| Zinc and zinc alloys are used for extrusions, castings, forgings, rolled sheet or drawn wire and of course as a protective coating. Two other major uses are as sacrificial anodes and in brass alloys. Application of Zinc Coatings Zinc coatings are either applied from a bath of molten zinc or from a dip of zinc/0.2% aluminium in a strip galvanising line or by thermal spray (or metallising), mechanical plating (called barrel plating after the tumbling process used to carry it out) or else, for thinner coatings, by electroplating. Casting of Zinc Alloys Zinc alloys have been used in die casting for over 60 years and white hypoeutectoid alloys with <5% aluminium were used for pressure die castings, the current practice is to use the range of higher strength hypereutectoid zinc/aluminium alloys. Alloy contamination must be minimised to maintain mechanical and corrosion resistance properties. Zinc alloys have low melting points, good fluidity and, because of the relatively rapid chill rate, are quite stable dimensionally. The alloys lose about 25 to 35 % of tensile and yield strength at 100°C. Gravity castings also require high purity stock. Dimensional stability may be assured by a stabilising treatment @250°C for 12 hours followed by furnace cooling. Specialised slush casting alloys are used for hollow objects. Zinc castings are often surface finished and treatments include phosphating prior to painting and anodising, or more commonly chromating, to provide corrosion resistance and prevent white rust during damp storage. Joining of Zinc Alloys Components may be soldered, brazed or even adhesively bonded. Use of mechanical fasteners requires adherence to recommended torques. Wrought and Superplastic Zinc Alloys Wrought zinc products include zinc or foil with typically <1% alloying addition of copper or lesser quantities of lead, titanium, manganese or cadmium. A superplastic alloy with 21 to 23% aluminium is readily formed into complex shapes. Corrosion of Zinc Alloys Zinc is resistant to corrosion by chemicals over a pH range from 6 to 12.5. It produces white corrosion product when kept wet especially when freshly exposed. |