The effect of pH on the pitting corrosion of copper in an open cooling system containing an inhibitor such as nitrilotris methylene triphosphonic acid (ATMP), benzotriazole (BTA) or zinc sulfate (ZnSO4) in the cooling water has been investigated by immersion test and electrochemical measurement. It was found that pitting corrosion tended not to occur, when pH rose from 3 to 7. A close relationship was found between the electrochemical properties (Ecorr (corrosion potential)/Eb (breakdown potential)) and the occurrence of pitting corrosion in copper such that when Eb≥ECORR pitting did not occur.