Inhibitor concentration on the corrosion behaviour of X70 steel in CO2 environments

Purpose - The purpose of this study is to investigate the corrosion behaviour of X70 steel in the presence and absence of various concentrations of inhibitor N-(2-o-Tolyl azophenyl)-acetamide (NTAA) in a CO2 environment. Design/methodology/approach - The temperature was set at 80 degrees C, and the flow velocity was 1.5 m/s. The inhibitor concentrations were 10, 20, 30, 60 and 80 ppm, and the CO2 partial pressure was 0.8 MPa. Weight loss method, pitting depth measurement, scanning electron microscopy and electrochemical techniques were used to investigate the inhibitory effects of the inhibitor NTAA. Findings - The results showed that a small peak emerged in the curve of the corrosion rate versus inhibitor concentration plot at 20-30 ppm. Polarisation studies revealed that the anodic Tafel slopes changed greatly in the presence of an inhibitor; NTAA behaved as an anode-type inhibitor. At concentrations of 20-30 ppm, the incomplete coverage of the metal surface by inhibitor molecules resulted in macroscopic galvanic corrosion. Originality/value - Corrosion behaviour of X70 steel in the presence and absence of various concentrations of an anode-type inhibitor was assessed. Cathodic Tafel slopes are almost unchanged, while the anodic Tafel slopes change significantly with the increase in inhibitor concentration. The corrosion rates of 20 and 30 ppm are almost three times of that of 10 ppm, which is because of the macroscopic galvanic corrosion caused by the inadequate coverage of inhibitor on steel surface.