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Abstract: Two microwave absorbing coatings (MACs) are applied onto magnesium alloy substrate and are studied using electrochemical impedance spectroscopy (EIS) during immersion in 3.5% NaCl solution. The diffusion behavior of electrolyte through the MACs was firstly studied. The result shows that electrolyte transfers significantly fast in carbonyl iron loaded coating than barium ferrite loaded coating. In order to account for the distribution of different absorbents on corrosion process, a modified equivalent circuit with a distribute transmission line model was proposed to fit EIS spectra of MACs. Simulated EIS diagrams agree very well with experimental spectra of 25% and 50% PVC MACs. Moreover, simulation data was studied to analyze degeneration of two MACs. The result shows that the corrosion of CI particles plays an important role for failure of CI coating, but the degeneration of BF coatings which were slower than CI coating was ascribed to defects within the coating. [Copyright &y& Elsevier]

Abstract: Protective composite coatings were prepared on magnesium alloy AZ91D by micro-arc oxidation (MAO) treatment plus a top coating with sealing agent using multi-immersion technique under low-pressure conditions. The corrosion resistance of AZ91D alloy with composite coatings was superior evidently to that with merely MAO film. SEM observations revealed that the sealing agent was integrated with MAO film by physically interlocking; therewith covered uniformly the surface as well as penetrated into pores and micro-cracks of MAO film. The anti-corrosion properties in 3.5% NaCl solution of the composite coatings were evaluated by using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. Based on the results of chronopotentiometric (E ∼ t) and EIS measurements for long time immersion in 3.5% NaCl solution, appropriate equivalent circuits for the composite coatings system were proposed. It follows that due to the blocking effect of the sealing agent in pores and cracks in MAO film, the composite coatings can suppress the corrosion process by holding back the transfer or diffusion of electrolyte and corrosion products between the composite coatings and solution during immersion. [Copyright &y& Elsevier]

• The PAM gels is a new corrosion-triggered inhibiting structure. • The inhibitor ions Ce4+ could change the morphology of the products film. • The absorption and bridging of PAM gels about the corrosion product. Biomaterial polyacrylamide (PAM) hydrogel was first studied for self-healing coatings of Mg alloys. In this work, a PAM gel with Ce4+ inhibitor was in-situ composited on the surface of micro-arc oxidation treated AZ31 Mg alloy. The results indicated that the scratched samples with the composite coating could avoid pitting corrosions and self-heal in neutral salt spray test (NSST). XPS and SEM-EDS analysis reveal that the self-healing performance was ascribable to the inhibition of Ce4+ and the bridge adsorption of PAM gels. Moreover, the results of open circuit potential and polarization curve indicate that the self-healing product film has good corrosion resistance. [ABSTRACT FROM AUTHOR]

Abstract: After being pre-plated a zinc layer, an amorphous Al–Mn alloy coating was applied onto the surface of AZ31B magnesium alloy with a bath of molten salts. Then the corrosion performance of the coated magnesium alloy was examined in 3.5% NaCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results showed that the single Zn layer was active in the test solution with a high corrosion rate while the Al–Mn alloy coating could effectively protect AZ31B magnesium alloy from corrosion in the solution. The high corrosion resistance of Al–Mn alloy coating was ascribed to an intact and stable passive film formed on the coating. The performances of the passive film on Al–Mn alloy were further investigated by Mott-Schottky curve and X-ray photoelectron spectroscopy (XPS) analysis. It was confirmed that the passive film exhibited n-type semiconducting behavior in 3.5% NaCl solution with a carrier density two orders of magnitude less than that formed on pure aluminum electrode. The XPS analysis indicated that the passive film was mainly composed of AlO(OH) after immersion for long time and the content of Mn was negligible in the outer part of the passive film. Based on the EIS measurement, electronic structure and composition analysis of the passive film, a double-layer structure, with a compact inner oxide and a porous outer layer, of the film was proposed for understanding the corrosion process of passive film, with which the experimental observations might be satisfactorily interpreted. [Copyright &y& Elsevier]