Base metal black oxide interaction

[metrat23]

I black oxide treat a casting made of 8620 steel. Periodically, the parts exhibit a redish color. I've been told that it is most likely cause by an interaction between the black oxide salts and the silicon content in the material. I've verified the silicon content is it's only about 0.3 wt%. I'm told taht as long as the silicon is below about 0.5 wt %, it should be fine.

Any info. on what else might cause a red color would be appreciated (non-metallics, process variations, etc.)

Thanks

 

[unclesyd]

It is probably the build up of colloidal iron in the bath that will cause mainly red, brown, or green films.
You need to get with you bath supplier and see if they have a rectifier for your bath. You will also need to maintain the bath with the proper additions recommended by the bath supplier.