Corrosion Behavior of AZ31B Magnesium Alloy in Potassium Nitrate and Sulfate Solutions
Authors: Chang Chin-Kai , Tu Chia-Hang , Lin Chao-Sung 
Published: February 2020

In light of low density, high specific strength, and good heat dissipation properties, magnesium alloys are promising for lightweight structure applications; specifically aerospace, automotive, and 3C devices [1]. However, most magnesium alloys have poor corrosion resistance due to the high chemical reactivity and the porous corrosion product, i.e., magnesium hydroxide [2]. Better understanding of the corrosion behavior of magnesium alloys is thus an urgent necessity. Different kinds of anions existing in the service environment generally affect the corrosion behavior of metals. For example, chlorine ions accelerate the corrosion rate of magnesium. Moreover, sulfate ions or the nitrate ions in chloride-containing solutions can inhibit hydrogen evolution. They are so-called the magnesium corrosion inhibitors. Sulfate ions serve as a physical barrier to hinder chlorine ions from attacking the magnesium substrate [3]; however, the hindrance effect of nitrate ions on hydrogen evolution on magnesium is still not well understood [3].
In this research, AZ31B magnesium alloys were immersed in potassium nitrate and potassium sulfate solutions, respectively, to investigate how NO3
-­ and SO4
2- anions influence the corrosion behavior and the microstructure of the resulting corrosion product. Cross-sectional transmission electron microscopy (TEM) characterization results show that the corrosion product layer formed in the potassium sulfate solution is porous and has a non-uniform thickness ranging from 80 nm to 240 nm. In contrast, the corrosion product layer formed in the potassium nitrate solution is uniform in thickness of about 60 nm. Moreover, a thin compact layer was observed at the interface between the AZ31B magnesium substrate and the magnesium hydroxide layer. Scanning TEM - high angle annular dark field line-scan and mapping as well as X-ray photoelectron spectroscopy analysis results further identify that the thin compact layer is magnesium oxide. It is likely that the oxidation of the magnesium substrate by nitrate ions results in the formation of magnesium oxide. Compared with magnesium hydroxide, this thin magnesium oxide layer is more compact; therefore, it can protect the magnesium substrate from corrosion, which, in turn, inhibits the hydrogen evolution. The resulting corrosion product layer is thus more uniform. Finally, EIS was employed to characterize the properties of corrosion products tested in 0.05 M NaCl and 0.1 M Na2SO4 solution. The EIS data were fitted using an equivalent circuit, Rs(Qc(Rc(Qdl Rct L))), which is slightly modified from the equivalent circuit of M. Curioni et al. [4] who studied the corrosion behavior of the pure magnesium in chloride-containing aqueous and a charge transfer resistance (Rct) was adapted to account for the process of electron transfer. This modification is based on the fact that the corrosion product on AZ31B magnesium formed in potassium sulfate or potassium nitrate solution provides to some extent corrosion protection in the test solution. It was found that the AZ31B after potassium nitrate treatment has a higher R­ct value than that after potassium sulfate treatment. This can be attributed to the thin, compact magnesium oxide layer formed at the interface. As a result, the presence of nitrate anions facilitates the formation of a thin compact MgO layer and the resulting corrosion product displays a larger charge transfer in 0.05 M NaCl and 0.1 M Na2SO4 solution compared to the counterpart formed in potassium sulfate solution.

References:

[1] A. A. Luo, Jom-The Journal of the Minerals, Metals & Materials Society, 42-48, 54, 2002
[2] M. Taheri, M. Danaie, J. R. Kish, Journal of the Electrochemical Society, 89-94, 161, 2014
[3] S.V. Lamaka, B. Vaghefinazari, D. Mei, R. P. Petrauskas, D. Hoche, M. L. Zheludkevich, Corrosion Science, 224-240, 128, 2017
[4] M. Curioni, F. Scenini, T. Monetta, F. Bellucci, Electrochimica Acta, 372-384, 166, 2015