Corrosion cells resulting from differential pH values have been investigated in the absence and presence of Cl − ions. Measurements of potential, galvanic current for separate and coupled electrodes, as well as experimental determination of Evans diagrams, were carried out. The results of coupling rvealed that the steel at lower pH (7 and 8) suffered from corrosion while that at the. higher pH (12 and 12·5) was completely protected even in presence of high chloride concentrations The increase in the pH range caused a relative enhancement of the anodic process. The presence of 10 −3 or 10 −1M Cl − ions increased the rate of the anodic reaction by ∼ 8- or 12-fold. The rate of the cathodic reaction was of the same order of magnitude both in presence or absence of Cl − ions. The % anodic and cathodic control were found to be 6·4 and 93.6, respectively, in the presence of Cl − ions as compared with 34·3 and 65·7 in the absence of Cl − ions.