Selective Alloy Oxidation and Iron Oxyhydroxide Reactivity: A Surface Chemistry Approach to Corrosion Control and Catalysis

Abstract

Corrosion control is the science of preventing selected chemical reactions while catalysis is the science of promoting selected chemical reactions. These processes may appear unrelated, however both are controlled by the surface chemistry of the materials. To better understand the surface chemistry governing these processes three different investigations were conducted on iron oxyhydroxide phases and a 20Fe-40Ni-10Mn-30Cr alloy.

The effects of aluminum on the formation, stability and transformation of Fe-oxides/oxyhydroxides from amorphous precipitates and the mechanism through which it affects these changes was investigated using infrared spectroscopy and x-ray absorption spectroscopy (XAS). It was observed that precipitation of amorphous Fe-oxyhydroxides in the presence of aluminum significantly retards the transformation to crystalline phases. XAS analysis shows amorphous Fe-oxyhydroxides primarily composed of Fe-Fe edge-sharing octahedra and suggests that aluminum stabilizes the amorphous phase and hinders transformation to goethite by preventing corner-sharing linkages between Fe-polyhedra.

The surface chemistry of particulate Fe-oxyhydroxides and their reactivity with CO and H2O was also explored. X-ray photoelectron spectroscopy (XPS) and IR-spectroscopy were used to characterize the surface chemistry of the particles under conditions ranging from high vacuum to near ambient pressures. A size dependence of O1s binding energy of the Fe-OH is observed. Additionally, exposure to H2O and CO reveals a size dependant adsorption of CO and a transformation to a formate species on the particle surfaces. The selectivity of formate formation vs. CO adsorption also correlates to O1s binding energy.

Lastly, the oxidation kinetics and mechanism of a 20Fe-40Ni-10Mn-30Cr alloy was investigated in a low pO2 environment for corrosion control applications. This knowledge would provide a better understanding of its corrosion resistant behavior and guide future alloy development. Analysis of the oxidized alloy was conducted using electron microscopy, XRD, XPS and thermogravimetric analysis. Oxidation resulted in the predominant formation of an "MnCr2O4" surface layer with MnO protrusions at on the MnCr2O4 surface. In selected regions, a two layer structure consisting of an outer layer of MnO and inner layer of MnCr2O4 was observed. The rate constant for MnCr2O4 growth was on the order of 10-14 g2cm-4s-1 at 1000K, and was the dominant process.