Studies on corrosion resistance of thick Ti/TiN multilayer coatings under solid NaCl–H2O–O2 at 450 oC

Abstract

Thick Ti/TiN multilayer coatings with different thicknesses of Ti sublayers were prepared on Ti–6Al–4V titanium alloy by arc ion plating technology. The corrosion resistance of coatings was investigated under solid NaCl, water vapor and oxygen at 450 oC. The results demonstrate that the as-prepared Ti/TiN multilayer coatings were consisted of Ti, Ti2N and TiN phases. The crystalline grains of TiN layers exhibited columnar growth, while Ti sublayers with equiaxed grains interrupted the continuity of columnar grains of TiN layers in Ti/TiN multilayer coatings. With the increasing in Ti layer thickness, more and larger macroparticles were embedded into the multilayer coatings. Ti/TiN multilayer coating with thinner Ti sublayer maintained integrity except for the formation of a continuous thin corrosion scale after corrosion tests and no obvious internal oxidation occurred, presenting favorable corrosion resistance. In contrast, locally severe internal oxidation occurred to Ti/TiN multilayer coatings with thicker Ti layer, deteriorating the corrosion resistance of multilayer coatings. Oxygen inwardly diffused from ambient environment to sublayers along the pinholes or columnar structure penetrating through TiN layer. This resulted in internal oxidation accompanied with volume expansion, then bending and cracking of sublayers, and finally promoting localized corrosion of multilayer coatings with thicker Ti sublayer. Besides, the existence of large macroparticles facilitated this localized internal oxidation. Internal oxide also occurred at the edge of TiN single layer coating, which was essentially detrimental for coating protection with prolonged exposure.