Official Answer
Answer
Black Oxide (MIL-DTL-13924) is a conversion coating formed by a chemical reaction with the surface of the carbon steel. Unlike electroplating, it does not change the dimensions of the ring significantly (typically $< 0.00001$ inches) and, most importantly, does not introduce Hydrogen Embrittlement. While it provides only marginal corrosion resistance (rated for approx. 24-96 hours in salt spray with oil), it is excellent for internal mechanical assemblies where the ring is submerged in oil. It also provides a non-reflective surface and increases the lubricity of the ring, which aids in installation and reduces the risk of galling in the groove.
