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Black Oxide Coating: What Is Black Oxide?

Black Oxide, sometimes called blackening, is the act of converting the top layer of a ferrous material with a chemical treatment. Treating fasteners with a black oxide coating not only adds a nice clean black look but can also add a mild layer of corrosion and abrasion resistance. To achieve the maximum in corrosion resistance, black oxide can either be waxed or oiled to add an extra layer of protection. The wax coating may dull the color of the treatment but provides the best resistance. The black oxide treatment is also referred to as gun bluing.

When fasteners are dipped into the black oxide bath materials (warm and hot processes) the harsh chemical components convert the top layer into magnetite. Black Oxide Stainless Steel Nuts Bolts and Washers are the most common form of black oxide fastener available. Prized both for the corrosion resistance of stainless steel with the added benefits of the black oxide treatment.

Materials That Can Get The Black Oxide Treatment

• Stainless Steel
• Copper
• Copper Based Alloys
• Zinc
• Powdered Metals
• Silver Solder

The Black Oxide Processes

Blackening can be achieved in a number of different ways. There are three main ways to do this. If you are interested in the specifics of each type check out this article from Wikipedia.

Hot Bath

The process of treating the fasteners in different chemicals in order to convert the surface material into magnetite. The process is completed by dipping the fasteners into different tanks and occasionally placed in water. Once the process of dipping is completed the top layer of the fastener is porous and a layer of oil is applied. The oil seeps into the material adding that extra corrosion resistance to rust. This process creates a black oxide bath that complies with MIL-DTL-13924, AMS 2485, ASTM D769 and ISO 11408 standards.

Cold Coating

The cold process applies a compound (copper selenium) onto the fasteners at room temperature. This process is more convenient for working in house as it does not require many chemicals at heated temperatures to apply. Once applied it is considered weaker than the other forms of oxidization until a layer of oil or wax is applied to the surface.

Mid-Temperature Bath

Like the hot bath treatment method but does not produce the same toxic fumes of the hot bath and the surface is converted to magnetite at a much lower temperature. This method can also meet the same military standards that the hot bath version produces.

Benefits Of Black Oxide Fasteners

• Does not significantly change the dimensions of the fasteners as hot dip galvanizing does.
• A cheaper alternative to other methods of corrosion resistance such as electroplating.
• An appealing look when a bright shine is not desired.
• The process can normally be done in large quantities which makes it faster and cheaper for smaller sizes.
• Reduces the risk of galling by adding an oil finish
• Decorative finish
• Adds a layer of mild corrosion and abrasion resistance
• Paint sticks well to the black coating if painting is required

Will Black Oxide Bolts Rust

Black Oxide adds a mild layer of corrosion and abrasion resistance to fasteners. Like any material, black oxide treated fasteners can rust in the right environments. It will depend on if the fastener is damaged or marred, the type of metal with the black oxide treatment and environmental conditions.

Disadvantages Of Black Oxide

Black oxide is not nearly as corrosion resistant as some of the other options available to improve corrosion resistance.

The black oxide finish can be easily rubbed off. To prevent this, put some painters tape over the tip of your bit to create a tighter fit and prevent scratching in the event of a slip out of the drive.

The Black residue from the fasteners may come off of them so for certain applications it is recommended that they are wiped down before use. This extra step can be labor-some if many fasteners are needed.

Black oxide fasteners are largely used in the automotive industry due to their sleek black look. This look can deteriorate very quickly though in hot environments. Often, after a black oxide coat is applied to a fastener, they are placed in an oil bath for an extra layer of protection. The oil residue that stays on the fasteners will begin to turn brown when in high heat environments. So many users find they bolts look like they rust prematurely. It is not recommended to place black oxide bolts on or near engines.

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