High Carbon Steel & Black Smut

Question:

We have problems when etching high carbon steels prior to hard chromium plating. A black layer forms that can be wiped off but is a major problem for us. Is there a procedure available that will minimize or, better yet, eliminate the formation of this black layer? B. N.

Answer:

Your e-mail doesn’t mention what type of acid you are using for this etch but I am willing to bet it is sulfuric acid. Sulfuric acid is not a good choice for this type of substrate. After electrocleaning in an alkaline cleaner, the part should be reversed etched in a separate chromic acid solution that is approximately the same concentration and temperature as your hard chromium plating bath. Resist the temptation to do the reverse etch in your chromium plating bath. It will work, but you will also build up the iron concentration in the bath, causing additional headaches.

The plating step should start with a current density higher than normal, as much as two times the normal. After a short period of time the current density should be dropped to your normal current density.