THE INFLUENCE OF THE STATE OF THE SURFACE ON THE ELECTROCHEMISTRY OF IRON AND CARBON-STEEL ELECTRODES IN AQUEOUS CO2 SOLUTIONS

Exposures of pure iron and a low carbon steel were carried out at 70 degrees C in a deoxygenated 3% NaCl solution saturated with CO2 at a partial pressure of 0.65 bar giving pH 4.2. The effect of surface finish and surface changes during exposure on the electrochemical kinetics was studied. Anodic polarisation was found to cause changes of the surface that increased its effective area and led to super-polarisation. The corrosive attack was non-uniform with regions without any detectable attack even after long exposure - an observation in conflict with established views of an active dissolution of iron in the environment studied. Localised attack, FeCO3 deposits in cavities and concentration gradients in the environment of the cavities led to polarisation data being the result of multiple phenomena.