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The effects of molybdate and dichromate anions on pit propagation of mild steel in bicarbonate solution containing Cl- were investigated by electrochemical measurements. MoO42- ion suppressed both pit nucleation and propagation. Cr2O72- ion Suppressed pit nucleation, but stimulated pit growth. The different effects of the two anions on pit propagation were explained by the opposite effects on pH value within pits. The pH value in molybdate-containing solution increased as a result of polymerization of MoO42-, while in dichrornate4 containing solution, pH value decreased due to hydrolysis of Fe3+, and Cr3+ which are the products of oxido-reducing reactions between Cr2O72- and Fe2+ ions. (C) 2002 Published by Elsevier Science Ltd.