LIQUID-FILM THICKNESS AND WEAK ACIDS IN THE CO2 PITTING OF STEEL

Flow-loop runs were conducted at 0.3 and 1.0 mm liquid film thickness in acetic acid solutions at different concentration. Maximum pit depth was found to be independent of liquid film thickness. Dilute, 3.5% NaCl solutions of acetic, formic, and chloroacetic acid at different pH values and at the same pH of 2 (adjusted with HCl) were used to study the effect of weak acids on the CO2 pitting of steel. Pitting was found to decrease with increase in weak acid concentration and to be independent of pH. The results indicate that pitting is less likely when the pH at which the corrosion products start precipitating corresponds to a region of high buffer capacity of the solution adjacent to the metal surface.