Research On Corrosion Behavior Of Bulk Nanocrystalline Copper

The corrosion behavior of nanocrystalline (nc) copper bulks with various grain sizes prepared from IGC(Inert Gas Condemsation) and vacuum annealing in comparison with conventional microcrystalline (mc) copper (as-rolled and electrolytic) in acid copper sulphate solution and neutral solution containing chlorides under free corrosion conditions and anodic polarizations has been studied using potentiodynamic polarization , potentiometric analysis , cyclic voltammetry and electrochemical impedance spectroscopy.X-ray diffraction was used to estimate the grain size of the annealed nc copper.Field emission gun scanning electron microscopy and X-ray energy-dispersive spectroscopy was used to characterize the surface morphology and analyze the surface composition after the polarization and potentiometric test of both nc and mc copper.In acid copper sulphate solution , the nc copper has a more negative corrosion potential which is decreasing with reducing the grain size and diffusion impedance was only found in the as-prepared and as-annealed at 180癈 nc copper due to the diffusion within the pores and channels on the surface.Both the Tafel plots and EIS study demonstrated that the nc copper exhibits a higher corrosion current and lower polarization resistance than its mc couterpart. A modified definition of polarization resistance calculated from the equivalent circuit of EIS make its value dependent on the grain size.In anodic polarization test(sweeing and stair-steps),the maximum(critical) current density increases while the passivation potential and breakdown potential lowers with the reduction of the grain size.FEGSEM images showed the grain growth and pitting corrosion of the nc copper surface ,uniform corrosion in the mc copper surface and the broken passivation layer in both surfaces. EDS found that the S and Cl concentrations in the pit are higher than the surface of the nc copper.In neutral solution containing chloridions, the nc copper has a more positive corrosion potentiaLAnodic polarization test revealed that the nc copper has a lower passivation potential,critical current density and passivation current density. Cyclic voltammetry indicated that the reaction mechanisms of micro-and nc copper are identical,but the anodic peaks current density of nc copper is much lower than that of mc copper.The potentiometric analysis demonstrated that the pitting corrosion seemed to happen earlier on nc copper surface than on the mc surface.The XRD found Cu2O on nc copper surface while no oxide was found on mc copper surface. EDS suggests the nc copper surface containing O and Cl,but no other element except Cu and C could be found on mc copper surface.FEGSEM images revealed that mc copper exhibits uniform corrosion but the pit on the surface of nc copper is discrete.Based on the results presented in this study, the following conclusions can be drawn:Grain size has an important effect on the corrosion behavior.Reduction of grain size results in enhancing the activity of surface atoms and intergranular atoms.But it is not the only factor which influence the corrosion behavior,otherfactors,such as surface conditions,impurities ,defects ,sensitivity to oxygen and some sorts of ions are also important. The effectiveness of protection of passive film in different solution are sometimes more crucial to determine the corrosion behavior.