The Effect of Deformation on the Corrosion Resistance of Nickel + Chromium Coatings: A Metallographic Study

Abstract

SummaryA study has been made of the incidence of cracking in deformed nickel + chromium plated panels and of the relation to cracking corrosion behaviour. After very severe deformation, only semi-bright nickel remained completely free from cracking. Bright nickel cracked readily, and in heavily deformed regions the cracks tended to propagate, by notch effect, through ductile semi-bright nickel or copper undercoats. In specimens less severely deformed, cracking terminated at the semi-bright nickel/bright nickel or the copper/bright nickel interface. Thick, initially crack-free chromium coatings cracked over a wider area and induced many more cracks in bright nickel substrates than did chromium of conventional thickness, whereas over similar substrates thick microcracked chromium coatings induced fewer cracks than conventional chromium. In all test environments, wide cracks which terminated at an active substrate such as steel or zinc usually showed corrosion, of the substrate only, but in stationary and mobile exposure tests corrosion of both the bright nickel and the active substrate tended to occur in narrow cracks. In acetic-acid/salt spray, however, corrosion in even the narrowest cracks was confined to the active substrate. With increasing nobility of substrate, in the order brass, copper, semi-bright nickel, the proportion of corrosion occurring in the substrate decreased and attack was confined to the bright nickel layer.