Effects Ti/TiAlN composite multilayer coatings on corrosion resistance of titanium alloy in solid NaCl-H2O-O2 at 600 °C

This paper investigates corrosion resistance of Ti-6Al-4V coated by Ti/TiAlN composite multilayer coatings with different Ti layer thicknesses in presence of solid NaCl, H2O and O2 at 600 °C. Effects of TiAlN single layer coatings with different thicknesses on the corrosion resistance of Ti-6Al-4V were also studied for comparison. The results showed that the thick TiAlN single layer coating and the Ti/TiAlN composite multilayer coating with Ti layer thickness of 0.15 μm exhibited good corrosion resistance in the above environment, while localized accelerated corrosion of substrate occurred for thin TiAlN single layer coating and Ti/TiAlN composite multilayer coating with Ti layer thickness of 0.3 μm coated specimens. It is proposed that improvement of corrosion resistance of titanium alloy by application of TiAlN single layer coating and Ti/TiAlN composite multilayer coating is attributed to suppression of active corrosion induced by gaseous metal chloride, hydrochloric acid and chlorine in the corrosion process. Accelerated local corrosion of thin TiAlN single layer coated specimen is related to existence of intrinsic through-coating defects such as pinholes in the coating. For the Ti/TiAlN composite multilayer coating with Ti layer thickness of 0.3 μm, volume expansion caused by oxidation of Ti in sublayers to TiO2 when corrosive media diffused into along through-layer defects in top TiAlN layer led to bending and cracking of the sublayers of the composite multilayer coating and eventually accelerated local corrosion of the coating and substrate.