(A) study on the effects of solution temperature and such anion additives as $SO_4^{2-}$, $NO_3^-$ and $HCO_3^-$ ions on the morphological change of pits formed on inconel alloy 600 in $Cl^-$ ion containing solution by using fractal geometry

Abstract

Effects of sulphate ($SO_{4}^{2-}$), nitrate ($NO_{3}^{-}$) and bicarbonate ($HCO_{3}^{-}$) ion additives on the morphological change of the pits formed on Inconel alloy 600 were investigated in 0.5M NaCl solution as functions of anion concentration and solution temperature using a potentiodynamic polarization experiment, a potentiostatic current transient technique, optical microscopy (OM) and image analysis method. As solution temperature increased, the value of pitting potential shifted to more negative values and the value of current density increased at any given anodic potential due to the decrease in the oxide film passivity. Potentiostatic current transients obtained above the pitting potential revealed that the anion addition to NaCl solution impeded the pitting process due to the competitive adsorption with $Cl^{-}$ ions. On the other hand, at 60 ˚ and 100 ℃, the addition of $SO_{4}^{2-}$ ions facilitates pitting corrosion. As a result, it is inferred that the addition of anions to NaCl solution has a inhibiting effect on pitting corrosion, while pitting corrosion is accelerated by $SO_{4}^{2-}$ ions in NaCl solution at elevated solution temperature. In addition, from the fact of the increase in the total charge consumed during the pitting process with rising solution temperature, it is realized that increasing solution temperature causes more serious pitting phenomena. After OM observation of the pits, the fractal dimension $D_{f}$ of the pits was determined as functions of solution temperature and anion concentration using perimeter area method. The value of $D_{f}$ 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 micro-branched shape. It was observed that the anion addition to NaCl solution reduced the value of $D_{f}$ of the pits. This means that anion additives diminish the ratio of pit perimeter to pit...