Corrosion protection of metallic components using hexavalent chromium coatings has been widely used for military and commercial application for decades. Hexavalent chromium is a known carcinogen and alternative corrosion inhibitors that do not have negative environmental and health impacts are needed. This project focused on development and evaluation of environmentally friendly trivalent chromium passivation on electroplated zinc nickel to replace hexavalent chromium passivation for electrical connectors.

Development of deposition processes was done to control the chemistry and morphology of trivalent chromium passivation to optimize corrosion protection of electroplated zinc nickel coatings on steel and aluminum substrates. Data from neutral salt spray exposure, electrical contact measurements, electrochemical testing, and materials characterization analysis of samples was used to adjust deposition conditions for optimal corrosion performance. This data was also used to better understand fundamental corrosion mechanisms and propose a protection scheme for how trivalent chromium passivation mitigates corrosion on electroplated zinc nickel coatings. Once the optimized processing conditions were identified, fully functional electrical connectors were coated, passivated, and tested per electronic industry association specifications and standards.

Novel trivalent chromium passivated zinc nickel coatings on steel and aluminum metallic substrates exposed to hundreds of hours of neutral salt spray exposure had minimal corrosion product and low electrical contact resistance values comparable to hexavalent chromium passivation reference samples. The passivated layers were measured to be less than 100 nanometers in thickness but able to provide active corrosion protection. Chemical composition analysis of the trivalent chromium passivation was found to be dependent on the deposition conditions and post-deposition processing. In particular, trace amounts of hexavalent chromium in the passivation could be correlated to the acid level in the deposition bath and heat treatment time and temperature after deposition. The appearance and uniformity, adhesion, torque lubricity, neutral salt spray performance, and shell conductivity of electrical connectors with trivalent chromium passivation on electroplated zinc nickel were comparable in value to hexavalent chromium passivation on zinc nickel and hexavalent chromium passivation on electroplated cadmium connector assemblies. A corrosion protection mechanism based on the oxidation-reduction reaction between trivalent chromium and hexavalent chromium and transport of species to active corrosion sites was proposed.

The technology developed during this project demonstrated for the first time that trivalent chromium passivation on electroplated zinc nickel coatings was capable of simultaneously providing corrosion protection and low electrical contact resistance on aluminum electrical connector assemblies. With further investigation and optimization, it is possible to replace the existing connection coating systems that utilize electroplated cadmium and hexavalent chromium passivation with a more environmentally friendly electroplated zinc nickel coating and trivalent chromium passivation.