Effects of water additions on the passivity of iron in dimethoxyethane solutions

The effects of trace water additions on the passive behavior of iron in an otherwise anhydrous dimethoxyethane/0.5 M LiAsF6 solution have been studied by electrochemical and surface analytical techniques. The results indicate that the passivity observed in the anhydrous solvent is maintained when small (less than 2000 ppm) concentrations of water are present as a contaminant. Passive current densities increase slightly, but water does not cause any catastrophic interference with the various passivation mechanisms. Water, when present at these levels, actually helps to extend the passive range of iron by several hundred millivolts in this system. This behavior is the result of a complex interplay between iron dissolution and solvent electropolymerization at increasingly anodic potentials. Water is believed to be the component responsible for the localized acidification of areas of active dissolution, which then results in optimal conditions for polymer film formation. This behavior is peculiar to the system of iron in dimethoxyethane/LiAsF6.