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Effect of Equal-Channel Angular Pressing on Pitting Corrosion of Pure Aluminum

DOI: 10.1155/2012/450854

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Abstract:

The effect of equal-channel angular pressing (ECAP) on the pitting corrosion of pure Al was investigated using electrochemical techniques in solutions containing 0.1?m?mol·dm?3 of Na2SO4 and 8.46?mol·dm?3 of NaCl (300?ppm?Cl?) and followed by surface analysis. The potential for pitting corrosion of pure Al was clearly shifted in the noble direction by the ECAP process indicating that this process improves resistance to pitting corrosion. The time dependence of corrosion potential and the anodic potential at 1?A·m?2 revealed that the rate of formation of Al oxide films increased due to a decrease in the grain size of the Al after ECAP. Since there exists a negligible amount of impurity precipitates in pure Al, the improvement in pitting corrosion resistance of pure Al by ECAP appears to be attributable to an increase in the rate of formation of Al oxide films. 1. Introduction Although aluminum is inherently an active metal, it shows excellent corrosion resistance over a neutral pH range of 4–8 due to its superficial oxide film. In solutions containing Cl?, however, pitting corrosion occurs locally where the oxide film is attacked by Cl? [1–7]. On the other hand, reducing the grain size of metallic materials to the submicron range or even the nanometer range using equal-channel angular pressing (ECAP), high pressure torsion (HPT), or severe torsion straining processing (STSP) is increasingly being studied with the aim of improving mechanical properties such as strength and ductility [8–13]. Although the literature is reasonably scarce with investigations reporting the effects of severe plastic deformation upon the pitting and general corrosion behaviour [14–20] of Al-based alloys, mainly AA1100 and AA5052 (Al-Mg), those reported their majority suggests that corrosion resistance decreases with decreasing the grain size [14, 15]. It should also be remarked that chloride content, pH and other characteristics and nature of solution significantly affect the corrosion behaviour. Considering the ECAP, it has been reported both increase and decrease on the corrosion resistance [15, 17, 18]. Ralston et al. [15] recently reported that finer grains of commercially pure (c.p.) grade Al samples provide significant gains into the corrosion resistance in acidic and alkaline sodium chloride media whilst having lesser, but noticeable, impact in near neutral electrolytes [15]. It was also reported [15] that ECAP has provided low corrosion rates by using a single cast ingot of ultrahigh purity Al. Precipitate compounds of Fe-Al and Si were found around the pitting area.

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