Black Oxide Clear Passivation of 303 Stainless Steel

I have recently received an enquiry to blacken a batch of small 303 SS screw. I understand the various blackening processes hot alkaline, selenium copper and zinc phosphate however I have the further requirement of having to "clear passivate" the component. Has anybody an idea what this type of passivation entails? We are setup to carry out nitric passivation but I'm concerned that this process may remove the black oxide.

Any ideas would be greatly appreciated...

I believe you are looking for the dichromate process. Very few shops run this process due to the high temperature and hazardous nature of chromic acid in the molten state. We can provide a company name here in the U.S that does this process if you would like.