Steady state electrochemical kinetics of BS 4360 50D structural steel in deaerated 3·5%NaCl solution of varying pH

Abstract

Anodic and cathodic polarization measurements have been carried out on the structural steel BS 4360 50D in deaerated 3·5%NaCl solution in the pH ranges 2·0–10··0 and 2·0–13·0 respectively. The solution temperature was 5°C to correspond to North Sea temperature, but in addition some cathodic polarization measurements in alkaline solution were conducted at 20 and 40°C. Tests were carried out using a rotating disc electrode; a stepwise potentiostatic method was employed and the current was allowed to attain a ‘quasisteady’ value at each potential. The kinetics of water reduction showed a dependence on pH in the approximate range 10·0–12·5 but appeared to be nearly independent of pH outside this range. In acid solutions, reduction of hydrogen ions was the dominant cathodic process. The calculated transfer coefficient and the reaction order with respect to hydrogen ion concentration were about 0·46. The anodic polarization current density increased with increasing pH up to pH 5 but decreased as the pH was increased beyond 5.