Research On Electrochemical Behavior Of AP65Magnesium Alloy In Sodium Chloride Solution

AP65with a nominal composition of Mg-6%Al-5%Pb (hereafter in mass fraction, except for special illustration) is a magnesium alloy used as anode for high-power seawater activated battery. When used for practical applications, AP65alloy possessing good performance is required to exhibit strong discharge activity, high Columbic efficiency, and operate at a negative discharge potential with a short activation time for the potential to reach the steady state. This paper investigates the discharge behavior of AP65alloy in sodium chloride solution using electrochemical techniques and microstructure characterizations based on the following five aspects:mechanism of activation, homogenization annealing, alloying, plastic deformation, and electrolyte solution. The aim of this paper is to enhance the comprehensive discharge performance of AP65alloy. The main work in this paper is summarized as follows:1) The activation mechanism for the main alloying elements, i.e., aluminium and lead, to the magnesium electrode is investigated. The results indicate that aluminium and lead cannot significantly enhance the discharge activity of the electrode when only one of the alloying elements exists in magnesium. However, the magnesium electrode can be dramatically activated when both of the alloying elements are added into the electrode. The activation mechanism for aluminium and lead to the magnesium electrode is dissolution-reprecipitation, and there is a synergistic effect between aluminium and lead for activating magnesium:the dissolved Pb2+cations during the discharge process can easily precipitate on the electrode surface in the form of lead oxides, and this process facilitates the precipitation of the dissolved Al3+cations in the form of Al(OH)3, which detaches the precipitated Mg(OH)2film via2Mg(OH)2·Al(OH)3and promotes the self-peeling of the discharge products; thus the discharge activity of the magnesium electrode can be enhanced.2) The effect of homogenization annealing on the microstructure and discharge behavior of AP65alloy is studied. The results show that the β-Mg17Al12phase in the as-cast alloy makes the electrode operating at a stable discharge potential and enhances its Columbic efficiency in the course of discharge at10mA/cm2. However, when the current density increases to180and300mA/cm2, the β-Mg17Al12phase hinders the discharge process and prolongs the activation time of the electrode. Moreover, a large number of the β-Mg17Al12phase particles are detached from the electrode surface during the discharge process, resulting in a decrease of the Columbic efficiency. When the as-cast alloy is homogenized at400℃for24h, the αMg17Al12phase dissolves into the magnesium matrix and the alloy presents as a single-phase equiaxed grain structure. As a result, the electrode operates at more negative discharge potentials, exhibits high Columbic efficiencies, and can be quickly activated during galvanostatic discharge at180and300mA/cm2.3) The effect of trace alloying elements on the discharge performance of the homogenized AP65alloy is investigated. The results reveal that the addition of1%zinc refines the grains of the alloy and cannot improve the discharge performance of the electrode at10mA/cm2. However, adding1%zinc shifts the discharge potentials to more negative values, shortens the activation times, and enhances the Columbic efficiencies when the electrode is discharged at180and300mA/cm2; The addition of1%tin also refines the grains of the alloy and cannot enhance its performance at10mA/cm2, however, tin strengthens the discharge activities, shortens the activation times, but decreases the Columbic efficiencies of the electrode during galvanostatic discharge at180and300mA/cm2; There is no obvious effect on the grain size of AP65alloy when added with1%indium, but indium sustains the negative discharge potential of the electrode at10mA/cm2and enhances its discharge activities and Columbic efficiencies together with shortens its activation times when the electrode is discharged at180and300mA/cm2; The addition of0.6%manganese promotes the formation of Al11Mn4and Al8Mn5phases in the alloy. The two phases cannot shift the discharge potential of the electrode to a more negative value at10mA/cm2but significantly enhace its discharge activities in the course of discharge at180and300mA/cm2. However, adding0.6%manganese prolongs the activation times and decreases the Columbic efficiencies of the electrode when discharged at180and300mA/cm2.4) The effect of multi-pass hot rolling together with subsequent annealing and single-pass hot extrusion on the microstructure and discharge behavior of AP65alloy added with0.6%manganese is studied. The results show that both multi-pass hot rolling at400℃and single-pass hot extrusion at450℃refine the grains, facilitate the compositional homogeneity of the magnesium matrix, fracture the Al-Mn phases, and produce the{0001} basal texture in the alloy. In addition, single-pass hot extrusion plays a greater role in refining the grains and fracturing the Al-Mn phases compared with multi-pass hot rolling. The dislocations arrange themselves homogeneously in the alloy and the density of dislocations decreases after single-pass hot extrusion. By contrast, multi-pass hot rolling produces a large number of dislocations and twins in the alloy. The subsequent annealing at150℃for4h decreases the dislocation density, sustains the fine grains and the{0001} basal texture caused by hot rolling. However, the subsequent annealing at350℃for4h enlarges the fine grains and weakens the{0001} basal texture. The fine grains, low density of dislocations, and good compositional homogeneity of the magnesium matrix produced by multi-pass hot rolling together with subsequent annealing and single-pass hot extrusion exert an effect on shifting the discharge potential to a more negative value, sustaining a stable discharge potential, and shortening the activation time when the electrode is discharged at a large impressed current density. Furthermore, the fine grains, fractured Al-Mn phases, low density of dislocations, and{0001} basal texture play a vital role in enhancing the Columbic efficiency of the electrode during galvanostatic discharge at a large current density.5) The effect of the salinity and temperature of the sodium chloride solution on the electrochemical corrosion behavior of the hot extruded AP65alloy added with0.6%manganese is investigated. The results indicate that the rising of the salinity enhances the discharge activities of the electrode at different current densities, making the electrode be quickly activated during the discharge process. However, the Columbic efficiencies of the electrode decrease with increased salinity. In addition, the increase of the salinity favors a uniform dissolution of the electrode during galvanostatic discharge. It is observed that when the electrode is discharged at300mA/cm2, local dissolution occurs in1.5%sodium chloride solution, and the electrode dissolves uniformly but suffers filiform corrosion when the salinity increases to3.5%, whereas the filiform corrosion disappears and only uniform dissolution takes place when the electrode is discharged in5.5%sodium chloride solution. Moreover, the temperature of the sodium chloride solution also exerts a significant effect on the electrochemical discharge behavior of the electrode. The rising of the temperature promotes the discharge activities of the electrode at different current densities, leading to a quick activation of the electrode in the course of discharge. However, when the electrode is discharged at10mA/cm2, the rising of the temperature causes a decrease of the Columbic efficiency. Besides, the electrode exhibits the lowest Columbic efficiencies in35℃electrolyte when discharged at the current densities of180and300mA/cm2. Furthermore, the decreasing in temperature favors a uniform dissolution of the electrode. It is observed that when the electrode is discharged at300mA/cm2, only small metallic particles are detached from the electrode surface in0℃electrolyte, and the electrode suffers filiform corrosion when the temperature of the electrolyte rises to25℃, whereas pitting corrosion occurs on the electrode surface when discharged in35℃electrolyte.