Titanium and zirconium are used extensively in the industry due to their exceptional corrosion resistance in most of the harsh environments [1, 2]. Ti is used in aerospace industry, biomedical implantations, and in several chemical industries due to its low density, high mechanical strength and corrosion resistance [1]. Zr is mainly used as a fuel cladding material in nuclear industries and in chemical industries due to its high resistance to corrosion and H₂embrittlement [2]. Although these materials exhibit high corrosion resistance in harsh acidic and alkaline environment, they readily dissolve in acidic fluoride media. The anodic polarization curves of Ti and Zr in HF [3,4] show that the active and passive regimes are clearly distinguishable . A comparison of these results also shows that Ti offers better passivation than Zr, in HF media.

In this work, the effect of varying the concentration of HF on the corrosion rate was investigated using potentiodynamic polarization, linear polarization techniques and electrochemical impedance spectroscopy at open circuit potential (OCP). A rotating disk electrode of Ti/Zr was used in a three electrode cell with Ag/AgCl (in 3.5 M KCl) as reference and Pt wire as counter electrode. HF concentration was varied from 10 - 1000 mM for the corrosion rate estimation of Ti. Zr corrosion analysis was performed in 5 - 200 mM HF. In all the experiments, Na₂SO₄was used as the supporting electrolyte and the electrode was rotated at 900 rpm to suppress the mass transport limitations. Potentiodynamic polarization experiments were conducted at a scan rate of 2 mV/s, starting at 250 mV below open circuit potential (OCP) and ending at 250 mV above OCP. In case of Zr electrode dissolving in 100 and 200 mM HF, the scan rate was maintained at 20 mV/s, since a lower scan rate resulted in excessive corrosion of the material during the experiments. The potentiodynamic polarization curves of the Ti and Zr respectively are presented in Fig. 1A and Fig. 1B. The cathodic and anodic branches of potentiodynamic polarization were extrapolated to estimate the corrosion current using Tafel kinetics.

HF is a weak acid and it forms multiple species in aqueous solutions. The concentration of these species was correlated and it is difficult to delineate the contribution of each species towards the corrosion current. Hence few experiments with addition of KF/H₂SO₄ were also carried out. The corresponding changes in the open circuit potential and corrosion current estimates were also analyzed. The concentration of various species, viz. remaining HF, H⁺, F^-, HF₂^- and H₂F₃^-, in the solution were calculated using the equilibrium constants [5]. The value of the polarization resistance (R_p) was calculated from the slope of linear polarization data. Electrical equivalent circuit fitting was used to calculate the polarization resistance from EIS spectra. The corrosion current was estimated using the values of R_p in all solutions. The values of the corrosion current estimated from the different techniques were compared. The correlation between the corrosion current and various solution species was also analyzed. The results indicate that the corrosion current is proportional to and where α and β are close to the exponents obtained from the mechanistic analysis of the polarization curves [3, 4]. The morphology of the electrodes after the passivation was studied using atomic force microscopy (AFM) and it shows less roughness compared to the active region of dissolution. The AFM images showed lesser passivation for Zr metal compared to Ti. These observations provide insight to the difference in the corrosion prevention of the two metals regardless of the same group in the periodic table.

 References:

1. R. Boyer, E. W. Collings, and G. Welsch, Materials Properties Handbook: Titanium Alloys, 1st ed., ASM International: Materials Park, OH (1994).
2. Chen Y, Urquidi-Macdonald M, Macdonald DD (2006) J Nucl Mater 348:133–147
3. M. S. Amrutha, R. Srinivasan, J. Solid State Electrochem., 10.1007/s10008-016-3342-0 (2016)
4. M. S. Amrutha, F. Fasmin, P. Ilayaraja, S. Chandran, and R. Srinivasan, ECS Trans., 72(17), 75 (2016).
5. B. Messnaoui, J. Solution Chem., 37, 715 (2008).