Effects of temperature and sulfate on the pitting corrosion of titanium in high-temperature chloride solutions

The effects of temperature (100-230°C) and sulfate concentration (0-0.5 mol L^-1) on the pitting corrosion of titanium were studied in de-aerated 1mol L^-1 NaCl solution using cyclic potentiodynamic polarization (CPP) and linear-sweep thermammetrymeasurements. 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 1mol L^-1 NaCl solution with 0.5mol L^-1 sulfate addition, titanium did not undergo passivity breakdown at 200°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.