Automobile Corrosion and Corrosion Control at Bimetallic Couples

Zinc coatings significantly improve the structural and cosmetic corrosion resistance of steels under automotive paint systems. The increasing use of metallic-coated steels in automotive manufacture has inevitably resulted in areas where the coated steel is joined to cold-reduced steel. Electrochemical reactions at these areas result in poor phosphate deposition, under- paint saponification and eventual cathodic disbondment of the paint. Laboratory phosphating experiments and outdoor weathering trials have highlighted these effects. Pre-coating the zinc surface, usually by pre-phosphating, prior to joining to the cold-reduced steel, has been found to reduce the extent of these undesirable reactions.