Degradation of nanocrystalline metals and alloys by environment

Most metals suffer corrosion as a result of their inherent thermodynamic instability in elemental form. However, the progress of corrosion is largely governed by one or a combination of the foUowing phenomena  

Diffusion of elemental/ionic species (a) in the metal substrate, and/or (b) through the layer of corrosion product or through the electrolytic environment iimnediately adjacent to the metal surface. 

The NC stmctuie can remarkably inilirence the nature and/or degree of both electrochemical non-homogeneity at the surface and diffusion in the birlk of metals. Besides, there may be other irtilirences due to the NC stmcture (the degree of which will vary from metal to metal). For example, the structure (such as grain size) and associated properties (such as mechanical property) of the corrosion films developed on NC substrate may be considerably different from those on a MC sirbstrate. Thirs, 

Early fundamental studies on the role of nanostmcture in electrochemical corrosion were carried out on Co, ]sfi 73.8i-84 Ni-based 

Electrochemical corrosion resistance of a NC surface of 316 stainless steel developed by surface mechanical attrition treatment was found to be considerably inferior to the MC unmodified bulk. This behaviour is attributed to the considerable increase in the fast diffusion channels for ions, i.e. grain boimdaries and triple junctions in the NC material. In another study, the grain refinement of stainless steels to a nanometric level is reported to improve the corrosion resistance, as suggested by the considerably extended passivation for NC in the polarisation plots. The authors have attributed this behaviour to the greater chromium diffusion in the NC structure, which may be argued given the very low diffusivity (i.e. 10 -10 s ) at ambient temperatures. 

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