Black oxide is unlike any other finish. No other process produces the same finish and combination of properties for less cost. Black oxide has certain properties and characteristics that make it ideal for certain applications.
What is it?
Black oxide is a conversion coating formed by a chemical reaction produced when parts are immersed in the alkaline aqueous salt solution operated at approximately 285 degrees Fahrenheit. The reaction between the iron of the ferrous alloy and the hot oxide bath produces a magnetite on the actual surface of the part. It is possible to oxidize non-ferrous metals under suitable conditions to form black oxides.
How is the process performed?
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 oxiding. 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.
The advantages of black oxide are:
No dimensional changes
The as-formed dimensions do not change (as they do when plated or painted). Black oxide is a coloring of the base metal, no metal is removed or deposited.
The surface of the part may be shiny or matte depending on the surface of the part and the after finish applied. For example, a polished surface will retain its luster after black oxidizing.
The finish will not chip, peel, flake, or rub off: Black oxide can only be removed by mechanically or chemically removing the finish itself.
Reduces light glare (reflection)
Black oxide makes an excellent finished surface for moving parts such as hand tools and machine parts. The reduction in reflectivity reduces eye fatigue and is less distracting.
Improved lubrication characteristics
Black oxidized parts have improved lubricity and anti-galling characteristics due to the after-finish (oil or wax) resulting in smoother running, mating parts.
No hydrogen embrittlement
The black oxide process does not require an acid activation nor is it an electro-process; therefore, hydrogen embrittlement is not a factor. If the parts are scaled or rusty and an acid pickle is required, any hydrogen that may have evolved will quickly dissipate in the black oxide tank (running temp. 285 degrees Fahrenheit). Any remaining hydrogen will be completely dissipated within 48 hours after processing.
Welding black oxided parts does not produce noxious fumes nor hamper the ease with which a part can be welded as may be encountered by welding plated or painted parts
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