Mechanism of Passivity Breakdown in Seawater

摘要

Natural marine biofilms shift the open circuit potential of passive alloys and enhance their cathodic properties. It has been shown that this affects both the initiation and propagation of localize corrosion of these alloys. The increase in cathodic properties also produces an increase of 5 to 8 times in the galvanic corrosion rate of anodic materials connected to passive alloys with biofilms. Critical pitting and breakdown potentials are also affected by biofilm formation but the mechanism for this effect is uncertain. Solid state microelectrode techniques using square wave voltammetry have been developed for measuring chemical profiles through the biofilm thickness. Laser confocal microscopy has been used to image the community of microbes at the sites of chemical measurement. The mechanism for these effects is believed to involve continual redox cycling of manganese dioxide within the biofilm matrix. It has been shown that a combination of chemical and biological reoxidation of reduced manganese species is able to account for the observed corrosion rates.