Cominco's Direct Alloying Process for Ni-Zn Galvanizing

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1 Background Custom galvanizers must adapt their processes to provide quality zinc coatings on a variety of products, while at the same time achieving competitive processing costs. Usually, the galvanizer receives very limited information on the characteristics and history of the work received for processing. Of particular concern are steels containing certain levels of silicon, the so-called "reactive steels", which exhibit high reactivity in the molten zinc, resulting in problems with excessive coating thickness, undesirable properties and higher galvanizing costs. An economic and effective means of dealing with these steels is needed. The Reactive Steel Problem The relationship between the silicon content of the steel and its reactivity is shown in Figure 1, which illustrates the effect of silicon on the galvanized coating weight (i.e. thickness) under typical galvanizing conditions. The coating thickness increases with increasing Si above about 0.05%, reaching a peak at about 0.1% (the Sandelin peak); it then decreases somewhat before rising again to a new peak at about 0.4% Si. Certain other constituents in the steel, notably phosphorus levels, may also influence reactivity. The effect of the high reactivity on the coating microstructure is shown in Figure 2. Under normal galvanizing conditions, the high reactivity causes excessive coating thickness, poor coating adherence and a tendency for unattractive grey coating surfaces, as shown in Figure 3. This latter effect is due to a roughened surface associated with the reactive structure, in which the alloy layer extends to the surface of the coating. Higher dross losses and increased zinc consumption are also experienced. The range of silicon causing high reactivity corresponds to the level often found in continuous cast steels, Cominco Ltd. Page 1 of 10

2 which now make up a large portion of structural steels. Thus, the problem is widely experienced within the galvanizing industry. Since the undesirable effects are due to excessive reaction between the steel and the molten zinc galvanizing bath, one approach to the problem is simply to provide less reactive conditions and reduced reaction time. This involves lowering the temperature of the bath and minimizing the immersion time. Such an approach can be helpful but it has significant limitations, especially where there are mixed steels and the reactive materials cannot be identified. Other approaches, involving more drastic process changes, have included high temperature galvanizing and new bath alloys (e.g. Polygalva), but they have had only partial success. The most practical and effective solution has been the use of an addition of nickel to the galvanizing bath. One of its principal merits is that it requires very little modification to normal galvanizing techniques and conditions. The addition of nickel can be used with either Prime Western Grade or lead-free zinc. The Ni-Zn Process The beneficial effect of nickel in counteracting the high reactivity of Si-containing steels was discovered in the early 1960s in research carried out in Canadian government laboratories under the auspices of the Canadian zinc, steel and galvanizing industries [1], and later supported by ILZRO. Further development and first commercial use of the process occurred in Europe in the 1980s, where the process was given the trade name "Technigalva". Since then, the Ni-Zn process has seen widespread commercial use in Europe, Australia and North America. The Ni-Zn process involves the addition of nickel, in the range of 0.05 to 0.14%, to a normal galvanizing bath. This addition has proved effective in reducing the detrimental effect of Si in steels, up to levels of about 0.2% Si. Figures 4 and 5 illustrate the effect of the nickel on the thickness and structure of coatings on reactive steels [2]. The alloy layer thickness is greatly reduced for Si levels below about 0.2%, but there is little effect Cominco Ltd. Page 2 of 10

3 at higher levels. The structures of the coatings on reactive steels from the Ni-Zn bath are similar to those formed on low silicon steels in conventional galvanizing. The coatings produced on reactive steels by the Ni-Zn process are of good quality. The surfaces are typically smooth and bright, in contrast to the grey mottled surfaces often encountered with conventional galvanized coatings on such steels. The coatings provide good corrosion protection, which has been found to be equivalent to that of conventional coatings [3]. They also exhibit good formability and resistance to mechanical damage. One disadvantage of the process, as compared to conventional galvanizing, is increased dross production. This is due to a reduction in iron solubility in the bath, associated with the introduction of nickel. Iron in excess of the solubility limit combines with the nickel and zinc to form solid intermetallic particles that report to the dross layer. Dross production has been estimated at about 15% more than with conventional galvanizing [5]. An associated problem has been the relatively large amount of nickel lost to the dross. Some plants using the Ni-Zn process have reported that typically about one-half (40-60%) of the nickel added reports to the bottom dross; a small proportion goes to the ash; and the remainder reports to the coating. Since both the intermetallic particles of the dross and the alloy layers of the coating are enriched in nickel, it is normally necessary for the makeup additions to the bath to include nickel at approximately four times the targeted bath level. However, as discussed below, dross production, nickel losses to the dross, and the extent of nickel enrichment in the coating are all very much dependent on the alloying method and the bath operating procedures. There can be additional disadvantages if the nickel content of the bath is at the high end of the range (>0.1%). In such cases, the coating growth on low Si steels can be reduced to the extent that there may be problems in achieving sufficiently thick coatings to meet the specifications. High nickel levels also contribute to high dross production and rough coatings due to entrapped dross particles in the coatings. For these reasons, there has Cominco Ltd. Page 3 of 10

4 been a tendency in recent years to keep nickel levels toward the low end of the range (0.05 to 0.08%). Nickel Additions and the Cominco Direct Alloying Process The most efficient method of introducing nickel is by direct alloying. In this technique, nickel is alloyed into the zinc bath in a specially designed mixing unit. The strong mixing action of the mixer results in complete dissolution of the nickel in a very short period of time. For example, the initial changeover of a typical conventional galvanizing bath to a Ni-Zn bath can be accomplished in a few hours; for ongoing makeup of nickel during operation, the nickel addition can typically be achieved in about 15 minutes. This rapid alloying is advantageous in allowing fast changes from conventional baths to Ni-Zn baths and also in facilitating frequent adjustments to the nickel level. The Direct Alloying Process has proven to be an easy and convenient means for initial bath alloying and for ongoing nickel level maintenance. Prior to the introduction of the Direct Alloying Process, nickel was usually added in the form of a master alloy containing about 2% nickel. The nickel in the master alloy is present as a NiZn compound (NiZn 8 ) within a zinc-rich matrix. Solution of the nickel from the intermetallic compound tends to be a slower process, and there is the added disadvantage of possible settling of the NiZn compound to the dross layer before solution can take place. The extent of this settling will be dependent on the fineness of the intermetallic particles and the bath turbulence, as well as other bath operating conditions. It has been reported that, in the initial alloying with master alloy, the proportion of nickel reporting to the bath is high; however, during ongoing operation of the bath, as more makeup master alloy is added, the nickel efficiency drops greatly and may be as low as 20 to 30% in some cases [4]. A Ni-Zn prealloy (e.g. 0.5% nickel) has also been used for making nickel additions. Compared to the master alloy, this method is advantageous because, in this case, the proportion of nickel intermetallics is much lower and the intermetallics are likely to be more uniformly distributed. The nickel efficiency for additions using a prealloy has been reported at 33%, compared to 20% when using a 2% master alloy [5]. The Direct Alloying Process provides the galvanizer with a cost advantage in comparison to the other alloying approaches. With both the master alloy and prealloy methods, there are prior zinc-nickel alloying costs that must be reflected in the price of the alloy. This means that the cost of the nickel added to the bath by the alloy approach is significantly greater than that for the Direct Alloying Process. A comparison with the prealloy approach, using estimated current costs and typical conditions, is shown in Appendix A. The example estimates savings and costs (in US dollars) for a plant processing about 15,000 tons of reactive steel annually. For the calculation shown, an Cominco Ltd. Page 4 of 10

5 annual nickel cost of $90,000 using the prealloy method compares to a cost of $28,000 using the Direct Alloying Process, a reduction in cost of about 70%. (An additional cost with the Direct Alloying Process is the initial investment in the mixer costing approximately $4,000.) Also shown in the calculation is the zinc savings realized from using the Ni-Zn process; this is estimated at about $75,000 as a result of the reduced zinc consumption. In view of the toxicological nature of nickel, it is essential that good industrial hygiene practice be employed when making the nickel additions. This is particularly true in the case of the nickel powder additions. Regulations and reliable guidance are readily available [6,7]. An independent air sampling program in a plant using the Direct Alloying Process showed that airborne nickel was less than 1% of the allowable levels. The investigators concluded that, for the conditions tested, "...occupational overexposure to nickel is unlikely to occur during the handling of nickel powder" and "background nickel exposure levels are not likely to exceed existing or proposed occupational exposure limits... "[8]. Ni-Zn Alloy Bath Management Recent research by Cominco and others has increased the knowledge of the Zn-Ni-Fe alloy system and helped to clarify factors influencing the operation of the process [9]. It has shown that the solubility of iron in the zinc bath at 450C decreases from 0.029% in a nickel-free bath to less than 0.01% at a nickel level of about 0.229%, as shown in Figure 6. When the combined levels of iron and nickel exceed the solubility limit, intermetallic phases are formed which then settle to the bottom dross layer or, in some cases, may be trapped in the galvanized coating. From the phase diagrams developed, and confirmed by observation, it has been established that, when the nickel content of the bath is below 0.06 %, the zeta phase is the intermetallic phase formed. When the nickel content exceeds 0.06%, a ternary Zn-Ni-Fe phase, gamma 1, is formed. The zeta phase contains less than 1% nickel, while the gamma 1 phase contains nickel in the range of 2 to 3%. Cominco Ltd. Page 5 of 10

6 This improved understanding of the phase diagram indicates factors important to the nickel use efficiency. The gamma 1 phase intermetallic, which forms when the nickel is >0.06% and contains a higher level of nickel, adds to the loss of nickel to the bottom dross. Furthermore, if the nickel level is much above 0.06%, the higher level of gamma 1 phase formed can result in intermetallic particles being trapped in the coating, resulting in the roughness and excessive coating thickness sometimes encountered. The effect of bath nickel level on the loss of nickel from the bath is illustrated in Figure 7. [9] This figure shows a plot of nickel content of two different operating galvanizing baths as a function of time. For Bath No.1, the nickel consumption was 45 ppm per day for a bath nickel above 0.06% and 11 ppm per day for a bath nickel below 0.06%. For Bath No.2, the nickel consumption was 48 ppm per day for bath nickel contents above 0.05% and 12 ppm per day for bath nickel contents below 0.05%. The reduced rate of nickel loss when the nickel content dropped below about 0.06% is believed to be due to the change in the intermetallic dross particles forming in the bath and the degree of nickel enrichment in the coating. As noted above, below the 0.06% level, the only intermetallic formed would be zeta phase, which has significantly lower nickel content than the ternary gamma 1 phase. Cominco Ltd. Page 6 of 10

7 As shown in Figure 6, the solubility limits of iron and nickel in liquid zinc are very much affected by the bath temperature. Reductions in temperature result in reduced solubilities. Thus, if the temperature of a Ni-Zn galvanizing bath falls significantly, the solubility of iron and nickel in the bath will be decreased, resulting in the formation of increased amounts of intermetallics. These will tend to settle to the bottom of the pot contributing to more dross losses. Increases in temperature would cause more iron and nickel to go into solution, but if the temperature was later reduced, then additional intermetallics would be produced. It is, therefore, advisable to avoid wide swings in temperature as much as possible. It has been found that as little as 0.02% nickel can partially suppress the reactive coating tendency [2] (Figure 4), while 0.1% can completely suppress it. Earlier Cominco work (Figure 8) showed that most of the reactivity suppression effect can actually be obtained with nickel levels as low as 0.06% [10]. For example, the results with a reactive steel containing 0.17% Si indicated that a 0.064% nickel addition achieved more than 80% of the coating weight reduction given by and 0.13% nickel baths. Thus, in this case, a doubling of the nickel level in the bath resulted in less than 20% further reduction in the coating thickness. It is clear from the above discussion that the optimal nickel content for the process is at the low end of the range that provides effective reactivity control. Keeping the nickel content low will contribute to process efficiencies and will avoid problems of belowspecification coatings on low Si steels. The recommended range for nickel content in the bath is 0.05 to 0.06%. The use of frequent additions of nickel powder, using the Cominco Direct Alloying Process, is believed to be the best method of maintaining this narrow range in order to optimize the process from the point of view of cost, efficiency and product quality. Cominco Ltd. Page 7 of 10

8 Conclusions and Recommendations The Ni-Zn process has been found to be an effective method of coping with the problems associated with the galvanizing of reactive steels. The Cominco Direct Alloying Process provides the most cost effective method of adding nickel to the bath and controlling the process for maximum efficiency. The following processing practices will help optimize the operation of the process: 1. Maintain bath nickel content in the range of 0.05 to 0.06%. This should give the best balance between product properties and process efficiencies. 2. Make small nickel additions frequently so that the recommended nickel level can be maintained without large variations above or below the target level. 3. Keep the bath temperature within a narrow range, as far as is practical; wide swings in temperature will contribute to increased dross production. Further Information More information on Ni-Zn galvanizing and the Cominco Direct Alloying Process can be obtained by contacting Cominco's Technology Sales Group. References 1. J.J. Sebisty and R.H. Palmer, "Hot Dip Galvanizing with Less Common Bath Additions", in Proceedings 7th International Conference on Hot Dip Galvanizing, Paris, France, June, 1964, Pergamon Press, 1967, pp B.D. Notowidjojo, N.F. Kenon and A.L. Wingrove, "Zinc-nickel Coating -- A New Galvanizing Technology", in Proceedings Step into the 90's Conference, Queensland, Australia, August 27-31, 1989, Australasian Institute of Metal Finishing, Parkville, Victoria, Australia, pp R. Sokolowski, "Zinc-nickel Baths: A Response to Developments in Steelmaking", in Proceedings 15th International Galvanizing Conference, Rome, Italy, June 5-10, 1988, Zinc Development Association, London, England, 1988, pp. GE1/1-GE1/8. 4. D. Stroud, "Galvanizing with Zinc-Nickel Alloy", in Proceedings 1st Asian-Pacific General Galvanizing Galvanizing Conference, Taipei, Taiwan, September 15-18, 1992, Asian-Pacific General Galvanizing Association, 1992, pp Cominco Ltd. Page 8 of 10

9 5. A.F. Skenazi. and D. Rollez, "Hot Dip Galvanizing of Semi-Killed Steels with the Zinc- Nickel Bath", in Proceedings 15th International Galvanizing Conference, Rome, Italy, June 5-10, 1988, Zinc Development Association, London, England, 1988, pp. GE2/1-GE2/5. 6. Safe Use of Nickel in the Workplace, Nickel Development Institute and Nickel Producers Environmental Research Association, May Nickel Powder, Material Safety Data Sheet, Westaim Specialty Products, August 29, M.M. Dillon Limited, Nickel Survey Report, Cominco Ltd., June N. Qiang, N.Y. Tang and G.R. Adams, "A Study of the Zn Corner of the Fe-Ni-Zn System", Project No , Report No. 2, Cominco Ltd., Product Technology Centre, Mississauga, ON, July 21, L. Battiston and G.R. Adams, "New NiZn Galvanizing Alloy for Plant Trial", Project No , Report No. 1, Cominco Ltd., Product Technology Centre, Mississauga, ON, January 24, Cominco Ltd. Page 9 of 10

10 APPENDIX A Ni-Zn Galvanizing Operation -- Example of Savings and Costs Prealloy Nickel Additions Compared to Direct Alloying Process Assumptions 1. 1,000 cu. ft. kettle processing 15,000 tons of reactive steel annually. 2. Nickel addition at 0.25% of zinc feed, to maintain bath nickel content at target level (0.06%). 0.5% Ni prealloy blended 1:1 with zinc feed. 3. Zinc consumption reduced from 7.0% of steel throughput, for a conventional bath, to 6.5% for a Ni-Zn bath. 4. Metal costs (in US dollars): Zinc $1,000 per ton 0.5% Ni prealloy premium $185 per ton Nickel powder $5.80 per lb Annual Savings and Costs (US dollars) Zinc Savings from Ni-Zn Process (7.0% - 6.5%) x 15,000 tons x $1,000=$75,000 Prealloy Method Nickel Addition Costs 6.5% x 15,000 tons x $185 x ½(blend ratio) = $90,187 Direct Alloying Process Nickel Addition Costs 6.5% x 15,000 tons x (0.25% x 2,000 lbs) x $5.80 = $28,275 (Additional one-time investment for special mixer = $4,000) Cominco Ltd. Page 10 of 10