General corrosion or uniform attack occurs when the anodic areas on the metal surface keep shifting to different sites. This continual shifting results in relatively uniform metal removal. This type of corrosion often can be predicted so that material loss can be taken into account.
Galvanic corrosion occurs when dissimilar materials are in contact in a conducting fluid ( water ) . The intensity of attack is related to the relative surface areas of the metals in electrical contact. Thus large cathodic areas coupled to small anodic areas will aggravate galvanic corrosion and cause severe dissolution of the more active metal.
Pitting corrosion is one of the most insidious form of attack. The pit develops at a localized anodic site on the surface and continue to grow because of a large cathodic area surrounding the anode. High concentration of metal chlorides often develop within the pit and hydrolyze to produce an acidic pH environment. This solution remains stagnant, having a high salt content and low oxygen concentration. The reaction within the pit become self- sustaining (autocatalytic) with very little tendency for them to be suppressed, ultimately causing penetration through the base metal.
Chloride and Sulphate ions are known to have a deleterious effect on steel. The antagonistic nature of chloride ions is due to their ability to adsorb on the metal surface and interfere with the formation where chloride ions have adsorbed are anodic to the large cathodic passive oxide surface. High current densities are generated at the chloride site. Once corrosion starts hydrolysis of the metal ions form the anodic reaction causes a decrease in pH which acclerates attack.