Official Answer
Answer
Black oxide (MIL-DTL-13924) provides minimal corrosion resistance (mostly for aesthetics and light oil retention) and has negligible impact on fatigue. Zinc Phosphate (heavy phosphate), however, provides a porous crystalline structure that holds much more corrosion-inhibiting oil, offering significantly better salt spray resistance. However, the phosphating process involves an acid pickling step which, like plating, can induce hydrogen embrittlement. Furthermore, for high-cycle fatigue applications, the crystalline structure of phosphate can act as a micro-abrasive if the ring is subjected to vibration. For maximum fatigue life in mildly corrosive environments, a simple oil-dipped finish or switching to 302SS is often preferred.
