Study on the frequency of different types of galvanized steel. Corbec Inc.

Transcription

1 Study on the frequency of different types of galvanized steel Corbec Inc. Éric Michaud

2 Table of contents: Introduction 3 Summary Recommendations 5 Participants in the study 6 Results 7 Part1: Galvanizing steel plates 7 Part2: Galvanizing angles 11 Part3: Galvanizing rebar 13 Part: Galvanizing cylindrical tubes 1 Part5: Assembly containing two types of steel 15 Annex 1: Extract from the report entitled: «Thickness of zinc coatings 1 Annex 2: Experimental protocol 21 Annex 3: MTQ Report

3 Introduction: Standards ASTM A123 and G16 mainly define the minimum thickness of zinc coatings required on galvanized steel taking into account the thickness of the piece of steel. Although ASTM A123 distinguishes between certain types of steel, the Canadian standard CSA G16 does not specifically address the frequency in the galvanized steels submitted to the galvanizing process. Thus, according to standard, CSA G16, the galvanizer must reach the same minimum thickness whether it is aluminumkilled steel or silicon-killed steel. Sandelin 1 as well as many scientific publications have clearly demonstrated that some steels have a reactiveness that will produce very thick zinc coatings while others such as aluminum killed steel develop a relatively thin zinc coating, despite a prolonged dip time in the zinc bath. The use of steels containing different silica contents to achieve the same type of parts and/or in the same assembly is a major problem for galvanizers. To attempt to obtain the minimum thickness on aluminum killed parts will increase the galvanizing time, in doing so, parts of which the silica content is situated in the peak reactive zone of the Sandelin curve will potentially develop a zinc coating over 500 µm, producing a coating subject to chipping. To overcome this problem the use of nickel, as an alloying element in zinc to minimize the effect of zinc overgrowth on reactive steel, is widely used in the galvanizing industry. Although the presence of Ni reduces the thickness of zinc on reactive steels, it is not necessarily to save on zinc that galvanizers use nickel as an alloying element. Its use is motivated by the need to have a zinc bath that adequately addresses all of the galvanized steel submitted to galvanizers, including reactive steel known to develop extremely hard and thick zinc coatings subject to chipping. In recent years, Corbec has documented many cases where obtaining the minimum zinc thickness was not possible, despite the prolonged dip times and/or re-galvanizing. In each case studied, the thickness problems were invariably associated with galvanized aluminum killed steel. Following a study conducted in 2005 by the Ministry of Transport (MTQ), the ministry reviewed the minimum requirements of the zinc coating on HSS tubes among others. Let s remember that HSS tubes are generally made from aluminum killed steel with low silica content. In 2005, they therefore took the decision to refer to ASTM A 123 rather than CSA G 16 to better reflect the reality inherent in HSS tubes. Customer satisfaction, as well as the end users is a constant concern for Corbec. This is why last March; Corbec initiated a Pan-American study involving eight galvanizing sites. The objective of this study was to document the ability to galvanize different types of steel subjected to a standard galvanizing protocol using methods of rust removal and various alloys. The results obtained during a series of tests are presented in the following pages. 1: Sandelin RW, (190). Galvanizing characteristics of different types of steel 3

4 Summary: This study, initiated by Corbec, uses different galvanizers located in the United States and Canada to document the behavior of various steels when subjected to the galvanizing process. Eight galvanizing plants took place in the study. Among the plants that participated, some use sulphuric acid for rust removal, while others use hydrochloric acid. Some galvanize in a kettle containing 1% lead while others have only traces of lead. In short, at the end of the study we find that only the presence of nickel (or other Sandelin peak suppressors), in the kettle and the silica content in the steel are the parameters that have a significant impact on the thickness of the zinc coating obtained during galvanizing. The results obtained corroborate with what the scientific literature predicted. Thus, steel which has a silica content situated on the right side of the Sandelin peak (> 0.15% Si), like frames, angles and certain plates, are easily galvanized and obtain a zinc thickness superior to the minimum standard without developing an excessive zinc thickness. The presence of nickel in the kettle does not significantly affect the zinc thickness obtained. For these steels, the galvanizing time remains the main factor governing the development of the zinc layer. Steels with silica content between 0.05 and 0.15%, have developed zinc layers above the required standard for the galvanizers that do not use nickel, while those using Ni struggle to achieve the minimum standard after 12 minutes of galvanizing. Considering these results, the use of nickel could be questioned, but would be a mistake taking into consideration the composition of the zinc coating obtained for these steels, as we notice the virtual absence of the Eta layer. In fact, the coatings obtained are said to be reactive and are characterized by an excessive and disproportioned Delta layer acknowledged to be relatively brittle to shocks: See Annex 1. The episodes of documented problems with chipping in 2010 confirm this. Major problems with chipping were observed on reactive galvanized steel plates in a zinc kettle containing no nickel and whose dip time had been extended to allow the steels low silica content to obtain a higher than standard zinc coating. In addition, the migration of iron in zinc layers formed on reactive steel may produce rust stains on the surface of the zinc coating. Aluminum killed steel generally has a silica content of 0.05% or less. As shown in the theoretical galvanizing curves, (figures 1a and 1b) and the experimental results, the development of the zinc layer is very slow in the presence or absence of nickel.

5 Recommendations: Based on the results obtained by all galvanizers who participated in the study and taking into account the diversity and complexity of the material that the galvanizer must provide in the most efficient way, Corbec, maintains the use of suppressing alloying elements on the Sandelin peak, like nickel in its kettles all while controlling the concentration to limit the suppressing effect only to the region of the Sandelin peak. Therefore and considering that : a) various types of steel can be used to make the same part, b) an assembly of parts are often composed of more than one type of steel, c) the company must be able to maintain an acceptable production rate; Corbec recommends : 1- That the easing of the G16 standard awarded since 2005 by the Ministry of Transport of Quebec for HSS tubes be recognized and extended to plates deemed to have the same silica content as the HSS tubes. See annex The possibility of obtaining a waiver when parts have been galvanized for more than minutes and do not meet the minimum required standard. 3- The preferential use of steel with silica content between 0.15 and 0.25% when the situation allows so. Alternatively, let s recall that the processing of steel parts by sand blasting before galvanizing is a proven method that allows the development of a proper zinc coating on steel with low silica content and to effectively control overgrowth associated with the Sandelin peak. But, this treatment requires an additional step of which the cost is not negligible. 5

6 Participants in the study: The galvanizers that participated in the study are: Corbec Montréal & Québec, Galvan Montréal, Pure Metal Toronto, Béton préfabriqué du lac, AZZ Arizona and V&S Columbus. For confidentiality reasons, galvanizing sites will be represented by numbers from 1to. Only Corbec plants will be identified. Table 1: List of participants in the study and information on the galvanizing process: # Plant Acid Al (%) Ni (%) Pb (%) Bi (%) Sn (%) HCl HCl HCl HCl HCl H 2 SO HCl H 2 SO Table 2: Analysis of the steel composition used in the study: (Concentration expressed in percentage) Scale Description C Si Mn P S Ni Cr Mo Cu Al 1 5mm Plate mm Plate < mm Plate <0.01 Angles <0.01 5mm 5 mm Tube mm dia Rebar <0.01 Analysis by Exova : Lab 15197, Certificate 1671, OES Method, Date

7 Results: Part 1: Galvanizing steel plates containing different concentrations of silica. As the Sandelin curve predicted (Figure 1), the zinc layer that develops on the 5 mm plates containing 0.22% silica, (figure 2), is superior to the minimum required by standard G16. For all galvanizers, a dip time of minutes is sufficient to produce a coating of more than 7 µm. Note that nickel does not have a significant effect on this steel, because galvanizers 1, 2, 5, 7 and use Ni, while galvanizers 3 and do not. The data shows that a prolonged dip time will produce a zinc coating over 300 µm which increases the risk of chipping. For steel containing 0.03% Si, aluminum killed steel, the results shown in figure 3 shows that with a dip time of minutes the standard is achieved in most cases. However, a nickel level higher than 0.05% appears to slow significantly the development of the zinc layer. In light of the results, the steel plate with a silica content of 0.06% (Sandelin peak area), seems the most problematic for galvanizers. When nickel is not used, the zinc coating thickness will easily exceed 300 µm with a dip time of only minutes and can even reach more than 600 µm which we can recall is associated with chipping problems with the zinc coating. On the other hand, when nickel is used as an alloy in the zinc, the same minute dip time is just enough to reach the 7 µm required by the standard. This is when the concentration of nickel must be controlled. The literature suggests the use of Ni between 0.02 and 1%. As the Ni concentration increases, the more significant the suppressing effect on the Sandelin peak. On the other hand, the use of a Ni content of less than 0.03% leads to problems of balance between the Zn, Ni and Fe present in the zinc bath. 7

8 Figure 1a: Duration of dip time in the zinc bath on the Sandelin curve Source: EVRAZ / Highveld: Figure 1b: Effect of galvanizing temperature on the Sandelin curve Source: Zinc Handbook, Frank Porter, p 220

9 p l 5 p o p l 6 p o Figure 2: Values obtained on 5mm plates containing 0.22% silica according to the dip time. B o x p l o t o f p l 6 p o % S i ( e c h 3 ) v s G a l v a n i s e u r ; te m p s t e m p s G a lv a n is e u r Figure 3: Values obtained on 5mm plates containing 0.03% silica according to the dip time. B o x p l o t o f p l 5 p o % S i ( e c h 2 ) v s G a l v a n i s e u r ; te m p s t e m p s G a lv a n is e u r

10 p l p o Figure : Individual values obtained on 5mm plates containing 0.06% silica according to the dip time B o x p l o t o f p l p o % S i ( e c h 1 ) v s G a l v a n i s e u r ; te m p s t e m p s G a lv a n is e u r

11 a n g le 0.1 Part 2: Galvanizing angles As for the angles, (Si 0.1%), the data obtained shows that all galvanizers receive a zinc coating thickness above the galvanizing standard: Figures 5 and 6. The minimum thickness required is easily reached by all galvanizers regardless of dip time and whether or not they use nickel as an alloying element. In fact, the silica content of the angles is typically located in the non-reactive peak of the Sandelin curve and out of range of the suppressive effect of the nickel. Figure 5: Values obtained on 5mm angles containing 0.1% silica according to dip time. B o x p l o t o f a n g l e 0. 1 % S i ( e c h ) v s G a l v a n i s e u r ; te m p s t e m p s G a lv a n is e u r

12 a n g le 0.1 Figure 6: Interval graphic for angles containing 0.1% silica according to the Ni concentration I n te r v a l P l o t o f f e r a n g l e 5 m m 0. 1 % S i v s N i 9 5 % C I fo r th e M e a n * Ni 12

13 t ig e Part 3: Galvanizing rebar All galvanizers who participated in the study managed to obtain zinc thicknesses that meet the galvanizing standard, whatever the dip time (figure 7) or whether or not nickel was present in the zinc bath. Figure 7: Values obtained on 15 mm rebar containing 0.19% silica according to dip time. B o x p l o t o f ti g e % S i ( e c h 6 ) v s G a l v a n i s e u r ; te m p s t e m p s G a lv a n is e u r

14 t u b e Part : Galvanizing mm cylindrical tubes For mm cylindrical tubes containing 0.01% silica, the nickel effect causes a slowdown in the development of the zinc layer. However, after eight minutes of galvanizing, the standards minimum thickness is reached but remains close to the minimum requirement. To significantly distance ourselves from the standards minimum limit, we must galvanize for a duration of twelve minutes: (figure ) Figure : Values obtained on mm tubes containing 0.01% silica according to dip time. B o x p l o t o f tu b e m m é p a i s s e u r % S i ( e c h 5 ) v s G a l v a n i s e u r ; te m p s t e m p s G a lv a n is e u r

15 Part 5: Assembly containing two types of steel and series of different steel parts. In many cases, certain assemblies are composed of more than one type of steel. Take for example the case in image 1 where thick plates containing 0.03% silica are welded to an H Beam, which the silica content is 0.22%. To meet the standards minimum thickness requirement, the dip time must be extended so that the zinc thickness sufficiently develops on the plates. Image 2 shows two assemblies consisting of HSS tubes and plates with silica contents of respectively 0.02 and 0.07%. In this case, the dip time is extended so the HSS tubes reach a sufficient thickness. If the zinc bath does not contain nickel, the zinc coating that develops on the plate will be 00 µm and may be prone to chipping. On the other hand, if the zinc bath contains nickel, the zinc thickness will be about 90 µm. Figure 9 shows that in the case of an assembly containing steel of different nature, the galvanizers not using Ni (galvanic 3 and in our case), are left with thicknesses of 00 µm if they used a dip time of eight minutes to allow the tube to achieve its objective. The galvanizers who use Ni will be faced with measurements reaching the minimum standard even after a dip time of twelve minutes. Figure 9: Results obtained if an assembly of tubes (0.01% Si) and plates (0.06%Si) were used. A s s e m b l a g e T u b e % S i e t p l a q u e p o % S i v s G a l v a n i s e u r ; te m p s tu b e p l p o te m p s G a lv a n ise u r In the case of image 3, we find a series of identical pieces including steel used to manufacture which shows different levels of silica that can be located at the extremes of the Sandelin curve. 15

16 Image 1: H Beam (0.22%Si) and steel plate (0.03%Si) assemblies Image 2: HSS tube (0.02%Si) and steel plate (0.07%Si) assemblies 16

17 Image 3: Identical parts (DGQ 162) from two different types of steel 0.02% Si and 0.22% Si. 17

18 Annex 1 Extract from the report entitled «Thickness of zinc coatings (case studies and search for solutions), November 2010, Eric Michaud» Image 5: View of the typical zinc layers. Source AGA Image 6: Si 0.02% HCL: Pb, Ni, Al Image 9 : Si 0.02% HCL : Pb, Al Image 12 : Si 0.02% H 2SO : Pb, Bi Image 7: Si 0.06% HCL: Pb, Ni, Al Image 10: Si 0.06% HCL : Pb, Al Image 13 : Si 0.06% H 2SO : Pb, Bi Image : Si 0.12% HCL: Pb, Ni, Al Image 11: Si 0.12 HCL: Pb, Al Warning: The images are not all on the same scale refer to tables 10, 11 and 12 for the thicknesses. 1

19 Table 10: Zinc thickness of plates shown in images 6- that were galvanized in Montréal (Alloy Pb, Ni, Al) Image Si (%) Al (%) Thickness (µm) Total Layer 1 Layer 2 Layer Table 11: Zinc thickness of plates shown in images 9-11 that were galvanized in Québec (Alloy Pb, Al) Image Si (%) Al (%) Thickness (µm) Total Layer 1 Layer 2 Layer Table 12: Zinc thickness of plates shown in images that were galvanized by another galvanizer (Alloy Pb, Bi) Image Si (%) Al (%) Thickness (µm) Total Layer 1 Layer 2 Layer Images 6 to 13 were produced by (CM) 2 from samples of steel galvanized in both plants and by another galvanizer. In images 6, 9 and 12 we clearly see the Eta zinc layer. This layer containing 100% Zn is characteristic of aluminum killed-steel. In the case of steel containing 0.06% Si, image 10 shows the reactive development of the zinc coating. We see the presence of two zinc layers and the near absence of the Eta layer. On the other hand, the plates in images 7 and 13 show the presence of the Eta layer. The presence of nickel and bismuth in these respective alloys explains the results observed. Images and 11 show the impact of Ni on reactive steel. The entire zinc coating in image is 71 µm compared to 263 µm on the steel in image 11. The Eta layer clearly noticeable in image is absente in image

20 Annex 2: Experimental protocol: Corbec Project: Study on the frequency of different types of steel Objective: Demonstrate the frequency of different types of steel. Galvanizing protocol: For the test, 6 types of steel were selected. The silica concentration of these samples is divided to represent the variations in the Sandelin Curve. Three dip times will be studied:, & 12 minutes Each galvanizer receives ten samples of each type of steel and for each of the dip times required. Methodology: 1- Attach the parts so they are submerged 12 inches below the zinc surface; 2- Treat the parts in an acid solution and flux as usual; 3- Galvanize parts by giving each package received the following dip times: minutes, minutes and 12 minutes. - Group galvanized parts according to their dip time 5- Return all parts to : Éric Michaud Corbec Inc. 00, George V Lachine, Quebec HS 2R7 (Canada) Éric Michaud Technical Director R&D Cell:

21 Annex 3: 2005 MTQ letter 21