Spalling oxide layer

Breakdown of the coherent oxide layer by cracking and spalling, with partial or full loss of its protective character. 

Y and RE Improves adherence and spalling resistance of oxide layer and improves oxidation, sulfidation, carburization resistance ... 

Fig. 2. Schematic diagrams showing the cracking and spalling of an oxide layer, which often occurs on cooling from the oxidation temperature and comparing the isothermal and cyclic oxidation kinetics.

Thermal barrier systems have a tendency to form an oxide layer between the TC and BC, which leads to significant stress increases in the system, resulting in mechanical failure, like spalling or buckling of coatings. 

Plastic deformation of the oxide layer or of the substrate can relieve the growth-related stress and thus prevent cracking or spalling to occur. It occurs more readily if the oxide film is thin. Thick oxide layers therefore deteriorate more easily by cracking or spalling. 

Figure 9.19 schematically shows the spalling of an oxide layer that forms during the oxidation of a metal wire. Gradually, as the reaction progresses, the diameter of the wire decreases, while the thickness of the oxide layer increases (a). As a consequence, stress relief between the layer and the substrate becomes more difficult and the layer begins to separate locally Irom the substrate (b). At this location, growth... 

Figure 9.19 Growth of oxide layer leading to spalling (schematic) (a) adherent layer (b) partital layer separation (c) growth limited to adherent part of layer and (d) formation of a new layer in the cavity.

The radionuclides incorporated in the oxide layer are in part released again to the coolant in a manner very similar to the release of corrosion products from corroding stainless steel. The exact mechanism of release is not certain, but it may be a combination of dissolution, diffusion, ion exchange and desorption or spalling of smaller oxide particles. The overall time constants for the Co activity release from the inner and the outer layer oxides were empirically determined from a number of reactors to be about 2 10 d and 8.6 10 d, respectively (Lin, 1990). 

The understanding and prediction of corrosion phenomena in the various reactor environments is important to assess the performance of the structural materials. Corrosion usually impacts the wall thickness, thus the load-bearing capability of the structural materials. It may also degrade their mechanical properties. Moreover the presence of an oxide layer at the steel surface needs to be carefully considered since it can reduce the heat transfer capability of the component and if it spalls from the material, it can impact the general process by generating solid impurities in the circuits. 

Spalling or flaking of the surface oxide layer can occur if the process temperature exceeds 570 °C (1060 °F) and process times exceed 4 h. The maximum thickness of the surface oxide layer should not exceed 7 xm (0.28 mil). Beyond this thickness, flaking can occur due to an increase in surface tensile stress. 

Figure 4.45. Iron Fe reacts spontaneously with the O2 of the air and is covered with a surface layer of iron oxide. At high temperatures, for example hot-rolling, this oxide layer may become thick, and part of it may spall off. The layer is called mill scale.

Layer formation cannot entirely be avoided in any of the samples. However, as shown in the SEM pictures of cross sections in Fig. 7 c) and f) the densest samples tend to form thicker oxides layers during redox treatment than samples sintered at lower temperatures. These layers tend to spall off during redox-treatment, as could be seen upon removal of the samples from the furnace. The samples sintered at lower temperatures (Fig. 7 a, b, d, e) do not show this behavior. Particles in specimens sintered at 900 C appear to have agglomerated during redox treatment considerably in comparison to those in the other samples. This could mean that the sintering temperature of 900 °C is not sufficient to obtain a structure that is stable under operating conditions. 

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