Black Oxide Coating; What? Why? How?

What is Black Oxide Coating?

Black Oxide Coating is a surface treatment that can be used to increase corrosion resistance and wear resistance of ferrous materials such as steel and iron.

Unlike other surface treatments such as powder coating or electro-plating which add a material blanket to the surface of the component, black oxide coatings are created by a chemical reaction. This chemical reaction occurs when the surface layer of the substrate is converted into its oxide when exposed to special oxidising chemicals. For ferrous materials this oxidised layer is called magnetite and is black by nature, hence the term black oxide coating.

The primary use for black oxide coatings is to increase corrosion and wear resistance. This surface treatment is often used on hand tools and other hardware such as fasteners for this reason. Another advantage of black oxide coating is the layer thickness on a finished part is typically between 1-2um. This negligible surface layer buildup makes black oxide coating great for high tolerance applications such as gears and electronic components.

Advantages and Disadvantages of Black Oxide Coating

Advantages

• Improved corrosion resistance
• Improved wear resistance, including anti-galling
• Improved surface layer hardness
• Low friction coefficient
• Negligible dimensional changes
• Smooth surface finish
• Provides a uniform black finish to component. Either glossy or matte finishes can be achieved depending on post-treatment sealants.
• Affordable

Disadvantages

• Not suitable for non-ferrous materials like aluminium or titanium
• If corrosion resistance is the primary reason for surface coating, other, cheaper alternatives more suitable.
• Lower durability than other coatings which can expose the substrate material
• Coating is difficult to repair once damaged
• If oil is used to seal the oxide layer post-treatment, this oil layer when exposed to hot environments can turn brown and appear like the part is rusting prematurely

Black Oxide Coating Process

There are three specifications of black oxide coating: hot, mild and cold. Hot black oxide process is by far the most common process and consists of seven stages. The ‘bathing’ processes used make it very suitable for batch processing.

1. Surface Cleaning
   Cleaned with an alkaline solution it ensures the removal of contaminants such as grease, dirt, dust and oils
2. Rinsing
   A water bath is used to remove the alkaline solution from the surface
3. Acid Pickling
   This step may be optional but acid pickling is used to remove any oxide film and rust stains from the material surface
4. Rinsing
   Part is rinsed again to remove acid solution from the surface
5. Black Oxide Chemical Bath
   Components are dipped into a hot bath of special oxidising chemicals. The extent of ‘blacking’ is dependant on the dipping duration.
6. Rinsing
   Part is rinsed again to remove any residue
7. Sealing
   This is an additional step that ensures any pores on the surface are filled for greater corrosion resistance. For a glossy appearance an oil sealant is used, for a matte finish a clear wax may be used instead. The sealant must be allowed to completely dry before the final component is ready for use.