Our finishing shop does black oxide primarily on fasteners. We are able to get good consistency from job to job with one exception. This particular customer buys fasteners offshore and has us do the black oxide. Our results vary from batch to batch, and some batches show surface rust almost immediately. What is the cause of this problem? Z.S.
The first thing to consider is whether the material used to manufacture the fastener is the same from batch to batch. You do not mention the type of fasteners involved but keep in mind that many fastener specifications are based on performance requirements and not finishing requirements. As the metal finisher, you normally do not have much control over material specifications. The best you can do is point this out to your customer.
The second thing to look at is your process bath chemistry. Do you monitor the chemistry of the bath on a regular basis? If you do not, you should start doing so. You did not mention in your e-mail what type of bath you are using. If you are using one of the newer proprietary baths, the vendor should be able to help you with this.
An article in the Products Finishing 2002 Directory And Technology Guide, 800-950-8020, www.pfonline.com/articles/pfd0025.html, discusses newer developments in blackening of ferrous materials.