CORROSION-RESISTANCE OF COMPOSITE TIN-TIC LAYERS DEPOSITED ON TOOL STEELS BY DIFFERENT TECHNIQUES

TiC layers were formed by the low-pressure chemical vapour deposition (LPCVD) technique on tool carbon steel (0.9 wt % C; 0.4 wt % Si; 0.5 w t % Mn), bare or nitrided by glow discharge with formation of zones co nsisting of (epsilon + gamma') and (gamma') phases. The composite TiN- TiC layers were formed by the deposition of TiN on the coated TiC laye rs, by a pressure-assisted chemical vapour deposition (PACVD) techniqu e or by TiC sputtering in nitrogen under glow discharge conditions. Th e electrochemical behaviour of the metal/coating has been established by potentiodynamic tests done in acid (20% H3PO4) and alkaline (0.1 N NaOH) solutions. The deposited layers protect the steel by decreasing the steel area exposed to the aggressive solution and by promoting ste el passivation. The protective ability of composite TiN/TiC layers is higher than that of corresponding single layers. Improvement of the pr otective ability of LPCVD TiC layers is achieved by the deposition con ditions, providing the formation of fine grains [111] texture. The ele ctrochemical behaviour of steel coated with complex TiN/TiC layers app roaches the intrinsic electrochemical behaviour of TiN.