Official Answer
Answer
Black oxide (MIL-DTL-13924) is a conversion coating that provides minimal corrosion resistance (typically 2-4 hours of salt spray) and is primarily used for aesthetics and to reduce light reflection. It does not change the dimensions of the ring. Zinc Phosphate (MIL-DTL-16232) is a heavier coating that provides significantly better corrosion protection (up to 72 hours salt spray with oil) and acts as an excellent base for lubricants. In automotive applications, Zinc Phosphate is preferred for rings exposed to the elements, but it adds a measurable thickness (3-10 microns). Neither coating is suitable for high-corrosion environments where stainless steel or specialized coatings like Magni or Geomet would be required.
