Analyses of electrochemical impedance spectra and electrochemical noises as examples of aqueous corrosion of pure aluminium and its alloys

Abstract

The present work involves an investigation of aqueous corrosion of pure aluminium (Al) and its alloys using electrochemical impedance spectroscopy and electrochemical noise (EN) measurement. Chapter III-1 is concerned with the analysis of electrochemical impedance spectra of the anodised pure Al foil used for Al electrolytic capacitors. The capacitance of an Al electrolytic capacitor is mainly dependent on the surface area of Al foil. In this regard, several articles have been devoted to the study of increasing the surface area by the formation of etch tunnels. Since the ions in the electrolyte diffuse through the etch tunnel for charge saturation of the electrolytic capacitor, the electrochemical behaviour of the Al electrolytic capacitor will be affected by the tunnel length distribution. In this chapter, for the numerical calculation of the total impedance of the electrode, the transmission line model was modified to reflect the etch tunnel length distribution representing the contributions of the etch tunnel length to the total impedance. From the agreement of the numerically calculated impedance spectra with the experimentally measured spectra, it was concluded that the deviation of the impedance spectra from the ideal capacitive behaviour in the high frequency region originates from the characteristic frequency dispersion which itself it dependent upon the etch tunnel length distribution. In chapter III-2, the effects of sulphate, thiosulphate and hydrogensulphate ion additives on the pitting corrosion of pure Al in 1 M NaCl solution were investigated as a function of solution temperature using double current step experiment. It is generally accepted that these ion additives have an essential role in increasing the surface area of Al foil used for Al electrolytic capacitors. From the analysis of the galvanostatic potential transients obtained from the double current step experiment, it was suggested that both sulphate and thiosulphate ions ret...