Q. I have a part made from Monel that was plated with nickel strike and silver plated. I need to strip the plating from the outer diameter and leave the inner diameter plated without attacking the base metal. Any suggestions? T.I.
A. Based on your description of an inner and outer diameter, I’m going to assume you’re plating something solid and tubular, probably a pipe. The ends will need to be sealed or capped in a way to eliminate the ingress of liquid from the stripping bath. If the workpiece is not solid, or if the inner diameter cannot somehow be sealed or isolated from the outer diameter, a liquid immersion stripping process is not feasible.
Monel is a nickel-copper alloy (typically a mixture of 70% nickel and 30% copper). There are a couple of potential options—one electrolytic, and the other not.
The electrolytic process would be performed with concentrated sulfuric acid with 30 g/L of sodium nitrate added to it. It can be used from room temperature to about 120ºF. The piece you want to strip should be the anode and you should use a lead cathode and approximately 3 V between the two.
The immersion process is performed with a combination of 95% sulfuric acid and 5% nitric acid at elevated temperatures (150-180ºF). It’s recommended that parts be dry when entering the bath, otherwise the solution would get even more aggressive locally. You’ll need to watch the parts closely to ensure they don’t pit excessively by this process or need further rework.