Hard Chrome on Stainless Steel

Question:

We are attempting to plate stainless steel with hard chromium. The process we use for doing this on mild and alloy steel parts is as follows: electropolishing, alkaline cleaning, reverse current etch and electroplating. The chromium plating bath is the standard chromic acid bath catalyzed by sulfuric acid. When we use this process for stainless steel the chromium deposit is spotty, thin or non-existent. G.L.

Answer:

The preparation of stainless steel for hard chromium plating is somewhat different compared to that of mild or alloy steel substrates. The key thing to keep in mind when dealing with stainless steels is that stainless steels contain nickel as a constituent. Nickel is easily passivated and care must be taken to prevent this from happening. The following procedure is one used with good results.

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1. Anodically clean
2. Rinse
3. Activate in a 20-50% sulfuric acid at 150°F for 1-2 minutes after gassing begins
4. Rinse well but rapidly to avoid possible repassivation of the surface
5. Plate

An alternate procedure is to use cathodic activation as a replacement for step three above. The cathodic activation uses 5-50% sulfuric acid at room temperature and a current density of 4-5 asf.

I should also add that there are procedures that use cathodic cleaning in place of the anodic cleaning step. Additional procedures can be found in the Electroplating Engineering Handbook, 4th edition, by Lawrence J. Durney. The book is out-of-print but copies can be purchased at www.amazon.com.