Study On Micro-galvanic Corrosion Behavior And Its In-situ Observation Of Mg-9Al Alloy With Rare Earth Addition

Magnesium aluminum alloy is one of the lighter metal structural materials in the world,and its density is only half that of aluminum alloy.The Mg-Al alloy structure material strengthened by alloying can reduce the weight of space vehicle by20%?30%,and at the same time maintain the characteristics of shock absorption and high damping.It has a broad development prospect in the fields of national defense military industry,transportation system and electronic devices.However,Magnesium element has high electrochemical activity,and there is a large potential difference between the secondary phase and?-Mg in the alloy,which is easy to form micro-galvanic corrosion,reducing the corrosion resistance of Mg-Al alloy.Micro-galvanic corrosion is a kind of corrosion form which does extensive damage and may cause serious losses.The in-situ observation of its corrosion behavior is an international frontier research topic.Therefore,how to accurately characterize the micro-galvanic corrosion behavior will help to reveal the influence mechanism of micro-galvanic corrosion on the corrosion resistance of alloys,which is an important basic scientific problem.The effects of surface micro–galvanic corrosion and corrosive film on the corrosion resistance of AZ91–x Nd alloys were evaluated by scanning electron microscopy(SEM),scanning Kelvin probe force microscopy(SKPFM),and electrochemical measurements.The microstructural morphologies showed that white acicular Al₂Nd and granular Al–Mn–Nd phases were observed in modified AZ91–x Nd Alloys.The addition of Nd to AZ91 alloy transformed?–Mg₁₇Al₁₂ phase from the agglomerated structure to the dendritic and granular structure.According to the results of SKPFM,the Volta potential of constituents in tested alloys revealed that the Volta potential of the Al–Mn–Nd(?275 m V)and Al₂Nd(?180 m V)intermetallic phases relative to?–Mg were significantly lower than Al–Mn(?560 m V)particles,but slightly higher than the?–Mg₁₇Al₁₂(?175 m V)phase.The polarization resistance(5129?·cm²)of AZ91–1.0Nd alloy was 3.9 times higher than that of unmodified alloy(AZ91,1313?·cm²),due to the suppression of micro–galvanic coupling corrosion between the secondary phases and the matrix.The main factors affecting the corrosion resistance of AZ91-x Nd alloys were as follows:(i)the suppression of surface micro-galvanic couple corrosion,since the Volta potential difference(secondary phases/?-Mg)was reduced after Nd addition,the effect of surface micro-galvanic corrosion was weakened,thereby enhancing the corrosion resistance of the tested alloy(AZ91);(ii)corrosive film,the uniform corrosive film containing Nd₂O₃ isolated the specimens and the electrolyte,which suppressed the penetration of harmful anions(OH^-,CO₃^2-)and inhibited the occurrence of surface micro-galvanic corrosion of AZ91-Nd alloys.The micro-galvanic corrosion behaviours of Mg-9Al-1Fe-1M(M=Nd,Gd)alloy were investigated by in-situ OM,quasi-in-situ SEM,in-situ AFM,scanning Kelvin probe force microscopy(SKPFM)and electrochemical measurements.Except for the conventional composition(?-Mg,?-Mg₁₇Al₁₂),the Mg-9Al-1Fe alloy consisted of some uniformly distributed Al Fe₃ phases.The addition of Nd/Gd element resulted in the formation of many Nd/Gd-rich phases(Al₂Nd/Al₂Gd,Al₈Fe₄Nd/Al-Fe-Gd)in the Mg-9Al-1Fe-1Nd/Gd alloy.Surface potential results in Mg-9Al-1Fe alloy demonstrated that the Al Fe₃ particles have the largest potential difference(?400 m V,relatively to?-Mg)and the potential difference of?-Mg₁₇Al₁₂ phase is?150 m V.With respect to?-Mg matrix,the potential difference of Nd/Gd-rich phases were significantly reduced,which were Al₈Fe₄Nd?250 m V,Al₂Nd?180 m V,Al₁₀Fe₂Gd?225 m V and Al₂Gd?195 m V,respectively.As the corrosion time went on,localised severe corrosion occurred near nobler Al Fe₃ particles at the same location,while slight corrosion occurred near Al₂Nd/Al₂Gd and Al₈Fe₄Nd/Al₁₀Fe₂Gd intermetallics.Immersion test and electrochemical tests showed that the addition of Nd and Gd significantly improved the corrosion resistance of Mg-9Al-1Fe alloy.The effect of Nd/Gd on the corrosion resistance of the studied alloys was mainly related to the following two factors:(i)the micro-galvanic corrosion between the secondary phases and?-Mg matrix;the addition of Nd/Gd reduced the potential differences between the secondary phases and?-Mg matrix,resulting in the weakening of the driving force for micro-galvanic corrosion,thereby inhibiting the micro-galvanic corrosion effect of the sample.(ii)the reduction in the volume fraction of?-Mg₁₇Al₁₂ phase;due to the consumption of Al element in the formation of Nd/Gd-rich phases,the volume fraction of?-Mg₁₇Al₁₂ phase decreased,which reduced the cathode area of the micro-galvanic couple and led to a decrease reduces the overall cathodic effect of micro-galvanic corrosion.