Answer
Black oxide (MIL-DTL-13924) is a conversion coating formed by a chemical reaction with the surface of a carbon steel (SAE 1070-1090) spiral ring. Unlike plating, it does not add significant thickness to the ring (less than 1 micrometer), meaning it does not interfere with the tight tolerances of the groove fit. It provides a moderate level of corrosion resistance, primarily by acting as a carrier for supplemental oil or wax coatings. One of its greatest advantages in high-strength springs is that it does not carry the risk of hydrogen embrittlement associated with electroplating. In automotive applications, black oxide is used for internal engine components where the ring will be continuously lubricated by oil, providing sufficient protection during storage and assembly.
