HIGH-TEMPERATURE CORROSION OF TI AND TI-6AL-4V ALLOY

Pure titanium and Ti-6Al-4V were exposed at 750 degrees C in an H-2/H2 O/H2S (P(O2)similar to 10(-18) Pa and P(S2)similar to 10(-1) Pa), H-2/ H2O (P(O2)similar to 10(-18) Pa) and air environments for up to 240 hr . The corrosion kinetics, obtained by the discontinuous gravimetric me thod, showed that the sulfidation/oxidation kinetics were linear for T i and linear-parabolic for Ti-6Al-4V in the H-2/H2O/H2S environment. B oth materials obeyed parabolic rate laws in the H-2/H2O atmosphere aft er a transient period, and linear-parabolic rate laws in air. After ex posure to the H-2/H2O/H2S atmosphere, the titanium specimen displayed a double scale of TiO2 with an intervening TiS2 film between the doubl e-layered scale of TiO2 and the substrate. Ti-6Al-4V also contained,a double layer of TiO2 together with a stratum consisting of Al2S3, TiS2 and vanadium sulfide at the junction of the inner TiO2 layer and subs trate. Some Al2O3 precipitated in the external portion of the outer Ti O2 layer. Following oxidation in the low-P-O2 atmosphere a double-laye red oxide of TiO2 scale formed on both Ti and Ti-6Al-4V. The scale on Ti-6Al-4V also contained an alpha-Al2O3 film situated between the oute r and inner (TiO2) layers. For both materials, multilayered scale form ation characterized air oxidation. In detail a multilayered oxide scal e of TiO2 formed on the air-oxidized Ti, while a multilayered oxide sc ale with alternating layers of Al2O3/TiO2 developed on Ti-6Al-4V oxidi zed in air.