Effect of Zn Existence on Mg Corrosion Due to Cu
Authors: Ishimaru Hiroaki , Morishige Taiki , Takenaka Toshihide 
Published: February 2020

INTRODUCTION

Mg alloys are attractive materials due to its lightness and excellent mechanical properties. Recycling of Mg alloy is meaningful because secondary Mg alloy is produced with few energy than its virgin metal. However, secondary Mg alloy easily contains some impurities elements which degrade the corrosion property. Cu is one of undesirable impurities, and its tolerance limit is 1000 ppm in pure Mg. Since it is difficult to remove Cu from Mg alloy because of it chemical affinity with Mg, another method to reduce the harmful influence of Cu must be considered.
In this study, the corrosion behaviors of Mg-Cu and Mg-Zn-Cu were investigated, and the inhibitory effect on Mg corrosion due to Cu by Zn was discussed.

EXPERIMENTAL PROCEDURE

Mg-Zn, Mg-Cu and Mg-Zn-Cu alloys shown in Table 1 were prepared by electric furnace. The specimens were immersed for 1h, 2h, 4h and 6h in 5 mass% NaCl solution, and the specimens before and after the immersion test were observed and analyzed by OM, SEM-EDX and XRD. The corrosion rate were also estimated by polarization measurement.

RESULTS AND DISCUSSION

Mg2Cu was detected by XRD in the Mg-Cu specimens, while the peak of Mg21Zn25 was observed in the Mg-Zn and the Mg-Zn-Cu specimens. Fig.1 shows the SEM images and the element distributions of the Mg-6mass%Zn-0.1mass%Cu specimens by EDX. In the specimens, the deposits containing Mg and Zn were seen, and Cu was condensed locally in the deposit. Considering the results of XRD and EDX, Mg21Zn25 containing Cu was formed.
After the immersion test for 1h, local corrosion was observed in the Mg-Cu and the Mg-Zn-Cu specimens by OM. The deposits remained on the surface, while the matrix around the deposit was corroded. This result indicates that the micro-galvanic corrosion occurred and that the deposit acted a cathodic phase. Only local corrosion was seen in the Mg-Cu specimens, while general corrosion occurred on the Mg-Zn-Cu specimens. After the immersion test for 6h, general corrosion were seen in all the specimen.
The corrosion rates of the specimens were estimated by polarization measurement as shown Fig.2. The more the Cu content was, the bigger the corrosion rate became. Zn addition decreased the corrosion rates overall. When the Cu content was below 2000 ppm in the specimen containing 6 mass% Zn, the corrosion rate was kept less than commercial AZ31.
Even though micro-galvanic corrosion occurred in both the Mg-Zn-Cu and Mg-Cu specimen, the corrosion rate of the former was smaller than the latter. The difference in the corrosion rate can be explained by the change in the deposit: Mg21Zn25 containing Cu in the former and Mg2Cu in the latter. From the electrochemical property of Zn and Cu, it is presumed that Mg21Zn25 containing Cu is baser than Mg2Cu. Under this consideration, the potential defference between the deposit and the matrix in Mg-Zn-Cu alloy should be smaller than that in Mg-Cu alloy, and so micro-galvanic corrosion could be inhibited. It is concluded that the corrosion property of Mg due to Cu can be improved by the existence of Zn.

CONCLUSIONS

In this study, the corrosion behaviors of Mg-Cu and Mg-Zn-Cu alloys were investigated and the inhibitory effect on Mg corrosion due to Cu by Zn was discussed. Mg2Cu deposit and Mg21Zn25 deposit containing Cu were found in Mg-Cu alloy and Mg-Zn-Cu alloy, respectively, and micro-galvanic corrosion occurred in both alloys. The corrosion rate of Mg-Zn-Cu was smaller than that of Mg-Cu, and the difference in the corrosion rate was considered due to the difference in the deposit in the alloys. The corrosion rate of Mg-6mass%Zn containing 2000 ppm-Cu was controlled less than that of AZ31.

Figure 1