Dissolution Behavior of Fe-Fe3 Structures as a Function of pH, Potential, and Anion – An Electron Microscopic Study

Abstract

Abstract A phenomenological study of the dissolution behavior of an Fe-Fe3C structure was conducted using electron transmission microscopy as the primary investigative tool. Four distinct modes of attack were observed, i.e., carbide attack, matrix attack, general attack, and interface attack. The morphology of dissolution was found to depend upon pH, potential, and anion present. The results are rationalized in terms of: (1) thermodynamic calculations of the stability of carbides in aqueous environments, (2) known effects of pH, potential, and anion on dissolution kinetics, and (3) the influence of complexes formed with oxidized or reduced species. There were a number of significant results observed: (1) Iron carbide will dissolve at potentials in the range of –1000 mVH where Fe is thermodynamically stable. (2) The dissolution behavior of pearlite at pH 4 can shift modes between preferential attack at the ferrite, the carbide, and the interface as a result of changing only the anion. (3) Despite the general thermodynamic similarities for the stability of carbon and of iron carbide, their kinetic processes are greatly different. (4) The general patterns of dissolution behavior do not suggest any particular synergistic effects between the ferrite, carbide, and interface.