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).

An Industrially Applicable Passivation Strategy for Significantly Improving Cyclability of Zinc Metal Anodes in Aqueous Batteries

“…For decades, TCCF have demonstrated outstanding corrosion resistance for various sacrificial coatings, including Zn and its alloys, during the protection of underlying metals such as steels. [36,38,39] Significantly, the low-toxicity TCCF exhibit zincophilicity, wettability, and an electronically insulated nature, [36] which are beneficial for suppressing Zn dendrite generation and alleviating parasitic side reactions that take place on the surface of Zn foils. [40][41][42][43][44][45][46] If superior passivation films can be converted into artificial interphases, diverse Zn plating passivation strategies might greatly stimulate the progress of surface modification approaches for Zn anode protection.…”

“…Industrially, the functional lifespan of Zn-plated steels is often prolonged by passivation films formed in situ on the surface of Zn coatings through a passivation treatment, involving simple immersion into a specific passivation solution. [36][37][38][39] Take the commercially available Cr(III)-based passivation technique as an example, the physically protective properties of trivalent chromium conversion films (TCCF) can be controlled by tuning the composition of the passivation bath, including the film former, oxidizer, inhibitor, sealant, surfactant, etc. [36,38,39] The asprepared TCCF with adjustable colors (e.g., black, bright blue, green, and colorful) are typically comprised of a complex mixture containing Cr(III) oxides, hydroxides, and hydrated oxides.…”

“…[36][37][38][39] Take the commercially available Cr(III)-based passivation technique as an example, the physically protective properties of trivalent chromium conversion films (TCCF) can be controlled by tuning the composition of the passivation bath, including the film former, oxidizer, inhibitor, sealant, surfactant, etc. [36,38,39] The asprepared TCCF with adjustable colors (e.g., black, bright blue, green, and colorful) are typically comprised of a complex mixture containing Cr(III) oxides, hydroxides, and hydrated oxides. For decades, TCCF have demonstrated outstanding corrosion resistance for various sacrificial coatings, including Zn and its alloys, during the protection of underlying metals such as steels.…”