Black Oxidization or Blackening is probably the most popular surface treatment for ferrous metal corrosion protection. Like many other surface treatment method, the process creates a thin but compact oxide layer which isolates the workpiece from the air and hence prevents corrosion. There are hot alkaline oxidation and low-temperature oxidation. Low-temperature oxidation does not work well with low carbon steel.

To obtain a perfect compact and smooth oxide layer, bath time, temperature and oxidant are the key elements. Black oxidization uses an aqueous solution of NaOH, NaNO2, and NaNO3 as oxidant with temperature range 130-145℃ and bath 50-80min. Low carbon steel needs a bit longer time to be oxidized, some steel alloy (for example steel alloy with a high content of cr) can form a protective layer on its own, and hence need a longer both time to have a good blackening result too.

A subset of black oxidization is called ‘bluing'. Instead of the aqueous solution, bluing applies a fused solution of NaOH, NaNO2, and NaNO3 at 550℃. At this temperature, Fe3O4 protection layer appears blue.

Not all black oxidized workpieces look ‘black', ferrous alloys content Mn looks more ‘dark red' or ‘coffee' color after black oxidization.

Black oxidize is the most popular anti-corrosion treatment for ferrous alloy parts. Although it only provides limited protection against corrosion, it costs low and only requires minimum equipment. As the protection layer created in this process is very thin (um level), the process has almost no effects on size or tolerance.

In some rare cases, stainless steel is required to be black oxidized (for example, optical instruments require parts with no light reflections), although it is called ‘black oxidization' but it is a salt-bath process.