Fe flux with the following composition (wt %) was used during the first trial. 5% HCl, 2% FeCl2.4H2O, 0.4% CuCl2.2H2O A Cu-Sn flux with the following composition (wt %) was during the second trial. 5% HCl, 5% SnCl2, 0.25% CuCl2.2H2O The flux deposits a thin Cu-Sn metallic film on the pickled steel surface by electro less plating after a simple immersion technique. A third zinc ammonium chloride-based flux developed by Teck Cominco Ltd and Ferrotech Inc., designated the G flux was used. The flux is proprietary to Teck Cominco and Ferrotech Inc. as it has not been patented yet. The cleaned samples were fluxed for a predetermined time, dried in a drying furnace maintained at 100 C before galvanizing. The flux coating protects the cleaned metal surface from oxidation. 2.5 Bath preparation The master alloys available were a) Zn23Al, b) Zn23Al1Si In an effort to get roughly 0.3 % Si in the bath 1 part by volume of Zn23Al1Si and 2 parts by volume of Zn23Al alloy were used to prepare the bath. Initially, the alloy ingots were added in large chunks. Later, addition of the alloy to the bath in small pieces was practiced in order to maintain the bath temperature at a set value, avoiding drastic temperature drops. 24

41 2.6 Analysis of the bath samples and the wet chemical analysis of the coating The Si content in the bath samples was determined in house with the ICP technique. The bath Fe and Al content were also determined in house. The sample was cleaned with acetone and dried thoroughly. The weight of the sample was determined to the nearest 0.1 mg. To dissolve the bath sample or the actual coating on the substrate, the sample was placed in 50 ml of 10% sulfuric acid with 0.5% Activol 1874 as inhibitor. The solution was transferred and rinsed well to a 100 ml volumetric flask with 5% HNO 3 content. In order to dissolve silicon, 1 ml of hydrofluoric acid (HF) was added to the flask with sample solution. To obtain the Al and Si contents in the coating, the stripping solution was analyzed using an atomic absorption method. In order to control ionization interferences, 0.1% potassium as potassium chloride was used for preparation of aluminum samples and corresponding standard solutions. 5% nitric acid content was maintained in all original and diluted samples. The sample was rinsed with acetone, dried, weighed and the coating weight was calculated. The analysis was carried out using the parameters listed in Table Bottom sample A sample from the bottom of the 30-kg crucible was drawn with the help of a glass pipette to analyze for any bottom dross present and its analysis. The bottom sample was analyzed for Si and Fe. The bottom sample was mounted and polished for metallographic examination. 2.8 Top dross Some mushy stuff was found floating on the bath surface whenever a new ingot was melted. This mushy stuff is called top dross. A sample of this top dross was mounted, polished and analyzed for metallographic examination. 25

42 2.9 Ingot analysis The master alloy ingots were cut, mounted and polished for metallographic analysis. The master alloy ingot samples were analyzed for their Fe, Si and Al content by wet chemical analysis Galvanizing in the laboratory (30-kg crucible) The fluxed steel panels were dipped in the galvanizing bath with the help of the Sample Insertion Machine (Appendix 1). The parameters that were controlled and varied for galvanizing were the bath composition, bath temperature and the dipping time. The speed of insertion and retraction was kept constant at 1 cm/s. After retraction the panels were allowed to cool in air Galvanizing in the shop (500-kg crucible) A bath of 400 kg was made in the bigger crucible (Appendix 2). Steels of various sizes, shapes and chemistries were dipped in this bath Coating thickness measurement Coating thickness was determined at ten locations on either face of the panel. The average of all the twenty readings is reported Microstructure investigation: scanning electron microscopy Diamond cutters were used to cut the samples. The samples were mounted with a conductive carbon-carbon moulding compound. The samples were polished using silicon carbide abrasive papers and finally subjected to cloth polishing using diamond paste. The cross sections were etched in Nital and then inspected in a Scanning Electron Microscope equipped with Energy-Dispersive X-Ray analysis. Both a JEOL JSM 5800LV equipped with an EDX type PGT PRISM instrument at PTC and a Philips ESEM model XL-30 microscope at the 26

43 University of Cincinnati were used. Of some samples secondary electron images of the surface were also taken. Energy Dispersive Spectroscopy (EDS) analysis was done at various locations on the coating of the samples Bend test The Zn23Al alloy is known to be super plastic [10-12]. To test the deformability of the coating structure, panels of the Zn23Al-coated and Zn5Al-coated coupons were bent in a vise over 180º. In order to check on the deformability of the Zn23Al0.3Si coating further, cross sections were prepared of the 180 º-bent panels, which were then analyzed in the electron microscope Hardness determination The Vickers hardness number (VHN) of the distinct layers of the coatings was determined in a LECO M-400-H1 hardness testing machine, where a load of 10 gm was applied for 10 s. The hardness was checked on the steel, the drag-out layer and the alloy layer. The alloy layer had two phases visible and there was a hardness difference among both phases. An average of 5 such measurements in each coating layer is been presented here. A 500 gm load was used for the hardness test done on the larger bath samples Salt spray test The method used was ASTM B117 salt spray test, which exposes samples to a continuous fog of a 5% sodium chloride solution at 35 C. The samples were compared with Zn5Al-coated coupons. The following types of tests were conducted in the salt spray chamber; 1) Exposure until a limited number of hours and analyzing the weight loss 2) Exposure till the formation of red rust 27

44 3) Exposure till the formation of red rust in the salt spray chamber at Ecosil Technologies LLC, Cincinnati, OH. 4) Exposure of samples with scribe marks compared with normal galvanized coatings to determine the cathodic protection characteristics of the Zn23Al alloy. 5) Exposure of the bent samples compared against bent Zn5Al-coated samples to study the corrosion behavior of the stressed alloy coatings. The tests would also confirm if there were any major cracks in the alloy coating developed due to the severe bending of the coupons Humidity test The samples were tested as per ASTM D2247. The Zn23Al coated coupons were compared with Zn5Al-coated coupons in the humidity test. The samples were inclined at 15 on a plastic rack. They were placed in a 0.3 m 3 chamber and held at 38 C and 100% humidity. The initial condition of the coupon before inserting in the humidity chamber and the final condition after removing from the test chamber was compared DC Polarization test The electrochemical activity of the surface of the coatings was measured by performing potentiodynamic polarization tests in an aerated 3.5 wt.-% NaCl solution. The potentiostat used was a Gamry CMS100 system equipped with an SR810 Frequency Response Analyzer. The counter electrode was a platinum mesh and a saturated calomel electrode (SCE) was used as the reference electrode. Both cathodic and anodic curves were recorded. It should be noted that the corrosion cell used in these measurements was a so-called flat cell, in which a large panel is clamped against a hole of about 1 cm 2 at one end of the cell. The electrolyte only contacted the sample through this hole. In this set-up any cutting and possible 28

45 delamination effects are avoided. Further, cut samples would have edge effects due to exposed steel at the edges. The IR drop effect, inherent in the use of a flat cell, was automatically compensated by the potentiostat. Along with the Zn23Al0.3Si coatings, controls of standard HDG and delta-galvanized steel (both provided by the Weert Groep on unknown steels), and Zn5Al-coated samples, prepared at PTC, were also tested. It should be pointed out here that the results of these electrochemical tests indicate the initial electrochemical reactivity (dissolution rate in NaCl) only. 29

46 3 RESULTS AND DISCUSSION 3.1 Fluxes Three cold-rolled steel coupons cleaned as per the procedure mentioned above were immersed in the Cu-Fe flux for 1 min, 2 min and 3 min. A characteristic orange red color of Cu was observed on the coupon. The coupons were furnace-dried at 110 C for 10 min. The steel coupons were immersed in the galvanizing bath made of 0.3 % Si (as calculated) maintained at 575 ± 5 C for 1 min and 3 min. However, it was found that there is no reaction between the steel and the galvanizing bath and the coupons had a largely discontinuous, loosely adherent solidified layer of the alloy. Later, cold-rolled steel coupons were immersed into the Cu-Sn flux. The cleaning procedure mentioned above was followed and the same Cu color was observed on the coupons. The steel coupons were immersed in the galvanizing bath maintained at 575 C for 1 min and 3 min. Again, there was no reaction between the steel and the galvanizing bath and the coupons had discontinuous, loosely adherent solidified layer of the alloy. A third Zn-ammonium chloride based flux developed by Teck Cominco Ltd. and Ferrotech Inc., designated the G flux was used. The cleaning procedure was kept constant. The immersion time in the flux was also kept constant. The steel coupons had a black color after they were drawn out of the G flux. The coupons were dried and immersed in the galvanizing bath for 1 min. Continuous coatings were obtained but there were quite a number of outbursts. The coating thickness was observed to be on an average 16 microns. 30

47 In an another trial the steel coupons cleaned as per the procedure mentioned above were immersed first in the Cu-Fe flux for 1 minute and then in the G flux for 1 minute and dried before immersing in the galvanizing bath (hereafter called as double flux). The coupons showed the characteristic orange red color after they were drawn out of the Cu Fe flux and were black in color when they were drawn out from the G flux. This coupon was immersed in the galvanizing bath maintained at 575 C for 1 min. The coatings obtained were smooth, continuous and without any outbursts. The average coating thickness was again 16 microns. However, as the immersion time in the bath was increased to 5 min, 8 min, 10 min, etc., the steel coupons immersed in the double flux as well as the G flux showed outbursts. It was noteworthy that the steel coupons immersed in the double flux showed fewer outbursts than the ones immersed only in the G flux. The same experiments mentioned above were repeated on the hot-rolled steel coupons and the same trend of results was observed with the hot-rolled steel coupons as well Working of the G flux A galvanizing flux retards the oxidation of the base metal during the crucial time between fluxing and galvanizing, thus preventing smaller amounts of iron decomposition products formed and the dross formed from this source. Ideally, a flux should decompose as it enters the galvanizing bath allowing contact between the bath and the reactive substrate. Thus, a galvanizing flux must be stable at the galvanizing temperatures and its properties should be so adjusted that the flux melts and vaporizes, thus facilitating the reaction between the galvanizing bath and the substrate within the dipping time allowed by the galvanizing process. The conventional galvanizing flux consists of zinc chloride, ammonium chloride and other additives to improve the wetting properties of the flux. Appendix 3 shows the phase diagram of Zn chloride and ammonium chloride. Ammonium chloride increases the melting point of the flux 31

48 [49]. Degree Baume is the measure of the specific gravity of the flux and it affects its wettability. The Baume of a conventional flux is in the range of 11-14, which helps attain the optimum wettability. The wetting features of the flux helps in drying the flux quickly to stop excessive sputtering and spitting as the steel enters the molten zinc. The wetting action also helps the molten zinc flow quickly and evenly over the work and problems such as warping of pipes is reduced because of this. The conventional chloride based flux is believed to be incompatible with high Al containing galvanizing baths because of the temperature at which these baths operate. The high temperature in these baths burns the conventional flux which forms a tightly adherent burnt layer on the steel substrate, preventing any reaction between the bath metal and the steel substrate. However, this is not entirely true because the G flux used during the current work revealed that this chloride based flux is functional in the Zn23Al eutectoid alloy. The G flux is modified to increase the stability of the flux so that the flux decomposes at higher temperature and does not burn even at a temperature as high as 600 C. This can be accomplished by increasing the ammonium chloride in the flux, which increases the melting point of the flux. Also, the substrate surface, after fluxing with the G flux, was observed to be wetted uniformly as opposed to the crystalline coat observed when a substrate is fluxed in the conventional flux. This suggests that the G flux is more watery, i.e., it has a lower degree Baume. The water in the flux improves the wetting characteristics of the flux and also its stability at higher temperature. The flux should also be composed of surfactants and additives to maintain the ph and reactivity of the flux. 32

49 3.2 Bath analysis The bath was formulated with the as-calculated Si content of 0.3 %. The bath samples drawn were initially analyzed in a laboratory outside Teck Cominco Ltd. and hence the bath results were offset by about a week. The initial immersion trials were thought to be at a bath Si level close to 0.3 %. A sample from the bottom of the crucible was taken for analysis after approximately 50 dipping trials in the bath. The sample was analyzed for Fe. The Fe content was 0.036%. The bottom sample was polished and investigated by electron microscopy for intermetallics. Black intermetallics were observed in the microscope and subsequent SEM-EDS analysis revealed that Si was tied down in these intermetallics. After approximately 50 dipping trials, a bath sample was taken (called B1). The bath temperature was 575 ± 5 C. After that % Si (as calculated) was added and another bath sample was taken called B2. After adding the alloy ingot to the bath the bath temperature dropped to 510 ± 5 C. The top dross formed after the new alloy addition was removed from the bath. A third bath sample B3 was taken after letting the bath stand for two days. The bath temperature was 550 ± 5 C. The results are summarized in Table 5. The anomaly in the expected results was attributed to the % Si in the bath being a function of bath temperature, i.e., the variation from the expected results is caused by the different solubility limits of the Si in the bath at different temperatures. Further, the effects of oxidation cannot be negated. The depletion of Si through dipping was not considered because no dipping trails were carried out when the bath samples were drawn. To ascertain the effect of temperature on the solubility of Si in the bath, another three samples each 33

50 from the bath and the bottom of the crucible were taken at three different temperatures, shown in Table 6. The results showed that the bath Si was fairly stable over a range of temperatures and that the solubility of Si in the bath did not vary with temperature. 3.3 Top dross The variation in the bath Si in the earlier experiment occurred because the top dross formed after a new addition of alloy ingot was removed from the bath. The top dross sample showed the presence of intermetallics which were found to be complex quaternary intermetallics of Si, Fe, Zn and Al. The composition of these intermetallics is listed in Table 7. The intermetallics had unusually high Fe content and as there were no dipping trials done after the addition of the new alloy ingot, the alloy ingots were the only source of Fe in the bath. Figure 14 shows the microstructure of a sample from the top dross which was mounted, polished and observed by SEM. The intermetallics are seen embedded in the Zn-Al alloy matrix. 3.4 Ingot analysis In order to ascertain the source of Fe in the bath and also to understand the phenomenon of top dross formation, both the master alloy ingots were cut, mounted and polished for metallographic analysis. The alloy ingots were analyzed for their Fe, Si and Al content by wet chemical analysis. The results are summarized in Table 8. The microstructure of the Zn23Al1Si master alloy is shown in the Figure 15 and compared with the microstructure of the Zn23Al master alloy (Figure 16). The microstructure of a sample cut from the Zn23Al1Si ingot shows pure Si particles embedded in the Zn23Al1Si alloy. When this alloy ingot is melted, Fe reacts with Si and Al and forms complex intermetallics which float on the top surface. The composition of the intermetallics was in close accordance with the AlFeSi α phase also known as tau5, with a sublattice model of Al 0.66 Fe

51 Si 0.05 (Al,Si) The density of the dross, calculated from its lattice parameters, is 4.10 g/cm 3 which are slightly lower than the melt density of 4.5 g/cm 3 at 550 C. However, noteworthy to mention the top dross was formed only when a new alloy ingot was added to the bath, i.e., when sufficient Fe was added to the bath. Because of their high melting point, the pure Si particles are present in a solid state in a bath maintained at 600 C and float on the top surface as their density is lower than the bath metal. These intermetallics, Si particles and the aluminum oxide together form the mushy top dross. Once formed, the top dross leads to the loss of Si from the bath. The dipping of steel coupons did not increase the Fe content of the bath and hence there was almost no top dross formation which is an advantage of this alloy. The above experiment led to the conclusion that the alloy ingots should be added to the bath in small pieces so as to avoid the drastic temperature drop. To dissolve more Si, stirring should be carried out during the preparation of a new bath. 3.5 Bath 2 A new bath was formulated with as-calculated 0.46% Si. Bath sample B8, bottom sample D8 and top dross sample T8 were drawn at 575 ± 5 C and analyzed. The results are shown in Table 9. Bath samples B9 from close to the surface and the other sample B10 from the middle portion of the bath were drawn. The samples were polished, etched and observed for metallographic analysis. It was found that the bath sample taken from close to the surface had a large amount of oxides. The bath samples were analyzed for their composition by EDS and it was found that the bath composition was around 22Al0.2Si. 35

52 3.6 Bath 3 A new bath was formulated with bath Si level as-calculated at 0.47%. The Zn23Al1Si alloy was cut into small pieces and added one by one with constant stirring to get as much Si dissolved in the bath as possible. The bath was constantly stirred for 3-4 hours. Bath sample and top dross sample were taken, samples B12 and T12, respectively. 0.1% Si as-calculated was then added to the bath three times in the same manner as mentioned above. The results are summarized in Table 10. Thus, the % Si recovery of this alloy ingot was assumed to be 50 %, the rest of which seemed to be lost in the intermetallics formed as top dross. 3.7 Depletion of Si in bath 3 After 10 dipping trials with 2 min immersion, each at 575 C, 550 C and 600 C, bath samples were drawn, viz., B16, B17 and B18. The immersion time was increased to 5 min and another set of 10 dippings was carried out. A bath sample was drawn at the end of the day for two days and referred to as B19 and B20 respectively. The depletion of Si level in the bath is reported in Table Galvanizing in the 500-kg crucible A bath of 400 kg was prepared. The as calculated bath Si was 0.8 % and thus, assuming 50% recovery, the bath Si was aimed at around 0.4%. However, the bath sample B21 showed the bath Si to be 0.76%. This can be attributed to the fact that the Si % in the Zn23Al1Si alloy ingot is not exactly 1%. Some ingots have less than 1% Si whereas the others have more than 1%. Thus, the addition of a large amount of the alloy ingots would have led to an average Si % in the bath. Thus almost 100 % recovery of Si in the bath was attained in this experiment. 36

53 Hot-rolled steel coupons were dipped in the bath with varying time and temperature and the coupons had a uniform coating without outbursts. 3.9 Dipping of various shapes and sizes of steels (Appendix 4) Two pipes, two large plates, three I-beam sections and three C-channels were dipped into the 500-kg bath at 0.75 % Si in the bath at 575±5 C. The coatings on the pipes and plates turned out smooth and continuous because of their less complex shapes. The I-beam sections and C- channels had a few blind spots and showed burnt flux residues at the blind spots Coating appearance of the Zn23Al-coated coupons dipped at various Si levels Effectively three sets of coupons were obtained. Coupons dipped in the bath with 0.175% Si, coupons dipped in the bath with 0.4% Si and the coupons dipped in the bath with 0.75% Si, referred to as Bath A, Bath B and Bath C, respectively. The coupons dipped in Bath A showed little or no outbursts at 1 min immersion time. However, as the immersion time was increased to 5 min and 10 min, outbursts were observed on the coating surface. The coupons dipped in Bath B and Bath C showed no outbursts at 1 min, 5 min or 10 min immersion in the two baths Coating Structure The surface of the coatings produced at 0.4% Si bath was smooth and generally did not show clear spangles. In the electron microscope, the secondary electron images (SEM) of the surface showed either an interdendritic phase system or two distinct phases, as shown in Figure This two-phase system was typically found for thinner panels that were cooled rapidly or galvanized at lower temperatures. Therefore, the primary phase of this two-phase system is suspected to be the eutectoid composition (Zn/Al = 77/23) Al-rich phase, that is formed upon cooling below 406ºC. The coarser secondary phase is formed from the remaining liquid 37

54 resulting in eutectic phase. This interdendritic phase should disappear at 348ºC, but if the cooling rate is fast, it still exists in a metastable form at the eutectoid temperature of 270ºC. Annealing the parts at temperatures between 275ºC and 350ºC should convert the coating to the eutectoid composition if desired. It is not known if the presence of that Al-rich interdendritic phase is beneficial or not. It could be envisioned that if one needs the ultimate mechanical properties in terms of plasticity, such annealing is warranted, e.g., of small parts, such as fasteners. Figure also depicts SEM images of cross sections taken from samples produced at various operating conditions. Figure 21 shows a coating exhibiting two continuous layers. The thin layer at the metal-coating interface was found to contain measurable levels of Fe, Al, Si and Zn and therefore will be denoted as Fe 2-x-y Al 5 Zn x Si y, with x and y as variables such that x+y <1. This layer is continuous and uniform with a thickness between 5-10 µm. The top layer is also uniform and has approximately the bath composition, so it can be interpreted as the drag-out layer of the bath. Under magnification it becomes apparent that this layer exhibits a distinct lamellar structure. Thus, it must be composed of the two phases into which the eutectoid composition decomposes, initially Zn/Al = 31/68 and 99/1, but, depending on the cooling rate experienced, the Al-rich phase can become richer in Al, as can be concluded from Figure 4. Figure 17, 18 shows a sample in which regions with the eutectoid composition are embedded in another phase, namely the phase seen in Figure 22 from the top. This is an Al-rich inter-dendritic phase which has not been completely converted to the two phases with rather narrow composition which narrows further when approaching the eutectoid, i.e., around Zn/Al = 75/25 and Zn/Al = 80/20 (Figure 4). In the eutectoid phase the lamellar structure is clearly discernable. The Si was found to be distributed throughout the entire coating. However, its level in the Fe 2-xyAl 5 Zn x Si y phase is higher than the overall bath composition. We can, therefore, conclude that, 38

55 indeed, the Si in the bath stabilizes the Fe 2-x-y Al 5 Zn x Si y layer, as the entire coating is devoid of Fe-Zn outbursts. As a result, the coating is of a simple structure, viz., that of a solidified drag-out layer on top of a reaction layer formed by the reaction of steel with the aluminum in the bath. Even in the case of a multiphase drag-out layer, the intermetallic layer remains continuous and does not vary significantly in thickness. Because that layer is formed at the high temperature of the liquid bath and further growth then seems to stop, it is self-limiting and only varies in thickness as a function of the bath temperature. It is therefore suggested that the structure of the Fe 2-x-y Al 5 Zn x Si y layer is not much dependent on the cooling rate, whereas the structure of the drag-out layer is Characterization of the coatings Coatings dipped at % Si level Figure 23 shows the through-thickness cross sections of the coupons immersed for 1 min (identified as 6-2), 5 min (identified as 6-10) and 10 min (identified as 6-12). The microstructure shows two distinct layers in the coating; 1) the inhibition or the intermetallic layer and 2) the top layer also known as the run-off or drag-out layer of the bath metal. The bath temperature was kept constant at 575±5 C. The phase analysis of the coupons in at% is as per Table 12. The coupon dipped for 1 min shows that the intermetallic layer is not completely formed and is still growing. This is confirmed by the EDS analysis of the intermetallic layer which reveals the composition close to the phase T 5 (cubic) reported by Ranjan having the composition of approximately 21at%Fe-68at%Al-3at%Si. [3]. As the immersion time was increased to 5 min for the coupon 6-10 the intermetallic layer i.e. the T 5 phase became enriched with Fe and the T 5 phase at the interface transformed to θ (FeAl 3 or its 39

56 ternary variant) phase, having a composition of 25at%Fe-67at%Al-5at%Si, which is confirmed by the results obtained outlined in Table 12. The interface layer exhibited 2 distinct layers, the one closer to the substrate composed of θ phase while the outer layer continued to have the metastable T 5 phase. In other words, part of the interfacial layer transformed into thermodynamically more stable θ phase. After sufficient Fe had diffused through this layer and with continued enrichment of the interface layer with Fe, the θ phase closest to substrate got transformed into η (Fe 2 Al 5 or its ternary derivative) phase, so that the interface can be comprised of three layers, each with a different phase composition, in the following order: η-θ-t 5. As the immersion time increased to 10 min, the intermetallic layer transformed completely to the η phase (Table 12) Coatings dipped at 0.4% Si level The coupons were dipped at three different temperatures 550, 575 and 600 C. The microstructures (Figure 24) show the through-thickness cross sections of the coatings dipped at the three different temperatures with the immersion time kept constant at 1 min. The phase analysis of the coupons in at% is as per Table 13 and 14. Table 14 shows the results of the EDS analysis of the coupons dipped for 5 min in this bath, done at University of Cincinnati s Advanced Materials Characterization Center. Figures 17, 18, 19 shows the microstructures of the coupons dipped for 5 min. The coupons dipped for 1 min at 550 C show the intermetallic layer composition close to the T 5 phase given the slow kinetics of the reaction at this temperature, whereas the coupons dipped at higher temperature, i.e., 575 C and 600 C shows that the intermetallic layer has transformed to η (Fe 2 Al 5 ) or its ternary derivative. As the bath Si increases the Si in the intermetallic layer also 40

57 increases and hence the phase in equilibrium with the bath at that temperature changes. The intermetallic layer in the coupons immersed for 5 min at 0.4% Si level at 575 C had the following composition at %Al, at %Fe, 8-14 at % Si and 4-6 at % Zn which could be formulated to the phase FeSiAl 5 or to its quaternary derivative as Fe(Si 1-x Zn x )Al 5. Owing to higher Si activity in the bath with reference to the current study the Fe atoms in the intermetallic layer η (Fe 2 Al 5 ) or its quaternary derivative are replaced by the Si atoms so that the equilibrium composition of the phase is FeSiAl 5 or its quaternary derivative. Thus the reaction could be formulated as η (Fe 2 Al 5 ) + Si FeSiAl 5 or Fe 2-x-y Al 5 Zn x Si y + Si Fe(Si 1-x Zn x )Al Coatings dipped at 0.75% Si level The coupons were immersed for 1 min and 5 min in a bath with 0.75 wt % Si at 575 C. Figures 25 and 26 show the microstructures of the coupons immersed for 1 min and 5 min respectively. The microstructures revealed that the intermetallic layer was absolutely uniform, compact and regular unlike any coupon dipped in the baths with lower Si level. Table 15 reveals the compositions of the constituent layers of the coatings. The EDS analysis of the coupons immersed for 1 min revealed that the intermetallic layer had a composition close to θ, i.e., FeAl 3 or its ternary derivative which can be explained on the basis of the lack of time available to transform completely into the thermodynamically stable Fe(Si 1- xzn x )Al 5 phase. With the immersion time increased to 5 min for the other coupon the intermetallic layer becomes enriched with Si and Al from the bath and the composition changes to the equilibrium Fe(Si 1-x Zn x )Al 5 phase. 41

58 3.12 Dipping of the special set of Si steels Three coupons each of special set of steels identified as 2, 3, 4, 7 and 10 were dipped (Table 3). The microstructures of these steels are shown in the figures 27 to 33. The compositions of the various layers observed in the coatings are summarized in Table 16. The intermetallic layer had the composition close to Fe(Si 1-x Zn x )Al 5. The drag-out layer is similar in all cases and is fairly continuous with occasional evidence for the Al-rich interdendritic layer mentioned earlier. This layer does not seem to depend on the steel composition. The steels required thorough cleaning because of the tightly adherent scales on them. The pickling time for these steels during cleaning was increased to 60 min from the normal minutes used for other steels. The steel surface was scrubbed intermittently with stainless steel wire brush during the pickling treatment. The steel substrate before immersing had a very rough and uneven surface after pickling because of the removal of the tight oxide scales which covered them. However, the coatings obtained were smooth, uniform and without outbursts. The average coating thickness after 5 min immersion at 0.4% Si in the bath was 30 microns irrespective of the steel chemistry. There was no effect of the silicon or phosphorus content on the total layer thickness. Since most of the layer is the solidified drag-out layer, such an effect could not be expected. Figures 34 and 35 show the cross section of Steel No 3 and 4 respectively having varying coating thicknesses because of the uneven nature of the substrate. The microstructure revealed a few bare spots where there was no intermetallic layer and the coating was composed of just the solidified bath metal (Figures 31 and 33). This could be because of the trapped burnt flux which sticks to the substrate and prevents any reaction between the substrate and the bath metal or it could be because the high Si or P level blocks the reactivity 42

59 of the steel to the Al in the bath. This was only a local effect and whether it affects the mechanical or corrosion properties of the coatings has not yet been investigated. These results show that the Sandelin effect associated with general galvanizing does not play a role when the Zn23Al0.3Si bath is used. Many qualities of steel can be effectively coated without effect on coating thickness or performance Coating thicknesses of the Zn23Al-coated coupons dipped at various Si levels in the bath Table 17 shows the average coating thickness obtained on hot rolled steel substrates at different Si levels and at different immersion times and temperature. The conclusions from the Table 17 are summarized in the following two sections Effect of immersion time and bath Si level on the coating thicknesses The coating thickness does not increase linearly with immersion time. The thickness of the intermetallic layer is limited by the availability of Fe atoms which diffuse through the intermetallic layer. At higher Si level in the bath, the at% Si in the intermetallic layer increases making the intermetallic layer more compact which retards the Fe diffusion through it. Thus with increasing Si in the bath the coating thickness reduces because Si not only decreases the intermetallic layer thickness, but also reduces the drag-out or run-off layer thickness because it increases the fluidity of the bath. At immersion times of the order of 1 min the intermetallic layer is formed of FeAl 3 as mentioned in the Table 15. As the immersion time increases to 5 min the composition of the intermetallic layer changes to Fe 2 Al 5 or Fe(Si 1-x Zn x )Al 5. This further consolidates the conclusions made by Ranjan that contrary to the popular view that Fe 2 Al 5 is the first phase to form during galvanizing 43

60 in an Al-containing Zn-bath, the sequence of phase formation in the coating obtained from a bath containing Si could be expressed as [3]: Liquid - Solidification of Al-rich phase - Cubic phase (T 5 ) - θ (FeAl 3 ) - η Fe 2 Al 5 At lower Si level in the bath the Si % in the intermetallic layer decreases resulting in a more open structured intermetallic layer. This results in an increased thickness of this layer because of more Fe atoms diffusing through it. Also, at low Si level since the bath fluidity is reduced the coatings dipped in a bath having low Si level have a thicker drag out layer which increases the overall coating thickness. However, because of the open structured, prolonged immersion in a bath with low Si level leads to outbursts Effect of bath temperature The microstructures of the through thickness cross sections of coatings dipped at 0.4 % Si level in the bath and dipped at three different temperatures were shown in Figure 24. It can be concluded from the microstructures of the coupons that the intermetallic layer is a function of temperature, i.e., the higher the temperature, the thicker the intermetallic coating, provided the immersion time is kept constant. However, since at higher temperature the bath is more fluid, the top layer thickness is inversely proportional to the bath temperature and hence the overall coating thickness remains constant Characterization of the top layer of the Zn23Al coating The EDS analysis of the cross section of the samples revealed that the top layer of the coating had around 70 wt% Al (90 at% Al). However, this high Al in the top layer of the coating is rather 44

61 unusual since the top layer was thought to be the drag-out layer of the bath metal. The anomaly in the results was attributed to the fact that if the cross sections of the Al containing coatings are exposed to the environment for a prolonged time then Al, because of its high affinity to oxygen, diffuses out from beneath the surface of the cross section and comes out to the surface. To avoid the diffusion of Al to the surface and in an effort to have a correct analysis of the top layer Al content, two new specimens were polished and immediately analyzed to obtain the composition of the top layer of the coating. Again the results indicated that the top layer has around 70% Al. The other reason for the erroneous results could be attributed to the fact that the top layer coating being extremely small, the EDS analysis of the top layer coating encompasses not only the top layer but also the surrounding area. After that an unpolished coated coupon surface was introduced into the SEM. The surface of the coupon was analyzed such that the coated surface was perpendicular to the electron beam and the composition of the coated layer was found to be 18 wt. % Al (Figure 22) Wet chemical analysis of the Zn23Al coating dipped at 0.175% Si in the bath at 575 ± 5 C To confirm the results of the experiment mentioned in section 3.16 the Zn23Al coating on a coupon was dissolved and the wet chemical analysis of the solution was done. The results revealed that the coating had a total of 31.7 wt % Al. The intermetallic layer consistently showed around 45 wt % Al as per the results of EDS analysis of the coupons. Thus the top Al layer was back calculated to be around 18% Al, which confirms the results of section Bath metallographic analysis Bath samples, one from close to the surface and the other from the middle portion of the bath, were taken from an undisturbed bath in order to check the composition of the bath metal. The 45

62 samples were polished, etched and observed for metallographic analysis. It was found that the bath sample from close to the surface showed black oxide particles The bath samples were analyzed for their composition by EDS and it was found that the bath composition was around 22%Al. Figure 36 shows the microstructure of the bath samples one from the middle portion and one from the top of the bath Spangle observed on the coupons A characteristic spangle (textured appearance) was visible on the coatings which were dipped at a temperature of around 540 C. The top surface of the coatings was observed under SEM to better understand this effect. The microstructures of the coating surfaces of the coupons dipped at 540, 575 and 600 C are shown in Figure 37. The spangle can be explained on the basis of the temperature difference, ΔT, between the bath temperature and the freezing point of the alloy. ΔT is directly proportional to the number of nuclei formed on the coating surface during the solidification of the drag-out layer or the top layer of the coating. The greater the temperature difference, the higher the number of nuclei formed per unit area on the surface. Thus, the coupons dipped at lower temperature have lesser nuclei and hence a more uniform structure per unit area, which explains the observed spangle Black spots on the coating surface In the latter part of the work at Teck Cominco Ltd the surface of the coupons dipped at high temperature (around 570 ± 5 C) showed black spots to the naked eye (Figure 38, 39). The cross sections of the coated coupons also sometimes showed a black layer. The EDS analysis of this black layer showed that they are mixtures of chlorides of Fe, Al and Zn (Table 18). Also, the EDS results were erratic which confirms that the black spots observed were not any phase but a 46

63 non-uniform mass. This can be attributed to residual flux that burned at high-temperature dipping trials. Also, it was found that if the flux layer was thick (increased immersion time in the flux) it was increasingly difficult to coat the samples and the half-burnt flux became trapped in the solidifying metal and showed on the surface as black spots. Later, the immersion time in the flux was reduced to 15 s and it was found that even with such short immersion time the flux worked fine and the coatings obtained were uniform and smooth. The flux seemed to work with 15 s immersion because of the resultant thin layer which is sufficient to prevent the coupon from oxidation and to remain reactive before it comes in contact with the bath and at the same time the thin layer of flux is easy to fake off and escape allowing bath metal and substrate contact without any flux burning and residue Mechanical tests performed on the Zn23Al coated coupons The bend test on the coatings Bend test on a Zn5Al-coated sample and a Zn23Al-coated sample was performed. The coupons were bent 180 (2T). Both coupons did not show any discontinuity to the naked eye in the top layer of the coating. The bent samples were then cut longitudinally, mounted, polished and observed under SEM. The coupons did not show any cracking in the top layer probably because of the plasticity of the Zn23Al. However, the intermetallic layer showed discontinuities in the region which was under tension and lateral relative displacement in the intermetallic layer in the region which was under compression (Figure 40, 41). It is seen that on the compression side of the panel the top layer of the coating has not cracked. On the tensile side of the deformed panel, it is observed that the drag-out layer has not cracked, but has become considerably thinner. The 47

64 interfacial reaction layer has not deformed, but has cracked. These results confirm that the Zn-Al drag-out layer is very ductile and that the iron-aluminide layer is not. The Zn5Al-coated specimen showed some discontinuities in the coating in the region which was under tension. The region under compression did not show any noticeable defect Hardness test The hardness of the bath sample was determined by a 500 g load. The top layer and the intermetallic layer of the coating were extremely thin to make an accurate reading by the equipment available. The smallest load available, i.e., 10 g was used to determine the hardness of these layers. The high hardness of the intermetallic layer made it even more difficult to make any discernible indentation on it and hence the results obtained were varied. The results obtained are listed in Table 19. The results suggest that the Zn23Al coating forms a very hard intermetallic layer on the substrate which is covered by a soft superplastic drag out layer of the Zn23Al eutectoid alloy. This imparts a very unique combination of a hard, brittle layer covered by a soft, ductile layer. The wear resistance of the coating was not tested which could be incorporated in future work. The hardest of the Fe-Zn intermetallic phases is known to have a hardness of about 510 HV [3]. The hardness of the quarternary intermetallic layer needs to be ascertained though Corrosion tests Salt spray test a) 1000 hrs exposure One 10 cm x 7.5 cm panel each of Zn5Al and Zn23Al-coated alloy were placed in the salt spray chamber (ASTM B117) for 1000 hours. Figure 42 shows the Zn5Al and Zn23Al- 48

65 coated coupons before placing them in the salt spray chamber. Red rust spots started appearing on the Zn5Al panel at 427 hours. They slowly became worse over the test period. The Zn23Al panel never showed red rust. The panel that had been in the 1000-hour salt spray test was removed and set aside (Figure 43). Other tests then proceeded. As the panel was drying on the counter, red rust spots appeared (Figure 44). Either these spots were present but hidden under the initial white rust coating, or they developed due to any small residual moisture. b) 2000 Hrs exposure at Ecosil Technologies LLC Three Zn23Al0.3Si, HDG and Zn5Al coated coupons were introduced in the salt spray chamber at Ecosil Technologies LLC located in Cincinnati, OH to check the repeatability and the validity of the results. Figure 45 shows panels of Zn23Al0.3Si panels after exposure in the B-117 test. The HDG was removed after 350 hours, the Zn23Al0.3Si after 2000 hours. The HDG panel had begun to form red rust. One of the Zn23Al0.3Si panels also showed one spot of red rust after 2000 hours, so the test was terminated. Panels of Zn5Al coated lasted about 600 hours in this test before red rust appeared. Thus, the Zn5Al coatings are a factor of 2 better than HDG, the Zn23Al0.3Si coating is a factor 6-7 better in the salt spray test. It should be noted that the coating thickness of the HDG panels was 75 µm, whereas that on both the Zn5Al and Zn23Al0.3Si was not more than µm. The results illustrate the enormous effect that Al has on the protection against red rust. c) Weight loss study One could argue that the corrosion resistance of Zn23Al0.3Si is only due to the interfacial reaction layer of iron-aluminides and that the corrosion resistance of the drag-out layer is similar to that of conventional HG. Therefore, the weight loss in the salt spray test was 49

66 measured quantitatively in another comparative test involving HDG, Zn5Al-coated and Zn23Al0.3Si-coated samples. One panel of each was removed after 48 hours of testing, and the others were removed after 114 hours of testing. The weight loss of each panel is as per Table 20. Figure 46 shows the Zn5Al and Zn23Al-coated coupons before and after weight loss study in the salt spray test. Figure 46 shows copious amounts of white rust that had been formed on both the HDG and the Zn23Al0.3Si panels in the salt spray exposure. It is seen that Zn5Al-coated loses, on average, 1.6 g/24 days, HDG loses 2.5 g/14 days and Zn23Al0.3Si loses only 0.46 g/24 days. On a daily basis these weight loss ratios are 1 : 3 : 9 for Zn23Al0.3Si, Zn5Al and HDG, respectively. These ratios are similar to the red rust appearance data reported above and they demonstrate that it is not only the interfacial ironaluminide layer but the Zn-Al drag-out layer as well that contributes to the outstanding corrosion resistance of Al-containing coatings, especially the Zn23Al0.3Si system. d) Scribe test and bent specimen tested in the salt spray chamber Two types of samples were tested using this condition: I. Panels with a 180 bend. The round top of the bend pointed upwards (referred to as the Bend test) II. Flat panels with a 7 cm long, diagonal scribe through the coating (referred to as the Scribe test). The coupons were scribed with the help of a silicon carbide cutting wheel. The cleaning procedure used was in accordance with ASTM G1 solutions (C.9.2 and C.9.5) to remove the zinc corrosion products from the panels after testing. Tables 21 to 24 are the sample identification and weight loss tables for the bend test and the scribe test. Galvanized panels in salt spray test: The zinc coating quickly developed red rust spots. The coating would have severely deteriorated under any further testing, so the panels were 50

67 removed. After cleaning it was seen that the top shiny, spangled coating was completely gone. There were several areas where additional zinc was lost and iron corrosion products were seen. Although no red rust was seen in the scribe marks, the scribe was completely covered over with zinc corrosion products. (Figure 47, 48) Zn5Al-coated panels in salt spray test: On both panels subjected to bend test, the coating dulled to a dark gray and developed red rust spots Zn23AlSi-coated panels in salt spray test: The panels in the bend test were a dull gray with no red rust spots, but several corrosion pits. The panels in the scribe test were a dull gray with some red rust which appeared to reside on the surface, but no corrosion pits were observed. There was never any red rust seen in the scribe mark. HDG can protect steel by cathodic protection of the steel exposed in a defect, as has been well documented. In this test, the cathodic protection performance of the new Zn23Al0.3Si system was investigated. It could be argued that Al by itself does not protect steel, as it tends to passivate. Hence, diluting zinc with aluminum could lead to a degradation of the cathodic protection effect. One can notice the difference in white rust formation between the two, although the Zn23Al0.3Si system was exposed for almost twice as long as the HDG panel. The HDG panel also shows some red rust. However, the scribe still seems to be protected in both systems. After cleaning, there were white corrosion products around the scribe in the case of the Zn23Al0.3Si system. They could not be removed in the acid. The presence of these products is interpreted as being indicative of a very high electrochemical activity of the Zn-Al topcoat. In the HDG system such tenacious products are not seen. 51

68 The purpose of the exposure of bent samples in the salt spray chamber was to verify whether the Zn23Al0.3Si system is, indeed, ductile. The performance criterion in this test was the appearance of red rust in the bend, i.e., the region of the highest tensile stresses. The HDG panels were exposed for only 3 days, as red rust had already become apparent. The Zn5Al coated and Zn23Al0.3Si panels were exposed for 24 days. It was observed that the HDG panel shows a large amount of white rust and has also formed red rust in the bend area (Figure 47). The Zn5Al coated panel began to break down as it showed local spots of red rust. The Zn23Al0.3Si panel showed less white rust than the other systems and not a single spot of red rust. Thus, it can be concluded that this coating can withstand severe deformations better than HDG and Zn5Al and that the corrosion performance does not suffer in the bend test, due to the strong protective action of the top layer Humidity test Three Zn5Al-coated and three Zn23Al-coated samples were tested as per ASTM D2247. The samples were inclined at 15 on a plastic rack. They were placed in a 0.3 m 3 chamber and held at 38 C and 100% humidity. Zn5Al panels in humidity test: All three panels remained fairly shiny, with a slight dulling over most of the panel. Low magnification (20x) showed about 50% coverage with a thin white rust layer (Figure 30). No corrosion products were seen under low magnification. Zn23Al alloy panels in humidity test: The panels were originally dull silver, so no change was seen after the test. Low magnification showed about 5% coverage with a thin white rust layer (Figure 30). No obvious corrosion products or pitting was seen under low magnification. Only a few, very small white rust spots were seen. 52

69 DC polarization From potentiodynamic polarization curves in an electrolyte, the initial corrosion rate of the metal i.e. the electrochemical activity of the surface is measured. In addition to the corrosion current i corr, which can be converted to the corrosion rate in mpy (mm per year), the corrosion potential E corr is measured. The lower the E corr, the more active the metal surface is. Zinc surfaces in salt solutions, as were used here; typically have an E corr slightly lower than -1 volt. Aluminum surfaces have a lower E corr, provided they are not passivated. In salt solutions Al will not easily passivate, as the Cl ion depassivates it. Table 27 gives the E corr and mpy values measured for HDG, Zn5Al and Zn23Al0.3Si. The values for delta-galvanized HDG, which is normally completely alloyed due to the high galvanizing temperature, were also measured. Figure 50 shows polarization curves for the Zn23Al0.3Si alloy and for HDG. There is no evidence for passivation of any of the alloys in this solution and the general shape of the curve for Zn23Al0.3Si is very similar to that of standard HDG. All other curves, e.g., for Zn5Al, were also similar to those of Figure 50. The E corr values shown in the Table do not vary significantly. They all are very close to the value for pure zinc and the variability between runs of the same material is about 20 mv. The differences between the i corr values are significant, however. They show that HDG has a higher corrosion (dissolution) rate than the other systems. Zn5Al is a factor of 1½-2 lower than HDG, but Zn23Al0.3Si is a factor of 5 lower than HDG and a factor of 3 lower than Zn5Al. The corrosion rate of delta galvanized HDG, if fully alloyed, is similar to that of Zn5Al and a factor of 1½-2 lower than regular HDG. The results of the B-117 test, described above, are different in that they indicate the resistance against red rusting only. The polarization tests can be quantified to give the corrosion rate which is the rate of the consumption of the alloy coating in the solution. The results of these two tests 53

70 do not necessarily have to agree. Weathering tests in outdoor exposure have not yet been carried out but are planned for exposure in Florida. The overall corrosion results demonstrate that the Zn23Al0.3Si system has outstanding cathodic protection properties, despite the high Al content and despite the lower reactivity to form white rust, as compared with conventional HDG. They further seem to indicate that where cathodic protection is needed, e.g., around a defect area, the electrochemical activity of the coating increases. It can also be concluded that the Zn23Al0.3Si alloy is electrochemically just as reactive as the standard HDG, but the rate of zinc consumption in a corrosive environment is much lower, so the coating will last longer. It should be noted that these ratios, 1: 3: 5 are similar to the ratios found in the weight loss measurements for these alloys. 54

71 4. SUMMARY AND CONCLUSIONS The novel Cu-Fe and Cu-Sn fluxes were tried during the initial part of the work at PTC but with limited success. A modified Zn ammonium chloride flux developed by PTC and Ferrotech Inc., Canada, was found to work perfectly well with Zn23Al-alloy bath. Smooth and uniform coatings were obtained with the use of this flux even in a high Al containing bath like Zn23Al-alloy. Black spots observed on the coating surface at higher temperature C are burnt and trapped flux residues. The blind spots on different sizes and shapes of steels make it difficult for the flux to escape and they show residues of burnt flux. If the immersion time in the flux is higher than 1 min, i.e., if the flux layer on the samples is thick it is increasingly difficult for the flux to escape resulting in black spots on the coatings. The flux seems to work with 15 s immersion because of the resultant thin layer which is sufficient to prevent the coupon from oxidation and to remain reactive before it comes in contact with the bath and at the same time the thin layer of flux is easy to fake off and escape allowing bath metal and substrate contact without any burning and flux residue. Proper cleaning and drying of the coupons is essential, as is the case generally with high-al galvanizing baths but it is not absolutely critical as thought earlier. Si in the Zn Al eutectoid bath, indeed, effectively suppresses the outbursts on the coatings when present in the range of wt %. Even and smooth coatings (without outbursts) were obtained at 0.15 %, 0.2%, 0.3 %, 0.4 % and as high as 0.75% Si in the bath. Stirring of the bath and addition of the alloy to the bath in small pieces helps the recovery of Si in the bath and reduces the Si lost in the form of intermetallics formed as top dross. 55

72 The bath does not form bottom dross. The amount of top dross formed is also very low, proportionate to the Fe that is present in the form of impurities in the ingot, and there is almost no dissolved iron in the bath. Thus, this alloy forms almost no dross and thus the losses are greatly reduced. Once the Si is dissolved in the bath the depletion rate of the Si is not drastic and the Si is stable in the bath even with temperature fluctuations. The new Zn23AlSi coating consists of two layers, an intermetallic layer or inhibition layer of Fe, Al, Zn and Si and the top drag-out or run-off layer. The thickness of the intermetallic layer is governed by the inter-diffusion of Fe and Si in the intermetallic layer. The thickness of the coating is about µm, i.e., considerably less than the currently used galvanized coatings. The coating has a simple structure consisting of an interfacial intermetallic layer (mainly Fe 2 Al 5 or Fe 2-x-y Al 5 Zn x Si y ) at the steel coating interface and a drag-out layer of approximately the bath composition. It is the intermetallic layer that provides an extraordinary corrosion resistance to the steel. Upon cooling the top layer separates into several phases, with the exact composition depending somewhat on the type of steel and on the cooling rate. The relative thickness of the two layers has slight temperature dependence. At very high galvanizing temperature (e.g., 600 C), the ratio of base layer to top layer is 10/20 µm, whereas at 510 C, which is the lowest temperature that gave good-quality coatings, the thicknesses were 5/25 µm, typically. Microstructures of the cross section of the coatings revealed that the intermetallic layer thickness is a function of temperature, i.e., the higher the temperature the thicker the intermetallic coating, but at higher temperature the bath is more fluid and hence the top layer thickness is inversely proportional to the bath temperature. As a result the overall coating thickness does not change with bath temperature provided the immersion time is kept constant. This example shows that the temperature of the bath is not critical in this new process. Further, it demonstrates that the steel is 56

73 very well protected by the iron-aluminum alloy. The base layer does not grow linearly with time or exponentially with temperature. An important implication of this observation could be that certain steel kettles could possibly also run this alloy system and not just ceramic kettles. This expectation is based on the strong passivation of the steel by the bath due to the Fe 2-x-y Al 5 Zn x Si y layer and the lack of further growth with time. Si is known to increase the bath fluidity and hence as the bath Si % increases, the coating thickness decreases. The coatings dipped at lower temperature show a characteristic spangle as opposed to ones dipped at higher temperature. The alloy works on steels with varying chemistries. The coating structure or thickness did not show any dependence on the carbon, phosphorus, silicon or any other content of the steel substrate. Thus, the so-called Sandelin effect, notorious in general galvanizing, has been eliminated. The coatings obtained on these steels were about 5 10 microns thicker than the hotrolled steel used in the study, with all other parameters kept the same. Coatings obtained on coldrolled steels were more uniform than the hot-rolled steels, under similar conditions. Bend test done on the coupons suggest that the coating is flexible and that the top layer does not crack under severe bending. The hardness of the coating is much higher than those of regular zinc coatings, yet the coatings are ductile. The salt spray test revealed that the new alloy of 30 µm thickness lasted for at least 2000 hours in the salt spray chamber as opposed to 350 hours for a regular zinc coating. The time elapsed before the appearance of red rust on Zn23AlSi-coated coupons is more than 4 times the time before the appearance of red rust on the Zn5Al-coated coupons, i.e., the red rust formation is delayed in the case of Zn23Al-coated coupons. 57

74 The weight loss study led to the conclusion that the Zn23Al-coated coupons lose half as much weight as Zn5Al-coated coupons. The weight lost by Zn5Al-coated bent samples is more than the Zn23Al-coated bent samples, also the Zn5Al-coated bent samples showed red rust spots after the end of the test, whereas there was no red rust formation on the Zn23Al-coated samples. Thus it can be concluded that the corrosion resistance of Zn23Al alloy is superior to Zn5Al. The scribe test on the samples revealed that there was no red rust formation in the scribed area on the Zn23Al-coated sample, whereas the scribed area was completely covered with zinc corrosion products on the zinc-coated coupon. Thus, it can be concluded that the galvanic protection of Zn23AlSi alloy is superior to the pure zinc coatings. This result was explained on the basis of the absence of passivation of the aluminum in the alloy. The humidity test on Zn5Al and Zn23Al-coated coupons had little effect on the coupons. Examination at low magnification revealed 5% coverage and 50% coverage of a thin, white rust layer on Zn23Al-coated and Zn5Al-coated coupons respectively. Weight loss was not calculated as the samples were virtually unaffected. Electrochemical tests indicated that the corrosion resistance compared to conventional HDG coatings in a dilute salt electrolyte ia a factor of 5 times better and 3 times better than the Zn5Al coating. One could argue, as far as economics of the new process are concerned, that the Zn23Al0.3Si bath has to be run at considerably higher temperatures than conventional batch galvanizing, viz., 510 C vs. 450 C. However, what one gets in return is a) a coating that at only 1/3 of the coating thickness of current HDG, exhibits a performance in many tests that is considerably better, e.g., a factor of 6-7 in salt spray resistance and other tests and b) considerably low drossing and run time losses. 58

75 5. FUTURE WORK A fresh batch of the G flux needs to be prepared and the galvanizing of the coupons immersed in the new flux should be tested to ascertain the origin of the black spots on the coatings. The prolonged exposure of the coupons in the bath also can be tested to check the stability and the durability of the intermetallic layer. If the intermetallic layer is passive to the bath metal, as it is thought to be, a low carbon stainless steel kettle could be used for the commercial batch galvanizing with this alloy and would eliminate the necessity of a ceramic kettle. Addition of other elements to the bath to increase the coating thickness can be experimented. The hardness of the intermetallic layer needs to be ascertained. Alternately, the wear test on the coatings could be done to study the durability of the coating under abrasive conditions. The performance of the coatings in erosion corrosion needs be analyzed. 59

76 6. REFERENCES 1. Anderson E.A, Reinhard C.E; The Corrosion Handbook, Ed. Uhlig H.H, 1948, p Lin K.L, Ho J.K, Jong C.S, Lee J T ; The minerals, metals and materials society; San Francisco, California; USA; 27 Feb.-3 Mar pp Development of Zn-Al alloy for Hot Dip Batch Galvanizing, Dr Madhu Ranjan, PhD thesis, p Massalski T. B; Ed: Binary alloy phase diagram; (Metals Park, Ohio), Vol.2, 1987, p Jordan C.E, Marder A.R; Journal of Materials Science Publisher: Springer Netherlands January 1997; Rangarajan V, Giallourakis NM, Matlock DK, Krauss G; J Materials Shaping Technolgy, 1989; Vol. 6, no. 4, pp Horstmann D; Proceedings of 14 th International Hot Dip Galvanization Conference, London; Zinc Development Association, 1986, p.6/1 8. Horstmann D, Peters F K ; Proc 9 th Inter Galva Conf London, Zinc Development Association, Industrial Newspapers Ltd , p M.Hansen: Constitution of Binary Alloys. (McGraw-Hill (New York 1958) p A. V. Yarovchuk, R. K. Aubakirova, and A. A. Presnyakov, Zavod. Lab., No. 5, (1976). 11. Torisaka, Y; Kojima, S Acta Metallurgia et Materialia. Vol. 39, no. 5, pp May A. A. Bochvar and Z. A. Sviderskaya, Izv. Akad. Nauk SSSR, Otd. Tekh. Nauk, No. 9, (1945) 60

77 13. Koster W, Godecke T; Das Dreistoffsystem Eisen-Aluminum-Zinc. Z Metallkde 1970, 61, Koster W, Godecke T; 9 Intern. Conf. on Hot Dip Galvanizing, Dusseldorf, June London, Industrial Newspapers Ltd., 1971, pp Tang N-Y;. J Phase Equilibria 1994; 15: Tang N-Y, Adams G.R, Kolisnyk P.S; GALVATECH 95, Chicago, IL; Iron and Steel Society, 1995, p Mackowiak J, Short N.R; Metallurgy of galvanized coatings; Int. Met Reviews 1979;1 18. Lepretre Y, Goodwin FE. Warrendale, PA: TMS, 1998, p Kanamaru T, Nakayama M; Mater Sci Res Int 1995;1: Isobe M; Initial alloying behavior in galvannealing process; CAMP-ISIJ 1992;5: Nakayama M, Kanamaru T, Numakura Y; CAMP-ISIJ 1992; 5: Tang N-Y; Met Mater Trans, 1995; 26A: Guttmann M; Galvatech 95, Chicago, IL; Iron and Steel Society, 1995, p Jordan C.E, Marder A.R; Met Mater Trans 1997; 28A: Guttmann M; Mater Sci Forum; 1994; 155 (156) : Hisamatsu Y; Galvatech 89, Tokyo; The Iron & Steel Institute of Japan, 1989, p Saito M; Tetsu-to-Hagane 1991; 77: Allegra L, Hart R.G, Townsend H.E; Met Trans 1983; 14A: Mercer P.D; Galvatech 92, Amsterdam; Stahl and Eisen, 1992, p Jordan C.E, Marder A.R; M Mater Sc, 1997, 32: Ghuman A.R.P, Goldstein J.I; Met Trans, 1971; 2A: Caceres P.G, et al; Mater Sci Technol, 1986;2:871 61

78 33. Ichiyama K; Proceedings of 14 th international hot dip galvanization conference, Munich; Zinc development association, 1985, p.9/1 34. Makimattila S.J; Scan J Metall, 1986; 15: Chen Z.W, et al; Met Trans, 1992; 23A: Yang Y, Yu Z; Acta Met Sin, 1993; 29A: Sharp R.M, Gregory T.J, Chen Z.W; Mater Forum, 1992; 16: Lin K.L, Ho J.K, Jong C.S, Lee J.T; In: Marder A.R, editor. The physical metallurgy of zinc coated steel. Warrendale, PA: TMS, 1994, p Selverian J.H, Marder A.R, Notis M.R; Met Trans, 1998; 20A: Selverian J.H, Marder A.R, Notis M.R; J Mater Engng, 1987; 9: Neemuchwala N.D, Hershman A.A; Proceedings 7th Int l Conf. Hot Dip Galvanizing, Paris, 1964, Zinc Development Association, Pergamon Press, Oxford (1967), Raynor G.V, Rivlin V.G, Phase equilibria in iron ternary alloys, The Institute of Metals, 1998, p Takeda S, Mutuzaki K, Tetsu-to-Hagane, 1940, 26, p Zoccola J.C, Townsend H.E, Borzillo A.R, and Horton J.B; ASTM STP 646, ASTM Philadelphia, 1978, p Carter V.E; Metallic coatings for corrosion control; Newnes-Buttleworths, Sevenoaks, Kent, England, 1977; p Van Ooij W.J, Prasanna V, High-Aluminum Galvanized Steel; US Pat. 6,372,296, April 16, Van Ooij W.J, Prasanna V, Fluxing Process for Galvanization of Steel; US Pat.6,200,636, March 13,

79 48. Van Ooij W.J, Ranjan M; 5 th Asia Pacific General Galvanizing Conference, Busan, Korea, October 21-25, 2001, p Dr. Thomas H. Cook; Metal Finishing Volume 101, Issue: 7-8, July-August, 2003, p

80 Table 1 Fe-Zn phase characteristics [4] Phases Formula Crystal structure VHN* (25mg) αfe Fe(Zn) BCC 104 Γ Fe3Zn10 BCC 326 Γ 1 Fe5Zn21 FCC 505 δ FeZn10 Hexagonal 358 ζ FeZn13 Monoclinic 208 ηzn Zn(Fe) HCP 52 * VHN Vickers Hardness Number Table 2 Ternary solid phases in Fe-Al-Si system [43] Phase Formula Composition (wt%) Symmetry Al Fe Si t1 Al 3 Fe 3 Si t2 Al 12 Fe 6 Si Monoclinic t3 Al 9 Fe 5 Si t4 Al 3 FeSi Tetragonal t5 Al 15 Fe 6 Si Cubic Hexagonal t6 Al 4 FeSi Tetragonal 64

81 Table 3 Composition of the special set of steels Steel No. % Silicon % Phosphorus Table 4 Parameters used for the in-house analysis of Si and Al Al Si Wavelength nm nm Lamp Al (HCl) at 25 ma Fe ( HCl) at 40 ma Slit Width 0.7 nm 0.2 nm Integration Time 2 s 2 s Flame Nitrous Oxide/Acetylene Nitrous Oxide/Acetylene 65

82 Table 5 Wet chemical analysis of the bath Si level Bath sample Bath temperature at which sample was drawn ( C) wt% Si ( Reported after chemical analysis ) B1 575 ± B2 510 ± B3 550 ± Table 6 Si level in bath and bottom samples Bath sample Bath temperature at which wt% Si sample was drawn ( C) B4 510 ± B5 550 ± B6 580 ± Bottom dross sample Bath temperature at which wt% Si sample was drawn ( C) D4 510 ± D5 550 ± D6 580 ±

83 Table 7 EDS analysis of the intermetallics found in the top dross wt% at% %Al %Si %Fe %Zn %Al %Si %Fe %Zn Table 8 Wet chemical analysis of the ingots Ingot wt% Al wt% Si wt% Fe Zn23Al Zn23Al1Si Table 9 Wet chemical analysis of the bath sample, the bottom sample and the top dross Sample wt% Al wt% Si wt% Fe Bath Sample B Top Dross T Bottom Dross D

84 Table 10 Wet chemical analysis of bath 3 Bath Sample wt.% Si in the bath (as calculated) wt% Si in the bath ( as reported after chemical analysis) B B13 B B14 B B15 B Table 11 Bath analysis showing the depletion of the Si level Bath Sample Bath Temperature wt% Si in the bath ( as reported after Chemical analysis) B B B B B B

85 Table 12 Characterization of the coatings dipped at 0.175% Si in the bath (at%).element Al Si Fe Zn Phase 6-2 Inhibition Layer T 5 (Cubic) 6-2 Top Layer Inhibition layer θ (FeAl 3 ) 6-10 Top layer Inhibition layer η (Fe 2 Al 5 ) 6-12 Top layer

86 Table 13 Characterization of the coatings dipped at 0.4% Si bath (at. %) for 1 min Carried out at Teck Cominco PTC Element Al Si Fe Zn 550 Inhibition layer top layer inhibition layer top layer inhibition layer top layer Table 14 Characterization of the coatings dipped at 0.4% Si in the bath (at. %) Carried out at UC AMCC Immersion Bath Time Temperature Layer Al Fe Si Zn Phase 5 min 550 C Top min 550 C Inhibition Fe(Si,Zn)Al 5 5 min 575 C Top min 575 C Inhibition Fe(Si,Zn)Al 5 5 min 600 C Top min 600 C Inhibition Fe(Si,Zn)Al 5 70

87 Table 15 Characterization of the coatings dipped at 0.75% Si in the bath (at%) Carried out at UC AMCC Immersion Bath Time Temperature Layer Al Fe Si Zn 1 min 575 C Top min 575 C Inhibition FeAl 3 5 min 575 C Top min 575 C Inhibition Fe(Si,Zn)Al 5 71

88 Table 16 Characterization of the special set of steels dipped at 0.4% Si in the bath (at%) Immersion Time:- 5 min Bath Temperature:- 575 C Carried out at UC AMCC Steel ID No Layer Al Fe Si Zn 2 Top Inhibition Fe(Si,Zn)Al 5 3 Top Inhibition Fe(Si,Zn)Al 5 4 Top Inhibition Fe(Si,Zn)Al 5 7 Top Inhibition (FeZn)SiAl 5 (?) 10 Top Inhibition Fe(Si,Zn)Al 5 72

89 Table 17 Coating thicknesses obtained at various immersion time, temperature and bath Si level Bath Si level Immersion time (min) Bath temperature ( C) Coating thickness (µm) % % % % % % % % % %

90 Table 18 EDS analysis of the black spots observed on the coating surface (burnt flux residue) (wt%) Element Al Si Cl Fe Zn Black Spot Black Spot

91 Table 19 Hardness Test performed on the Zn23Al bath sample and Zn23Al coated coupon Layer Load (g) Vickers Hardness Number Average VHN (VHN) Substrate , 140, 139, Bath Sample , 85, 87, Top Layer 10 97, 125, 115, 102, Intermetallic layer , 579, 620, 646,

92 Table 20 Weight loss in the Galfan-coated and Zn23Al-coated coupons in the salt spray test Sample Hours on test Original wt. (g) After testing Weight loss, (g) Galfan Galfan Galfan % Al % Al % Al

93 Table 21 Sample identification table for coupons used in the bend test ID Coating Test Days in test Z1 galvanized Bend (note 1) 3 Z2 galvanized Bend 3 G2 galfan Bend 24 G3 galfan Bend 24 S1 Zn23Al alloy Bend 24 S2 Zn23Al alloy Bend 24 Table 22 Weight loss table for coupons used in the bend test ID Coating thickness, µm Initial weight, g Weight loss, g Z Z G G S S Note 1: Galvanized panel Z1 was not bent. It remained flat and was used as a reference for weight loss in this particular test, as compared to the bent panels. 77

94 Table 23 Sample identification table for coupons used in the scribe test ID Coating Test Days in test Z3 galvanized Scribe 14 Z4 galvanized Scribe 14 Z5 galvanized Scribe 14 S3 Zn23Al alloy Scribe 24 S4 Zn23Al alloy Scribe 24 S5 Zn23Al alloy Scribe 24 Table 24 Weight loss table for coupons used in the scribe test ID Coating, µm Initial weight, g Weight loss, g Z Z Z N/A (note 2) S S N\A (note 2) S Note 2: These two panels (Z5 and S4) were not cleaned to view the condition of the corroded panel and the scribe mark in the as removed condition. 78

95 Table 25 Sample identification table for coupons used in the humidity test ID Coating Test Days in test G4 galfan Humidity 48 G5 galfan Humidity 48 G6 galfan Humidity 48 S6 Zn23Al alloy Humidity 48 S7 Zn23Al alloy Humidity 48 S8 Zn23Al alloy Humidity 48 Table 26 Weight loss table for coupons used in the humidity test ID Coating, µm Initial weight, g Weight loss, g G N\A (note 3) G N\A (note 3) G N\A (note 3) S N\A (note 3) S N\A (note 3) S N\A (note 3) Note 3: The panels in the humidity test did not develop any visual corrosion products, and were, therefore, not cleaned 79

96 Table 27 Corrosion rate calculated with DC Potentiodynamic method Experiment No. Coating Type Corrosion rate (mpy)* Average Corrosion rate, i corr E corr (Volts) (mpy) Zn23Al Galfan Regular HDG Delta HDG** * mils (25 µm) per year ** depending on whether complete alloying had occurred; these test were performed with thin panels which did not fully alloy in the center; the higher value was obtained with the unalloyed regions 80

97 Figure 1 Iron-zinc equilibrium diagram [4] 81

98 Figure 2 Microstructure of zinc coating formed after 5 mins in a 450 C zinc-bath on a steel substrate (1) gamma (2) delta and (3) zeta phase (4) eta phase (from the bottom to top) [5] 82

99 Figure 3 A schematic representation of Fe-Zn pha se layer formation in zinc galvanizing bath, to corresponds to zero time and t1<t2<t3<t4 [5] 83

100 Figure 4 Al-Zn system [9] 84

101 Figure 5 Primary deposition areas in the zinc corner [13, 14] 85

102 l + ζ + δ l + ζ l+δ l + δ + η Fe 2-x Al 5 (Zn x ) l l + η Fe 2-x Al 5 (Zn x ) Figure 6 Zn-rich corner of the 460 C isothermal section of the Fe-Zn-Al phase diagram [16] 86

103 Figure 7 Ternary Fe-Al-Zn phase diagram at 575 C [5] 87

104 Figure 8 Growth Kinetics of Fe 2 Al 5 layer at 470 C for various bath Al content [20] Figure 9 Effects of bath Al content and alloying temperature on incubation period [21] 88

105 Figure 10 Schematic diagram showing the Fe-Zn outburst growth behavior [26] 89

106 Figure 11 Schematic of the Fe-Zn phase layer formation in 0.20 wt% Al-Zn galvanizing bath. to corresponds to zero time, and development occurs according to time such that to<t1<t2<t3<t4 90

107 Figure 12 Ternary Fe-Al-Si phase diagram at 600 C [42] 91

108 Average Corrosion Loss (mils) Figure 13 Corrosion losses with time on atmospheric exposure for Al, Al-Zn and Zn hot dip coatings on steel [44] 92

109 100µm 50µm Figure 14 SEM image of the intermetallics found in top dross T8. 100µm 50µm Figure 15 Microstructure of the Zn23Al1Si alloy ingot showing black Si particles embedded in the Zn23Al matrix 100µm 50 µm Figure 16 Microstructure of the Zn23Al alloy ingot showing various phases of Zn and Al 93

110 Figure 17 Hot rolled steel dipped in 0.4% Si bath at 600 C for 5 min Mag 2000X Figure 18 Hot rolled steel dipped in 0.4% Si bath at 575 C for 5 min Mag 2000X 94

111 Figure 19 Hot rolled steel dipped in 0.4% Si bath at 550 C for 5 min Mag 2000X Figure 20 Hot rolled steel dipped in 0.75% Si bath at 575 C for 1 min Mag 2000X 95

112 Figure 21 Hot rolled steel dipped in 0.75% Si bath at 575 C for 5 min Mag 2000X Figure 22 SEM image of the top surface of the Zn23Al coating dipped at 0.175% Si in the bath at 575±5 C (1000X) 96

113 6-2 1 min immersion 2500 X min immersion 2500 X min immersion 2500 X Figure 23 SEM images of cross sections of the coatings dipped at 0.175% Si in the bath with varying immersion time 97

114 550 C 1500X 575 C 1500X 600 C 1500X Figure 24 SEM images of cross sections of the coatings dipped at 0.4% Si in the bath with varying bath temperature. Immersion time kept constant at 1 min. 98

115 Figure 25 Hot rolled steel dipped in 0.75% Si bath at 575 C for 1 min Mag 1000X Figure 26 Hot rolled steel dipped in 0.75% Si bath at 575 C for 5 min Mag 1000X 99

116 Figure 27 Steel No. 2 dipped in 0.4% Si bath at 575 C for 5 min Mag 2000X Figure 28 Steel No. 3 dipped in 0.4% Si bath at 575 C for 5 min Mag 2000X 100

117 Figure 29 Steel No. 4 dipped in 0.4% Si bath at 575 C for 5 min Mag 2000X Figure 30 Steel No. 7 dipped in 0.4% Si bath at 575 C for 5 min Mag 2000X 101

118 Figure 31 Steel No. 7 dipped in 0.4% Si bath at 575 C for 5 min Mag 1000X The microstructure shows no presence of an intermetallic layer. Figure 32 Steel No.10 dipped in 0.4% Si bath at 575 C for 5 min Mag 1000X 102

119 Figure 33 Steel No.10 dipped in 0.4% Si bath at 575 C for 5 min Mag 250X The microstructure shows a fairly large area without any intermetallic layer. Figure 34 Steel No. 3 dipped in 0.4% Si bath at 575 C for 5 min Mag 1000X Microstructure shows varying coating thickness (34.97µm, 49.77µm) 103

120 Figure 35 Steel No. 4 dipped in 0.4% Si bath at 575 C for 5 min Mag 250X Microstructure shows varying coating thickness (41.5µm, 24.6µm, 19.98µm, 38.43µm, and 27.68µm) 104

121 Mag1000X Mag1000X Figure 36 SEM images of the samples from the bath surface (top) showing black oxides and from the middle portion of the bath (bottom) 105

122 Coupon surface dipped at 540 C 75X Coupon surface dipped at 575 C 75X Coupon surface dipped at 600 C 75X Figure 37 SEM images of the top surface of the coatings dipped at different bath temperature 106

123 Figure 38 The coupons showing black spots (burnt flux residues) Black spot on the coating surface 1500X Figure 39 SEM image of the top surface of the coating showing the black spot 107

124 Zn23Al Edge in Compression 750X Edge in tension 750 X Figure 40 SEM images of the bent Zn23Al-coated coupons showing the edges in tension and compression Galfan Edge in Compression 1000X Edge in tension 1000 X Figure 41 SEM images of the bent Zn5Al-coated coupons showing the edges in tensio n and compression 108

125 Figure 42 Figure showing the Zn5Al-coated (top) and Zn23Al-coated (bottom) coupons before the salt spray test Figure 43 Figure showing the Zn5Al-coated (left) and Zn23Al-coated (right) coupons after 1000 hrs in the salt spray test chamber 109

126 1000 hrs 3 weeks later Zn5Al-coated panels after 1000 hrs salt spray test 1000 hrs 3 weeks later Zn23Al-coated panels after 1000 hrs salt spray test Figure 44 Zn5Al-coated and Zn23Al-coated coupons after 1000 hrs in the salt spray chamber (left) and 3 weeks after they were removed from the salt spray chamber (right) 110

127 Figure 45 Zn23Al coated samples after 2000 Hrs in the salt spray chamber at Ecosil Technologies Inc. 111

128 Figure 46 Zn5Al and Zn23Al-coated coupons before (left) and after (right) weight loss study in the salt spray test 112

129 Zn5Al Galvanized Zn23Al-coated Figure 47 Bent Zn5Al, galvanized and Zn23Al-coated coupons after the salt spray test 113

130 scribed panel Z3 after the test scribed panel S5 after the test scribed panel Z3 after cleaning Galvanized Panel scribed panel S5 after cleaning Zn23Al coated panel Figure 48 Figure showing galvanized and Zn23Al-coated coupons before the salt spray test, immediately after the removal from the salt spray test and after cleaning the corrosion products 114

131 Zn5Al-coated panel after 48 days in humidity chamber showing thin white rust spots Zn23Al-coated samples after 48 days in the humidity test chamber showing white rust spots Figure 49 Zn5Al and Zn23Al-coated samples after 48 days in the humidity test chamber (20x) 115

132 a) Zn23Al coated sample b) Commercial HDG sample Figure 50 DC Polarization curves for a Zn23Al coated and a commercial HDG sample 116

133 Appendix 1: The Galvanizing Laboratory Laboratory furnace showing the stirring system, the sample immersion machine fume hood and the temperature controller in the background. 117

134 Cleaning and fluxing area in the laboratory The Drying furnace in the Lab 118

135 Appendix 2: Galvanizing in the Shop The 500 kg shop furnace showing the thermocouples, stirring system, sample immersion system and the temperature controller. Drying furnace. The cleaning and fluxing area can be seen in the background. 119

136 Appendix 3: Binary phase diagram of Zn-chloride and ammonium chloride 120

137 Appendix 4: Dipping of different size and shape of steels Coating on an I-beam 121