Corrosion microscopic

In reality, the identification of one or more forms of corrosion requires visual observation, nondestructive inspection methods, optical microscopic examination, and sometimes electron scanning microscopy, etc. The first study of the corrosion appearance of a case should divide corrosion into uniform and localized corrosion. Localized corrosion can be further identified as macroscopic or microscopic local corrosion. Microscopic attack refers to a minute amount of dissolved metal, accompanied by considerable damage, before the phenomenon becomes visible to the naked eye. 

Finally, although pitting is regarded as the least damaging form of corrosion, microscopic studies of corrosion in situ have shown that the more severe modes of corrosion, such as SCC, frequently initiate and grow from pits. 

Optical Techniques. The most important tool in a museum laboratory is the low power stereomicroscope. This instmment, usually used at magnifications of 3—50 x, has enough depth of field to be useful for the study of surface phenomena on many types of objects without the need for removal and preparation of a sample. The information thus obtained can relate to toohnarks and manufacturing techniques, wear patterns, the stmcture of corrosion, artificial patination techniques, the stmcture of paint layers, or previous restorations. Any art object coming into a museum laboratory is examined by this microscope (see Microscopy Surface and interface analysis). 

Evaluation of attack if other than general, such as crevice corrosion under suppoi t rod, pit depth and distribution, and results of microscopic examination or bend tests... 

Oxygen corrosion involves many accelerating factors such as the concentration of aggressive anions beneath deposits, intermittent operation, and variable water chemistry. How each factor contributes to attack is often difficult to assess by visual inspection alone. Chemical analysis of corrosion products and deposits is often beneficial, as is more detailed microscopic examination of corrosion products and wasted regions. 

The visual and microscopic appearance of the cracks, coupled with the presence of chlorine-containing corrosion products on the cracked surfaces, identifies this failure as chloride SCC. The circumferential orientation of... 

Microscopic examinations revealed that the cracks were unbranched and transgranular, t3q)ical of corrosion-fatigue cracks. These examinations also... 

Microscopic examinations revealed tight, unbranched, transgranular cracks originating on the external surface. Many of these cracks originated at shallow pockets of corrosion. 

Examination of surface profiles in these pitted regions under a high-power microscope revealed jagged, undercut profiles free of deposits or corrosion products. This appearance is typical of cavitation damage. 

The tube was transversely sectioned, polished, and viewed with various microscopes. Large porous corrosion-product plugs extended entirely through the 0.050-in. (0.13-cm)-thick wall (Fig. 13.10B). The depressions contained 94% copper, 5% zinc, and 1% tin. 

Removal of deposits and corrosion products from internal surfaces revealed irregular metal loss. Additionally, surfaces in wasted areas showed patches of elemental copper (later confirmed by energy-dispersive spectroscopy) (Fig. 13.12). These denickelified areas were confined to regions showing metal loss. Microscopic analysis confirmed that dealloying, not just redeposition of copper onto the cupronickel from the acid bath used during deposit removal, had occurred. 

Occasionally, corrosion of this type produces large cavities covered by a thin outer skin of weld metal (Fig. 15.5). Even close examinations of such sites under a low-power microscope may fail to reveal the cavities. Compare Figs. 15.6 and 15.7. Generally, such sites are detected either by fluid leakage or by nondestructive testing techniques such as radiography and ultrasonics. 

An irregular trough of metal loss is apparent along the circumference of the ring (Fig. 16.4). Metal loss is severe near the nozzle holes (Fig. 16.5). The corroded zone is covered with light and dark corrosion products and deposits. Analysis of these revealed substantial quantities of copper and zinc. Microscopic examinations revealed exfoliation of the aluminum ring in corroded regions. 

Apart from the application of XPS in catalysis, the study of corrosion mechanisms and corrosion products is a major area of application. Special attention must be devoted to artifacts arising from X-ray irradiation. For example, reduction of metal oxides (e. g. CuO -> CU2O) can occur, loosely bound water or hydrates can be desorbed in the spectrometer vacuum, and hydroxides can decompose. Thorough investigations are supported by other surface-analytical and/or microscopic techniques, e.g. AFM, which is becoming increasingly important. 

Cavitation corrosion occurs when a surface is exposed to pressure changes and high-velocity flows. Under pressure conditions, bubbles form on the surface. Implosion of the bubbles causes local pressure changes sufficiently large to flake off microscopic portions of metal from the surface. The resulting surface roughness acts to promote further bubble formation, thus increasing the rate of corrosion. 

Heterogeneities associated with a metal have been classified in Table 1.1 as atomic see Fig. 1.1), microscopic (visible under an optical microscope), and macroscopic, and their effects are considered in various sections of the present work. It is relevant to observe, however, that the detailed mechanism of all aspects of corrosion, e.g. the passage of a metallic cation from the lattice to the solution, specific effects of ions and species in solution in accelerating or inhibiting corrosion or causing stress-corrosion cracking, etc. must involve a consideration of the detailed atomic structure of the metal or alloy. 

In this section the interaction of a metal with its aqueous environment will be considered from the viewpoint Of thermodynamics and electrode kinetics, and in order to simplify the discussion it will be assumed that the metal is a homogeneous continuum, and no account will be taken of submicroscopic, microscopic and macroscopic heterogeneities, which are dealt with elsewhere see Sections 1.3 and 20.4). Furthermore, emphasis will be placed on uniform corrosion since localised attack is considered in Section 1.6. 

Cron, C. J., Payer, J. H. and Staehle, R. W., Dissolution Behaviour of Fe-Fe3C Structures as a Function of pH, Potential and Anion an Electron Microscopic Study , Corrosion, 27, 1 (1971)... 

Crevice corrosion and pitting have a number of features in common, and it has been stated that pitting may be regarded as crevice corrosion in which the pit forms its own crevice however, whereas a macroscopic heterogeneity determines the site of attack in crevice corrosion, the sites of attack in pitting are determined by microscopic or sub-microscopic features in the passive film (5 Sections 1.3 and 1.5). 

Gray, H. R., Ion and Laser Microprobes Applied to the Measurement of Corrosion-Produced Hydrogen on a Microscopic Scale , Corrosion, 28, 47 (1972)... 

Stickler, R. and Vinckier, A., Electron Microscope Investigation of the Intergranular Corrosion Fracture Surfaces in a Sensitised Austenitic Stainless Steel , Corros. Sci., 3, 1 (1963) von Schwenk, W. and Buhler, H.-E., Beoboshtungen an einem Kornzerfallsen falligen Austenitischen Cr-Ni-Stahl im Aktivzustand , Corros. Sci., 3, 145 (1963)... 

Janik-Czachor, M., Electrochemical and Microscopic Study of Pitting Corrosion of Ultra-pure Iron , Br. Corros. J., 6, 57 (1971)... 

In mechanistic studies of stress corrosion and also in the collection of data for remaining-life predictions for plant there is need for stress-corrosion crack velocity measurements to be made. In the simplest way these can be made by microscopic measurement at the conclusion of tests, the assumption being made that the velocity is constant throughout the period of exposure, or, if the crack is visible during the test, in situ measurements may be made by visual observation, the difficulty then being that it is assumed that the crack visible at a surface is representative of the behaviour below the surface. Indirect measurements must frequently be resorted to, and these... 

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