Effect Of Ag On Corrosion Behavior Of Magnesium Alloys

Magnesium alloys have a wide range of applications in the fields of aviation,aerospace and automotive due to their high specific strength.However,poor corrosion resistance limits its application.As an important alloying element,Ag is widely used in magnesium alloys including QE22A alloy which has been used in aircraft and missile castings.In addition,Ag also can be used as an antibacterial element in medical devices.However,regarding the effect of Ag on the corrosion rate of magnesium alloys,many literatures have reported very little and only indicated that Ag can improve the corrosion rate of magnesium alloys.The specific corrosion mechanism and the corrosion products of Ag are not clear.In order to clarify these reasons,Mg-2Mn-xAg alloys were prepared by melt casting and their solution treatment was performed in this study.The degradation behavior of the magnesium alloys with different compositions and states in 3.5 wt.%NaCl solution and SBF solution were studied.In addition,OM,SEM,XPS,hydrogen evolution weight loss device and electrochemical workstation were used to study the effects of Ag on the microstructure,phase structure and corrosion properties of the as-cast and solid solution alloys.Corrosion behavior and mechanism were analyzed and discussed.Microstructure observation and analysis show that Mg-2Mn alloy is composed of ?-Mg matrix and Mn-rich phase.Mg-2Mn-2Ag and Mg-2Mn-4Ag alloy are composed of ?-Mg matrix,Mn-rich phase,Mg-Ag phase and Ag-rich phase.The grain size of the alloy gradually decreases with the increase of Ag element content.After solution treatment,Mg-Ag phase and Ag-rich phase in the alloy disappear,but a fine acicular and granular Mn-rich phase precipitates appear in the Mg matrix.Corrosion property tests show that the corrosion behavior of as-cast and solid solution Mg-2Mn-xAg(x=0,2,4)alloys in 3.5 wt.%NaCl solution and SBF solution is related to the content and presence of Ag element.The Mg-Ag phase and Ag-rich phase in the alloy form galvanic corrosion with the Mg matrix,thereby reducing the corrosion resistance of the alloy.The larger the Ag content in the alloy,the faster the corrosion rate.Although the potential is different between the Mg and Ag element,when the Ag element content is less than 2 wt.%,the corrosion rate does not greatly increase.On the contrary,when the Ag element is over 2 wt.%,the corrosion rate increases quickly.Through observation and analysis of the corrosion process and corrosion morphology of the alloys,it is found that the presence of Ag in the alloy enhances the cathode hydrogen evolution capability of magnesium alloys.However,the cathode hydrogen evolution capability does not increase all the time due to the corrosion products on the surface of the alloy hinder the cathode reaction.In addition,corrosion products can hinder the anode reaction of the alloy.Ag atoms are oxidized to Ag+during the corrosion process.Part of Ag+reacts with Mg and form elemental Ag that is deposited on the alloy surface,and then forms nano-sized Ag particals.Another part of Ag+reacts with Cl-to generate AgCl particals that is deposited on the alloy surface.