The aims of this study were to investigate the effects of Zr addition and potentiodynamic polarization on the microstructure and corrosion resistance of Ti-22Nb and Ti-22Nb-6Zr alloy samples. The corrosion tests were carried out in 0.9% NaCl at 37 8C and neutral pH value, utilizing the OCP, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) techniques. The results of XRD and optical microscopy indicated that the addition of Zr stabilized the b phase, which plays a crucial role in the corrosion resistance improvement of the Ti-22Nb-6Zr alloy. From the polarization curves, it can be seen that the alloys exhibited a wide passive region without the breakdown of the passive films and also low corrosion current densities. In addition, the values of the corrosion current densities and passive current densities decreased with the addition of 6 at% Zr into the Ti-22Nb alloy. The EIS results of these two alloy samples after 1-h immersion in 0.9% NaCl solution, and being fitted by R S (Q P R P ) model, suggested that the corrosion resistance of the passive films improved with the addition of Zr and only a single passive film formed on the surfaces. However, two time constants were observed for the Ti-22Nb and Ti-22Nb-6Zr alloy samples after potentiodynamic polarization, the spectra of which can be fitted using the R s (Q o (R o (Q b R b ))) model. In addition, the corrosion resistance of the two alloy samples was reinforced significantly because of polarization when compared to the immersed samples. All these observations suggested a nobler electrochemical behavior of the titanium alloys with the addition of Zr element and after polarization.

The aim of this study was to investigate the effects of Hf content and immersion time on the electrochemical corrosion behavior of the Ti-22Nb-xHf (x ¼ 0, 2, 4, and 6 at%) alloy samples in 0.9% NaCl solution at 37 8C and neutral pH range, utilizing the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. From the polarization curves, all these alloys exhibited typical passive behavior, which was indicated by a wide passive region without the breakdown of the passive films and low corrosion current densities. In addition, the values of the corrosion current densities and passive current densities decreased with increase in the Hf content. The EIS results, fitted by R S (Q P R P ) model, exhibited capacitive behavior (high corrosion resistance) with phase angles closed to À808 and high impedance values at low and medium frequencies, indicating the formation of a highly stable film on these alloys in the test solution. The resistance of the passive films improved with increase in the Hf content and immersion time. All these observations suggested a more noble electrochemical behavior of the Ti-22Nb-xHf alloys compared to the Ti-Nb binary alloy.