Abstract
In this work, the effects of the microstructure and phase constitution of cast magnesium alloy ZK60 (Mg-5.8Zn-0.57Zr, element concentration in wt.%) on the corrosion behavior in aqueous NaCl (0.1 mol dm−3) were investigated by weight-loss measurements, hydrogen evolution tests, and electrochemical techniques. The alloy was found to be composed of α-Mg matrix, with large second-phase particles of MgZn2 deposited along grain boundaries and a Zr-rich region in the central area of the grains. The large second-phase particles and the Zr-rich regions were more stable than the Mg matrix, resulting in a strong micro-galvanic effect. A filiform corrosion was found. It originated from the second-phase particles in the grain boundary regions in the early corrosion period. The filaments gradually occupied most areas of the alloy surface, and the general corrosion rate decreased significantly. Corrosion pits were developed under filaments. The pit growth rate decreased over time; however, it was about eight times larger than the general corrosion rate. A schematic model is presented to illustrate the corrosion mechanism.
Highlights
Magnesium (Mg) alloys have been widely applied as lightweight engineering materials due to their unique properties [1,2,3,4,5,6,7,8]
A thin oxide film can be formed on the surface of the cast ZK60 alloy in a moist atmosphere at room temperature according to Reactions (5) and (6), which may have a bilayer structure with an inner room temperature according to Reactions (5)–(6), which may have a bilayer structure with an inner layer of MgO and an outer layer of Mg(OH)2 [72,73]
The microstructure of the cast ZK60 alloy was composed of an α-Mg phase and large second-phase particles (MgZn2 ), which mainly deposited along the grain boundaries, and a Zr-rich region existed in the central area of the grains
Summary
Magnesium (Mg) alloys have been widely applied as lightweight engineering materials due to their unique properties [1,2,3,4,5,6,7,8]. The microstructure [13,14], mechanical properties [15,16,17], and biological applications [18,19] of ZK60 alloys have been studied widely in recent decades. The weak corrosion resistance of ZK60 alloys limits their further applications. The microstructure of Mg alloys, especially their second phases, has an evident impact on their corrosion behavior [3,5]. In Mg-Al alloys, when the amount of aluminum (Al) is low (e.g., Mg-5Al), the β-phase (Mg12 Al17 ) is relatively discontinuous in the Mg matrix and mainly acts as a cathode
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