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

Abstract In this paper we report on the influence that the deposition of an uppermost TiO 2 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 TiO 2 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) SO 2 -polluted atmosphere ([SO 2 ] = 0.04% v  /  v ). This result appears to be correlated with the rapid dissolution (and transformation into Ti 4+ sulphate) of the TiO 2 layer. However, the TiO 2 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 TiO 2 layer does not occur.