EFFECT OF NB COATING ON THE SULFIDATION OXIDATION BEHAVIOR OF TI AND TI-6AL-4V ALLOY

The environmental response of Nb-coated Ti and Ti-6Al-4V alloy was studied at 750 degrees C in an atmosphere of pS(2) similar to 10(-1) Pa and pO(2) similar to 10(-18) Pa. By acting as a diffusion barrier and through the formation of a Nb1-xS scale the Nb coating deposited enhanced the corrosion resistance of both Ti and Ti-6Al-4V alloy. The corrosion products generated on uncoated titanium in the same environment and temperature were characterized by a double layered oxide scale of TiO2 beneath which a TiS2 layer was formed. For the Ti-6Al-4V alloy, alpha-Al2O3 was precipitated in the external portion of the outer-layer of TiO2 whilst a layer containing Al2S3, TiS2 and vanadium sulphide (possibly V2S3) was idenitified underlying the inner TiO2 layer. After prolonged exposure (168 h), the Nb coating deposited on Ti and Ti-6Al-4V alloy was consumed. A scale following the sequence of TiO2/TiO2 + NbO2 + Nb2O5/Nb1-xS/TiO2/TiS2/(substrate) was observed on the surface of the Nb-coated Ti, whilst a scale with sequence of TiO2/V2S3/TiO2 + NbO2 + Nb2O5/Nb1-xS/TiO2/Al2S3 + TiS2/(substrate) characterized the corrosion products formed on the Nb-coated Ti-6Al-4V alloy.