Surface oxidation of steels at low oxygen pressure and elevated temperature: Impact on gas nitriding

Abstract

Low alloy steels are covered with a thin oxide layer containing Fe₃O₄ adjacent to the metal and Fe₂O₃ at the interface with the atmosphere. It can be shown by means of modern surface analysis techniques that such layers are usually less than 10 nm thick. The formation of Fe₃O₄ is held to be a precondition for the formation of Fe₂O₃, the latter being responsible for corrosion protection and the slight passivity of iron and low alloy steels. Passivity can be significantly increased by increased chromium concentration, which in the case of stainless steels means by chromium contents exceeding 13 wt-%. Stainless steels form an oxide layer at the surface enriched in chromium oxides (Cr₂O₃). When this layer is formed spontaneously near room temperature, it too is thinner than 10 nm. At elevated temperatures, for example during heating in a nitriding furnace to 500 or 600°C, oxide layers tend to grow fast when oxygen is present, even at low partial pressure. The result is oxide layers which are at least an order of magnitude thicker than oxide layers formed at room temperature, their thickness depending not only on oxygen partial pressure but also on kinetic parameters such as heating rate and time. Experimental investigations have been carried out to study the formation of oxide layers at a range of temperatures and oxygen partial pressures, and to see how this impacts on subsequent nitrogen uptake in an ammonia atmosphere. The objective of the investigation was to explain contradictory and irreproducible results in practical gas nitriding and, more generally, in heterogeneous surface treatment processes in gas atmospheres at elevated temperatures.