Oxide scale inner layer

Since the paper by Pilling and Bedworth in 1923 much has been written about the mechanism and laws of growth of oxides on metals. These studies have greatly assisted the understanding of high-temperature oxidation, and the mathematical rate laws deduced in some cases make possible useful quantitative predictions. With alloy steels the oxide scales have a complex structure chromium steels owe much of their oxidation resistance to the presence of chromium oxide in the inner scale layer. Other elements can act in the same way, but it is their chromium content which in the main establishes the oxidation resistance of most heat-resisting steels. 

In 1929 Pfeil" published a most interesting account of the way layered structures form and the manner in which they influence oxidation rates. From detailed studies of the growth and composition of scales he was able to show clearly how the formation of barrier layers reduced scale formation by hindering outward diffusion of iron through the scale. Naturally, this work had to be largely based on the study of scales of sufficient thickness so that the mechanism of the early stages of oxidation could not be studied in this way. Pfeil analysed the outer, middle and inner layers of scales formed... 

Barrett and his colleagues , and Kosakhave summarised existing information on the scales formed on nickel-chromium alloys. Up to about 10% Cr, the thick black scale is composed of a double layer, the outer layer being nickel oxide and the inner porous layer a mixture of nickel oxide with small amounts of the spinel NiO CrjOj. Internal oxidation causes the formation of a subscale consisting of chromium oxide particles embedded in the nickel-rich matrix. At 10-20% Cr the scale is thinner and grey coloured and consists of chromium oxide and spinel with the possible presence of some nickel oxide. At about 25-30% Cr a predominantly chromium oxide scale is... 

Both rust and oxide scales are usually mixtures of iron oxides vith other Fe (e. g. siderite) and non-Fe compounds (CaCOs). In some cases there is a more or less random mixture of components, vhereas in others, the different oxides are arranged in layers to form duplex or triplex scales. Layer-type rust arises as a result of potential or chemical gradients across the film. As these gradients vary ivith film thickness, the composition of the rust changes with the distance from the metal. On the whole, if Fe " and Fe" are present, the oxide containing Fe" is found in the inner layer of the rust. 

Towards the inner oxide scale a porous layer containing TiOz and titanium nitrides (TiN,Ti2AlN) follows. At the metal/oxide interface A127039N is predominantly formed as results from electron diffraction indicate. Contrary to the observations withTi36Al... 

Extensive studies of the oxidation behaviour of Ni-Cr alloys have been published by a number of investigators. Alloys in this system with low Cr contents show internal oxidation of Cr forming Cr203 islands within a matrix of almost pure Ni. An outer scale of NiO is formed with an inner layer, sometimes porous, of NiO containing NiCr20a islands as shown in Figure 5.13. 

Iron oxide scale Oxidation of iron or low-alloy steels at temperatures >570 C (wustite stable) leads to a scale composed of an inner thick layer of wustite, Fei j,0, and two outer thinner layers of magnetite Fe3 04 and hematite Fe203. The disorder of Fei yO has been described in Sect. 6.2.2.1.3, its very high-iron vacancy concentration being the reason for fast outward cation diffusion and rapid growth. 

For the linear equation, the rate of oxidation is constant, or dy/dt = k and y = kt + const, where k is a constant. Hence, the thickness of scale, y, plotted with time, t, is linear (Fig. 11.3). This equation holds whenever the reaction rate is constant at an interface, as, for example, when the environment reaches the metal surface through cracks or pores in the reaction-product scale. Hence, for such metals, the ratio MdInmD is usually less than unity. In special cases, the linear equation may also hold even though the latter ratio is greater than unity, such as when the controlling reaction rate is constant at an inner or outer phase boundary of the reaction-product scale for example, tungsten first oxidizes at 700-1000 °C, in accord with the parabolic equation, forming an outer porous WO3 layer and an inner compact oxide scale [14]. When the rate of formation of the outer scale becomes equal to that of the inner scale, the linear equation is obeyed. 

On oxidation, aluminium forms the highly refractory, and hence protective, AljOj. However, addition of aluminium to steels can cause embrittlement problems. There is a need, therefore, to realise the protective benefit of aluminium at as low a concentration as possible within the steel. At aluminium concentrations below 2.4 wt%, bulky stratified scales, comprising FcjOj and Fe304, with an inner layer of AljOj or FeAl204 are formed at 800°C . At 2.5% aluminium, large areas of AIjO, were always observed with iron oxide nodules. Formation of these iron oxide nodules is only suppressed once the aluminium content exceeds approximately 7 wt% . 

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