Development of Trivalent Chromium Passivation for Zn Platng with High Corrosion Resistance after Heating

Kawaguchi, Hiroshi; Funatsumaru, Osamu; Sugawara, Hiroyoshi; Sumiya, Hiroshi; Iwade, Takanobu; Yamamoto, Tomitaka; Koike, Tatsuhiko; Kashio, Ryuta

doi:10.4271/2016-01-0542

Cited by 2 publications

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“…Investigations into trivalent chromium passivation (TCP) layers have shown excellent anticorrosion performance on many substrates − including ZnNi alloys. − TCP layers utilize Cr(III) compounds for corrosion protection, which are the same as the products formed when hexavalent chromium passivation layers react to protect damaged portions of the passivation layer. TCP layers may also generate some Cr(VI) species during deposition or use, although a lesser degree of active corrosion protection has been demonstrated for TCP layers compared to chromate conversion coatings. ,, Many different chemical solutions can be used to deposit TCP layers, but solutions that contain cobalt lead to improved corrosion performance of TCP layers in salt spray testing. , Despite the improvement seen by additions of cobalt, a hexavalent chromium alternative that does not contain cobalt is needed since the European Union already has some industrial restrictions on cobalt with an expectation that even stricter regulations will be passed in the future…”

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confidence: 99%

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

Foster

Claypool

Fahrenholtz

et al. 2021

ACS Appl. Mater. Interfaces

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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).

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