PASSIVITY OF IRON AND ITS BREAKDOWN IN ALKALINE SULFIDE SOLUTIONS

The passivity of iron in borate buffer solutions and its breakdown in presence of sulfide ions was studied using standard electrochemical techniques (e.g., galvanostatic, potentiodynamic, potentiostatic) and complemented with microscopic examination and electron microprobe analysis of the surface. Mass transport effects and the nature of dissolved species were investigated using rotating disc and ring-disc electrodes. The electronic properties of the oxide film, and their relationship with the breakdown of passivity, were studied using photoelectrochemical techniques. Initial passivation of iron in sulfide-containing solutions is due to coverage of the surface by an iron(II) oxide phase, which competes with the formation of a non-protective iron sulfide (mackinawite) film. The passive layer is gradually converted into non-stoichiometric (gamma)-Fe(,2)O(,3). This oxide is an n-type semiconductor. At higher positive potentials, mackinawite is oxidized to Fe(,2)O(,3) and S. Sulfur can also be produced by direct oxidation of HS('-), and FeS(,2) (pyrite) can precipitate from the solution. The resistance of the passive film to breakdown is affected not only by its thickness, but also by its composition (expressed as deviation from stoichiometry). Both are controlled by the potential at which the electrode is passivated and the time of passivation prior to sulfide injection. The mechanism of localized corrosion on passive iron is associated to the formation of mackinawite patches on the surface. The extent of corrosion is determined by the kinetics of nucleation and growth of mackinawite. The nucleation of mackinawite patches, is a fast process related to the adsorption of HS('-) on weak spots of the passive film. The growth of the nuclei occurs by dissolution of the adjacent oxide, and its rate is controlled by the diffusion of Fe(II) species away from the surface. Pitting corrosion occurs underneath the mackinawite deposits by a mechanism similar to that of crevice corrosion