Microstructural and corrosion studies of Ti alloy (IMI 834) in acidic solutions

Abstract

The electrochemical polarization behaviour of a titanium alloy (IMI 834) was studied in different concentrations of aqueous and ethylene glycolic solutions of hydrochloric and phosphoric acid at 30 °C. The influence of furnace cooled (FC), air cooled (AC) and water quenched (WQ) heat treatments and microstructure on polarization behaviour was also studied. An active passive transition and a large range of passivity potential were observed in aqueous and glycolic solutions of the acids. The magnitudes of the critical current density (ic) and passive current density (ip) were higher for hydrochloric acid in comparison to phosphoric acid. Values of ic and ip were lower in ethylene glycol–hydrochloric acid up to 7 M and were higher above 7 M hydrochloric acid–ethylene glycol solutions as compared to the corresponding hydrochloric acid concentrations. However, these values were always lower in ethylene glycol–phosphoric acid than in phosphoric acid solutions. The microstructure of the alloy after solution treatment (1080 °C, 30 min; FC, AC and WQ) showed single phase transformed β structure. The increase in the cooling rate enhances the fineness of the β laths. The working electrode surface was examined by scanning electron microscopy (SEM) and scanning tunnelling microscopy (STM).