| The passivation of iron in both anhydrous ({dollar}<{dollar}1 ppm water) propylene carbonate (PC) and binary mixtures of PC and water has been studied. The objective of this work was to determine the mechanism(s) by which iron passivates in the absence of water, and to delineate the role of water in the passivation process. Iron was chosen because of the widespread use of ferrous materials as construction materials in lithium batteries, many of which use PC as the basis for their electrolytes.; Electrochemical tests and surface analysis were used to study the passivation process. In the anhydrous solutions, the air-formed film was found to be stable and protective over a wide range of applied potential. Scratched electrode tests showed that bare iron can also passivate in anhydrous solutions. This passivation occurs via the oxidative chemisorption of PC molecules at potentials below the oxidation potential of PC, and via a precipitated Fe(ClO{dollar}sb4)sb2{dollar} salt film at higher potentials. Small water additions ({dollar}<{dollar}500 ppm) were found to be deleterious to passivity, leading to a uniform activation of the surface. Higher levels of water prevented passivation altogether. In fact, between 30 mole % PC and 99.9 mole % PC, stable passivity does not occur for iron. Instead, an unstable passivity results which leads to massive localized attack of the surface. However, a passive film formed in aqueous solution was found to be stable when exposed to the mixed aqueous/non-aqueous solution, though a removal of the film formed in aqueous solution by cathodic reduction created bare surface site which, when polarized back to the passive film formation potential, were sites for localized attack of the metal.; These observations are explained in terms of the competition between the different passivation mechanisms for organic and aqueous solutions. This competition between the incompatible mechanisms leads to localized metal loss because in the mixed solutions, neither mechanism can dominate. |
