Black Oxide, Cold Blackening, and Surface Finish

A. True black oxide sounds ideal--but room temperature blackening processes are not black oxide, they are the application of a black copper selenium coating. The ones I've seen were a rather poor substitute for true black oxide, being 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 could cause a surface finish to go out of tolerance.

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

A. 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.

A. 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.

A. 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 temperature blackening is 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.

A. Hi Rolfe. Artists usually don't have to be concerned about tolerances & dimensional issues, so that helps. And neither black oxide nor cold blackening offer much corrosion resistance, so almost any substitute finish will offer at least as much.

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

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

Regards,

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

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 ⇦ on eBay or Amazon [affil link] immersion may work.

Ed. note: We thank Brendan, but remind readers that liver of sulfur is for blackening copper, brass, or silver; it won't blacken steel.