Black oxide, or Blackening is a conversion coating for ferrous materials, stainless steel, copper and copper based alloys, zinc, powdered metals, and silver solder. It is used to add mild corrosion resistance, for
appearance and to minimise light reflection. To achieve maximum corrosion resistance the black oxide must be impregnated with oil or wax.
Chemical blacking of steel provides a uniform glossy black finish on steel, the film is a particular state of iron oxide which is black in appearance, it has very little protective value without a supplementary treatment of oil to retard corrosion. In fact a dry coating will quickly revert to the more typical brown/rusty coloured oxidation state of iron and corrode very quickly. The film is of neglible thickness and makes no discernible changes to the dimensions of the parts treated, however with the hot caustic blacking process; parts must be able to resist processing temperatures of up 1500 C.
It is not feasible to process mixed materials, for instance components that include copper such as bronze bushes, brazed, as the copper will not blacken and acts as a poison to the blacking solution. It is also known as Blackodising, Chemical blacking, Chemi black.
Black oxide for copper, sometimes known by the trade name Ebonol C, converts the copper surface to cupric oxide. For the process to work the surface has to have at least 65% copper; for copper surfaces that have less than 90% copper it must first be pretreated with an activating treatment. The finished coating is chemically stable and very adherent. It is stable up to 400°F (204°C); above this temperature the coating degrades due to oxidation of the base copper. To increase corrosion resistance the surface may be oiled, lacquered, or waxed. It is also used as a pre-treatment for painting or enamelling. The surface finish is usually satin, but it can be turned glossy by coating in a clear high gloss enamel
Hot Black Oxide for Stainless Steel is a mixture of caustic, oxidizing, and sulfur salts. It blackens 300 and 400 series, and the precipitation hardened 17-4 PH stainless steel alloys. The resulting finish complies with military specification MIL-DTL-13924D Class 4 and offers abrasion resistance.
Room temperature blackening for stainless steel occurs via auto-catalytic reaction of copper-selenide depositing on the stainless steel surface. It offers less abrasion resistance and the same corrosion protection as the hot blackening process. An excellent application for room temperature blackening is in architectural finishes (patina for stainless steel).
Black oxide for zinc is also known by the trade name Ebonol Z. Another excellent product is Ultra-Blak 460, which blackens zinc plated and galvanized surfaces without using any chrome and zinc die-casts.