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#rustproofing

Hot Black Oxide Finish on Steel (Iron)

A hot bath of sodium hydroxide, nitrates, andnitrites, at 285 °F (141 °C), are used to convert the surface of the material intomagnetite (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 amicrometre thick) it is far cheaper than similar corrosion protection systems, such as paint andelectroplating

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.