CORROSION PROTECTION BY PECVD-SIOX AS A TOP COATING ON TIN-COATED STEEL

Amorphous SiO(x) was applied by plasma-enhanced chemical vapour deposition as a top coating on TiN-coated steel in order to evaluate its corrosion resistance as a function of the process parameters by an a.c. impedance test in 3.5% NaCl solution. Experimental results showed that the substrate and the CO2/SiH4 flow ratio were the two most influential coating parameters. A high passivation resistance value was obtained when specimens were prepared with the ps source at a low CO2/SiH4 flow ratio, which favours radical-molecule reaction in the plasma state; as a consequence, a nodule-like area together with the film thickness of the SiO(x) deposit are responsible for the increase in corrosion resistance. An equivalent circuit is proposed to elucidate the R(p) value of SiO(x)-coated specimens as a function of the CO2/SiH4 ratio; the trend was found to be consistent with results evaluated by a salt spray test and an atmospheric corrosion test. SiO(x) coating gave additional protection on TiN-coated low carbon steel as well as on TiN-coated stainless steel.