Blackening, also known as black-oxide coating or black oxide, is a process used to create a black, corrosion-resistant layer on the surface of various metals, particularly ferrous metals such as steel. This coating provides both aesthetic appeal and improved durability.

The blackening process typically involves immersing the metal part in a hot alkaline solution containing various chemical compounds, such as sodium hydroxide, nitrate, nitrite, and sometimes copper or selenium salts. The exact composition of the solution may vary depending on the desired outcome and the specific metal being treated.

Corrosion resistance The black oxide layer acts as a protective barrier, reducing the metal's susceptibility to rust and other forms of corrosion.

Reduced light reflection The black oxide coating absorbs light rather than reflecting it, making it useful for applications where glare reduction is important, such as optical equipment or camera components.

Blackening is widely used in various industries, including automotive, aerospace, firearms, and general manufacturing. The process can be performed in-house by manufacturers or outsourced to specialized service providers with the necessary equipment and expertise.