The corrosion mechanism of AZ31 magnesium alloy used as automobile components and the influence of the concentration of Cl- ion in simulated acid rain(SAR)were studied by electrochemical tests and SEM.The results show that pitting corrosion happens around the AlMn phases locating at the grain boundary.The corrosion of AZ31 magnesium alloy in SAR is controlled by the rate of anodic dissolution and hydrogen evolution,and the corrosion rate of AZ31 increases with increasing concentration of Cl- ion.However,the Cl- ion in SAR is not the main influencing factor inducing the pitting corrosion.
An environmentally friendly method for synthesizing a dawsonite conversion film was developed to improve the corrosion resistance of AZ31 Mg alloy. The film was prepared by two steps： the AZ31 alloy was first immersed in an Al 2 （SO 4 ） 3 solution venting CO 2 gas to form a precursor film, and then the precursor film was treated in a Na 2 CO 3 solution dissolved with Al to obtain the dawsonite film. The surface morphology of the conversion film was observed with an environmental scanning electronic microscope. The chemical composition of the conversion film was analyzed by energy dispersive X-ray spectroscopy and X-ray diffractometry. Electrochemical and immersion tests were carried out to evaluate the protection effect of the conversion film on AZ31 alloy. There are some network-like cracks on the surface of the film. The conversion film is mainly composed of dawsonite NaAlCO 3 （OH） 2 , Al（OH） 3 and Al 5 （OH） 13 （CO 3 ）·5H 2 O, which can increase the corrosion potential and reduce the corrosion current density of the Mg substrate. After immersion tests, the film almost keeps intact, except for the localized narrow areas with several corrosion pits, while the bare material undergoes serious general corrosion. It is indicated that the dawsonite film can provide good protection to the magnesium alloy.
The formation processes of a composite ceramic coating on AM50 magnesium alloy prepared by plasma electrolytic oxidation （PEO） in a K 2 ZrF 6 electrolyte solution were studied by scanning electron microscope （SEM） and energy dispersive X-ray spectroscope （EDX）. Electrochemical impedance spectroscopy （EIS） tests were used to study the variation of the corrosion resistance of the coating during the PEO treatment. The results show that the coating formed on Mg alloy is mainly composed of MgO and MgF 2 when the applied voltage is lower than the sparking voltage, and zirconium oxides start to be deposited on Mg substrate after the potential exceeding the sparking voltage. The corrosion resistance of the coating increases with increasing the applied voltage.
In this study,an environment-friendly hydrotalcite film has been deposited on AZ31 Mg alloy by a twostep technique.To improve conversion film technique and control film properties,batch studies have been carried out to address various process parameters such as immersion time,pH value,and temperature of the treatment solution.The morphologies and chemical composition were characterized by scanning electron microscopy(SEM) and energy dispersive X-ray spectroscopy(EDS).The corrosion resistance of the samples with the various final films was then further compared by polarization curves.It can be concluded from the results that,at low value of pH,temperature,or long deposition time,the precursor film is mainly composed of a cracked layer.The transformation of hydrotalcite film is largely influenced by the pH value.There are optimum values of pH,temperature of the coating bath and immersion time for the film formation process to achieve the best quality and corrosion resistance of the hydrotalcite film.The optimum process is as follows:the sample is first immersed in the pretreatment solution with a pH value of about 8 at a temperature of 60 ℃ for 30 min to form a precursor film and then this precursor film is immersed into the post treatment solution with a pH of 10.5 at 80 ℃ for 1.5 h to obtain the Mg—Al hydrotalcite conversion film.
Product/metal ratio (PMR) was introduced as a novel criterion for the evaluation of electrolytes on micro-arc oxidation (MAO) of Mg and its alloys. The criterion initially sprang from Pilling-Bedworth ratio (PBR), focused on the roles of electrolytes for the compactness of the fabricated coatings, and took attention on properties of reactants/products during MAO. Meanwhile, based on our experiments as well as the results from literatures, the effects of electrolyte additives on morphologies and com-positions of the fabricated MAO coatings of Mg alloys were exploited for verification and supplement of the initial criterion. In combination of the initial PMR criterion and experimental verification, PMR could be represented by special mode (PMRs=Voxide products/Valloy substrates) and general mode (PMRg= PMRs+ PMRd). The ideal PMRs should be between 1 and 2, while PMRd is related to the coating deposition during MAO. PMRd is a supplement to PMRs when the effect of the overlaying prop-erty (O) of the coatings and the effective deposition (D) of electrolyte composites are considered (PMRd=f(O, D). O is related to the melting point (MP) and boiling point (BP) of the MAO products. D is related to the effective reactions between alloy substrates and electrolytes during MAO.
SONG LaiWenSONG YingWeiSHAN DaYongZHU GuoYiHAN EnHou
