Black oxide hex socket cap screw is carbon steel socket head fastener with black oxide finish. Black oxide is chemical conversion coating instead of electroplating. It forms matte black surface, provides slight rust protection and good lubricity during tightening. Salt spray resistance is low, suitable only for dry indoor environment. Common strength classes: 8.8, 10.9, 12.9. Compact head design allows installation in recessed space. Widely used in molds, machinery, automotive, hydraulic equipment.