Hard Chrome Plating
What is causing teardrop-shaped pits on parts plated in a standard hexavalent chromium bath?
Q. We are a landing gear repair station that hard chrome plates in a standard hexavalent chromium bath. Recently, some of the plated parts have shown rough areas with pits that are teardrop shaped and have a tail that goes up. We have tried a number of different adjustments to our plating bath to no avail. The chemistry has remained the same and within specifications given by our chemical vendor. Do you have any ideas as to what is causing this problem? R.R.
A. The two most common causes of pitting and services in a hard chrome plating bath are contaminants on the surface of the part and sticking bubbles. Since you mentioned that the tear-shaped pits have tails that go up, I think that your problem is sticking bubbles that are caused by the evolution of hydrogen gas that is always part of the hard chrome plating process.
You should try increasing the flow of plating solution on the surface of your parts by pumping of the solution. You should also try “jiggling” on a regular basis to remove the bubbles from the surface. If possible, you might also consider using a modern mist suppressor designed for use in hard chromium plating. You would have to discuss this with your chemical vendor.
I also am sending a copy of The Chromium Tank Doctor that may help you troubleshoot problems with your chromium bath.