Q. Are there different grades of black oxide? If so, are some tougher than others? R.D.
A. I don’t know if you can truly say there are different “grades” of black oxide, but there are certainly different types. The “classic” black oxide is what is generally called a hot alkaline black oxide. Steel parts are placed in a hot alkaline nitrate solution that is operated at about 290°F. The finish is jet-black in color and attractive, but does not offer much in the way of corrosion resistance unless sealed with oil or some other material.
Another class of black oxides is known as room-temperature black oxides. These formulations are not true chemical conversion coatings; the blackening affect comes from the formation of an amorphous selenium/copper/iron compound. These black oxides do not offer any corrosion protection unless sealed with a suitable oil, and the finish from this process may not adhere to the base material as well as the classic black finish. These are best considered as cosmetic finishes. The room-temperature finishing process is easier to control than the classic process.