The present work involves the studies on the effects of various ion-additives on the localized corrosion of pure aluminium (Al) and Al-Zn-Mg alloy in aqueous chloride solutions.
Part A is concerned with the studies on the effects of hydroxide ion ($OH^-$), electrolytical charged hydrogen, i.e., proton ($H^{+}$), sulphate ($SO_4^{2-}$) ion additives on pitting corrosion of pure Al by using electrochemical quartz microbalance (EQCM) technique.
In chapter Ⅲ, effects of OH^- ion addition on the anodic dissolution of pure Al were investigated in chloride ion ($Cl^-$)-containing solution using potentiodynamic polarisation experiment, optical microscopy, ac impedance spectroscopy and EQCM technique combined with open circuit potential (OCP) transient technique. The addition of $OH^-$ ions to NaCl solution raised the anodic dissolution rate of pure Al in value and at the same time extended the constant anodic current density region in width on the polarisation curves. This implies that the pitting corrosion is preceded by uniform thinning of the Al oxide film due to the chemical dissolution by the attack of $OH^-$ ion additives, which is validated by optical microscopy. The mass change rate enhanced by the addition of 0.5 M $Cl^-$ ions during immersion in $OH^-$ ion-free 0.01 M NaCl solution indicates that $Cl^-$ ions incorporate into the growing Al oxide film even at steady-state OCP. From the lower fresh inner oxide film resistance $R_{inn,ox}$ values in $OH^-$ ion-containing chloride solution than those in $OH^-$ ion-free chloride solution below the pitting potential ($E_{pit}$), it is suggested that the chemical dissolution of the fresh inner oxide film accelerated by the attack of $OH^-$ ions inhibits the formation of the $Cl^-$ ion-incorporated outer film, what is responsible for the suppression of the pitting corrosion of pure Al.
In chapter Ⅳ, effect of prior cathodic polarization in the pitting corrosion of pure Al has been investigated in aqueous 1 M HCl solution (pH = 0) using a potentiostatic current transient technique combined with an EQCM technique, ac impedance spectroscopy and scanning electron microscopy (SEM). From the analysis of the first period of combined cathodic current transient and electrogravimetric curves, it is suggested that the hydrogen evolution current starts obeying the Tafel relation via proton absorption, formation of such hydrogen-containing species as $OH^-$ ions and/or $H_2O$ molecules within the native oxide film and proton reduction at the metal/oxide interface. During the second period appearing at potentials -1.5 and $-1.7 V_{SCE}$, hydrogen evolution may occur at an enhanced rate at the bottom of micro-pits, formed due to local breakdown of the oxide film. This breakdown was ensured by the more abrupt decrease in oxide film resistivity $\rho_{ox}$ values with increasing prior cathodic polarization, calculated at -1.5 and $-1.7 V_{SCE}$, from the analysis of the measured impedance spectra as compared to that reduction in $\rho_{ox}$ calculated at $-1.0 V_{SCE}$. The surface area of the pitted specimens was quantitatively estimated from the measured capacitance values to increase with rising prior cathodic polarization, as evidenced by SEM. This means that the exposed metal surface at the bottom of micro-pits formed during prior cathodic polarization serves as a preferential site for $Cl^-$ ion attack during the following anodic polarization. Hence, a sufficient prior cathodic polarization for the formation of the micro-pits is a necessary condition for the promotion of pitting corrosion of pure Al.
In chapter Ⅴ, effects of $SO_4^{2-}$ ion additives on the pitting corrosion of pure Al have been investigated in aqueous 0.01 M NaCl solution as a function of $SO_4^{2-}$ ion concentration using potentiodynamic polarisation experiment, ac impedance spectroscopy, EQCM technique and abrading electrode technique. The addition of $SO_4^{2-}$ ions to NaCl solution raised the $E_{pit}$ of pure Al in value and simultaneously the anodic current density at potentials much higher than the $E_{pit}$ on the polarisation curves. This implies that $SO_4^{2-}$ ions impede the initiation of pit on pure Al surface below the $E_{pit}$, but enhance the growth of pre-existing pits, which is validated by optical microscopy. It was found that the values of the $Cl^-$ ion-incorporated outer film resistance R_{out,ox} in $SO_4^{2-}$ ion-containing chloride solutions were much lower than those in $SO_4^{2-}$ ion-free solution, obtained from the impedance spectra measured at potentials below the $E_{pit}$. The chloride peak disappeared from the Auger spectra in $SO_4^{2-}$ ion-containing solutions. The mass decay rate and pit growth rate b were observed to increase in values once the pits were formed in $SO_4^{2-}$ ion-containing chloride solutions. Based upon the above experimental results, it is suggested that $SO_4^{2-}$ ions retard the oxide film breakdown by $Cl^-$ ion incorporation into the film, while they accelerate the Al metal dissolution through the instantaneous formation of tunnels at the bottom of the pre-existing pits after the exposure of bare surface above the $E_{pit}$.
Part B was aimed at investigating stress-corrosion cracking (SCC) of Al-Zn-Mg alloy in a chloride solution.
In chapter Ⅵ, effect of chromate ($CrO_4^{2-}$) addition to a chloride solution on crack growth in pre-pitted samples of Al-Zn-Mg alloy AA-7039 has been investigated using potentiodynamic polarization experiment, OCP transient during slow strain rate test (SSRT), SEM, abrading electrode technique and ac impedance spectroscopy. The addition of $CrO_4^{2-}$ ion to NaCl solution raised the OCP in value on the potentiodynamic polarization curve. The OCP transients and SEM fractographs revealed that the initiation of SC crack from pits occurs in $CrO_4^{2-}$ ion-free chloride solution during SSRT, but mechanical crack develops in $CrO_4^{2-}$ ion-containing chloride solution. From the analyses of data obtained by using abrading experiments and ac impedance spectroscopy, it was found that the repassivation of newly exposed bare surface is enhanced by adding $CrO_4^{2-}$ ion to chloride solution, which is ascribed to the formation of protective chromium oxide film on the surface. This is responsible for the hindrance to pit-to-SC crack transition of AA-7039 in $CrO_4^{2-}$ ion-containing chloride solution. Based upon the experimental results, it is suggested that the occurrence of a stress-assisted dissolution just after the mechanical exposure of the pit bottom at the transition time $t_tr$ is a necessary condition for proposed pit-to-SC crack transition mechanism.
In appendix AI, pitting corrosion of sensitized 316 stainless steel has been investigated as a function of the degree of sensitization in aqueous 0.01 M NaCl solution at room temperature. The squared rod specimens of 316 stainless steel were thermally annealed at 700℃ for various durations (0 h : non-sensitized specimen A; 8 h : moderately sensitized specimen B; 96 h : severely sensitized specimen C) and the degree of sensitization was assessed by using electrochemical potentiokinetic reactivation (EPR) test technique. The pitting corrosion resistance of the three kinds of specimens was evaluated by the potentiodynamic anodic polarization method, abrading electrode technique and ac impedance spectroscopy. The measured potentiostatic decay current transient obtained just after interrupting the abrading action showed that the repassivation rate of the oxide film on the fresh bare surface of the specimen decreased in the order of specimens A, B and C in the early stage of the film formation. From the results of ac impedance spectroscopy, the charge transfer resistance $R_ct$ and film capacitance $C_f$ of specimens B and C were evaluated and found to be lower and higher, respectively, than those of specimen A. Based upon the combined results of the abrading experiment and ac impedance measurement, it is indicated that the instantaneous formation of the passivating oxide film is more retarded by the impoverished chromium (Cr) concentration over the wider Cr-depleted zone adjacent to the grain boundaries in sequence of specimens A, B and C. This means that the thin oxide film formed on the sensitized specimens B and C is less protective than that formed on the non-sensitized specimen A.
In appendix AII, effects of lead oxide (PbO) addition on the electrochemical properties of alloy 600 have been studied in aqueous 1M NaOH solution at room temperature using surface analytical techniques and electrochemical methods. For the comparative study on the different roles of the surface layers in the electrochemical properties of alloy 600, two kinds of electrodes were potentiostatically prepared at the anodic and cathodic potential peaks observed in cyclic voltammograms obtained in PbO-containing caustic solution: lead dioxide ($PbO_2$) film-covered and lead deposit-covered alloy electrodes. The crystal structures and morphologies of the two kinds of the surface layers were characterized by X-ray diffractometry and SEM. From the OCP transients and galvanic current transients, it is inferred that nickel (Ni) and iron (Fe) among alloy 600 constituents as anodic sites are selectively dissolved in caustic solution due to the lower OCP value where there is no $PbO_2$ film compared to that of $PbO_2$ film-covered surface on the alloy. Based upon above experimental results, it is suggested that an enhanced dissolution of Ni and Fe in alloy 600 in caustic solution at room temperature is a consequence of $PbO_2$-induced corrosion.
