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Corrosion protection for aluminum blade shaft on variable pitch fan

I am a mechanical engineer. We make a variable pitch fan, and use an aluminum shaft to mount our blades on. Due to fatigue issues with anodizing, I want to get away from it. The fan may be exposed to salt (radiator fan on highway trucks). What options do I have for coating the exposed area of the shaft. I am looking at 6061-T6 or 7075-T73 alloys. A simple spray on coating would be ideal.

Jon McCallum
- Edmonton, Alberta, Canada
2003

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1. Chromic Acid Anodizing is the best answer. The aircraft industry still uses it to get corrosion protection without fatigue problems. There are still a few chromic anodizing shops around.

2. Second best is a light chromate followed by powder coat. Do not put the chromate on too heavy as the coating will shear and the paint peel.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2003

What about electroless nickel plating? Does EN have any effect on Fatigue? If we get a scratch through the coating, will the aluminum sacrifice for the nickel?

Jon McCallum
- Edmonton, Alberta, Canada
2003

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If the electroless nickel coating becomes compromised, yes, it will cause galvanic corrosion of the aluminum. However, millions of aluminum automobile wheels are successfully electroless nickel plated before decorative nickel-chrome plating. I would not expect electroless nickel plating to severely impact fatigue strength, but I think Mr. Probert offered some sound options.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey