Role of Hydrogen in the Pitting of Passivating Films on Pure Iron

Abstract

The pitting initiation behavior of hydrogen-charged passive films on pure iron in 0.15 Na2B4O-0.15 N H3BO4 solution of pH = 8.4 at 30-degrees-C has been investigated by using gas-phase hydrogen charging and an electrochemical detection technique. The pitting induction time has been measured as a function of the input hydrogen-charging pressure and of the time elapsed prior to the addition of chloride ions after the cessation of hydrogen charging. The pitting induction time decreased with increasing input hydrogen-charging pressure and increased with increasing lapse of time at which chloride ions are added following the stopping of hydrogen charging. Measurements of the change of capacitance of the passive film with aging time, and of the magnitudes of electric charge involved in the current transients occurring with step changes of potential, show that the hydroxyl and water contents of the film decrease with increasing potential and with aging time at constant potential. The data lead to the inference that hydrogen reduces resistance to pit initiation by chloride ion by increasing the OH- and H2O contents of the film. both of which can easily exchange with Cl-.