Zinc Alloy Coated Steel Sheets

It is not easy to find the composition of an alloy which gives the optimum corrosion resistance and applicability of paint. The third and fourth additives may then be considered. With multi-layer coatings more combinations are possible. The Fe--Zn/Zn--Fe two-layer coating has already been in use. The Zn--Fe alloys and the Zn--Ni alloys give hard coatings, which cannot easily be worked. Microscopic cracks result in powdering. Flaking may be due to the fact that the coating is not firmly attached to the base metal or to the fact that flakes built up on the tool are crushed and scattered on the surface. Small metal pieces attached to the tool may cause damage to the coating during press working. The Zn coated steel is more susceptible to this galling than is Zn alloy coated steel. Spot welding is essential for welding automotive steel sheets. It requires suitable weld current density. The pure zinc coated steel, whose characteristic resistance is smaller than that of iron, requires 20% more current than the cold-rolled steel. Alloy coated steel, such as with Zn--Ni, Zn--Fe, has greater contact or characteristic resistance than pure zinc coated steel. 43 ref.--S.W.