Nitride Layer on TC4 Alloy Prepared by Ultrasonic Surface Rolling and Plasma Nitriding and Its Effect on Corrosion Resistance in LiBr Solution

Titanium-nitride layer on TC4 alloy was prepared by ultrasonic surface rolling process (USRP) and plasma nitriding (PN) to promote corrosion resistance of alloy in 90 oC LiBr solution. The microstructure and electrochemical properties of PN treated titanium alloy were investigated by scanning electron microscopy (SEM), open circuit potential (OCP) test, potentiodynamic polarization, and electrochemical impedance spectrum (EIS). The results showed that the thickness of PN layer was 6.5 mu m to 750 oC PN treatment, and its value was 10 mu m to USRP+650 oC PN multiple treatment. The USRP could dramatically promote the thickening of PN layer even at a lower temperature. The PN layer significantly increased the OCP value, and decreased the corrosion current density of titanium in 90 oC LiBr solution. A thicker PN layer showed a higher film ohmic resistance and a bigger charge transfer resistance in electrolyte. The PN layer prepared by USRP+650 oC PN multiple process had the biggest thickness and exhibited the best corrosion resistance.