On Corrosion and Stress Corrosion Cracking Behavior of Nanocrystals

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Corrosion tests under a constant potential, stress corrosion cracking (SCC) tests were performed on the ultra fine grained (UFG) copper (avarage grain size : 200 nm) manufactured by Equi-Channel Angular Extrusion (ECAE) technique and on conventional copper specimens. The surface of the UFG copper showed a typical general corrosion. However, the UFG copper has a very high corrosion resistance compared with the conventinal copper (average grain size : 20μ). By Severe Plastic Deformation (SPD), all lattice dislocations flowed out to the grain boundary, so the extremely high dislocation density was formed at the grain boundary of UFG copper. The corrosion potential at the grain boundary of UFG copper was increased by the high dislocation density. On the otherhand, a very strong strain field was induced in the grain interior, so the corrosion potential became high, too. Disolution is controlled by the difference of potential between grain boundary and grain interior.The susseptibility to stress corrosion cracking of UFG copper was very low compared with conventinal copper and annealed UFG(the same grain size). The gain boundary slidings (GBS) were observed during the tests at room temperature. This GBS acts as a stress relieving sites on the SCC behavior of the UFG copper.