THE CORROSION behaviour of copper in sodium nitrate solution of pH 2 was investigated using open-circuit potential measurements, polarization, and electrochemical-impedance spectroscopy. The results showed that the metal dissolves under open-circuit conditions as Cu~+ which transfer either by disproportionation or further oxidation to Cu2~(2+). Anodic polarization measurements showed Tafel behaviour with an average slope equal to approx.37mV per decade which indicates that the dissolution of Cu takes place in two steps. The overall dissolution reaction is controlled either by the second electrochemical step (Cu~+ = Cu~(2+) + e), or by a disproportionation reaction (2Cu~+ = Cu + Cu~(2+)). The impedance spectra depend on the applied potential. At E < -0. i V (she), an inductive loop was recorded, which can be attributed to electroadsorption of the corrosion products. In the potential range -O.IV < E < 0.27V, the spectra consisted of depressed semicircular arcs. At low frequency, a diffusional tail appears, indicating that the copper dissolution is accompanied by a diffusion-controlled process. The results showed that the charge transfer resistance, R_t, increases gradually with increasing potential reaching a maximum at E = 4SmV (she) due to passive film formation. At higher potentials, R_t decreases markedly due to metal dissolution. A mechanism for the copper corrosion in acidic nitrate solution was suggested and equivalent circuit models to describe the electrode processes under different conditions were proposed.
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