The Synthesis And Corrosion Behavior Of Bulk Nanocrystalline Copper

Anodic polarization, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) electrochemical methods and XRD (X-ray diffraction), EDS (energy-dispersive spectroscopy), FEGSEM (Field emission gun scanning electron microscopy) technique were used to experimentally investigate the corrosion behavior of bulk nanocrystalline (nc) copper prepared by IGCWC (inert gas condensation and in situ warm compress) in acid copper sulphate solution (CUSO₄+H₂SO₄), neutral solution containing chlorides (Na2SO₄+NaCl), Na₂SO₄ and NaCl solutions. The influences of sulfate ions, chloride ions, pH value and temperature of solution, grain size of nc copper and defects produced in the fabrication of the sample on the corrosion behaviour of nc copper were studied. Meanwhile, the differences in corrosion behavior between nc copper and coarse copper at the same corrosin condition were analysed.The conclusions are listed below.1. Grain size of nc copper played an important role in its corrosion behaviour. With decreasing grain size, passive current density Ip, critical current density for passivity I_cr and transpassive potential E_tp of nc copper increased but primary passive potential E_cr of nc copper decreased. In addition to self-corrosion potential E_corr of nc copper decreased and the corrosion current density increases notably as grain size decreased. Those results showed that the corrosion resistance of nc copper decreased as grain size reduced. which was mainly attributed to the high activity of surface atoms and intergranular atoms resulted from the reduction of grain size.2. In acid copper sulphate solution, I_p and I_cr of nc copper were greater than that of coarse copper but E_corr of nc copper was negative than that of coarse copper indicating that nc copper had lower corrosion resistance than coarse copper. EIS study demonstrated that R_p of nc copper was lower than that of coarse copper and the original pitting corrosion seemed to happen earlier on nc copper surface than on the coarse copper surface. Compared with coarse copper, nc copper decreased in resistance to corrosion.3. In neutral solution containing chlorides solution, I_p and I_cr of nc copper were lower than that of coarse copper but E_corr of nc copper was positive than that of coarse copper indicating that nc copper exhibited higher corrosion resistance than coarse copper. The formation of the stable passivity film was restrained by chloride ions, leading to a narrowing of the range of passivation of nc copper in neutral solution containing chlorides solution than in acid copper sulphate solution. Because of thenano-scale effect nc copper was much easier to passivate than coarse copper. The protective film on the surface of nc copper had a better protection than that on the surface of coarse copper. CV study demonstrated that nc copper exhibited a higher corrosion resistance than coarse copper.4. In neutral solution containing chlorides solution, nc copper exhibited uniform dissolution of the surface and discrete localized corrosion typically, but conventional coarse copper exhibited considerable preferential attack along the grain boundaries with signs of pitting corrosion dispersed throughout the surface uniformly. In addition to the grain growth of nc copper in solution was observed. The intrinsic grain growth of nc copper was much slower than the apparent growth in solution.5. Nc copper was much sensitive to the oxygen content in the solution. This sensitivity is mainly attributed to the high activity of surface atoms and intergranular atoms. The high activity of surface atoms and intergranular atoms, resulting from the reduction of grain size, leads to an enhancement of free transfer of oxygen. In addition to there was accumulation of sulphur on the sarface of nc copper but there was no on sarface of coarse copper.6. For nc copper the diffusion effect in the polarization was important For coarse copper the polarization was mainly controlled by the transfer but not by diffusion. The intermediate or soiption results had a great effect on the polarization of coarse copper.7. In 0.47wt.% NaCl solution, nc copper exhibited a different anodic polarization behavior from coarse copper. There was no second passivity for nc copper, while there was a notable second passivity for coarse copper. The second passivity was was attributed to the formation of the two steps protective films. In addition to the self-corrosion potential and the Tafel slope of nc copper almost equated to that of coarse copper but I_P, I_cr and E_cr were higher than that of coarse copper. It revealed that nano-scale effect accelerate the dissolution of nc copper.8. In Na₂SO₄ solution the sulfate ion concentration played an important role in the anodic polarization behaviour of nc copper. With increasing sulfate ion concentration, I_cr increased but E_cr decreased. The relationship between loglcr and 1/T fitted a linear equation of logI_cr=0.01158[SO₄^2-] +0.14142.9. Adding 10⁴ ppm Cl^- in 0.3wt.% Na₂SO₄ solution, the anodic polarization behavior of nc copper changed significantly. I_cr decreased and the decreasing rate of anodic current intensity lowed notably in the range of activity-passivity zone. These changes were attributed to the CuCl protective film on the nc copper.10. The decreasing rate of anodic current intensity lowed notably in the range of activity-passivity zone. A model of "static ion congeries" was proposed to explain this notable change of the decreasing rate of anodic current intensity and the case of I_cr increased with increase of sulfate ion concentration. Kinetics analysis revealed that the dissolution current of nc copper was dependent on potential, sulfate ion concentration, pH value, temperature, the diffusion coefficient of Cu+ in the oxidation film and the thickness of the oxidation film on the surface of nc copper.11. Temperature T played an important role in the anodic polarization behaviour of nc copper. With increasing temperature T, I_p, I_cr and E_cr of nc copper increased. The relationship between logI_cr and 1/T fitted a linear equation of logI_cr= —669.94 (1/T) + 2.4388. The CU₂O film on the surface of nc copper after polarization became loose but the grain size of Cu₂O film became small with increasing temperature.12. With increasing pH value of the solution, I_cr and I_p of nc copper increased and the relationship between logI_cr and pH fitted a linear equation of logI_cr = —0.0287pH+0.25. Due to the reaction 2Cu_sur⁺ +OH^-(?) Cu₂O + H⁺ was restrained as pHvalue of the solution increased. In other words, the formation of the oxidation film on the surface of nc copper was restrained, therefore, the protective effect lowered leading to an increase in the rate of dissolution.13. Defects such as micro-gap produced in the fabrication of nc copper sample had great effect on the overall corrosion performance of nc copper sample, which were weakness to corrosion...