INVESTIGATION OF CHEMICAL AND ELECTROCHEMICAL CHANGES WITHIN CORROSION CRACKS——Ⅰ.Interrelations between Currents,C1~- migration and pH values

A simulated occluded corrosion cell was used to study the changes within corrosion cracks or crev-ices of 1Crl8Ni9Ti austenitic stainless steel in NaCI solution.As anodic currents passed through thecell,a series of changes took place,the most important of which were pH fall and CI-migration.Itwas found that the pH fall is proportional to current density and duration of current flowing,consequentlyit is a function of the quantity of electricity passing the occluded anode.The pH in the occluded cellmight be negative value when current density was 3.5mA/cm~2 after 168 hours.Meanwhile the CI-migratedinto occluded cell increased the CI-concentration 18 times as much as the original concentration.A linear relationship existed between CI-migration in g-equivalents per cm~2 of anode area and currentdensity,and also a linear relation existed between CI-migration per hour and current intensity.Conse-quently,the number of equivalents of CI-migration into crack tip was proportional to the quantity ofelectricity passing through it.It was interesting to discover that for each faraday passed almost always1/2 g-equivalent of CI-migrated into the occluded cell.This relationship did not change whatever werethe concentration of bulk solution,current density,temperature,and time elapsed.Two empirical for-mulas regarding the relationship between pH and Cr~3 dissolution or Cl-migration respectively werederived based on the relationships between quantity of electricity passed and other variables found in thepresent experiments.