Surface analysis of oxides and corrosion products formed on surfaces of iron-based alloys

Materials are, more or less, interacted with atmospheric gases and liquids, and thereby the materials are sometimes degraded. Typically, iron-based alloys are oxidized and corroded in air and water. Surface analysis methods are used for characterizing chemical products formed through solid/liquid and solid/gas reactions on the alloy surfaces. The characteristic features of the distribution of several elements in ironbased alloys annealed under varying partial pressure of oxygen were discussed based on the results obtained by the surface analytical methods in this chapter. Some alloying elements, typically chromium, enriched at the surface layer were shown to suppress the oxidation of iron-based alloys. Oxides of a less noble element, such as silicon oxides, are formed in the surface layer of ironbased alloys during annealing under a low partial pressure of oxygen. The formation of such oxides induces a change in the microstructure of the surface layer. A number of relevant phenomena in iron-based alloys were discussed along with the thermodynamic characteristics of their elements. In the analysis of corrosion products of iron, it is generally difficult to analyze directly hydrogen in electron spectroscopy. Nevertheless, oxygen bound with hydrogen reveals a characteristic XPS spectrum, which is due to a chemical change in the oxides. Hydrogen is also detected by the mass analysis of the corrosion products, although residual gas may interfere with mass spectra to some extent. In addition, the chemical reactions of corrosion products formed via gas and liquid are studied by analyzing the mass spectra of corrosion products, which are labeled with stable isotopes. Thus, surface analytical methods are effectively utilized in the understanding of the microscopic mechanism of oxidation and corrosion, whereas morphological and structural results of oxides formed by oxidation and corrosion are complementary to the elemental information. This is because the structures of polymorph compounds, like FeOOH, are distinguished using a structural method. By a combination of the surface analytical method with morphological and structural methods, all the characteristic features of microscopic processes of oxidation and corrosion as well as the roles of foreign elements in the processes become clear.