![]() However, mid-temperature black oxide blackens at a temperature of 90–120 ☌ (194–248 ☏), significantly less than hot black oxide. Like hot black oxide, mid-temperature black oxide converts the surface of the metal to magnetite (Fe 3O 4). The process is usually repeated several times. Iron(III) chloride (FeCl 3) may also be used for steel blackening by dipping a piece of steel into a hot bath of 50% FeCl 3 solution and then into a hot boiling water. Alternate specifications include AMS 2485, ASTM D769, and ISO 11408. The oldest and most widely used specification for hot black oxide is MIL-DTL-13924, which covers four classes of processes for different substrates. ![]() It is far cheaper than similar corrosion protection systems, such as paint and electroplating.The blacking process creates a layer about 1 μm thick. There is no significant dimensional impact.Blackening can be done in large batches, which is ideal for small parts.There are many advantages of blackening, including: ![]() The combination prevents corrosion of the workpiece. 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 NaOH (caustic soda) and elevated temperature cause Fe 3O 4 (black oxide) to form on the surface of the metal instead of Fe 2O 3 (red oxide rust). These tanks contain, in order, alkaline detergent, water, sodium hydroxide at 140.5 ☌ (284.9 ☏) (the blackening compound), and finally the sealant, which is usually oil. The workpiece is usually dipped by automated part carriers for transportation between tanks. Hot blackening involves dipping the part into various tanks. Water must be periodically added to the bath, with proper controls to prevent a steam explosion. Hot baths of sodium hydroxide (NaOH), nitrates such as sodium nitrate ( Na N Oģ), and/or nitrites such as sodium nitrite (NaNO 2) at 141 ☌ (286 ☏) are used to convert the surface of the material into magnetite (Fe 3O 4). 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. They are of interest historically, and are also useful for hobbyists to form black oxide safely with little equipment and without toxic chemicals. Traditional methods are described in the article on bluing. Modern industrial approaches to forming black oxide include the hot and mid-temperature processes described below. Ferrous material Ī standard black oxide is magnetite (Fe 3O 4), which is more mechanically stable on the surface and provides better corrosion protection than red oxide (rust) Fe 2O 3. One of its advantages over other coatings is its minimal buildup. To achieve maximal corrosion resistance the black oxide must be impregnated with oil or wax. It is used to add mild corrosion resistance, for appearance, and to minimize light reflection. For traditional methods of forming black oxide, see Bluing (steel).īlack oxide or blackening is a conversion coating for ferrous materials, stainless steel, copper and copper based alloys, zinc, powdered metals, and silver solder. This article is about modern black oxide coatings.
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