Pit formation and growth of alloy 600 in Cl ion-containing thiosulphate solution at temperatures 298-573 K using fractal geometry

Pit formation and growth of alloy 600 has been investigated in aqueous 0.1 M Na₂S₂O₃+0.1 M NaCl solution at elevated solution temperatures 298-573 K and at pressures 0.1-8 MPa in terms of fractal geometry using potentiodynamic polarisation experiment, potentiostatic current transient technique, scanning electron microscopy (SEM), image analysis method and ac-impedance spectroscopy. From SEM observation, it was realised that pit morphology changed from cylindrical shape developed at 60 °C to highly branched shape formed at 150 °C. Furthermore, corrosion pits formed and further grew without any morphological change during the whole pitting process below 200 °C. On the other hand, above 200 °C, the morphology of the pits changed from highly branched shape in the early stage of the pitting process to widely grooved shape in the later stage. After SEM observation of the pits, the fractal dimension of pits was determined as a function of solution temperature 60-150 °C using perimeter-area method. The value of the fractal dimension of the pits increased with increasing solution temperature. This is caused by the increase in the ratio of perimeter to area at higher solution temperature, indicating the formation of pits with highly branched shape. The appearance of specific shape and fractal dimension value of the pits at each solution temperature implies that the formation and growth of pits proceed with the typical fractal geometry throughout the whole pitting process irrespective of pit size. In addition, constant phase element (CPE) behaviour observed from the impedance spectra is discussed in terms of the fractal dimension of pits.