Surface Properties of Steel

Stainless steels have versatile mechanical properties. The corrosion resistivity makes steel unique among the engineering materials. As such the corrosion rate of ferritic stainless steels decreases drastically within a narrow concentration interval making the transition from iron-type to non-corrosive behaviour quite abrupt. The type of the oxide layer formed on the surface depends on the oxygen pressure, temperature and the alloy compositions in the vicinity of the surface. Later on, the oxidation process assumes transport of metal and oxygen ions through the initially formed oxide scale. The ion transport is controlled by diffusion, which in turn is determined by the defect structure of the oxide layer. The high mobility of Fe in Fe oxides, especially in FeO, which is the dominant oxide component on pure iron above 570°C, explains the corrosive nature of Fe. The passivity in Fe-Cr, on the other hand, is attributed to a stable Cr-rich oxide scale. Above the critical concentration, a pure chromium layer is formed on the surface which effectively blocks the ion diffusion across the oxide scale. The lack of Cr at the surface of Ferric alloys is a direct consequence of the anomalous mixing of Fe and Cr at low Cr concentrations, which in its turn has a magnetic origin. This finding has important implication in modern materials science as it offers additional rich perspectives in the optimization of high-performance steel grades. Chromium oxide gives good corrosion protection at usual operating temperatures but since Cr forms volatile compounds at high temperature the corrosion protection at elevated temperatures requires the more stable Al oxide scales on the alloy surface. The Cr2O3 scale is protective up to 1000-1100 °C whereas Al2O3 scales up to 1400 °C. Unfortunately, for most of the Fe alloy applications the straightforward procedure to improve high-temperature corrosion resistance by increasing the Al content in bulk, is not an acceptable solution. This is because the high Al content makes Fe-Al alloys brittle which poses a natural upper bound for the Al content in these alloys regarding most of the applications. Fortunately, the additional alloying of Fe-Al with Cr boosts the formation of the Al oxide scale on the surface up to such a level that the Al content in bulk can be kept within the acceptable limits regarding the required mechanical properties of the alloy. This phenomenon, called the third element effect, is still considered a phenomenon without a generally accepted explanation.