Black oxide comes out red, need jet black finish

A. You don't say if you or an outside metal finisher is doing the black oxide process. Regardless, as the black oxide tank becomes aged, a build-up of iron occurs (from the steel). This is the red you see. Years ago, we could complex this to prevent it. The chemical was a carcinogen, don't think anyone uses it now. Best to contact the supplier of the black oxide salts, ask him what to do.

Black oxide is made up at 7 1/2 lbs./gal. of water, operates at 295 degrees F. Can be deadly if you do not have proper equipment and know-how to operate. There are cold blacks that are equivalent to hot black oxide. Suppliers support this web site. An alternative possibility is zinc electroplate followed by black chromate. More expensive. Really depends on what application the finished parts see.

A. I'm a big fan of black oxide, so maybe looking at bath maintenance would help solve your problem. That said, you could also look at black zinc phosphate. Phosphate will help hide some imperfections in the steel that black oxide would leave exposed. Hope this helps.

A. Hi. If you are 100% sure that a process is properly controlled and it still doesn't give you what you are looking for, it's time to look for a replacement. But the fact that parts are coming out red isn't evidence that black oxiding isn't viable, but that you are not operating it correctly.

In addition to the points raised by Bill and Dan, a very simple concept that I didn't get until it was explained by Rod Henrickson (search our site for his name) is that the black oxide tank must be at a rolling boil and must also be at the right temperature. That is, you cannot control the temperature by modulating the heat input, you can only control it by modulating the water content. To be overly brief, Rod says red parts can indicate that the solution is not boiling or the temperature is too high, so you need to add water. Good luck.

Regards,