Effect of pH on CO₂ Corrosion of Mild Steel at Elevated Temperatures Available to Purchase

Electrochemical and surface analysis techniques were used to study the mechanisms of CO₂ corrosion of mild steel over the temperature range of 80-200°C. Experiments were carried out in a 4-liter autoclave equipped to perform in situ electrochemical measurements using linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS). The electrolyte was 1 wt.% NaCl saturated with CO₂ at 0.030 M of [CO₂]_aq. Corrosion tests were conducted at temperatures of 80, 120, 150 and 200°C; pH values of 4.0 and 6.0 were used. After the experiments, sample surfaces were characterized via scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results showed that the corrosion rates decreased with time due to formation of corrosion products. Corrosion rates at pH 4.0 are higher than those at pH 6.0 for each temperature. For the test series at both pH 4.0 and pH 6.0, the final corrosion rates decreased with temperature because increasingly protective layers formed on the steel surface. Analysis of the corrosion products confirmed that FeCO₃ layers were observed for all the tests and that there was a mixture of Fe₃O₄ and FeCO₃ at temperatures above 150°C.