I do not know of any way of preventing oxidation of non-noble metals - oxides must form when exposed to air. How about reverse thinking? Is the dark coating unacceptable to your customer? If it is being formed consistently then it is useful proof of coverage! That is the reverse of using lacquer coatings that have to be blasted off to prove coverage.

Last edited by Socrates; 12/12/19 07:00 PM.