Black Oxide

AKA Gun Bluing, Black Magic, Black Penetrate or Black Passivate

About Black Oxide

For over 65 years Electro Polish has provided fully certified black oxide finishing. Our two tank Dulite solution provides for deeper penetration giving you a more resilient, richer black finish that is ready for extended service…all of this with no effect on the dimensions of your parts.
Whether it’s hardened tool steels, stainless steels, or cast iron you will not see the red and brown finishes inherent to the other black oxide methods. At Electro Polish Co. we will get your parts blacker than you ever imagined. You’ll see the difference!

Black oxide is a process that chemically alters the outer layer of certain metals to create a protective coating. This coating is integral to the surface of the part and therefore will not chip, peel, flake, or rub off. Additionally, no dimensional change of the part occurs during the process. This is to be contrasted with applied coatings, such as paint, that do change the dimensions of a part and will chip or peel. A black oxide coating provides an excellent pretreatment for paint, increasing the adhesion qualities of the metal. Black oxide is typically applied to ferrous alloys like steel, where the iron in the material reacts with oxidizing salts to form magnetite (Fe304). Electrical conductivity is not affected by this chemical compound.

A black oxide coating can be formed in either a hot or cold solution. The “cold” bath is usually at room temperature, but can be at a slightly elevated temperature (under 200°F). This process doesn’t meet military or automotive standards because the blackened material isn’t a true oxide. It doesn’t provide uniform coloring and easily rubs off. Our company employs the “hot” bath solution, where the temperature ranges from 200°-300°F. This type of process is known by industrial trade names like Black Magic, Black Penetrate, Black Passivation, and Gun Bluing. All our black oxide processes use Dulite chemicals and conform to the Society of Automotive Engineers (SAE) specification AMS-2485. We also conform to military specification MIL-C-13924.

Black Oxide
Black Oxide
Black Oxide
Black Oxide

The Process

The process begins with a thorough cleaning of the part. This step is critical to creating an even finish over the surface. Once the part has been properly cleaned, it is dipped into an alkaline oxidizing bath at an elevated temperature. The elevated temperature is the catalyst needed for the chemical reaction to occur. Black oxide penetrates the metal and creates a coating 10 millionths of an inch above and below the surface. No hydrogen embrittlement occurs during this step because the parts are not exposed to an acid activator. The parts are then removed from the bath, rinsed, and dried to prevent “flowering” or bleeding out of leftover chemicals from “blind” holes.

The final step in the black oxide process is applying a supplementary coating to the part. Black oxide coatings by themselves are easily corroded, resulting in flash rusting. This final finish is necessary to provide corrosion protection to the part. The type of final finish applied determines the type of color the part will receive. An oil-based finish is applied when no specific finish is requested, and provides a glossy color. A wax or lacquer finish will create a matte color. Both types of finishes leave the part with a deep black color that doesn’t reflect light, provides improved lubricity, and imparts anti-galling.

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