Oxide scale outer layer

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... 

Internal oxidation was observed only at 900°C in samples with less than 10 at.% aluminium, whereas at 1000°C and 1100°C even 5 at.% aluminium was still enough to form a complete alumina outer layer. The internal oxides generate stresses, which cause formation of outer iron protrusions (Fig. 8). The pore formation beneath the oxide scales which is caused by Al-consumption and inward diffusion of Fe, is more marked for the ordered phases near 50 at.% and less for the disordered alloys with low A1 content. These, however, at 900 °C show a tendency to internal oxidation. Al203 is growing inward while Fe protrusions are pressed outward in the process [6,7],... 

In Fig. 3 a TEM micrograph and a schematic illustration of the complete oxide scale and of the subsurface layer of Ti36AI after oxidation at 900 °C in air for 0.5 h is presented. The oxide scale consists of an oxide mixture of A1203 and TiOz in the outer part of the scale. Beneath this oxide mixture a porous partial layer rich in TiOz is found. At... 

Initially, from to the duplex scale grows at a constant rate until time h, when the activity of the metal at the scale surface has fallen to the value given by identity in Equation (7.33b). After this point, only oxide is stable and can form on the scale surface. After time h, the duplex scale is covered with an outer layer of oxide only, which slows down the reaction rate. Consequently, the metal activity at the duplex-layer-oxide-layer interface rises, reflecting the lower rate of transfer of iron ions... 

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. 

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