Black Oxide Technical Information

Black oxide can be produced on ferrous metal using four basic methods:

1. Hot alkaline aqueous black oxide processes typically operate at temperatures in the range of 138^o - 143^o C (280^o - 290^o F). The solutions are primarily made up of sodium hydroxide with nitrates, nitrites and other additives to assist in the blackening process. The exact chemistry of the blackening solutions can vary with the manufacturer. These processes produce a deep, rich black finish which is uniform in appearance. Hot black oxide processes are the most commonly used for industry and they comply with most black oxide specifications.
   
   Added corrosion protection can be achieved by submerging the black oxide coated parts into a post treatment sealer. The sealer is absorbed into the pores of the oxide coating, which increases corrosion protection of the overall coating. While there are various post treatments available for black oxide, the most common is an oil treatment.


2. Mid-temperature black oxide processes also produce magnetite (Fe₃O₄) coatings very much like the hot processes. Temperatures for the mid-temperature processes range from 104^o - 118^o C (220^o - 245^o F). The resultant black oxide finish has similar properties to those produces the hot temperature processes.


3. Cold black oxide processes, as the name suggests, operate at ambient or room temperature. Cold black oxide is not a true black oxide conversion coating. The cold black coating consists of a copper selenium compound rather than the magnetite deposit. While it may be a good choice for limited uses, the coating does not have the same properties as the black oxide finishes produced by the hot and mid-temperature processes.


4. Molten salt processes operating at 315^o C (600^o F) and above. The molten salt baths are difficult to control and they do not consistently produce a uniform colour. The molten black process is a specialty process and not readily available.

Dimensional Stability

One of the advantages of black oxide is that it produces no significant dimensional change. Because the process is strictly a chemical reaction, there are no high or low current density areas to cause uneven coating thicknesses, as is typical with electro-deposited finishes. Black oxide coatings produce a uniform coating over the entire surface area of the metal. The oxide formation penetrates into the surface of the metal from 5 to 10 millionths of an inch.

While black oxide makes claim that no dimensional change occurs, the fact is that there is a minute build up that takes place. This build up is about equal to the depth that the coating penetrates into the surface of the metal (5 to 10 millionths of an inch). This increase is so small, however, that it is unlikely to affect most dimensional tolerances. The black oxide finish will not chip, peel, or rub off.

Black Oxide Applications

While black oxide finishes are not suitable for severe corrosion environments and can not replace phosphate coatings or plated finishes, except where lower corrosion resistance is allowed, it is a cost-effective process with aesthetic appeal for the proper application.

Black oxide has a range of unique properties and benefits that can be applied to a variety of industries.

 

Supplementary Post Treatments

Applying a supplementary post treatment after the black oxide finish increases the corrosion protection properties of black oxide coatings. Alone, black oxide offers only a very mild corrosion resistance. A properly applied post treatment, allowing full absorption of the supplementary coating into the pores of the black oxide finish, enhances the corrosion protection to the metal, while producing a deeper black appearance. An appropriate post treatment also increases the resistance to abrasion of the black oxide where break-in of mating parts are required.

Oil post treatments are frequently used after black oxide coating. The oil can be either water-soluble or solvent-based and can be controlled to allow a heavy oil film on the part or so that the part is virtually dry to the touch. Of course, less oil means less corrosion protection. A clear wax or acrylic may also be used as alternatives where absolutely no oil film is desired.

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