The effects of molybdate and dichromate anions on pit propagation of mild steel in bicarbonate solution containing Cl~-

摘要

The effects of molybdate and dichhromate anions on pit propagation of mild steel in bicaronate solution containing Cl~- were investigated by electrochemical measurements. MoO~2- _4 ion suppressed both pit nucleation and propagation. Cr_2O~2- _7 ion suppressed pit nu- Cleation, 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 MoO~2- _4, while in dichromae- Containing solution, pH value decreased due to hydrolysis of Fe~3+ and Cr~3+ which are the Products of oxido-reducing reactions between Cr_2O~2- _7 and Fe~2+ ions.