Some Physical Characteristics of Nickel and Chromium Deposits and their Relation to Corrosion Resistance

SummaryAs part of a continuing effort to improve the durability of decorative plated automobile parts, a deleterious type of chromium cracking and the changes in the microstructure of nickel brought about by addition of a sulphur-containing additive have been studied. Type Y cracking of 0·02–0·05 mil chromium deposits was found to result from interaction of the internal stress of the deposit with tensile stress induced by directional abrasion of the nickel substrate. A similar type of cracking was found to occur in a bright nickel plus chromium plate on directionally abraded steel. Reduction of the stress by annealing or removal of the stress layer by electropolishing eliminated this type of cracking. The size of the columnar structure of a Watts deposit was found to decrease when sodium allyl sulphonate was added to the bath. With increasing concentration, the columns gradually disappeared leaving visible only a laminar structure. The transition from columnar to laminar structure occurred well below the normally used additive concentration. In a fully bright deposit, perpendicular faults associated with scratches in the basis metal were observed. These were duplicated in deposits on electro-formed precision roughness standards. The effects of these phenomena on corrosion of decorative plated coatings are discussed.