2.3 - Black Coatings

There are a variety of black coatings available to the manufacturer. The following is a comparison of different types:

Black Oxide is a coating applied to steel in a high temperature alkaline solution at about 200 F. Black oxide is useful because of its deep black color with NO dimensional build up on machined parts. The downside of black oxide is it has very poor corrosion properties; therefore, an oil, wax, or water lacquer sealer is routinely applied to enhance shelf life. Once these secondary coatings are removed, the black oxide wears and corrodes rapidly. Black oxide normally is not applied to stainless steel, nickel, or copper base alloys. Recent advances in chemistry permit some blackening to be done at ambient temperature. Many machine shops have their own on-site facilities for this type of application.

Black Zinc and Black Cadmium coatings are first plated, then followed with a chromate dip which turns the original silver-colored plating black. The final chromate imparts excellent corrosion protection to pass a 96 hour or better salt spray test. However, there is at least a .0003/5” dimensional build-up on parts. Also, the thin, black chromate coating exhibits a slight iridescence since the base coatings tend to be naturally semi-bright. This rainbow effect can be reduced if the parts are media blasted or deburred before plating. Moreover, a final sealer can be applied as in the case of Black Oxide. These types of coatings are also electrically non-conductive.

Black Nickel is similar to electrolytic nickel plating except that impurities such as zinc or cadmium are added to the solution to cause a color change to black. The color obtained is not a true black, but rather a deep charcoal grey. If the parts are blasted, the color will approach true black. This coating is useful in optics, electronics, and computers where electrically conductive dark coating is necessary. To enhance corrosion protection of black nickel, a base plating of copper and nickel is applied. If a final sealer is used in this case, the conductivity will be reduced. Black nickel is also a soft coating which wears off during heavy use.

Black Chromium is plated from a chromium chemical solution which results in a dark grey coating that approaches black. This coating was very much “in vogue” for military applications in the 1940’s and 1950’s. The finish is electrically an insulator, and still has demand in the decorative industry- on such items as handrails and clothing racks due to its exceptional hardness and wear properties. Lately, it has found some demand in solar reflectors because of its good light ray absorption. This coating, however, is difficult to apply with consistency.

Black Electroless Nickel is a dark grey coating especially resistant to corrosion and wear. These coatings are finding increased usage in solar reflectors and optical equipment. The final result is achieved by exposing regular electroless nickel to a nitrate solution during processing.

Black Anodizing is a black dyed, anodic coating which has dimensional build up and is applied to aluminum alloys only. There is an electrolytic process called 2-Step Black Anodizing which has excellent lightfastness properties and will not fade, even in a harsh solution like anti-freeze.

Black Paint is applied as an electrocoat or a powder coat, and has excellent color and wear properties. With these particular applications, however, chipping can occur when exposed to vibrations. The base material must also receive extreme preparations in surface cleaning, otherwise adhesion becomes a reoccurring problem.

Black Ebonol is a proprietary deep black coating supplied by Ethone/OMI that can be applied directly on copper parts or steel parts which have been copper plated. It has excellent light absorption properties, will not fade when exposed to UV rays, and better corrosion protection than black oxide. The application of black ebonol conforms to MIL-F-495 for use on precision aerospace optical parts.

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