Damaged surface layer metals

In spite of the fact that only a comparatively modest amount of systematic work has been carried out on what is undoubtedly a complicated subject, it is now possible to assemble a reasonably complete and basically sound description of the damaged layer produced during the machining and polishing of metal surfaces. It is immediately apparent that some caution must be exercised in applying the term damage in a strictly literal sense. The surface is certainly altered, and often is drastically altered, but this does not always constitute damage. The likely influence of the alterations in the surface layers must be appraised in any particular set of circumstances. 

Aluminium and A1 alloys react with oxygen and water vapor in the air to produce a thin, conqtact surface oxide film which protects the underlying metal from further attack (Fig. 3.1-80). The surface layer contains mainly amorphous AI2O3 in several layers. The so-called barrier layer has an extremely low conductivity for electrons and ions and thus acts as an insulator in any interfacial electrochemical reactions. It thus affords effective protection against corrosion. If mechanical damage of the protective layer occurs, or if the layer is removed by pickling, it re-forms immediately. Aluminium and A1 alloys thus exhibit good corrosion resistance to chemicals, seawater, and the weather. 

Electrons of 30 keV energy do not penetrate far into the surface of the specimen. Unless the irradiated material evaporates, so that the beam drills a hole in the sample, the unaffected bulk holds the damaged thin surface layer in place and the effect on the image is limited to fine scale details. To reduce these effects, the thickness of the surface layer that is irradiated should be reduced. Either the metal coating can be made thicker or the accelerating voltage reduced. Both... 

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