Our plating shop uses a black chromate conversion coating for a number of customers. Lately we have had problems with off colors, streaking and general poor appearance of the finished part. Can you give us any pointers on getting this process to work properly? F.G.
Your e-mail does not give much detail as to the exact nature of your problems, but I am sure the cause is the usual culprit—poor process control of the chromate conversion bath.
The first step is to get a copy of the data sheet that goes with the bath. Your chemical vendor can help you with this. After reviewing the data sheet, determine how close you are to the recommended values. In general, the factors that most affect the chromate conversion bath are concentration, immersion time, temperature and pH.
In the case of black chromate, the most common causes of the coating not being black are not enough silver being present, out of control pH and poor rinsing.
An excellent publication, Chromate Conversion Coatings, (American Electroplaters and Surface Finishers Society, 407-281-6441) has a very good section on troubleshooting of chromate conversion baths. The Metal Finishing Guidebook and Directory, 2002 ed. (212-633-3100) has two sections on chromate conversion coatings. One discusses the classic hexavalent baths and the other the newer trivalent baths.