What is Black Oxide?

Black oxide is the conversion coating formed by a chemical reaction with the iron in the metal to form an integral protective surface. (Contrasted to an applied coating which bonds to the metal but does not react chemically.) A black oxide conversion coating is applied to ferrous alloys when oxidizing salts react with the iron to form magnetite (Fe304), the black oxide of iron.

Similar results can be obtained on non-ferrous metals which oxidize under suitable conditions to form black oxides. Black oxide coatings are offered under a variety of trade names including Black Magic®, Black Penetrate®, Black Passivation, Gun Bluing and others. While some can be applied at room temperature, most are applied at elevated temperatures.

All the benefits of black oxide are only available from the "hot" oxidizing process.

Where is black oxide regularly used?

Black oxide conversion coatings are widely used in the machine tool, automotive, appliance and general metal forming industries. Screw machine parts, forgings and stampings are commonly black oxide coated.

What determines the appropriate black oxide treatment for a given part?

To evaluate a part for black oxide conversion coating, the following are examined:

• Ambient storage conditions (airborne corrosives, vapors, humidity, temperature range)

• Length of protection required

• Type of base metal

• Post-coating handling conditions (pH limits, oil contamination concerns, adhesion to other parts)

• Subsequent finishing operations

• Final application environment

The above ranges of values were derived from Cleveland Black Oxide testing of various parts and applications. We recommend conducting corrosion resistance testing on your black oxided parts to obtain specific values.

• While the high sulfur content of 303 Stainless (for machining purposes) makes it more difficult to black oxide, corrosion resistance equivalent to 300 hours of salt spray is possible with proper post-treatment. 400 Series Stainless is not highly corrosion resistant and does not meet Mil C-13924-4. However a protective post-treatment will impart significant corrosion resistance.

What is room temperature/cold black oxide?
"Cold" black oxide, applied at room temperature is often used on small lots or as a touch-up on larger parts. The so-called "cold" black oxide process for steel are not true oxides but soft deposits that offer less corrosion and abrasion resistance than the "hot" oxide processes.

The cold black oxides oftentimes display color variations on the same part or from part to part. The cold process does not meet automotive or military specifications for black oxide coatings.