Burned Chrome Deposits
Q: Our company does hard chromium plating. Quite often, we’re faced with the problem of burned chromium deposits. What are some of the causes of chrome burn? How can I eliminate this problem? F. C.
A: There are many different causes of chrome burn. Here’s a list of some common causes:
|Etching in the alkaline cleaner||Primarily a problem if parts are anodically electrocleaned prior to plating. If the alkaline cleaner is not concentrated enough, your parts can be etched.|
|Improper fixturing||Parts may require robbers or shields to reduce current density in high-current-density areas of the part.|
|Bath temperature too high or too low||Chromium plating is very sensitive to temperature changes. Your hard chromium plating bath should be controlled in the range of 2–3 degrees F.|
|Low concentration of chromic acid||Chromic acid concentration should be monitored on a regular basis. A simple Baume’ reading using a hydrometer is adequate for day-to-day measurements.|
|High current density||Knowing your surface area is critical. The current required in chrome plating is very high compared with other types of plating baths and it’seasy to end up with current densities that are much too high.|
|Improper ratio of chromic acid/sulfate||If the ratio of chromic acid to sulfate is lower than 70:1, you are likely to burn your deposit.|
There are other causes of burning not mentioned in this table. To get rid of these, you must operate your chrome tanks in a tight range. To help you with your hard chromium troubleshooting, I’m e-mailing you a copy of the Chromium Tank Doctor, a simple troubleshooting table developed by my father in 1955. The problems with hard chromium plating haven’t really changed in more than 50 years!
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