Zinc-aluminum alloy coating and method of hot-dip coating

#The present invention relates generally to a zinc-aluminum alloy coated ferrous metal strip and more particularly to a ferrous metal strip having a smooth bright zinc-aluminum alloy hot-dip coating which exhibits improved resistance to intergranular corrosion when exposed for prolonged periods to a high humidity atmosphere and which is further characterized by good formability properties and the absence of blisters both before and after prolonged exposure to a high humidity atmosphere, by a markedly reduced susceptibility to the formation of white rust, and by a reduced rate of general surface corrosion without any diminution in the mechanical properties of the coating.

#In a continuous process of producing hot-dip galvanized sheet material in which an endless ferrous metal strip is continuously passed through a molten bath comprised mainly of metallic zinc so as to protect the ferrous metal against corrosion it has been found advantageous to include at least a small amount of aluminum in the zinc bath. Thus, adding from 0.15 to 0.3 wt.% aluminum to a zinc hot-dip galvanizing bath prevents forming a thick inter-metallic layer on the ferrous metal surface and improves the formability of the coated strip. It has also been found that adding larger amounts of aluminum to the zinc coating bath (i.e. from about 4 wt.% up to about 17 wt.%) further improves the resistance of the coating to surface corrosion without interferring with good formability.

#When an endless steel strip is hot-dip coated with a zinc or a zinc-aluminum alloy in a modern continuous coating line, particularly when coating at low line speeds, the fluidity of the bath is such that it is difficult to form a smooth, ripple-free hot-dip coating having good paintability properties and an attractive appearance. In order to obtain a smooth, attractive hot-dip coating it has heretofore been considered necessary to include in the zinc-aluminum hot-dip coating bath a small but definite amount of lead to impart to the bath the required low surface tension so that a smooth ripple-free coating will be formed. The addition of lead to the coating baths also aids in the formation of spangles, particularly in the zinc coatings containing a small amount of aluminum (i.e. around 0.2 wt.% aluminum). In order to form a smooth hot-dip coating at least 0.06 wt.% lead is required in a hot-dip coating bath containing between about 0.2 wt.% and about 17 wt.% aluminum with the balance being essentially zinc and in commercial practice at least about 0.1 wt.% lead is used.

#Other objects of the present invention will be apparent to those skilled in the art from the following detailed description and claims when read in conjunction with the accompanying drawing, wherein:

#FIG. 1 is a plan view at 9X magnification of the unetched surface of a hot-dip coated ferrous metal panel (i.e. 20 gauge rimmed steel) after four weeks exposure to a condensing humidity atmosphere at a temperature of 130.degree. F wherein the hot-dip coating is a 5 wt. % aluminum-zinc alloy containing 0.1 wt. % lead with the balance essentially zinc;

#FIG. 2 is a vertical sectional view of the unetched panel of FIG. 1 showing the microstructure at 600X magnification of one portion of the hot-dip coated ferrous metal panel;

#The several objects of the present invention are achieved by continuously hot-dip coating a ferrous metal sheet in a zinc-aluminum alloy hot-dip coating bath which has a low lead content (i.e. a maximum of 0.02 wt. % lead), and which contains between about 0.2 wt. % and about 17 wt. % aluminum, and between about 0.02 and 0.15 wt. % antimony with the balance being essentially zinc. Whereas it has heretofore been considered essential to use at least about 0.06 wt. % lead and up to about 0.15 wt. % lead in a zinc-aluminum alloy hot-dip coating bath containing between about 0.2 to about 17 wt. % aluminum in order to reduce the surface tension sufficiently to form a ripple-free surface and provide a coating having an attractive appearance, it has now been found that by maintaining the lead content of the bath at a maximum of about 0.02 wt. %, and preferably not more than 0.01 wt. % lead, and adding antimony to the bath in an amount between about 0.02 wt. % and about 0.15 wt. %, the coating bath will have a surface tension required to form a smooth ripple-free hot-dip coated surface, will have the desired bright smooth appearance and, most significantly, will not exhibit significant intergranular corrosion nor form blisters caused by intergranular corrosion when the hot-dip coating is exposed to a high humidity atmosphere for a prolonged period. Furthermore, it has been found that the addition of antimony to an aluminum-zinc alloy coating bath has a greater effect in reducing the surface tension of the bath and produces, particularly in the 0.2% aluminum-zinc coatings, a significantly larger flatter grain or spangle size than the same concentration of lead provides in an otherwise identical zinc-aluminum coating bath and without causing any of the adverse effects of lead disclosed herein. It has also been discovered that by the having antimony in the zinc-aluminum hot-dip coating in the amount specified herein while the lead concentration is maintained at a maximum of about 0.02 wt. %, the susceptibility of the zinc-aluminum alloy coating to white rust is markedly reduced and the rate of general surface corrosion of the hot-dip coated ferrous metal sheet is also reduced without causing any adverse effects on the mechanical properties of the hot-dip coating.

#To illustrate the invention a series of 5 wt. % aluminum-zinc alloy hot-dip coating baths were prepared from pure aluminum and pure zinc to provide coating baths containing 5 wt. % aluminum, with antimony and lead contents as indicated in the following Table I, and with the balance being essentially zinc. Each bath was saturated with iron to provide an iron concentration of about 0.02 wt. % (which corresponds to the normal iron build up in a continuous hot-dip galvanizing bath). A series of 20-gauge rimmed steel panels (4 .times. 8 inches in size) were hot-dip coated in the baths. The steel had a chemical composition as follows: about 0.08% carbon, 0.29% to 35% manganese, 0.01% to 0.011% phosphorus, 0.019% to 0.020% sulfur, and 0.04% copper, with the balance essentially iron. All the panels were precleaned by oxidizing in a furnace at 1650.degree. F for 30 seconds, and the oxidized panels were then transferred into a laboratory "dry box" which contained the coating baths and laboratory galvanizing equipment. The reducing atmosphere inside the "dry box" comprised 10% hydrogen with the balance nitrogen. The dew point inside the dry box was always kept below -15.degree. F during the hot-dip coating operation. The clean panels were preheated at 1700.degree. F for 3 minutes in the reducing atmosphere of the dry box to effect removal of all surface oxides and then cooled while being maintained within the reducing atmosphere of the dry box to the hot-dip coating bath temperature of about 820.degree. F. The immersion time in the coating bath for each panel was about 5 seconds to provide an average coating weight of about 0.5 oz. per sq. ft. The several hot-dip coated panels, after two weeks exposure to a 92% R. H. atmosphere and at a temperature of 176.degree. F, were examined to determine the degree of intergranular corrosion and blistering, and the result for the several coatings are tabulated in the following Table I:

#TABLE I ______________________________________ Wt. % Aluminum-Zinc Alloy Coating Characteristics After Coating Bath.sup.(1) Exposure To High Humidity Additives (Wt. %) Atmosphere.sup.(2) ______________________________________ 1. 0.01% Lead, No Antimony No intergranular corrosion - No Surface Blisters 2. 0.02% Lead, No Antimony No Intergranular Corrosion - No Surface Blisters 3. 0.04% Lead, No Antimony Slight Intergranular Corrosion - No Surface Blisters 4. 0.05% Lead, No Antimony Intergranular Corrosion - Few Surface Blisters 5. 0.06% Lead, No Antimony Intergranular Corrosion - Suface Blisters 6. 0.08% Lead, No Antimony Severe Intergranular Corrosion - Surface Blisters 7. 0.1% Lead, No Antimony Severe Intergranular Corrosion - Surface Blisters 8. 0.02% Antimony, No Intergranular Corrosion - No <0.01% Lead Surface Blisters 9. 0.05% Antimony No Intergranular Corrosion - No <0.01% Lead Surface Blisters 10. 0.10% Antimony No Intergranular Corrosion - No <0.01% Lead Surface Blisters 11. 0.15% Antimony No Intergranular Corrosion - No <0.01% Lead Surface Blisters 12. 0.15% Antimony, Severe Intergranular Corrosion - 0.1% Lead Surface Blisters 13. 0.1% Antimony, No Intergranular Corrosion - 0.02% Lead No Surface Blisters 14. 0.1% Antimony, Intergranular Corrosion - Few 0.06% Lead Surface Blisters 15. 0.1% Antimony, Severe Intergranular Corrosion - 0.1% Lead Surface Blisters ______________________________________ .sup.(1) Before additives the coating bath contains 0.02 wt. % iron with balance essentially pure zinc and aluminum. .sup.(2) 92% R. H. atmosphere at 176.degree. F for two weeks.