Black oxide hex bolt is hexagon head bolt made of carbon steel 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. It has low salt spray resistance, only suitable for indoor dry environment. Commonly available strength classes: 8.8, 10.9, 12.9. Widely used in machinery, mould, automotive, indoor steel structures.