I am experiencing loss of adhesion in the field with powder coating over 304 stainless steel. The adhesion is terrific after it is painted. After the product is mounted to a truck and driven through snow, salt and sand, the non-stainless steel components begin to rust as expected. Unexpectedly though, is the loss of powder coat adhesion around the stainless steel components attached to the non-stainless steel parts. On these areas the paint comes off in chunks, and on the bare stainless steel surface there appears to be a white residue creeping under the paint from the non-stainless steel parts. We process our parts through a five-stage washer using iron phosphate and a non-chrome rinse. Why is the powder coating coming off the stainless steel parts? T.R.
I suspect that the non-stainless steel parts and the stainless steel parts create a galvanic reaction due to the dissimilar metals. This causes the non-stainless steel parts to corrode at a faster rate and causes under-film corrosion on the stainless steel. Replacing the non-stainless steel components with similar stainless steel components should correct the problem.