The stability of passive film growth on copper in anaerobic sulphide solutions

The corrosion behaviour of oxygen-free copper in anoxic 0.1 M NaCl + 2 × 10⁻⁴ M Na₂S·9H₂O solution (pH = 9.0) was investigated under potentiostatic polarisation for different times. Electrochemical methods, including electrochemical impedance spectroscopy, Mott–Schottky analysis, localised electrochemical impedance spectroscopy (LEIS) and scanning electron microscopy observations, were conducted. The results indicated that the corrosion resistance of oxygen-free copper decreased with increasing applied potential, whereas it increased with increasing polarisation time. The passive film growth kinetics obeyed a logarithmic law (lnD = alnt + b, where D is the layer thickness, b is a constant taken as the initial growth rate, t is the polarisation time and a is the time exponent). Subsequent to the formation of a compact and coherent passive film, the thicker the film was, the more difficult for ion to migrate, which further resulted in a slower film growth rate. The passive film displayed p-type semiconductor behaviour and the acceptor density (cation vacancy) was approximately 10²² to 10²³ cm⁻³. The LEIS results showed that the passive film achieved relative stability after 24 h of immersion under natural conditions, which was longer than the duration of potentiostatic polarisation (4 h at −0.6 V_SCE).