Galvanic Corrosion Mechanism Of Magnesium Alloy

The magnesium alloy plays an important role in automobile,aviation and other fields because of their excellent properties such as low density,high specific strength and easy processing.However,the standard electrode potential of magnesium alloy is extremely low,and galvanic corrosion is prone to occur when contacting with other metals.In this paper,we established the galvanic system of AZ91D magnesium alloy and 2002 aluminum alloy and explored mechanism.The main research works and results were as follows:1.The galvanic corrosion behavior between the AZ91D magnesium alloy and2002 aluminum alloy in the pure water was explored by using the electrochemical method and surface analysis method.The results showed that the sum of the diameters of capacitive reactance arcs under the coupling condition was significantly larger than the sum of the diameters of capacitive reactance arcs under the uncoupling condition and the impedance modulus value increased accordingly.The main reason for this phenomenon was that the galvanic effect between them caused more Mg（OH）₂film formation.With the prolongation of immersion time,the galvanic current density between the magnesium alloy and aluminum alloy first decreased and than increased after coupling and the galvanic potential shifted positively and then gradually shifted negatively.The decrease of galvanic current density might be attributed to the increase of cathode and anode resistance by the accumulation of corrosion products on the cathode and anode.The results obtained by the SEM and XPS indicated that the corrosion products of AZ91D magnesium alloy in pure water were composed of dense internal corrosion product films and relatively loose external corrosion product films under the coupling condition.2.The galvanic corrosion behavior between the AZ91D magnesium alloy and2002 aluminum alloy in chlorine-containing solution was explored by electrochemical method and surface analysis method.（1）In the simulated tap aqueous solution（0.5ml/L Na Cl solution）,Their corrosion potentials shifted positively and the corrosion rate increased under the coupling.The main reason for this phenomenon was that the galvanic effect between them accelerated the cathodic process of galvanic corrosion and inhibited the formation of passivation film on the surface of aluminum alloy.With the prolongation of immersion time,the galvanic potential between them first shifted positively and then gradually shifted negatively,the galvanic current density first increased and then decreased,and finally increased gradually and reached a stable state.The main reason for this phenomenon was that the coupling effect between them promoted the corrosion of magnesium alloy.With the prolongation of immersion time,the corrosion products on the surface of magnesium alloy continued to accumulate and the reduction of active anode surface caused a reduction in corrosion rate.During the later stage of immersion,the corrosion product film on the surface of magnesium alloy was ruptured by the cathode hydrogen evolution,at the same time,the p H of the solution increased and the hydrogen generated by the dissolution of aluminum led to the destruction of the surface corrosion product film,which accelerated its galvanic corrosion.（2）In simulated seawater solution（3.5%Na Cl solution）,The corrosion potentials of magnesium alloy and aluminum alloy shifted significantly negatively and the corrosion rates increased under the coupling.It indicated that the increase in chloride ion concentration significantly accelerated their anodic oxidation processes.Moreover,the potential difference between them decreased under the coupling condition,the main reason for this phenomenon was that the significant negatively shifted in the corrosion potential of aluminum alloy.In the initial stage of corrosion,the galvanic corrosion between them was accelerated due to the galvanic effect between them.With the prolongation of immersion time,the anodic process was gradually inhibited by the continuous accumulation of corrosion product films on the surface of magnesium alloy,which resulted in a slow positive shift of galvanic potential and a decrease in the galvanic current density.With the further prolongation of immersion time,on the one side,due to the accumulation of corrosion products,on the other side,the corrosion product film on the surface of magnesium alloy by the cathodic hydrogen evolution and chloride ions,the galvanic current density gradually reached a relatively stable state eventually.3.The effect of corrosion products on the galvanic corrosion behavior between AZ91D magnesium alloy and 2002 aluminum alloy was explored by electrochemical method and surface analysis method.The results showed that the corrosion products formed by pre-corrosion have a certain protective effect at the initial stage of corrosion,which led to a positive shift of corrosion potential,The galvanic effect between them accelerated the corrosion of magnesium alloy when the pre-corroded magnesium alloy and aluminum alloy were coupled.As the corrosion continued,the galvanic corrosion between them was accelerated by the cathodic hydrogen evolution and the cathodic corrosion of aluminum alloy led to loosening of corrosion products.