Adhesion Failure on Aluminum

We powder coat aluminum bench seats in 6061 and 6063 alloy and structural steel parts on the same line. We have had trouble with periodic adhesion failure on the aluminum parts.


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Q. We powder coat aluminum bench seats in 6061 and 6063 alloy and structural steel parts on the same line. We have had trouble with periodic adhesion failure on the aluminum parts. The parts pass an adhesion test in our shop but fail in the field. We use a five-stage cleaning process and an iron phosphate product with fluoride additive for the aluminum. 

Is there a better product that will treat both metals in the same washer? How can we test the aluminum so we can see a problem before we ship it out? T.L.

 
 
A. This issue has been discussed many times in print. Treating aluminum with iron phosphate is OK if the product is going to be used in a non-corrosive environment, but it is not reliable for outdoor products. 
 
If you need good performance for outdoor products, you should be using a conversion coating made for aluminum. Chrome treatment or a suitable non-chrome product is OK, as long as it is a conversion coating for aluminum. 
As far as a better treatment for both metals, you should consider the new transition metal treatments that feature zirconium or similar metals in nano-scale products. Be aware that these products usually require stainless steel construction and very clean water, and they do not work for everyone or in every washer. However, these products can usually meet or exceed iron phosphate performance on steel and provide more reliable treatment for aluminum. They are not as effective as the aluminum conversion coatings, but probably superior to the iron phosphate. 
 
For testing of aluminum, I like the boiling wet adhesion test described in the AAMA 2604 specification. The part is scribed and immersed in boiling de-mineralized water for 20 min. After is has been dried and cooled, you do a standard tape-pull test. If the pretreatment is not as good as it needs to be, you will often fail this test.
 

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