Effect of Organic Catalyst Addition on Pitting Evolution at Hard Chromium Plating

Abstract

Electrolytic hard chromium plating has been extensively used in commercial and military applications to impart enhanced wear and corrosion resistance to steel. However, a common problem on the surface of hard chromium plating is pitting defects which typically reduce mechanical properties and the visual appearance of the plating. While there are many factors that influence pitting in hard chromium plating, the use of a secondary catalyst is one of the major contributors. In the present study, two different chromium layers were plated on commercial St37 steel using two different mixed-catalyst baths at 55 +/- 1 degrees C: (i) an electrolyte CrO3/H2SO4 solution with 30 g/L as a commercial organic catalyst and (ii) CrO3/H2SO4 solution without any organic catalyst. The organic catalyst increased the throwing power of chromium and caused the formation of pit defects, trapped bubbles, and microcracks without affecting the plating-substrate interface bonding. The trapped bubbles were readily visible to the unaided eye after chromium plating, and they transformed into hemispherical pits at the end of the post-plating finishing treatment. The results of this study show that plating parameters and organic catalyst additives need to be carefully controlled in order to minimize pitting in hard chromium coatings.