A Study of Cathodic Polarization and pH Changes in Metal Crevices

Cathodic polarization studies of crevices in Type 304 stainless steel and copper in 0.6M NaCl solution show that crevices with a distance to crevice opening ratio of 12,000:1 can be polarized and that the pH shifted in the alkaline direction to the domain on the potential-pH diagram (Pourbaix) where water is unstable and hydrogen discharge would be expected. The potential was shifted to values where experience has shown that cathodic protection has been achieved. These potential and pH shifts occurred with cathodic polarization even when the solution initially was acid (pH 1.9) simulating the condition in an actively corroding stainless steel crevice. One can, therefore, deduce from the data presented that cathodic protection can both prevent and stop crevice corrosion to great distances within deep, narrow crevices.