| In recent years, the corrosion performance of nanocrystalline (NC) metals has attracted extensive attention. The investigation on corrosion behavior and mechanism of NC metal material could provide theoretical basis for their safe application. Copper is an important, structural, functional and ornamental metal material, which is widely used in many fields such as architecture, national defense, electric and machinery. However, copper could be easily corroded by typical anion (Cl-,SO42-NO3-) to form chemical corrosion and occur electrochemical corrosion in moist environment. NC copper presents even more potential applications because of the nanostructure. In this study, immersion test, potentiodynamic polarization measurements, electrochemical impedance spectroscopy (EIS), field emission gun scanning electron microscopy (FEGSEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS) were used to experimental investigate the corrosion behavior of bulk NC copper prepared by Inert gas condensation and in situ warm compress (IGCWC) method in 0.1 mol/L (M) NaOH and HNO3 solutions. The effect of grain size, temperature of solution, benzotriazole (BTA) inhibitor and micro-defects produced in the fabrication of the NC sample on the corrosion behavior were studied. Meanwhile, the difference in corrosion behavior and mechanism between NC copper and coarse grain (CG) copper were also analysed. The conclusions are listed as followed:1. NC copper shows a typical active-passive-transpassive corrosion behavior, which is associated with formation of duplex passive films in 0.1 M NaOH solution. The duplex passive films are Cu2O and CuO/Cu(OH)2, respectively. Cu2O forms on the substrate of NC electrode, CuO/Cu(OH)2 with bamboo-like flakes structure develop on the surface of Cu2O film. Compared with CG copper, high activity of surface atoms and high grain boundary density lead to an enhancement of passive ability and an increase of dissolution rate of passive film in alkaline solution, which decrease the stability and protection of passive films.2. The dissolved oxygen in NaOH solution promotes passive ability of NC copper and enhances the dissolution kinetic of passive films. In NaOH solution, from polycrystalline to nanocrystalline, grain size variation(48,68,92 nm) shows little effect on the overall corrosion resistance of copper samples, which is due to the unique duplex passive behavior. In de-aerated electrolyte, the corrosion resistance of NC copper decreases with grain size reduction to NC range.3. NC copper displays an active dissolution behavior in 0.1 M HNO3 solution. NC copper provides more copper atoms at step position to participate in electrochemical reaction. In comparison with CG copper, NC copper shows considerable preferential attacks along certain grain boundaries, which is mainly attributed to the micro-defects such as microvoids and weak links existing on certain grain boundaries to accelerate corrosion.4. The inhibition efficiency (IE) of BTA for NC copper is higher than that of CG copper in 0.1 M HNO3 medium. IE of NC copper increases with the concentration of BTA increasing to high range. Because of fine grain size, NC copper provides plenty of adsorption sites for BTA inhibitor. BTA forms [Cu(II)BTA]ads and [Cu-BTA]ads complex films in nitric acid electrolyte, [Cu(II)BTA]ads and [Cu-BTA]ads mainly adsorb on NC copper surface by physisorption process.5. In 0.1 M HNO3 solution, with increasing temperature to 50℃, the corrosion rate of CG copper gradually increases, but the corrosion rate of NC copper decreases continually. At high temperature, the dissolved oxygen in nitric acid solution is decreased. Oxygen-deficiency greatly reduces cathodic reaction process. However, the IE of BTA for NC copper gradually decreases with temperature increasing to high range in 20 mg/L BTA+0.1 M HNO3 electrolyte. [Cu(II)BTA]ads and [Cu-BTA]ads complex films become more unstable at high temperature to decrease their protection characteristics.
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