Characterization of cobalt containing and cobalt-free trivalent chromium passivations on γ-ZnNi coated steel substrates

Abstract

The corrosion performance was studied for standard trivalent chromium passivations and cobalt-free trivalent chromium passivations on steel substrates. The steel substrates were either polished or unpolished and coated with γ-phase ZnNi prior to passivation. For unpolished substrates, cobalt-free specimens had 50 ± 8 cracks/mm, a thickness of 69 ± 4 nm, and notable porosity compared to cobalt-containing passivations which had no cracks, a thickness of 40 ± 5 nm, and no porosity. For polished substrates, cobalt-free passivations had 50 ± 3 cracks/mm, a thickness of 38 ± 2 nm, and notable porosity compared to cobalt-containing passivations with 16 ± 2 cracks/mm, thickness of 48 ± 2 nm, and no porosity. After 1000 hours of salt-spray exposure, cobalt-free passivations had visible white rust on 4% of the surface area while cobalt-containing passivations had 22% of the surface area covered by white rust. The thickness of both passivations increased on both polished and unpolished specimens after salt spray exposure. Electrochemical testing showed that the cobalt-free passivation had the least change before and after salt spray exposure. The difference in corrosion performance was attributed to the greater porosity in the cobalt-free passivations influencing the amount of corrosion inhibiting species present.