Study of inhibition mechanism and efficiency of indole-5-carboxylic acid on corrosion of copper in aerated 0·5M H2 SO4

Abstract Inhibition of the corrosion of copper in aerated 0·5M sulphuric acid solutions containing various concentrations of indole-5-carboxylic acid was studied in the temperature range 25–55°C using potentiodynamic curves (Tafel lines), weight loss, analytical methods, and determination of double layer capacitance. The corrosion rates reveal good corrosion inhibition, up to 95% in the concentration range of 1 × 10-4–4 × 10-3M. Tafel anodic slopes in inhibited acid solutions are considerably higher than those in uninhibited acid solutions. This points to a change in the corrosion mechanism of copper in the presence of indole-5-carboxylic acid. Under these conditions, copper could electro-oxidise primarily to Cu+ rather than to Cu2+, forming slightly soluble [Cu–(indole-5-carboxylic)n ]+(ads) complexes. Corrosion rates determined by the weight loss method in both the absence and presence of inhibitor are much higher (on average by a factor of about 6) than those obtained with the potentiodynamic method. This points to a limitation of the Tafel line extrapolation method in corrosion rate determination. The double layer capacitance–potential curves indicate considerable adsorption of the inhibitor over a wide potential range (-600 to +200 mV with regard to Ecorr ).