(A) study on the relationship between passivity and pit morphology of oxide films formed on inconel alloy 600(75wt.%Ni-15wt.%Cr-8wt.% Fe) in aqueous $Cl^-$-Ion containing thiosulphate-based solutions at elevated temperatures 298 to 573 K and under elevated pressures 1 to 80 atm

Abstract

The present work involves a study on the relationship between passivity and pit morphology of oxide films formed on Inconel alloy 600 in aqueous $Cl^{-}$-ion containing thiosulphate-based solutions at elevated temperatures 298 to 573 K and under elevated pressures 1 to 80 atm. In chapter III, the effect of solution temperature on passivity of Inconel alloy 600 in aqueous 0.1 M $Na_{2}S_{2}O_{3}$ solution was investigated using potentiodynamic polarization experiment, abrading electrode technique, ac-impedance spectroscopy and atomic force microscopy. The potentiodynamic polarization curve showed that the value of current density rose at any given anodic potential with increasing solution temperature 25℃ to $0℃. The repassivation rate of the oxide film decreased with solution temperature, which is attributable to the formation of anodic oxide film with many defects as easy paths for the charge transfer in the early stage of repassivation. From the analyses of measured impedance spectra, it was found that the oxide film resistance decreased and the oxide film capacitance increased as temperature increased. This is due to the enhanced charge transfer through the less-protective anodic oxide film and the increase in the surface area of anodic oxide film, respectively. AFM images revealed that increasing solution temperature induces more roughened surface, which originates from the formation of porous film with a large number of defects. Based upon the experimental findings, it is deduced that the loss of passivity of anodic oxide film at elevated solution temperature is mainly caused by the formation of the defect-containing oxide film. In chapter IV, pit formation and growth of alloy 600 has been investigated in aqueous 0.1 M $Na_{2}S_{2}O_{3}$ + 0.1 M NaCl solution at elevated solution temperatures 298 to 573K and at pressures 0.1 to 8MPa in terms of fractal geometry using potentiodynamic polarisation experiment, potentiostatic current transien...