Letter 13097

Black Oxide problem 

We have a customer that has their parts black oxided by an outside source. The parts come back to their plant after black oxiding and are then dipped in a mineral spirits [link is to product info at Rockler] based rust preventive (after removal of the oil applied by the black oxider). The parts are then packaged in boxes with VCI paper for storage/shipment.

The problem is that the many of the parts have a visible white residue after being in storage. We suspect that the problem is due to incomplete rinsing of the caustic solution from the black oxider. Is this likely and if so how can this be verified?

Curtis Seichter
- Oak Park, MI, USA

I don't think that's it. The caustic black oxide solution is not corrosive to steel and the corrosion products would be typical red rust, not white. I suppose the white stuff could be a salting out of some kind.

The black oxided and oiled parts should exhibit some resistance to corrosion, so to me the question is why remove the oil?

 
Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

First, it may be whatever is being utilized to remove the soluable oil from the black oxide, leaving a residual film before mineral spirits is applied. Second, why not have the black oxider apply this material instead of removing the oil???

Bill Hemp
- Grand Rapids, MI, USA

Curtis, you could confirm the source of the white residue by carefully scraping some of it off the parts and analyzing it. If the residue is sodium or potassium salts, it is likely that it came from the black oxide process.

Another possibility is it may be from the rust preventative oil applied over the black oxide. Many rust preventative oils contain calcium based materials. If you are using solvent to remove the oil, the calcium based materials will be left on the surface of the parts. Without the other ingredients that were present in the original oil, the calcium materials migrate to the surface and form the white residue.

A third possible source is from your cleaning step. In the above scenario, I have assume you are using solvent to clean the original oil off the parts. If you are using an aqueous cleaner, the white residue could be salts from the cleaner.

From the outside looking in, you may be able to solve the residue problem and simplify your process by getting the shop that applies the black oxide to apply the oil you use to the parts instead of the oil they currently use. That would safe you the cleaning and reoil steps. Hopefully, I have given you some things to think about.

Roy Nuss
GE Water and Process Technologies - Trevose, PA, USA

I have the same problem, why I should remove the black oxide is because of coating purpose. I think, the black oxide that we receive from customer is to protect their fastener from corrosion while it is on board for export. That why we receive a black oxide fasteners and I have to remove it for coating purpose. The problem is what is the suitable chemical to remove the black oxide and what is black oxide content?

John Gerry
- Japan

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