Effects of Temperature and Sulfate on the Pitting Corrosion of Titanium in High-Temperature Chloride Solutions

The effects of temperature (100-230 degrees C) and sulfate concentration (0-0.5 mol L-1) on the pitting corrosion of titanium were studied in de-aerated 1 mol L-1 NaCl solution using cyclic potentiodynamic polarization (CPP) and linear-sweep thermammetry measurements. With increasing temperature, the breakdown potential E-b of titanium decreased, while the repassivation potential E-rp of titanium remained constant in 1 mol L-1 NaCl solution. The presence of sulfate ions shifted both the E-b and E-rp to higher values, implying that the pitting of titanium was inhibited. In 1 mol L-1 NaCl solution with 0.5 mol L-1 sulfate addition, titanium did not undergo passivity breakdown at 200 degrees C, even up to 9 V vs. SHE. X-ray photoelectron spectroscopy results showed that the inhibition effect of sulfate on the pitting could be attributed to the competition for adsorption sites on the oxide surface. A metastable pitting temperature threshold (MPTT) was defined for titanium as a function of sulfate to chloride mole ratio using linear-sweep thermammetry measurements. The MPTT results were consistent with those obtained in CPP measurements. The results confirmed that the MPTT of titanium was elevated by the addition of sulfate ions. (C) 2015 The Electrochemical Society. All rights reserved.