**2.2. Oxidation of steel alloys**

Due to the presence of alloying elements and impurities in Fe-C steel alloys, the oxidation rates, phase development and morphologies of oxide scale are dramatically different from pure iron under various processing parameters. The reader can be referred to some published books [1], with regard to these four factors, alloying elements, oxidation kinetics, resulting oxides and their microstructure. More attention will be given here to the relationships between theories associated with grain boundaries of oxide scale.

The oxide scale formed on low carbon steels generally consists of a typical three-layered microstructure with a thin outer layer of hematite (Fe<sup>2</sup> O3 ), an intermediate layer of magnetite (Fe<sup>3</sup> O4 ) and an inner layer of wustite (Fe1−xO, 1− *x* = 0.84–0.95) adjacent to the steel substrate [4]. This three-layered microstructure remains until a eutectoid point of the Fe-O phase equilibrium diagram is reached if oxygen is available at a temperature of 570°C. When the temperature drops below 570°C, the wustite phase becomes unstable and will decompose into eutectoid products of magnetite and ferrite. During the cooling from high temperature, the morphology and composition within oxide scale will be changed significantly. This variation mainly depends on heat treatments, atmosphere of the gas and chemistry of steels.

Analogous to the alloying strength in steel substrate, the alloying additions modified the oxidation quite dramatically. Normally, the amount of silicon and chromium aims to form a protective oxide scale, whereas small additions of nickel, copper, niobium, molybdenum and vanadium led to a greatly increased adherence of oxide scale. For instance, silicon generally enriches and forms an anchor-like morphology at the oxide scale/steel interface or grain boundaries of the oxides. Manganese is normally used as a solvent, while the transport of carbon is via defects such as pores rather than lattice or grain boundary diffusion. Because manganese has a stronger affinity for oxygen than iron does, manganese is normally spread sparingly over the entire oxide layer in the cross-sectional direction [3]. In addition to alloying elements, the atmosphere of gas as it interacted with oxides, especially water vapour, has made our understanding of the overall situation elusive [5], while the different behaviour of steel alloys in air-moisture mixtures has further complicated the set of observations.

The thickness or weight change in oxide scale with time is generally used to assess oxidation rates of the metallic alloys. In a linear or parabolic growth rate, the mixed para-linear kinetics is widely accepted to deal with the short-time growth of oxide scale during high-temperature metal processing. With the oxidation of steels below 727°C (the eutectoid point of the Fe-C system [4]), the oxidation kinetics is similar to pure iron due to without decarburisation at various atmospheres and follows approximately parabolic kinetics. In a case of oxidation in pure iron at 700–1200°C, the thickness ratio between 100:5:1 and 100:10:1 (FeO:Fe<sup>3</sup> O4 :Fe<sup>2</sup> O3 ) can be obtained, whereas at 400 and 550°C without wustite, it can be 10:1 to 20:1 (Fe<sup>3</sup> O4 :Fe<sup>2</sup> O3 ) [6]. The phase development during the high-temperature oxidation of steel alloys concentrated mostly on the evolution of wustite during isothermal holding because wustite will decompose into magnetite and ferrite below 570°C.

The oxide scale consists of most magnetite and hematite at room temperature. Three research directions can be: wustite formation above 700°C, magnetite and hematite below 570°C and wustite decomposition between two temperature ranges. This chapter here only focuses the morphologies of oxide scale at room temperature cooling from high-temperature processing. It is noted that this scheme, using microstructures at room temperature to deduce what happened at high temperature, can thus far be subject to the current characterisation techniques. If one *in situ* technique is available to observe oxidation behaviour at high temperatures around 1000°C, especially during a long term, some current results could be improved significantly.
