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 stu died at 750 degrees C in an atmosphere of pS(2) similar to 10(-1) Pa a nd pO(2) similar to 10(-18) Pa. By acting as a diffusion barrier and t hrough the formation of a Nb1-xS scale the Nb coating deposited enhanc ed the corrosion resistance of both Ti and Ti-6Al-4V alloy. The corros ion products generated on uncoated titanium in the same environment an d temperature were characterized by a double layered oxide scale of Ti O2 beneath which a TiS2 layer was formed. For the Ti-6Al-4V alloy, alp ha-Al2O3 was precipitated in the external portion of the outer-layer o f TiO2 whilst a layer containing Al2S3, TiS2 and vanadium sulphide (po ssibly V2S3) was idenitified underlying the inner TiO2 layer. After pr olonged exposure (168 h), the Nb coating deposited on Ti and Ti-6Al-4V alloy was consumed. A scale following the sequence of TiO2/TiO2 + NbO 2 + 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 + NbO 2 + Nb2O5/Nb1-xS/TiO2/Al2S3 + TiS2/(substrate) characterized the corro sion products formed on the Nb-coated Ti-6Al-4V alloy.