**1. Introduction**

Metallic oxides are inevitable to generate on the surface of steel products during metal forming processes at high temperature. If its thickness is larger than nanometre size, the oxide layer can be termed as *oxide scale* in industrial manufacturing. The formation of oxide scale or the oxidation of industrial materials depends, not only on the properties of their component crystals, but also on those of the boundaries between those crystals, in particular the structure and chemical composition of the boundaries.

Grain boundaries have distinct properties relative to bulk material in terms of atomic coordination, reactivity and diffusion rates. Phase contact creates interfaces that represent changes

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

in composition with disrupted atomic bonding. Grain boundary refers to the interface zone between grains of the same phase, while interfaces are boundaries between dissimilar phases. It represents the narrow zone where atomic bonding is disrupted by misalignment of the crystalline grains. This disrupted bonding at the grain boundary is about 5–10 atoms across. Grain boundaries have nanoscale spatial dimensions, which can generate substantial resistance to ionic transport due to dopant (or impurity) segregation. This diffusion provides active paths for atomic motion, particularly at high temperature, during the diffusion-controlled oxidation or corrosion. Thus, composite properties are sensitive to the interface structure and chemistry to large potential variations.

This crystallographic structure of a metal alloy is one of the important parameters in determining the oxidation or corrosion behaviour. The characters of grain boundaries in oxide layers formed on substrates influence adhesion and friction behaviour, surface fracture and wear during high temperature steel processing. However, the effect of grain characters on the oxidation behaviour is not fully understood yet. There are still many challenges, one of which is how to engineer grain boundaries to optimise the oxidation resistance of these materials. For this reason, detailed understanding of the processing-structure-property relationships that focus on grain boundaries and interfaces is critical to advanced manufacturing of metals. Furthermore, it is necessary to modify the grain boundary characteristics of this alloy which affect its oxidation resistance.

In this chapter, an attempt has been made to explore the role and behaviour of grain boundaries in the oxide scale formed on the steel surface during metal processing. In doing so, two things we need to consider for such high-temperature plastic deformation are diffusion mechanism at grain boundaries and resulting boundary migration in the growth of grains.
