Hideo Numata1, Isao Tomizuka1 and Tokuzou Tsujimoto2
1National Research Institute for Metals, Tsukuba
Effect of cathodic polarization (CP) times on the polarization curves was studied for TiAl-base alloy in 2.5 kmol/m3 H2SO4 at 293 K. CP was carried out at -900 mV vs SCE for 10 min or 130 min. Effects of CP time on change of anodic current density (Ia) versus holding time at -530 mV and 0 mV were also studied. The results were compared with those of pure Ti and Al. The anodic reaction product was examined by XRD, EDX, SEM and hydrogen (H) analysis.
The cathodic current density (Ic) and Ia of the alloy increased with the CP time, while those of pure Ti and Al were not affected by the CP time. The decrease of Ia with time of the alloy at 0 mV did not obey the parabolic rate law in contrast with Ti. Ia of Ti did not change with time at -530 mV but Ia of the alloy decreased. It is considered that H was absorbed in the alloy during CP and enhanced the cathodic and the anodic reactions, which result in preferential dissolution of Al and concentration of Ti and H in the alloy during anodic polarization (AP). The electrochemical characteristics of the alloy were induced not by degradation of the passive film but by absorption of H during CP.
XRD revealed the formation of two kinds of hydrides with a tetragonal and a hexagonal structure on the reaction surface of the alloy after 130 min CP. The hydrides were not observed after 10 min CP, but they were formed by AP after the CP. An electrochemically affected layer was formed on the alloy surface, which is 10 μm thick and enriched by H.
(Received April 6, 1995)
cathodic polarization, anodic polarization, TiAl-base alloy, sulfuric acid, hydride, hydrogen absorption
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