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Corrosion of titanium and titanium alloys in aqueous acid electrolyte Reid, Peter W. A.

Abstract

The corrosion characteristics of three dilute Ti alloys (ASTM 1, 2, and 7) in aqueous sulphuric acid electrolytes were studied using open circuit potential measurements, potentiodynamic techniques, and long term immersion testing. The effects of acid concentration, alloying addition, temperature, and the iron and chloride content of the electrolyte were examined. The open circuit potential measurements resulted in Ti grade 7 coming to a higher open circuit potential with the electrolyte at approximately 0.07 V[sub SHE], while the rest potentials for Ti grades 1 and 2 were approximately -0.50 V[sub SHE]- The difference is believed to be the result of Pd alloying additions, which act as a catalyst for reduction reactions on the surface of the active Ti alloys during crevice corrosion. By decreasing the required over voltage for the reduction of species such as H⁺ to occur, a more oxidizing overall mixed potential of the system is achieved. Potentiodynamic scanning around the corrosion potential of titanium alloys in sulphuric acid (H₂SO₄) yielded maximum corrosion rates of 4.9 x 10 ⁻⁵ mm/yr (0.002 mpy). In all cases corrosion rates increased with lower pH and higher Cl⁻ content. Potentiodynamic testing of the [Fe³⁺]/[Fe²⁺] redox couple shows limiting currents to be controlled by concentration polarization, and the kinetics of the overall reactions were faster on the surface of the more noble Pt electrode. Slopes were on the order of 150-300 mV/decade, with exchange current densities of 10 ⁻⁷ and 10 ⁻⁷ for Ti and Pt respectively. All corrosion potentials were in the range of 0.5 to 0.7 V[sub SHE]. During the long-term immersion testing, Ti grade 7 proved to be more corrosion resistant than the commercially pure alloys due to the formation of a visibly thicker oxide layer. Preferential attack, when visible, was initiated on metallic iron and iron oxide impurities in the alloys and was observed on all immersion coupons.

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