Corrosion behavior of Mg-Gd-Zn based alloys in aqueous NaCl solution

Abstract

The corrosion behavior of Mg-10GdexZn (x ¼ 2, 6 wt.%) alloys in 0.5 wt.% NaCl solution was investigated. Microstructures of both the alloys consisted of (Mg,Zn)3Gd phase and lamellar long period stacking ordered (LPSO) phase. The morphology of the second phase at the grain boundary differed in both alloys: it was a continuous network structure in Mge10Gde6Zn, whereas it was relatively discrete in Mge10Gde2Zn. The dendrites were finer in size and highly branched in Mge10Gde6Zn. The corrosion results indicated that the increase in Zn content increased the corrosion rate in Mge10GdexZn alloys. Micro-galvanic corrosion occurred near the grain boundary in both alloys initially as the grain boundary phase was stable and acted as a cathode, however, filiform corrosion dominated in the later stage, which was facilitated by the LPSO phase in the matrix. Severe micro-galvanic corrosion occurred in Mge10Gde6Zn due to the higher volume of second phase. The stability of the second phase at the grain boundary was altered and dissolved after the long immersion times. Probably the NaCl solution chemically reacted with the grain boundary phase and de-stabilized it during the long immersion times, and was removed by the chromic acid used for the corrosion product removal. Copyright 2014, National Engineering Research Center for Magnesium Alloys of China, Chongqing University. Production and hosting by Elsevier B.V. Keywords: MgeGdeZn alloys; Micro-galvanic corrosion; Polarization; Electrochemical characterization 1. Introduction Magnesium alloys have many attractive properties such as low density, high specific strength, good castability, excellent machinability and weldability. However, its low resistance to creep and corrosion are two important drawbacks. Magnesium alloys have poor corrosion resistance compared to aluminum alloys as magnesium is a reactive metal. Designing magnesium alloys with high corrosion and creep resistance, therefore, is a challenging task. The most widely used MgeAl based alloys such as AZ91, AM50, exhibit high corrosion resistance, but, the poor creep resistance limits their applications at elevated temperature. Many alloys developed for high temperature applications such as power train components in automobiles failed to make an impact as most of these alloys display poor corrosion resistance. Rare earth (RE) containing Mg alloys exhibiting superior high temperature properties are considered as potential candidates for automobile applications [1]. However, the corrosion behavior of Mg-RE alloys is not well understood. It is well documented that the RE additions improve the corrosion resistance of MgeAl based alloys [2,3]. Addition of RE reduces the b-Mg17Al12 phase in these alloys thereby reducing the micro-galvanic sites that results in improved corrosion resistance. Additionally, RE can react with impurities such as Fe and Cu resulting in a ‘cleaning effect’ of the melt [4]. * Corresponding author. CSIR-National Institute for Interdisciplinary Science and Technology, Pappanamcode (P.O), Thiruvananthapuram 695 019, India. Tel.: þ91 471 2515248; fax: þ91 471 2491712. E-mail addresses: srininiist@gmail.com (A. Srinivasan), carsten.blawert@ hzg.de (C. Blawert), yuanding.huang@hzg.de (Y. Huang), chamini.mendis@ hzg.de (C.L. Mendis), karl.kainer@hzg.de (K.U. Kainer), norbert.hort@hzg. de (N. Hort). Peer review under responsibility of National Engineering Research Center for Magnesium Alloys of China, Chongqing University. HOSTED BY Available online at www.sciencedirect.com ScienceDirect Journal of Magnesium and Alloys 2 (2014) 245e256 www.elsevier.com/journals/journal-of-magnesium-and-alloys/2213-9567 http://dx.doi.org/10.1016/j.jma.2014.08.002. 2213-9567/Copyright 2014, National Engineering Research Center for Magnesium Alloys of China, Chongqing University. Production and hosting by Elsevier B.V. Open access under CC BY-NC-ND license