Black oxide

Ferrous material

A standard black oxide is iron(II,III) oxide (Fe₃O₄), also called by the name of the ore composed thereof (magnetite); it is more mechanically stable on the surface and provides better corrosion protection than red oxide (rust) Fe₂O₃. Modern industrial approaches to forming black oxide include the hot and mid-temperature processes described below. Traditional methods are described in the article on bluing. They are of interest historically, and are also useful for hobbyists to form black oxide safely with little equipment and without toxic chemicals.

Low temperature oxide, also described below, is not a conversion coating—the low-temperature process does not oxidize the iron, but deposits a copper selenium compound.

Hot black oxide

Hot baths of sodium hydroxide (NaOH), nitrates such as sodium nitrate (NaNO
₃), and/or nitrites such as sodium nitrite (NaNO₂) at 141 °C (286 °F) are used to convert the surface of the material into magnetite (Fe₃O₄). Water must be periodically added to the bath, with proper controls to prevent a steam explosion.

Hot blackening involves dipping the part into various tanks. The workpiece is usually dipped by automated part carriers for transportation between tanks. These tanks contain, in order, alkaline detergent, water, sodium hydroxide at 140.5 °C (284.9 °F) (the blackening compound), and finally the sealant, which is usually oil.

The NaOH (caustic soda) and elevated temperature cause Fe₃O₄ (black oxide) to form on the surface of the metal instead of Fe₂O₃ (red oxide; rust). While it is physically denser than red oxide, the fresh black oxide is porous, so oil is then applied as post treatment to the heated part, which seals it by "sinking" into it. The combination prevents corrosion of the workpiece. There are many advantages of blackening, including:

• Blackening can be done in large batches, which is ideal for small parts.
• There is no significant dimensional impact. The blacking process creates a layer about 1 μm thick.
• It is far cheaper than similar corrosion protection systems, such as paint and electroplating.

The oldest and most widely used specification for hot black oxide is MIL-DTL-13924, which covers four classes of processes for different substrates. Alternate specifications include AMS 2485, ASTM D769, and ISO 11408.

Iron(III) chloride (FeCl₃) may also be used for steel blackening by dipping a piece of steel into a hot bath of 50% FeCl₃ solution and then into a hot boiling water. The process is usually repeated several times.

Mid-temperature black oxide

Like hot black oxide, mid-temperature black oxide converts the surface of the metal to magnetite (Fe₃O₄). However, mid-temperature black oxide blackens at a temperature of 90–120 °C (194–248 °F), significantly less than hot black oxide. This is advantageous because it is below the solution's boiling point, meaning there are no caustic fumes produced.

Since mid-temperature black oxide is most comparable to hot black oxide, it also can meet the military specification MIL-DTL-13924, as well as AMS 2485.

Cold black oxide

Cold black oxide treatment, also known as room temperature black oxide, is applied at a temperature of 20–30 °C (68–86 °F). It is not an oxide conversion coating, but rather a deposited copper selenide (Cu₂Se) compound. Cold black oxide is convenient for in-house blackening.

This coating produces a similar color to the one the oxide conversion does, but tends to rub off easily and offers less abrasion resistance. The application of oil, wax, or lacquer brings the corrosion resistance up to par with the hot and mid-temperature. Applications for cold black oxide process include tooling and architectural finishing on steel. It is also known as cold bluing.