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Coated aluminium is not withstanding phosphoric acid

We are a manufacturing company that produces some aluminium parts for washes. We are seeing corrosion on these parts which is going to ultimately cause failure. We have worked with the plating company that we use and already tested anodizing and Iridite/ Alodine. All of our tests have failed.

The problem is that these are being subjected to a mixture which includes phosphoric acid.
Is there a plating out there for aluminium that will hold up in these conditions?

Failure is first appearing where the parts meet. There is not a huge gap and these spots. They are wrench tight however there is some surface area at these locations do to chamfers, etc...

Please any suggestions would be appreciated.

Dan Cibulskis
metal manufacturer - Aurora, Illinois
March 30, 2009

I'm not sure how much help this is but I can tell you that when we do our coating weight tests for anodized parts that phosphoric acid is one of the main components used in the stripping of the anodized coating. So with out some other kind of surface treatment possibly nickel cadmium or something along those lines, but any untreated anodized part which is placed in contact with a phosphoric acid solution is going to fail. It may be possible to find a seal for your anodized coating that will prevent this problem for you. Hope this helps a little at least.

Jeff Garrett
Aluminum anodizing, dyeing and painting - Elgin, Oregon, US
May 26, 2009

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My opinion: Even dilute phosphoric acid is going to attack your common aluminum finishes as stated in the previous post. It will also attack cadmium and zinc. I suspect that it will in the very long term attack chrome plating.
It is not cheap, but I think that you will need to go with electroless nickel. Personally, I would go with 0.0015". You may be able to get a decent life out of 0.001". Anything less than that will fail sooner, but may be long enough if your need is for months rather than years.

James Watts
- Navarre, Florida
May 27, 2009