Corrosion metallographic analysis

Metallographic analysis allows the determination of the surface reduction along the fracture and defines the fragile or ductile nature of the fracture. By employing a longitudinal section, it was possible to evaluate the magnitude of corrosion close to the surface of the fracture. 

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. 

ASTM A 262) provide relative susceptibility only for austenitic stainless steels. These tests have little usefulness in freshwater environments. A metallographic analysis performed after test samples are exposed provides the most useful information about susceptibility to intergranular corrosion (ASTM E 3 and E 80). 

Quenched melt samples taken after each experiment were analyzed to determine the content of the elements of interest using the ICP MS method (Elan 9000, Perkin Elmer) and X-ray fluorescence spectroscopy (ARL QUANT X). The surface of the steel samples after the corrosion tests was examined using metallographic analysis (Olympus GX-71F) and X-ray microanalysis (Jeol ISM 6490 with Inka Dry Cool attachment). 

Figure 24.10 shows that increasing the chromium content of steels from the 0% of 15Mo3 to 1% in 13Cr, 2% in lOCr or even 10% in X20 did not lead to a reduction in corrosion rates. However the stainless steel Esshete 1250, containing 15% Cr, showed very low corrosion rates. Detailed metallographic analysis of the stainless steel has yet to be performed. 

Static test results may be evaluated by measurement of change of mass or section thickness, but metallographic and X-ray examination to determine the nature and extent of attack are of greater value because difficulty can be encountered in removing adherent layers of solidified corrodent from the surface of the specimen on completion of the exposure, particularly where irregular attack has occurred. Changes in the corrodent, ascertained by chemical analysis, are often of considerable value also. In view of the low solubility of many construction materials in liquid metals and salts, changes in mass or section thickness should be evaluated cautiously. A limited volume of liquid metal could become saturated early in the test and the reaction would thus be stifled when only a small corrosion loss... 

As in the case of corrosion failures, the sequence of steps involved in analyzing wear failures are initial examination of the failed component including service conditions to establish the mode or combination of modes of wear failure, metallographic examination to check if the microstructure of the worn part met the specification, both in the base material and in the hardened case or applied surface coatings, existence of localized phase transformations, shear or cold worked surfaces, macroscopic and microscopic hardness testing to determine the proper heat treatment, X-ray and electron diffraction analysis to determine the composition of abrasives, wear debris, surface elements and microstructural features such as retained austenite, chemical analysis of wear debris surface films and physical properties such as viscosity and infrared spectral determination of the integrity of lubricants and abrasive characteristics of soils or minerals in the cases of wear failures of tillage tools. 

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. 

As for immersion tests, also in cabinet tests the corrosion effects are evaluated by weight loss, appearance, metallographic examination, changes in electrical and mechanical properties, with evaluation criteria prescribed by standards. It is worthwhile to note that the evaluation of corrosion effects based on visual appearance often is subjective. Some objective method should be used, such as image analysis, with the aim of minimising the influence of human factor in the post-test evaluation. 

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. 

Corrosion test specimens are used to evaluate average corrosion rate over the exposure period and eire also useful for assessment of crevice corrosion, pitting, and end grain attack, and may be used for metallographic examination of the corrosion test specimen or analysis of any deposits. Special corrosion test specimens may be prepared with welds to assess corrosion problems particular to weld material or heat affected zones. SCC may be monitored with specially mounted and loaded corrosion test specimens. 

Our own investigations on different cast duplex stainless steels were performed by electrochemical measurements, accompanied by metallographic inspection and chemical analysis of the alloys. At first the influence of different levels of chloride and fluoride, pH, and temperatures, and the combined influence of chloride and fluoride on corrosion resistance were evaluated. Secondly, suitable alloys were identified which would have sufficient corrosion resistance even under extreme conditions. 

Identification Upon metallographic examination of the internal surface of the pipe it was found that the pipe was seriously corroded. The corrosion was of uniform type. A chemical analysis of the water showed the leaching of iron from the pipe. 

Analysis of the products of corrosion has its historic origin in the light-microscopic studies of metallographic cross sections that started in the 1950s. The.se were supplemented with TEM and SEM investigations in the 1960s of... 

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-... 

Chemical analyses on the remains of these plates are not yet available, Seven other ternary alloy plates (without defects), which had satisfactorily withstood screening tests and some further corrosion testing, have been returned for sectioning and resealing to yield a section from each plate for metallographic examination, core alloy corrosion testing, and chemical analysis, The resealed plates will then be tested with deliberate defects. 

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