Oxide scale failure

However, there are some possibilities of oxide scale failure, most frequent and momentous being the conversion of the chromium oxide in carbides [29-31], This conversion will occur at the iimer wall of cracking tubes when... 

In the present study the characterization of the effect of specimen geometry and the effect of exposure under thermal cycling conditions on the oxide scale failure mechanisms were the main objectives. Kinetics of corrosion product formation as well as changes in the chemical composition of both spalled oxide scale segments and internal corrosion products were determined to evaluate the material s resistance to high-temperature corrosion attack. 

U. Krupp, S. Y. Chang and H. J. Christ, Microstructural changes in the sub smface area of Ni-base superalloys as a consequence of oxide scale failure. Materials Science Forum, 2001, 369-372, 287. 

M. Krzyzanowski and J. H. Beynon, Compositional effect on oxide scale failure during hot rolling of steel . Metal Forming 2000, M. Pietrzyk et al. (eds), 2000, Balkema, Rotterdam, 331-334. 

M. Krzyzanowski, J. H. Beynon and C. M. Sellars, Analysis of secondary oxide-scale failure at entry into the roll gap , MetaU. Mater. Trans. B, 31B, 1483—1490 (2000). 

Ward et a/."" have shown that, under cyclical loading, the oxidation rate of steels is similar to that under unstressed isothermal conditions, provided the fatigue stress is below the stress required to exceed the scale failure strain. If, however, the failure strain is exceeded, the oxidation rate is accelerated due to repetitive scale failure, and linear kinetics are observed. 

The most common failures are associated with oxidation, carburization and metal dusting, sulfidation, chlorination, and nitridation. The most common high-temperature degradation mode is oxidation, and the protection against oxidation, in general, is given by the formation of a chromium oxide scale. The presence of a small amount of aluminum or silicon in the alloy may improve the resistance against oxidation of a... 

The mechanical behaviour of oxide scales has been investigated by a complex of appropriate techniques (cf. for example [12]), where the attention has been focused on the analysis of the stress development in the scale, as well as the measurement of the scale adherence. In the case of weak scale adherence, spontaneous scale failure is often observed during oxidation or cooling of specimens. For systematic investigations of the fracture-mechanical properties of oxide scales, scale failure is induced by a controlled loading of the scale which is produced by an appropriate deformation of the whole specimen. 

To this end, different bend tests in combination with acoustic emission analysis are widely applied [13], For example, tensile failure of oxide scales in dependence on the... 

One of the difficulties of a quantitative analysis of scale failure is the determination of the stress state in the scalers it results from the oxide growth process and from additional substrate deformations. The application of X-ray diffraction provides usually only a limited lateral spatial resolution. For alumina scales consisting of the a-Al20, phase, Lipkin and Clarke [20] have shown recently that stresses can be measured at RT also by means of optical fluorescence spectroscopy (OFS) employing the piezo-spectroscopic effect [21]. This method permits a high spatial resolution of stress measurements in the micrometre region and is therefore especially useful for the analysis... 

The compressive failure of oxide scales has been investigated comprehensively in [6-8], According to the analysis by Evans and Lobb [6], there are two main routes to spalling - scale buckling and wedging. 

At high-temperatures the corrosion failure of a material system results from failure of its protective oxide scale. Different researchers have proved that addition of a small amount of... 

Complex rate laws are usually observed for alloys where different oxides are simultaneously formed on the surface leading to changes in the oxidation rate. For example, if at the beginning a more or less nonprotec-tive oxide scale is formed which is undergrown by a protective partial layer then the oxidation rate will initially be higher than later in the oxidation period in other words, sub-parabolic behavior will be measured. Complex rate laws will also follow when gas phase transport through pores or cracks in the scales is involved. This case, however, is not a protective situation and actually represents at least partial failure of a protective scale. 

The ratio is characteristic for an alloy for determining its ability to form a protective scale. In this case is the metal recession rate constant due to oxidation. This dependence of the critical concentration in the alloy on the k /D ratio is illustrated by the graph in Fig. 2-12. Field 1 in this figure shows the area in which no protective oxide scale can be formed, since the amount of chromium is insufficient for a continuous scale and instead internal oxidation of chromium in the base material may occur. As under practical conditions cracking and spalling of protective oxide scales may often occur, there must be a reservoir of the protective scale forming element in order to reestablish the scale after failure, which is dealt with by fields 2 to 4. The meaning of these fields is as follows ... 

As already mentioned, mechanical scale failure takes place when a critical stress level Of. is reached. This critical stress level can be converted into a critical strain fij. by simply dividing the stress by the Young s modulus Ox if it is assumed that elastic behavior dominates in scale failure. Strain values are better suited for discussion than stress values, as they can easily be determined by experiments. Therefore the following discussion, which is covered in more detail in the literature (Schiitze, 1995), will be based on critical strain values for failure of the oxide scales. Generally, two failure cases are conceivable depending on the stress situa-... 

Figure 2-23. Scale failure diagram of tensile deformation according to Eq. (2-39) for several oxides (Schiitze, 1995). The critical strain is given in absolute values. For percentage values a factor of 100 has to be applied.

Figure 2-25. a-f) Schematic diagram of the different failure situations in oxide scales under tensile or compressive stresses (SchUtze, 1998). 

Some data on scale failure have been determined experimentally and are illustrated in Fig. 2-26 for tensile stresses, which is regarded as the more critical situation. Figure 2-26 a shows the situations for an alumina scale on a high alloy steel and for a chromia former. Figure 2-26 b shows the results for nickel oxide on nickel. The data are plotted versus the applied strain rate at a variety... 

The spallation of oxide scales generally involves the separation of cracked scale either by decohesion at the alloy-scale interface or by fracture on planes within the scale itself (Evans, 1995 Chan, 1997). The failure mode which predominates depends on the stress state in the scale and on the extent of adhesion at the alloy-scale interface. Evans and Lobb (1984) showed that, for scale with good adhesion and a thickness exceeding ca. 1 pm, scale-metal decohesion should occur by a wedging fracture process when the temperature drop is greater than a critical value, AT, given by ... 

In some circumstances the oxide scale that forms will be protective. Such scales form a continuous layer over the surface of the metal and serve as a barrier between the metal and the environment. They grow (thicken) at such a low rate that conversion of sufficient cross-section of the metal to oxide to cause failure does not occur within an acceptable lifetime. Depending on the application, this lifetime can vary from as short as minutes in rocket applications to 50000 h or longer in gas-turbine applications. 

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