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Some More Blackening Problems

Our shop specializes in blackening of 300 and 400 series stainless steel. We sporadically have trouble with smut, staining and color differences on the parts. What can we do to minimize these problems?
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Q: Our shop specializes in blackening of 300 and 400 series stainless steel. We sporadically have trouble with smut, staining and color differences on the parts. What can we do to minimize these problems? P. S.

 

A: You haven’t give me very much information about the process that you are using. However, there are some good common sense rules that you can follow to reduce the problems that you mention.

  1. It is not unusual that different alloys require different operating conditions. Separate the parts to be blackened according to alloy type. You no doubt will have to do some experimentation to determine the best conditions for each type of alloy. Once you have done this, you can run each batch of alloy using the conditions you have determined to be best
  2. Residual iron salts on the surface of your parts can lead to the formation of smut. It is important that you use a strong alkaline descaler prior to the blackening step.
  3. Temperature uniformity can play a role in the process also. Non-uniform temperature conditions will give a patchy black finish. Your bath must have good agitation to minimize this problem.
  4. Many of the alkaline baths contain sulfur compounds to produce the black color. The appearance of the black color is affected by the concentration of these sulfur compounds. The concentration of the sulfur compounds should be monitored and additions made as required.
  5. Last but not least, contact your chemical vendor and ask for assistance in solving your problems.

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