In situ aqueous stability of Mg-Li-(Al-Y) alloy: role of Li

Abstract

The role of lithium in the mechanisms of aqueous stability of Mg-Li alloys was explored by combining in situ and ex situ surface and solution characterization. In situ surface evolution of a corrosion resistant Mg-Li(-Al-Y-Zr)-alloy in aqueous NaCl solution was studied by confocal Raman Microscopy and Kinetic Raman Mapping [1], real time solution analysis was made with Atomic Emission SpectroElectroChemistry [2]. Additional ex situ surface characterizations by Photoluminiscence Spectroscopy, Auger Electron Spectroscopy and Glow Discharge Optical Emission Spectroscopy were made immediately after the aqueous exposure and after exposure to ambient air. In situ analyses demonstrated that both Li and Mg dissolved in aqueous solutions from visually intact anodic areas, leaving a Li-depleted metallic layer under an approximately 100 nm thick Li-doped MgO. Interestingly, in these areas the growth of magnesium hydroxide (Mg(OH)2) was very slow than in pure Mg, suggesting that the kinetics of the transformation MgO→Mg(OH)2 was strongly inhibited. On the cathodic areas, local accumulation of Li2[Al2(OH)6]2·CO3·nH2O (Li-Al layered double hydroxide), LiAlO2, Y2O3 and Mg(OH)2 was observed. Li2CO3, previously considered as a component of a protective film responsible for corrosion resistance of Mg-Li alloys [3], was not present in situ on the surface evolving in aqueous solution and was detected only ex situ after the exposure to ambient air. The proposed corrosion mechanism attributes the improvement of aqueous corrosion resistance of Mg-Li alloys to the increase of the chemical stability of MgO doped by Li+ [4]. The latter could be formed thanks to selectively leached in the solution Li+. Additionally, cathodic activation of Mg can be reduced on Li-doped MgO and Li-Al layered double hydroxide, detected in situ on cathodic areas. Pilling Bedworth ratios (PBR), calculated for lithium doped MgO film on Mg-Li alloys with different Li content, demonstrated that the condition of a protective film on Mg-Li alloy (PBR>1) requires a minimal Li concentration in the alloy close to 15-18 at. %. [1] A. Maltseva, V. Shkirskiy, G. Lefevre, P. Volovitch, Corrosion Science, 153 (2019) 272-282. [2] Y. Yan, P. Zhou, O. Gharbi, Z. Zeng, X. Chen, P. Volovitch, K. Ogle, N. Birbilis, Electrochemistry Communications, 99 (2019) 46-50 [3] L. Hou, M. Raveggi, X.-B. Chen, W. Xu, K.J. Laws, Y. Wei, M. Ferry, N. Birbilis, Journal of The Electrochemical Society, 163 (2016) C324-C329 [4] Y.M. Yan, A. Maltseva, P. Zhou, X.J. Li, Z.R. Zeng, O. Gharbi, K. Ogle, M. La Haye, M. Vaudescal, M. Esmaily, N. Birbilis, P. Volovitch, Corrosion Science, 2020, 164, 108342