Post treatments for Black Oxide -- Chromic Acid Dip?

A. What are you black oxiding? I know that with copper a chromic acid dip will passivate the surface and protect it against attack from acids.

A. Probably to passivate internal diameters, as for tubular work (to prevent bleed-out rust).

A. As a specification writer, it's needed to meet the spec. Also, it's needed to meet the 96-hour salt spray requirement. Without it, don't pass. Also, caustics can be very nasty to rinse off and the chromic acid will aid in neutralizing it.

Great answer, Milt -- thanks!

Regards,

A. I have never heard of this. What kind of parts are you referring to?

A. Hi, John. I agree with Tom. I personally have never seen anything like that. Please tell us what kind of edges of what kind of parts would benefit from this kind of process. Thanks!

Regards,

A. Neither hot nor cold black is a corrosion resistant finish, and some people claim neither offers any corrosion resistance at all -- that only the post-treatment offers corrosion resistance. There are water soluble waxes that you may find unobjectionable. Hot black is a nicer finish on almost all accounts and is no more expensive, but it can be dangerous because it operates at well above the boiling point of water such that raw water entering the tank can explosively flash to steam.

A. I will respectfully disagree with Ted on the cost issue. I did cold black oxide out of pickle buckets with expected results. Hot black oxide requires heat which is not cheap as well as somewhat more special tanks. I think that it shows surface flaws more than cold does. I think that hot is a better finish. Talk to your cold product tech services. It may be as simple as dumping the tanks and remaking the solutions. Also, look at some different oils. Most use a dry to the touch "oil" which leaves a very very thin coat. Properly used, some of the waxes look great and last longer.

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January 2014

Thanks Jim. Cold blackening offers significantly lower capital cost, so for very low volume, you are certainly right that it's cheaper and sometimes appropriate for machine shops. I am limited to book knowledge on this, and your hands-on experience certainly trumps it, but for high volume I lean towards thinking hot black oxide (a true oxidation process) is less costly than cold black oxide (the deposition of a selenium sulfide coating). And the hot black oxide not only looks better, but it doesn't rub off as a smut.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Agreeing with Ted and adding some additional thoughts. Again, ALL the protection comes from the top coat. In the case of so called "cold black oxide" (which it is not) the coating without a top coat is corrosion promoting. Top coats come in three general classifications:

- OILS in various viscosities, water based and water displacing, do "heal" when scratched, but erode away in outside exposure.
- LACQUERS which do not heal if scratched but stand up better outside. And finally the third class is
- WAXES which are half way in between the healing properties and the outside corrosion properties of oils and lacquers.

Water Displacing oils sometimes leave the water on horizontal "shelves" and this spot rust the next morning. Water displacing oils are not reliable inside of drilled and/or tapped holes. Water soluble oils get into holes and over and under shelves. Once dry, the soluble oils do not have a tendency to redissolve but they do tend to wash off in outside exposure.

A. Hi Abrie,
Phosphate coatings are slightly porous to some degree. A dilute chromic acid rinse helps "seal" these pores and passivate the exposed base metal. As a result corrosion resistance is improved. This is the primary function.

To address your question, yes they do more than passivate. Acid rinses can remove soluble residues on the part surface from previous steps. In this case, the hot chromic rinse is thought to remove nitrates and phosphate ions. The result is improved paint adhesion. You'll note that a lot of phosphated mil spec work is pre-paint.

Outside of the MIL-SPEC there are other ways to passivate this step. There are some modified trivalent/hexavalent rinses that form a chromate that acts like a gel in the pores. I've heard this referred to as a "reduced chromium rinse" but I don't have any personal experience with it. The previously discussed hot chromic rinse is far more common.