Official Answer
Answer
Black oxide (per MIL-DTL-13924) is a conversion coating formed by a chemical reaction with the iron in the steel. Unlike plating, it does not add significant thickness to the part (typically $< 1 \mu m$), making it ideal for wave springs where tight tolerances on thickness $t$ are critical for maintaining the spring rate $k$. However, black oxide offers very limited corrosion resistance and must be supplemented with a rust-preventative oil or wax. It is primarily used for aesthetic purposes or to reduce light reflection in optical assemblies. For applications requiring more than $24-48$ hours of salt spray resistance, a zinc-flake coating or switching to a stainless steel material is mandatory.
