Abstract
Cast iron and carbon steel become passive in caustic hydraulic luburicants of aqueous glycol solution with help of slightly dissolved oxygen from the air. They have performed well over 10 years in practical uses, but have some service experiences of crevice corrosion in the environments, as observed for stainless steels in neutral solutions. In this report causes of crevice corrosion are examined in terms of pH and critical potentials. pH values of A-B measured for solutions in practical cases of crevice corrosion were higher than depassivation pH (pHd), which shows that the pH values are not responsible to the corrosion. Spontaneous electrode potential (ESP) for the steels passivated in the solution was determined to be about -0.15V vs. SCE. Critical potentials, VCREV, above which crevice corrosion initiates and continues to grow over 20μm in depth were measured as a function of Cl- content of the solution and found to coincide to repassivation potential, ER*, CREV for crevice corrosion shallower than 20μm. Cl- more than 10ppm makes VCREV or ER*, CREV less noble than ESP and might lead to the crevice corrosion in the cases studied. To control Cl- content blow 10ppm is an effective way to avoid the crevice corrosion.
