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

The corrosion behaviour of oxygen-free copper in anoxic 0.1M NaCl + 2 x 10(-4) M Na(2)S9H(2)O 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 10(22) to 10(23) cm(-3). The LEIS results showed that the passive film achieved relative stability after 24h of immersion under natural conditions, which was longer than the duration of potentiostatic polarisation (4h at -0.6V(SCE)).