Effect of Surface Preparation on Cracking and Structure of Chromium Deposits

Abstract

Conventional bright chromium deposits (0.02-0.05 rail thick) on buffed nickel plates often develop a directionally oriented crack pattern which may adversely affect appearance and corrosion resistance. This cracking was found to occur by interaction of polishing-induced, directional residual stress in a surface layer of the nickel with isotropic internal stress of the chromium deposit. Removal of the stress-containing layer by electropolishing or stress relief by annealing eliminates this cracking. Similar cracking can occur in bright nickel-chromium composites plated on polished steel. Chromium deposits on electropolished nickel are apparently strongly epitaxial. Observed variations in appearance, porosity, and corrosion behavior of deposits on individual grains of wrought nickel correspond to similar variations symmetrically distributed in chromium plated on spherical nickel single crystal. The significance of these observations is discussed.The use, efficacy, and electrochemistry of microcracked chromium in controlling corrosion of copper-nickel-chromium plated coatings for decorative automobile trim have been discussed by a number of authors (1-6). This type of cracking [Type M in Lindsay's (2) classification] is described by Lovell (1) as "... a fine network of cracks extending through the chromium and invisible to the unaided