Fractal analysis of pit morphology of Inconel alloy 600 in sulphate, nitrate and bicarbonate ion-containing sodium chloride solution at temperatures of 25–100 ℃

摘要

Pit morphology of Inconel alloy 600 in sulphate (SO_4~(2-)), nitrate (NO_3~-) and bicarbonate (HCO_3~-) ion-containing 0.5 M sodium chloride (NaCl) solution was analysed in terms of fractal geometry as functions of solution temperature and anion concentration using the potentiostatic current transient technique, scanning electron microscopy, image analysis and ac-impedance spectroscopy. Potentiostatic current transients revealed that the pitting corrosion is facilitated by the increase in solution temperature, irrespective of anion additives, and that it is hindered by the increase in NO_3~- and HCO_3~- ion concentration, regardless of solution temperature. Above 60 ℃, it was also found that the addition of SO_4~(2-) ions impedes pit initiation, but enhances pit growth. The value of fractal dimension D f of the pits increased with increasing solution temperature and with decreasing NO_3~- and HCO_3~- ion concentration. Moreover, the alue of D f increased above 60 ℃ with increasing SO4 2- ion concentration. This is caused by the increase in the ratio of pit perimeter to pit area, implying the formation of pits with micro-branched shape due to the acceleration of the local attack in the pits. From the decrease of the depression parameter with increasing solution temperature, it is inferred that the roughness of the pits increased with increasing solution temperature. In addition, the depression parameter was found to increase with increasing NO_3~- and HCO_3~- ion concentration. But, above 60 ℃, in the case of SO_4~(2-) ion addition, the depression parameter decreased with increasing SO_4~(2-) ion concentration. From the experimental findings, the three-dimensional pit morphology is discussed in terms of the values of D f of the pits and the depression parameter, with respect to anion concentration and solution temperature.