Corrosion of Steel in Dilute Solutions Of Sodium Salts of Weak Acids

It was found that the exposure of steel specimens to dilute solutions of common anodic inhibitors such as sodium carbonate, sodium silicate, disodium phosphate and trisodium phosphate, free from and containing a small concentration of potassium chloride, could result in either general attack, localized attack or pitting—the latter being obtained only in carbonate/chloride solutions. It was suggested that the nature of the corrosion product largely controlled the form and rate of corrosion. A corrosion product of high permeability and low resistance, such as lepidocrocite, permitted general attack, under cathodic control, at practically a constant rate, whereas more impermeable corrosion products of higher resistance, such as goethite and vivianite, greatly reduced the rate of corrosion which was mainly under anodic control. Experiments carried out in a divided cell, designed to simulate conditions present during localized attack and pitting, indicated that the effect of the small concentration of chloride was to greatly reduce the internal resistance of the corrosion cells. This effect was considered to be due to the greater ease of migration and diffusion of chloride ions through the corrosion products which, in effect, resulted in a decrease in their resistance.