Dr. Dmitri Kopeliovich
Black oxide is a conversion oxide coating formed on the metal surface as a result of a chemical reaction of the metal atoms with an oxidizing agent (air, aqueous solution, molten salts).
Conversion coating is a film of a chemical compound formed in the reaction of the substrate substance with another substance. This reaction distinguishes conversion coating from a conventional coating applied on the substrate surface without changing its chemical state.
Examples of conversion coating are Anodizing (electrochemical process of growing oxide film on the surface of anodically connected metal in an acidic electrolyte solution) and Phosphating (coating consisting of an insoluble crystalline metal-phosphate salt formed in a chemical reaction between the substrate metal and a phosphoric acid solution).
Black oxide coating is used mostly for Carbon steels, Alloy steels, Tool and die steels and Cast irons. Stainless steels and Copper alloys may also be black oxide coated.
Black oxide coating of steel is composed of magnetite - ferrous-ferric oxide Fe3O4 (FeO*Fe2O3), which is formed as a result of oxidation of iron by Oxygen of air or from oxidizing ions (in aqueous solutions or molten salts).
Hot alkaline process is a formation of black oxide coating on the surface of the part immersed into a boiling aqueous alkaline solution of oxidizing salts.
Hot alkaline process is the most popular method of black oxide coating.
The temperature of hot alkaline solution should be in the range 275-295ºF (135-146ºC). The boiling temperature is determined by the salts content. At the temperatures above 295ºF (146ºC) the obtained oxide coating turns red and rusty. The temperatures below 275ºF (135ºC) indicate low concentration of salts resulting in too slow oxidizing process. Boiling temperature is adjusted by either addition of salts (temperature increase) or dilution of the solution (temperature decrease).
Typical compositions of hot alkaline solutions:
Stages of hot alkaline process
Cold black oxide is a process of non-conversion black finish coating deposited from phosphoric acid solutions containing selenium and copper compounds.
Cold black oxide coating is not really oxide. It is softer than conventional black oxide coating. The color of cold black oxide coating is not consistent and may be rubbed off from the surface. Cold black oxide coating is used as a simpler and less hazardous substitute of hot black oxide process.
Molten salts process involves molten oxidizing salts, Oxygen of which reacts with iron atoms forming a film of black oxide.
A mixture of sodium nitrate (NaNO3) and potassium nitrate (KNO3) in equal quantities is used for the molten baths preparation. Melting point of such eutectic composition is about 485ºF (250ºC).
Typical operating temperatures of the molten salt baths are 550-650ºF (288-343ºC). Stainless steels may be treated at the melt temperature 900ºF (482ºC).