Removal of heat scale from annealed electroless nickel deposit

I am looking for a solution to clean the coloration produced on the mid phosphorous electroless nickel deposit on copper pieces. Medium phosphorous electroless nickel (about 300 microinch) was deposited on about 1 square inch copper pieces and then heat treated at 400 °C for 1 hour. Unfortunately, the heat treatment at 400 °C tarnished the electroless nickel plating (to a bluish grey color). I need to remove this heat scale so that the normal nickel finish is restored on these slugs.

Can anyone please help me in finding a method to remove this scale without damaging the underlying nickel and the base metal?

Thanks,

Dear Sandy:

Unfortunately, there is no chemical method for removing the discoloration on EN coatings produced by heat treatment at temperatures over 300 °C. It can only be removed by abrasion cleaning by blasting, thumbing, etc.

You can sometimes plate a flash coating of new EN over the old coating to hide the discoloration, but it probably will not adhere.

Sorry.

I agree with Ron Duncan. It is difficult to remove the discolouration formed during heat-treatment.

Maybe you can try to heat-treat your components in inert atmosphere?