Influence of a deposited TiO2 thin layer on the corrosion behaviour of TiN-based coatings on iron

In this paper we report on the influence that the deposition of an uppermost TiO2 layer on top of different sequences of Ti/TiN-based coatings has on the corrosion behaviour of these materials. The results show that this top TiO2 layer fails to block the pinholes or micropores existing in the nitride coatings when they are submitted to an accelerated corrosion test in a constantly humid (98% relative humidity) SO2-polluted atmosphere ([SO2] = 0.04% v / v). This result appears to be correlated with the rapid dissolution (and transformation into Ti4+ sulphate) of the TiO2 layer. However, the TiO2 layer appears to enhance the protective character of the nitride layers under the corrosive conditions of a Prohesion test in which, in a cyclic manner, periods of spraying of the samples with ammonium sulphate and sodium chloride salts at room temperature are alternated with dry periods at 35 °C. Under the latter conditions the dissolution of the TiO2 layer does not occur.