Key words: aluminum-magnesium alloy; sacrificial anode; anode phase; electrochemical performance
Aluminum anodes have the advantages of large theoretical power generation, abundant resources, and simpler manufacturing processes, and are theoretically good sacrificial anode materials. However, aluminum anodes are susceptible to passivation during use, resulting in lower current efficiency [1]. An effective way to improve the performance of aluminum anodes is to add active elements to pure aluminum. The main alloying elements added are Zn, In, Cd, Sn, Mg, Bi, Hg, and the like. However, some alloying elements such as Cd and Hg can pollute the environment. Therefore, the selection of suitable additive elements has become one of the research focuses of aluminum anodes at home and abroad [2-4]. The Al-Zn-In anode is an active aluminum anode. Based on this, it adds other alloying elements such as Si, Sn, and Mg, and has also developed a series of anode materials [5-8]. The addition amount of Mg is generally w (Mg) ≤ 2% [9-11], and there are few reports on the study of the effect of Mg element on aluminum anode activation.
In this paper, based on the principle of alloy phase electrochemistry [12], an easily activated and dissolved Mg element (ESCE=-1.5 V) is added to pure aluminum (ESEC=-0.78 V) that is easily passivated on the surface to produce an oxide film. The microstructure, open-circuit potential and polarizability of the alloys were investigated with increasing Mg content to investigate the electrical behavior of α(Al) solid solution and intermetallic Mg5Al8 phase in Al-Mg alloy.
1 Test method
1.1 Aluminum alloy smelting
The test raw materials used industrial pure aluminum l A85 and l pure magnesium. Samples No. 1 to No. 5 are respectively (the number before the element symbol is the mass fraction of this element): Al-1Mg, Al-5Mg, Al-10Mg, Al-20Mg, Al-30Mg. According to the mass fraction, alloy elements were weighed, the aluminum ingots were placed in a crucible, placed in an electric resistance furnace, completely melted at 760° C., and magnesium ingots were added, and a flame retardant covering agent was quickly sprinkled evenly on the melt surface. After the Mg is completely melted, it is stirred evenly, refined with ZnCl2, cast into an ingot of Φ20 mm×120 mm, and naturally cooled.
L.2 Microstructure analysis
The tissue was observed using an optical microscope.
1.3 Measurement of Open Circuit Potential
The sample size is Φ20mm×10mm. A hole is drilled at one end of the sample to draw a copper wire. The other end surface was leveled and smoothed with 240#, 360#, 500#, and 800# water sandpaper, and then deoiled with acetone to leave a working area of ​​100mm2. The rest of the specimen was sealed with paraffin. The measuring device adopts a three-electrode measuring system. The auxiliary electrode is a Pt electrode, and the reference electrode is a saturated calomel electrode. The test medium is NaCl solution, C (NaCl) = 3.5%.
1.4 Measurement of Anodic Polarization Curves
Beijing Pei company PS-268A electrochemical measurement system. After the open circuit potential of the sample is stabilized, the polarization curve is measured by a potentiometric scanning method. The electrode area was set to 100 mm2, the scanning speed was 10 mV/min, and the scanning interval Ek was -20 mV to -500 mV.
2 test results and analysis
2.1 Microstructure
The solubility of magnesium in aluminum is very large. It can be seen from the Al-Mg binary state diagram [13] that at the eutectic temperature, the larger the solubility is 17.4%. At 450° C. at w(Mg)=35.0%, a eutectic reaction occurs: L=α(Al)×β(Mg5Al8). When the Mg content is high, the microstructure characteristics at room temperature are α(Al) phase and β(Mg5Al8) phase eutectic. FIG. 1 is a metallographic photograph of No. 1 to No. 5 specimens. It can be seen from the figure that the second phase β is dispersedly distributed on the α solid solution, and the second phase is either granular or branched. With the increase of Mg content, the number and area of ​​the second phase also increase greatly, and the interval between phases decreases. Especially when w(Mg)≥20%, the second phase distribution is more dense. Because the intermetallic compound is brittle, when the β(Mg5Al8) phase is uniformly and uniformly distributed in the aluminum matrix, it is an ideal microstructure state, otherwise the ingot is brittle and has adverse effects on the use properties of the aluminum anode material. .
Fig. 1 Micrograph of the microstructure of Al-Mg anode material
2.2 Measurement of open circuit potential
Figure 2 shows the open circuit potential of each sample in NaCl solution at room temperature. As can be seen from the figure, the open circuit potential of the alloy gradually shifts negatively with increasing Mg content. The open-circuit point of sample No. 5 with w(Mg)=30% can reach -1.880V. At present, the open circuit potential of aluminum alloy anode materials used in China is between -1.88 VSCE and -1.10 VSCE [14]. The open circuit potential of the aluminum anode has been brought to this range when the Mg content is high. The Mg5Al8 phase formed in the aluminum-magnesium alloy has an anodic character with an ESCE of -1.24V [15]. When the amount of Mg in the aluminum matrix is ​​high, the area of ​​the Mg5Al8 phase increases, and the potential of the α(Al) solid solution containing Mg also negatively shifts, thereby negatively shifting the anode potential, indicating that the alloying element Mg has a better effect on the aluminum anode. Activation.
Figure 2 Open circuit potential of different magnesium content samples
2.3 Open circuit potential stability
The potential stability test results of the high Mg aluminum anode in NaCl solution are shown in Fig. 3. The potential of No. 5 sample in the medium was stable to -1.18 V. After a measurement of as long as 50 h, the change was slight. The reason is that when w(Mg) (0.6%), the oxide film structure formed on the surface of the material is MgO solid solution in Al2O3 and is easily passivated. When w(Mg)=1.0%~1.5%, the oxide film is made of aluminum. In combination with magnesium oxide, the compactness deteriorates, and the higher the Mg content, the worse the compactness of the oxide film, which makes the surface of the anode material less susceptible to passivation, which also indicates that when the Mg content is high, the aluminum anode is activated.
Fig. 3 Change of open circuit potential of Al-30Mg sample at room temperature over time
2.4 Measurement of Anodic Polarization Curves
Figure 4 shows the polarization curve of each sample. It can be seen from FIG. 4 that the starting potential of the anode material is negatively shifted as the Mg content increases. With the increase of potential, the current density of each anode increases sharply and enters the activated state, and the curves of #4 and #5 samples are relatively flat, the activation area is longer, and the polarizability of the alloy is lower, indicating the content of Mg elements. When it is larger, it has a good activation effect on the aluminum anode. When w(Mg)=10%, the polarizability is large and passivation occurs when the current is small. The activation regions of Al-10Mg and Al-5Mg are short and the potential fluctuates greatly. As the potential increases, each anode sample enters the passivation zone. In the passivation region, Al-10Mg has a small passivation current density, followed by Al-5Mg, which can be considered as a more stable film at this time. The Al-30Mg has a larger blunt current density, and is less easily passivated than Al-10Mg and Al-5Mg, and has better performance. This proves that when w(Mg)=10%, it has an activating effect on the aluminum anode.
Fig. 4 Polarization curves of Al-Mg alloys in 3.5% NaCl solution at room temperature
The inflection points of Al-20Mg and Al-30Mg when entering the passivation zone indicate that the active dissolution of these two samples is not uniform enough. This is mainly due to the unbalance between the anode phase of the compound β(Mg5Al8) and the microscopic phase of the α(Al) matrix in the alloy. Selective dissolution occurs in the medium [12], and the surface potential or negative phase of the surface potential is dissolved first. This, in turn, leads to a rise in the potential, after which the positively charged active source begins to dissolve.
3 conclusions
(1) As can be seen from the as-cast microstructure of the prepared Al-Mg alloy, the precipitated Mg5Al8 phase in the alloy increases with increasing Mg content, and is distributed uniformly.
(2) Adding alloying element Mg in pure aluminum, the potential is negatively shifted, and it is relatively stable. The negative transfer value of potential is related to the content of Mg. When w(Mg) = 30%, the open circuit potential reached - 1.180 V, indicating that the alloying element Mg has an activating effect on the aluminum anode.
(3) When the alloy is polarized, the local potential in the microstructure of the alloy with higher Mg content is unbalanced, and the part with a more negative potential preferentially dissolves, resulting in a significant inflection point on the polarization curve. This requires the addition of other activating elements and homogenization treatments based on the Al-Mg binary alloy in future studies, and it is hoped that this phenomenon can be reduced or eliminated.