Corrosion Resistance of Chromium-Plated Zinc Alloy Die Castings

Abstract

SummaryThe benefits of thicker, crack-free chromium have been confirmed in several exposure tests. Production trials have demonstrated that the high-temperature high-ratio process involves no major technical difficulties and no large increase in cost. No significant improvement in performance was observed using duplex instead of single-layer bright nickel; sacrificial corrosion of the upper layer of the duplex coating occurred, but delayed penetration only briefly and gave rise to surface pits which were themselves unsightly. A duplex coating might show to greater advantage on steel articles, both because its superior ductility enables it to resist distortion and because the postponement of rusting is perhaps more important than a delay in the first appearance of zinc corrosion product. Dual, microcracked chromium showed variable corrosion resistance. At its best, it greatly delayed the occurrence of basis metal corrosion; it suffered a slow permanent loss of lustre, however, and was liable to stain in humid atmospheres. Sandwich chromium coatings were also variable in performance, and most samples showed at least a few sites of basis metal attack. The shallow pits formed in the upper layers of these coatings were unobtrusive so long as they were not too numerous. No doubt, a more consistent performance could be achieved, but the cost of this process must remain high.