Aluminum Pretreatment

Q. I've been reading PF for the past two years and really enjoy your column. I haven't found any articles relating to an area of concern for our company, which is painting over electrocoated aluminum castings or extrusions. Specifications require the top-coated parts to pass a 336-hr salt spray-test. Our normal procedure is to require aluminum parts supplied already e-coated which we run through our internal powder coat line for the final topcoat. We receive e-coated parts from various suppliers and have had widely varying results in our subsequent salt-spray testing. Through research we have found the pretreatment processes used before e-coating vary at our suppliers from a clean/clean and etch/zinc phosphate process all the way to Class II anodizing. In my experience many years ago at a metal fabricator, the only acceptable processes for adequate surface treatment on aluminum were chromate conversion coating or anodizing. We are confused by the number of pretreatment processes being used by contract e-coaters and wonder about their adequacy. In your opinion, what pretreatments are adequate for e-coating and/or painting on aluminum substrates for exterior applications? D.P.

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A. Welcome to the Painting Clinic, D. P.! Aluminum pretreatments have been discussed in previous clinics, but not in the past two years. I agree with your assessment of pretreatments for aluminum. Anodizing and chromate conversion coating pretreatments are best, and zinc phosphate pretreatments may be adequate.

However looking at the problem from a different angle, we shouldn't be too harsh on your suppliers who provide aluminum parts having alternative pretreatments. If they pass the 336-hr salt-spray test and you are not having customer complaints, these pretreated parts are adequate. Presumably, the salt- spray requirement was set based on projected exposure conditions for your products. If this requirement is too low, raise the value; if too high lower the value.

Do you provide a performance specification or a materials specification to your suppliers? If you want and feel that you need anodized or chromate conversion coating pretreated parts, you must specify such in a materials specification. Otherwise, don't worry-be happy!