Passivation Behavior of Ultrafine-Grained Pure Copper Fabricated by Accumulative Roll Bonding (ARB) Process
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COPPER and its alloys are widely used in the chemical and petrochemical industries.[1,2] Based on the Pourbaix diagram for copper, in the passive domain (at pH values ranging from 6.8 to 12.8), the surface film (known as passive film) formed on copper is protective.[3] Indeed, this film forms a perfect barrier against the dissolution of this metal. Therefore, the passive behavior of copper in this domain needs to be fully understood. Moreover, the passive behavior of the copper has been investigated in relation to the protective characteristics of passive films and the electrochemical production of copper oxide layers.[4–6] Generally, ultrafine-grained (UFG) structure of metals and alloys show eminent high mechanical behavior than their coarse-grained counterpoint. These behaviors have drawn much attention to UFG refining of metallic materials. Recent studies have focused on the application of large strain or severe plastic deformation (SPD) as a method of producing UFG metals and alloys.[5–8] Several SPD methods such as high pressure torsion, cyclic extrusion compression, multiaxial forging, torsion straining, equal channel angular pressing, and ARB have been used for fabrication high strength metals and alloys.[9–12] Among these methods, ARB is a high productivity ARASH FATTAH-ALHOSSEINI, Dean of Materials Engineering Department, and OMID IMANTALAB, Ph.D. Student, are with the Department of Materials Engineering, Bu-Ali Sina University, Hamedan 65178-38695, Iran. Contact e-mails: arash.fattah@gmail. com, [email protected] Manuscript submitted March 28, 2015. Article published online November 11, 2015 572—VOLUME 47A, JANUARY 2016
process. Also, there is no need to apply large load and make expensive dies. Therefore, this method is excellent for manufacturing UFG plates and sheets.[13,14] The evolution of microstructure and mechanical properties during ARB process of copper and its alloys were investigated.[7,8,15,16] However, little principled study has been performed on the passivation behavior of UFG copper and its alloys fabricated by ARB method. The answer to the question about the impact of UFG refinement of metallic materials on the passivation behavior is not straightforward. Still this subject is under study of many researchers. It was found that the comprehension of passivity and its protective character against corrosion were basically related to the semiconducting behavior of the passive films. Moreover, it was shown that the semiconducting behavior was affected by structure of the passive film.[16] The literature review clearly apparent the passivation behavior of UFG pure copper has been rarely investigated. Therefore, the main goal of this work was to evaluate the effect of ARB process on the passivation behavior of the passive films formed on pure copper. To investigate the passivation behavior of the specimens produced by ARB process, EIS and Mott-Schottky analysis was carried out. Also to determine the evolution of microstructure as a function of ARB process, TEM observations were used.
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