Black Oxide – RCT-1 Raven Drop Point Serrated Blade
440C Stainless Steel
What is Black Oxide?
Black Oxide is a controlled rusting of the very top layer of the steel, it adopts the properties of the steel that it is applied to.
Black Oxide or blackening is a conversion coating for ferrous materials, stainless steel, copper and copper based alloys, zinc, powdered metals, and silver solder. It is used to add mild corrosion resistance, for appearance and to minimise light reflection. To achieve maximal corrosion resistance the black oxide must be impregnated with oil or wax. One of its advantages over other coatings is its minimal buildup.
Black Oxide for Stainless steel
Hot black oxide for stainless steel is a mixture of caustic, oxidizing, and sulfur salts. It blackens 300 and 400 series, and the precipitation-hardened 17-4 PH stainless steel alloys. The solution can be used on cast iron and mild low-carbon steel. The resulting finish complies with military specification MIL-DTL–13924D Class 4 and offers abrasion resistance.
Hot black oxide application process
Hot baths of sodium hydroxide, nitrates, and nitrites 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 cleaner, water, caustic soda at 140.5 °C (the blackening compound), and finally the sealant, which is usually oil. The caustic soda bonds chemically to the surface of the metal, creating a porous base layer on the part . Oil is then applied to the heated part, which seals it by “sinking” into the applied porous layer. It is the oil that prevents the corrosion of the workpiece. There are many advantages of blackening, mainly:
blackening can be done in large batches (ideal for small parts),
no significant dimensional impact (the blacking process creates a layer about a micrometre 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.
This is the process used to blacken wire ropes for theatrical applications and flying effects.