**2. High-temperature oxidation**

This section is devoted to the fundamental issues of oxidation mechanism that should be defined and summarised before specific problems are confronted. During metal processing at elevated temperatures, oxidation occurs inevitably on the surface of products. In the case of the pure iron, the oxide scale formed on is a complex mixture of three iron oxide phases: hematite (Fe<sup>2</sup> O3 ), magnetite (Fe<sup>3</sup> O4 ) and wustite (Fe1-xO, *x* = 0.84–0.95) [1]. This is because iron has divalent and trivalent ions (Fe2+ and Fe3+). The complete oxidation of iron can be divided into three main steps, where iron oxidises to the lowest valence ion Fe2+ and forms the first sub-layer of wustite (Fe1-xO) next to the metal. Then, some of Fe2+ ions oxidise further to Fe3+ and contain both valence iron ions as the intermediate sub-layer of magnetite (Fe<sup>3</sup> O4 ). Under conditions of sufficient oxygen, the outer sub-layer of hematite (Fe<sup>2</sup> O3 ) only consists of the highest valence iron ion Fe3+. This is the case above 570°C (the eutectoid point of the Fe-O system) in the diffusion-controlled growth of multilayered scales on pure iron. Below 570°C, the wustite phase is unstable, and the oxidation of iron directly results in magnetite. In steel, various Fe-C alloys, their oxidation at high temperature can be more complex than pure iron, in particular the segregation of different element at grain boundaries.
