Answer
Black oxide (MIL-DTL-13924) is a conversion coating that provides minimal corrosion resistance (mostly for aesthetics and oil retention) but does not change the dimensions of the ring or introduce hydrogen embrittlement. Zinc plating (ASTM B633) provides significantly better corrosion protection through sacrificial anode behavior. However, zinc plating carries a high risk of hydrogen embrittlement and adds thickness ($0.0002$ to $0.0005$ inches), which can interfere with the fit in precision grooves. Furthermore, zinc plating can 'flake' under the high-stress coiling and installation of a spiral ring. For high-fatigue applications, black oxide with a rust-preventative oil is often preferred to avoid the risk of embrittlement-induced cracking.
