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Test to determine if scratch has gone through the plating?

I'm in the process of setting up a small Metallurgical/Physical Testing Lab. I received a call from a prospective client. A scratch was found in the hard chrome plating over the stainless steel substrate. How can I determine in the "scratch" has penetrated the chrome and exposed the stainless steel? Is there an etchant that I can treat the surface with to determine if the scratch has exposed the Stainless Steel? The client talked about using a potassium ferrocyanide ⇦this on eBay or Amazon [affil links] etch, but I'm not really equipped to handle poisons, I was thinking silver bromide or ammonia. Any suggestions? Thanks

Thomas Patterson
Test / Inspection - Roebuck, South Carolina
September 15, 2009

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September 16, 2009

I think that it is potassium ferricyanide not ferro. It is not a bad poison since the iron forms a very difficult bond to break. It is a listed hazmat however and various quantities trigger various reporting methods and handling.
This is a very sensitive test. copper sulphate ⇦this on eBay or Amazon [affil links] might work, but it is not very sensitive resulting in under reporting vs over reporting for the first one.
I think that it would take sophisticated equipment to measure the amount of chrome left in a narrow crevice. If it were wider, you might be able to make an epoxy mold of the crevice and measure that on an Opticon and then measure the thickness of the nearby chrome with moderate sophisticated measuring device.
Being a new company, I would not want to pick up a poor rep on anything less than positive information. Politely pass and say next year when we get some high dollar equipment that we do not have yet.

James Watts
- Navarre, Florida

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September 28, 2009

A drop of muriatic acid will react over chrome producing a dark stain and a green residue in a matter of seconds-minutes. Not so over 440SS. Another idea, depending on the particular geometry and delicateness of the surface, and of course of the certainty you are looking for and demanding posture of your client, you may test it with a very sharp knife (doctor blade type). Gently re-scratch inside the actual scratch. It takes practice and good sense of tact but if there is still chrome in there, the blade will just slip. On bare SS it will not.
Above all, why does it matter if it's plated or not? What difference does it make?

Guillermo Marrufo
Monterrey, NL, Mexico