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topic 27235

Black Oxide and Surface Finishes

A discussion started in 2004 but continuing through 2018


We recently had a rejection on some cast iron parts that received a black oxide coating using a room temperature process, there were surface ground areas on the parts that required a 32 surface finish, the parts were checked before and after, and the surface finish fell out of tolerance after coating them. Would this represent an error in the process or is Black oxide not the correct application for maintaining a good surface finish in this case.

Matthew J Martin
'Quality Manager' - Mt. Healthy, Ohio, United States


Black oxide sounds absolutely ideal--but my understanding is that room temperature blackening processes are not black oxide, they are the application of a black copper selenium coating. The ones I have seen were a poor substitute for true black oxide, being very smutty, and even gritty. I hope a supplier of room temperature blackening solutions can offer you a better answer and some encouragement, but I personally am not surprised that a room temperature blackening process would cause a surface finish to go out of tolerance.

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
We need "Aloha" now more than ever


My "guess" is they used a muriatic acid pickle which opened up "pores" that had been smeared over in the grinding process. Now that I have blamed the grinder, take 3 parts and run them thru the black oxide process, thru the pickle step. Rinse dry and recheck the surface finish. I quite strongly do not think that it is the black oxide step in the process. You might also try filtering the oil preservative as it may contain particles that would be fairly strongly attached to the part.

James Watts
- Navarre, Florida


True Black Oxide Fe3O4 will not change the surface finish.

Any prep that etches the metal would surely change the surface finish as the above report stated.

As Ted said, room temperature "black oxide" is not black oxide and it will definitely change the surface finish. It is a phosphate coating loaded with selenium and the phosphate takes iron out of the surface and uses it in the coating, so by removing some iron it is changing the RMS of the surface.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como
and co-author of The Sulfamate Nickel How-To Guide


Operating at high solution temperature, steel blackening processes are not the most pleasant things to be around. In order to reduce the hazards of hot blackening, and to save energy, proprietary cold blackening solutions have been developed. They are operated at room temperature and are based on different chemistries, so they are substantially less hazardous. Room temperatures blackening are not a true black oxide process. Rather it involves the application of a copper-selenium compound. This compound is not an acceptable substitute for black oxide, as it does not look as durable as the one obtained with the hot blackening process. I saw an article in December AESF issue, compound-layer blackening of steel at room temperatures. But it is a compound layer, which consists of a phosphate coating and a blackening coating, formed separately in a two-step operation and there are several chemicals used in both processes.

Ravi Chandran, Ph.D
New Brunswick, New Jersey

Alternative chemistry to selenium black oxide finishing

October 23, 2018

Q. Looking for blackening process without hazardous waste (Copper Selenium) or safety (Hot black oxide 290 °F people killed) that could be used in small art shops.

Rolfe Parsloe
I'm researching question for a bunch of hobbyist art shops - Spokane, Washington USA

October 2018

A. Hi Rolfe. Artists usually do not have to be concerned about tolerances and dimensional issues (in the ten thousandths of an inch range), so that helps. And neither black oxide nor cold blackening offer much corrosion resistance, so almost any substitute finish will offer as much.

A black phosphate may be acceptable to some artists, and black paint or powder coating may satisfy others.

Black chrome plating, black nickel, or black zinc plating all ought to be fine in terms of attractive corrosion resistant finishes -- but they are not free of 'hazardous waste' because that is a matter of statute rather than opinion. Several layers of burned on oil may give a black finish such as on a frying pan, but that doesn't strike me as a safe or environmentally friendly approach. A black porcelain ceramic finish sounds possible, but the equipment cost is probably beyond the means of small art shops.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

October 24, 2018

A. Depending on what material you are working with and what kind of output you need (i.e., is it just appearance, or do you need corrosion protection as well, for example), a simple liver of sulfur immersion may work.

Brendan McNamara
- Rochester New York USA

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