Corrosion And Protection Of Mg/Al Material On Atmospheric Environmental

As green engineering material,magnesium and magnesium alloys are the lightest metal structural materials in the world and have important applications in many fields such as aerospace,3C,automotive industries.However,the poor corrosion resistance is the main problem restricting the development of magnesium and its alloys and the corrosion of magnesium and its alloy exposed to atmospheric environment accounts for more than half of the toal corrosion.As well known,the nature of the atmospheric corrosion of materials is the electrochemical behavior under thin electrolyte layer（TEL）.Hence,discussing the effects of air pollutants on the corrosion behavior of magnesium alloy is necessary and important.Chemical conversion film is a common surface treatment technology to magnesium alloy,which can solve effectively the poor corrosion resistance of magnesium alloy.The main contents are listed as follows:1.The influences of thin electrolyte layer thickness,NaHCO₃ concentration and immerse time on the corrosion behavior of AZ91D magnesium alloy under thin electrolyte layers（TEL）were discussed by means of cathodic polarization curve,electrical impedance spectroscopy（EIS）,and characterized using scanning electron microscopy（SEM）,energy dispersive spectrometry（EDS）and X-ray diffractometer（XRD）.The results showed that the cathode and anode reactions were both suppressed,and the corrosion rate was depressed with the TEL decreasing.Under TEL（200μm）,increasing the NaHCO₃ concentration accelerated the corrosion of AZ91D alloy.After24 h immersion,the corrosion morphology in bulk solution and TEL were general corrosion and localized corrosion respectively.Combined with the corrosion products analysis,the corrosion model for AZ91D under TEL containing NaHCO₃ was proposed to explain the effect of NaHCO₃ on the corrosion behavior of AZ91D alloy.2.The effects of NH₄⁺content,different anions of ammonium salts on the electrochemical corrosion behavior of AZ91D magnesium alloy under bulk solution and TEL（200μm）were comparatively analyzed by means of cathodic polarization curve,EIS and SEM.The results showed that increasing the NH₄⁺content of bulk solution and TEL aggravated the corrosion of AZ91D alloy and there was a synergistic effect on corrosion of AZ91D alloy between NH₄⁺and Cl^-,SO₄^2-,NO₃^-respectively.Meanwhile,based on the corrosion behavior of AZ91D alloy under different thin electrolyte layer thickness containing NH₄⁺and the results of SEM and XRD,the corrosion of AZ91D alloy was divided into three regions:general corrosion area,localized corrosion region and corrosion inhibition area.The correspondent corrosion models for AZ91D under the three regions were proposed.3.The effect of processing parameters to calcium phosphate coating on the morphology and corrosion resistance were investigated by SEM,potentiodynamic curve and EIS.The results indicated that the leaf-like coating on AZ91D alloy possessed the best corrosion resistance when the AZ91D was immersed in phosphating bath of Ca（NO₃）₂·4H₂O（25 g/L）:NH₄H₂PO₄（15 g/L）=5:3（pH=2.8）for 20min at static condition of room temperature.The EDS analysis and XRD showed the main composition of the calcium phosphate conversion coating was CaHPO₄·2H₂O.Based on the above results,the formation mechanism of phosphate conversion coating on AZ91D magnesium alloy was proposed.