What is Black Oxide? Black Oxide is a form of coating used as a finish for ferrous metals, stainless steel and copper and its alloys. Black Oxide, like other coatings is used to provide protection to the part, achieve a required appearance or achieve other desired properties.
However it is different from other coatings in the way that the process is a conversion type coating. This means that there is no depositing or coating of another metal as in the case of galvanization on the part surface but the surface metal itself is converted into a black oxide coating.
Types of Black Oxide Finish: There are different types of black oxide finish that can be produced on stainless steel depending on the type of process that has been used to produce it. These coatings are categorized based on the temperature of the process.
The types of Black oxide finish are Hot Black Oxide, Mid-Temperature Black Oxide and Cold Black Oxide. Hot Black Oxide and Mid Temperature Black oxide are the most preferred types of coating since they are bonded better to the metal and are more resistant to abrasion.
Process of Black Oxide Finishing: Black Oxide Finishing is carried out by dipping the part into a bath of alkaline aqueous solution consisting of Sodium Hydroxide, oxidizing salts and sulfur salts at a temperature of 141 degree Celsius in case of hot black oxide.
The iron on the surface of the metal reacts with the compound and gets converted to magnetite which forms a porous layer on the surface of the metal. A suitable coating of oil, wax or lacquer is then applied. The corrosion resistance of Black oxide finish is derived from this after finish coating which penetrates into the porous layer of magnetite. This process is suitable for 300 and 400 series of Stainless steel and other low to medium carbon alloys of steel. If the part contains some rust or scaling is present on the part then it has to be cleaned using alkaline descaling or acid pickling before the process can be carried out.
Advantages of Black Oxide Finish: Black Oxide Finish provides many advantages for Stainless steel and other metallic parts. It provides protection from corrosion for the part, as well as from abrasion. The oil or wax used as after finish not only improves the visual features of the part but also helps in lubrication and better mating. The coatings can be made glossy or matte as required. Black Oxide finish is also applied to a variety of tools, surgical instruments and parts which are used in bright light to reduce reflectivity and reduce eye fatigue.
One of the main feature of black oxide finish is that no metal is deposited or removed from the part, therefore the dimensions of the part remain virtually unaffected after the process. This is in contrast to other protective methods such as plating or painting so it is beneficial for close fitting parts. Finally the biggest advantage of the Black oxide finish is the low cost of the process. It is significantly cost effective compared to other methods and the process can be performed to parts in large batches at a time.