Oxidation metallographic analysis

The metallographic analysis of the reactors with the optical microscope shows not worth mentioning trace of corrosion in the high temperature section of the reactor The elemental analysis of the exposed surface revealed a layer reach in Ni and Fe oxides and depleted in Cr and Mo. These changes affect only a thin layer. In account to the long exposure time of more than 1000 h and the high (up to 0.2 mol/kg) HCl concentration, corrosion rates of alloy 625 in high temperature - low-density supercritical water solutions are definitely low. For such HCl and Oj solutions Ni-base alloys, similar to alloy 625, can be used. 

The first and foremost step in failure analysis of ceramics consists of identifying the fracture origin and the type of cracking, which throws light on the type of failure such as failure due to impact, residual stress combined with load, thermal shock, improper machining, oxidation and corrosion. This is aided by micro- and macrofracto-graphy, examination of microstructure by SEM, chemical analysis and metallographic examination. 

The high temperature oxidation of (J-NiAl, undoped and doped with Ce, Y and Hf was studied in situ by thermogravimetry in He with p(02) = 5 10 6bar at 1000°C and by high temperature X-ray diffraction at 950 and 1000°C in air. After the in situ experiments the samples were analysed metallographically by optical microscopy and by scanning electron microscopy (SEM) with energy dispersive analysis (EDX). 

The results of metallographic investigations are in agreement with the gravimetric measurements. It can be seen in Fig. 6 that alloy 800H shows significant internal carburisation, while the Fe-Al-Cr alloys are entirely unaffected by carbon. The oxide scales of the Fe-Al-Cr alloys were too thin to be investigated by microprobc analysis after exposure in CH4/H2 it is however, very likely that they consist of pure alumina. 

To get mechanistic information, gravimetric measurements should always go together with metallographic and analytical investigations aimed at the smdy of the morphology and the composition of the corrosion products and the metal in the vicinity of the surface. For this, a scanning electron microscope equipped with an X-ray microanalysis system is particularly well suited. Transmission electron microscopy in conjunction with electron diffraction, and conventional X-ray diffraction methods are also frequently applied. Surface analysis methods such as XPS, AES and SIMS permit the study of the oxide-metal interface and of initial oxide growth. 

The analysis of corrosion films in cross section requires preparation procedures that take into account the friable and discontinuous nature of corrosion oxides. Some areas of the oxide will be less stable mechanically than others due to growth mechanisms. Therefore, it is important that any mechanical procedure for. sectioning the oxide preserves all portions of it. The section could be prepared by polishing in a standard metallographic preparation, with the require-... 

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