What is the impact of 'Black Oxide' coating on the dimensional stability and corrosion resistance of carbon steel rings?

Black oxide (per MIL-DTL-13924) is a conversion coating that does not significantly change the dimensions of the ring (typical buildup $< 0.0001$ inches). This is ideal for high-precision spiral rings with tight tolerances on $t$ and $b$. However, black oxide provides very limited corrosion resistance (essentially only preventing rust during storage) and must be supplemented with a rust-preventative oil. Unlike zinc plating, it carries no risk of hydrogen embrittlement. For automotive transmissions, the oil-retaining properties of the black oxide finish are beneficial for the sliding contact during assembly and operation.