Black Oxide, blackening, oxidizing, oxiding, black passivating, gun bluing . . . these terms all refer to the process of forming a black iron oxide on the surface of ferrous metals.
The black oxide process is a chemical conversion coating. This means that the black oxide is not deposited on the surface of the substrate like nickel or zinc electroplating. Instead, the black oxide coating is produced by a chemical reaction between the iron on the surface of the ferrous metal and the oxidizing salts present in the black oxide solution. These oxidizing salts include penetrates, catalysts, activators and proprietary additives which all take part in the chemical reaction. The result of this chemical reaction is the formation of black iron oxide, magnetite (Fe3O4), on the surface of the metal being coated.
- Corrosion Protection – Depending on the post-treatment applied, black oxide coating can give corrosion resistance equivalent to 144 hrs of salt spray exposure. Other applications are designed to provide extended shelf life for stored parts prior to their use or to prepare them for further surface treatment (painting).
- Dimensional Stability – Black oxide adds no more than 5 to 10 millionths of an inch to the dimension of a part (it also penetrates to an approximate depth of 5 to 10 millionths). For all intents and purposes, the as-formed dimensions do not change (as they do when painted or plated). This provides corrosion protection for critical-size parts.
- Improved Lubricity – Oil post-treatment not only protects against corrosion, but the oil itself results in smoother running, mating parts.
- Anti-galling – Where break-in of mating parts is necessary, the anti-galling surface lets the outer lubricating layer be sacrificed during initial contact and abrasion while work-hardened surfaces are formed. The black oxide itself has a hardness of 5.5 on the Mohs’ Scale.
- Decorative Finish – The resulting lustrous black finish adds visual appeal and a “quality” image at a very low cost. Harder parts, 40 Rockwell C and above, tend to have glossier finished. Softer parts tend to have matte finishes.
- Reduced Light Glare – Moving machine parts, hand tools, and surgical instruments coated with black oxide reflect less light and reduce eye fatigue when used under bright lights.
- Pre-treatment for Paint – Black oxide greatly improves the adhesion qualities of the metal, allowing paint and other finishes to hold better, last longer. It also minimizes paint chipping and flaking.
- Conductivity –Black oxided parts experience no more than a 1% reduction in conductivity, making it a suitable treatment for electrical parts.
- No Embrittlement – Unlike painted or plated parts which are oftentimes acid etched, no hydrogen embrittlement accompanies black oxiding. (However, if parts are already scaled or rusted, they may require pickling prior to black oxide and some embrittlement may occur.) Baking of the parts within four hours after black oxide application will relieve this problem. On extremely hard parts, 55 Rockwell C or above, and very thin springs, there is a possibility of “caustic” embrittlement. This condition can be minimized by careful control of the black oxide cycle time.
- No Welding Fumes – Welding black oxided parts does not cause the release of noxious fumes as may be encountered by welding painted or plated parts.
- Cost Competitive – It is very difficult to achieve the same protection, appearance and performance qualities from other finishing processes for the same low cost as black oxide.