Black Oxide Coating

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.


The purpose of the specification is to provide requirements for a thin film of Black oxide to provide corrosion protection. This coating is primarily used on fasteners, but is also appropriate for other small parts made from ferrous material.

Referenced Documents

ISO 11408  :  Black oxide coating on iron and steel

The Step-by-Step Zinc Plating Process

The five basic steps for the black oxide conversion coating are clean; rinse; black oxide; rinse; supplementary coating (after-finish). If rust or scale is present on the part, additional steps such as acid pickling or alkaline de- scaling may have to be added before oxidising. Neutralizing and/or further rinsing may be necessary on assemblies and parts with blind holes to eliminate "flowering" or bleed-out. Black oxide cannot be produced over plated parts (zinc, nickel, chromium, cadmium, phosphate). This plating must be stripped prior to the black oxide process.

The supplementary coating (after-finish) will dictate the final appearance and function of the part. When a print or drawing specifies "Black Oxide" without any specific after-finish, it is interpreted as Black Oxide and Oil. It is recommended that an after-finish is always applied, as black oxide without an after-finish has very poor corrosion protection. However, with an oil, wax, or lacquer, it is possible to achieve excellent indoor corrosion protection (100+ hrs. in a humidity cabinet). Black oxides on steel are not suitable for severe outdoor applications or corrosive environments, but they can provide superior humidity cabinet results with proper supplementary coating. Black Oxides on Stainless steel and/or brass alloys will yield excellent corrosion protection, primarily due to their inherent properties.

The after-finish is usually determined by the part configuration and the end-use. If a "dry-to-touch" finish is needed, it is important to specify either Oil spin dry to touch, wax or lacquer. The oil after-finish will generally be a glossy finish, whereas the wax will be more matte. It is possible to use a torque/tension wax to provide added lubricity and reduced drive torque.

In order to determine the appropriate after-finish, you must first evaluate which after-finish to use. The following factors should be considered:

• Length of protection required
• Desired finished appearance (gloss/matte)
• Storage conditions (humidity, vapor, temperature)
• Final application

The Benefits of Black Oxide

Applying a black oxide finish offers many essential benefits to ensure smooth operations of equipment, devices, components and much more.

Protection From Corrosion - When oiled or waxed, a black oxide finish helps to prevent corrosion. This protection from corrosion can be vital for error-free functioning, providing reliable and long lasting use.
A High Quality, Black Appearance - For discretion, essential functioning and more, sometimes your metal needs to feature a black appearance. Black oxide provides a high quality, black aesthetic at low production costs. Black oxide offers a durable way to make a metal black, avoiding chipping and other forms of corrosion that can occur from other blackening methods.

Dimensional Stability - A black oxide finish is extremely thin, providing 10 millionths of an inch at maximum to the dimensions of your metal. This minuscule addition does not effectively change your metal's dimensions, providing the benefits of a black oxide finish while maintaining your intended function and appearance.
A Cost Conscious Solution - Black oxide provides all of the above benefits at a reduced expense, making black oxide a great solution for organizations that are working with tight or fixed budgets.


Protective coating has no adverse effects on fastener life.
A simple thermal immersion test with constant heating rate is shown to be an effective method for investigating hydrogen uptake from the decomposing lubricant. It is proven that the black oxide coating can be used as a barrier to hydrogen uptake. Detailed microstructure and composition characterisation of the black oxide layer were also performed.



Photographs of the sample parts, before inspection must be included. The photograph or image Caption must clearly include the part reference, measured values and exposure conditions.

Final Remarks

The technical staff member and the laboratory manager must sign and certify the results presented in the report at the end of the document