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Nickel Acetate vs. Nickel sulphate anodizing seals

While evaluating samples from a new supplier we discovered that they used a Nickel sulphate seal on the Oxalic anodized Aluminum. Our print specifies a Nickel Acetate seal over the anodized surface.

What is Nickel sulphate and how does it differ from Nickel Acetate?

Cory Doble
product engineer - Caro, Michigan
2007

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The initial patent on nickel- and cobalt-containing anodize seals mentions both nickel acetate and nickel sulphate as well as nickel ammonium sulphate. Nickel acetate is considered preferable to nickel sulphate due to a more stable solution pH (less pH change when nickel precipitates from a weak acid salt solution than from a strong acid salt solution). 'Treatment of Coatings' - Alcoa, US Patent 2008733 (1935).

Nickel sulphate apparently hasn't been used commercially in anodizing seals in the English-speaking world; The Surface Treatment and Finishing of Aluminum and Its Alloys mentions its use in Italy. Nickel sulphate is nevertheless included in recent US Patents, and chromium sulphate is included as a source of trivalent chromium in a TCP anodize seal [US Patent 6511532, remarkably similar to the TCP chem film process], so the sulphate anion does not seem detrimental to sealing.

The real question is whether the anodizing meets application requirements. Have the nickel sulphate-sealed parts passed any dye stain, salt spray or other corrosion tests?

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2007

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Thanks for the information. Their initial samples provided with the Nickel sulphate sealing didn't pass the corrosion resistance testing. They are in the process of changing back to Nickel Acetate and modifying the bath temperature.

What process parameters can be modified to obtain an optimum seal? I am asking this because our new supplier is in no way an expert in this area.

Cory Doble
- Caro, Michigan
2007

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Rather than embark on a long distance, trial & error learning curve, use a commercial anodize sealing product such as Anoseal from Henkel. It includes nickel acetate, wetting agent, anti-smutting additive and instructions. Mix with DI water and adjust pH (probably 5.8-6.0) and temperature as specified. Also, rinse prior to sealing with DI water to minimize contamination.

What anodizing specification are you using? It should testing. ASTM B136 is a quickie stain resistance test. Salt spray testing per ASTM B117 for 336 hours is required by MIL-A-8625 / MIL-PRF-8625 [on DLA]F.

Mr. Probert's 'Aluminum How-To' book should benefit both you and your vendor.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2007