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 Amazon] 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, Michigan

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.

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 Brick, New Jersey

First, it may be whatever is being utilized to remove the soluble 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
tech svc. w/ chemical supplier - Grand Rapids, Michigan

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
water treatment - Trevose, Pennsylvania

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

I am facing a similar problem in my work as well.

I need to remove the oil from the black oxide for secondary work to be done on the metal.

Hence, I would like to know if the "de-oiled" metal, with just the black oxide coating, is sufficient for rust prevention under normal indoor environment?

Also, what is the most suitable method of removing the oil from the black oxide?? and what solvent should I use??

Hope to get a reply soon!!! Thank you!!

Jay Tan
Procurement - Singapore

^-- I want to contact this person and my company is a supporting advertiser of finishing.com

Hi, Jay. In my estimation it will not work. I have been to firearms factories where they remove the oil from blued (black oxided) guns for certain rework, and often suffer flash rusting within a few hours. It is only my opinion but I believe the idea of an oil-free black oxide finish is doomed to failure.

Regards,

Ted Mooney finishing.com Brick, New Jersey

Very simple answer to your question. I have the same problem when I am bluing firearms and it is close to quitting time. Like cockroaches bluing salts literally find their way into every nook & cranny. Dang stuff is insidious and will hide in the darndest places and wait until the customer has picked up his part or gun and will then wait another three weeks before quietly creeping out over the metal surface. When the guy brings it back in he is generally freaking out as he has already found that no amount of oil will stop it.

Whose fault is it? Well it's the fault of the guy that blued the parts. All parts that are caustic blued MUST be boiled out for at least a half hour after they come out of the bluing solution. If the parts have nooks and crannies the solution will hide there. If the parts have been sand blasted the danged stuff will hide in the bottoms of the tiny craters the sand blasting creates. Forget oil! Bluing salts are made from sodium nitrate or ammonium nitrate if you can stand the smell and sodium hydroxide. None of these chemicals are oil soluble. Bluing salts will crawl out of a hole and walk over an oiled surface like a baby crawling across the living room rug.

Crawling salts are simple to spot. They can appear as a light snow on the surface or an ugly white mould spot that just keeps growing. It can also take on a light bluish tinge if the chemicals happen to be new. The only way to avoid it is to boil the parts in fresh water for thirty minutes after bluing and then oil or wax everything down. Waxing is the best that I have found. Keep a separate hot water tank and toss a cup of any paste floor wax in it. The wax will melt and float on top. Dip the part in and out several times and then rinse with cold water and dry. The wax will fill the nooks and crannies. After this apply some light gun oil. Don't worry about the part rusting in the boil out tank. As long as it is under water it won't rust in the thirty minutes it is in there. When you take it out of the tank then you can start to worry as rust will form in less than 15 minutes. Go straight from the boil out tank to the wax tank or get oil on it right away.
Some people add water soluble oil to their boil out tanks. Does that really help? Well, I have tried 101 different hangover cures and short of getting drunk again they never seemed to work either. I suppose if you believe water soluble oil helps, far be it from me to tell you anything different. Rod

Rod Henrickson
- Edmonton Alberta Canada