The Corrosion of Decorative Nickel + Chromium Coatings: A Metallographic and Potential Study

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Abstract
The morphology of pits in decorative nickel+chromium coatings resulting from service corrosion in an industrial atmosphere has been compared with that of pits produced in accelerated corrosion tests. Corrosion in the three accelerated tests examined and in the early stages of service corrosion was frequently characterised by formation of hemispherically shaped pits in the nickel layer: these occurred at discontinuities in the chromium top-coat. In the later stages of service corrosion, say, after several years’ exposure, pits were more often of irregular shape. In copper+nickel+chromium coatings, none of the accelerated tests gave close reproduction of the type of pit produced by corrosion in service. In double-layer nickel+chromium coatings, however, the type of pitting experienced in service was reasonably well reproduced by the acetic acid/salt spray and the Corrodkote tests, but not by the SO2 test. In order to obtain a better appreciation of the electrochemistry of the pitting corrosion observed, measurements were made of the anode polarisation characteristics of electrodeposited nickel and copper foils and of wrought steel, in electrolytes simulating those used in the accelerated corrosion tests. The results of these measurements are discussed in relation to the metallographic observations.

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