Characterization of Cobalt-Containing and Cobalt-free Trivalent Chromium Passivation Layers on γ-ZnNi-Coated Al6061-T6 Substrates

Abstract

The corrosion performance and electrical contact resistance were investigated for a trivalent chromium passivation layer and a cobalt-free version of that same passivation layer on γ-ZnNi-coated Al 6061-T6. Both passivation layers had a similar surface morphology, were amorphous, had similar thicknesses, and contained pores within the passivation layer. The cobalt-containing passivation layer initially had an exchange current density of 9.5 × 10^-4 A/cm² and a polarization resistance of 290 Ω/cm². The cobalt-free passivation layer initially had an exchange current density of 10.6 × 10^-4 A/cm² and a polarization resistance of 116 Ω/cm². After 500 h of exposure to neutral salt spray, the cobalt-containing passivation layer showed no visible corrosion and had an exchange current density of 2.9 × 10^-4 A/cm² and a polarization resistance of 136 Ω/cm². The cobalt-free passivation layer showed uniform corrosion and had an exchange current density of 5.2 × 10^-4 A/cm² and a polarization resistance of 80 Ω/cm². After 500 h of exposure to neutral salt spray on specimens which were scribes down to the Al substrate, the cobalt-free passivation layers were uniformly corroded, but scribed specimens with the cobalt-containing passivation layers were only partially corroded. Both the cobalt-containing and cobalt-free passivation layers were found to be viable alternatives to hexavalent chromium as per the requirements of cobalt-containing MIL-DTL-81706 offering protection comparable to hexavalent chromium and cobalt-free offering less. The presence of cobalt in the TCP layer was found to improve corrosion performance and suggested that an intermediate species such as cobalt is beneficial to the oxidation of Cr(III) to Cr(VI).