Black oxide lock nut is carbon steel prevailing torque lock nut with black oxide finish. Black oxide is chemical conversion coating rather than electroplating, forming matte black surface. It offers minor rust resistance and good friction performance. Salt spray resistance is low, only for dry indoor conditions. The built‑in nylon insert creates friction to prevent loosening under vibration. Common strength classes: 8,10. Widely matched with high-strength bolts for machinery, molds and automotive assemblies.