Official Answer
Answer
Black Oxide (MIL-DTL-13924) is a conversion coating that provides a dark appearance and a small degree of corrosion resistance, primarily intended for indoor applications or where the ring will be submerged in oil. Zinc Phosphate (heavy) provides better corrosion protection and acts as a substrate for lubricants. For spiral rings, the coating must be thin enough to not interfere with the ring's ability to flex during installation. Carbon steel rings (SAE 1070-1090) must be carefully processed to avoid hydrogen embrittlement if these coatings involve an acid pickling step.
