A Closer Look at the Role of Nanometer Scale Solute-Rich Stacking Faults in the Localized Corrosion of a Magnesium Alloy GZ31K

Abstract

The localized corrosion of as-cast Mg-3Gd-1Zn-0.4Zr (GZ31K) was investigated herein. Assessment of this alloy was carefully selected owing to the presence of chemically distinct stacking faults (SFs) in the α-Mg matrix. The role of a nanometer scale solute-rich SFs on localized corrosion was realized by quasi in-situ scanning transmission electron microscopy (STEM), supplemented by scanning electron microscopy (SEM) and electrochemical methods. It was determined that chemically distinct SFs were highly enriched in Gd and Zn. When the Mg-3Gd-1Zn-0.4Zr alloy was exposed to quiescent 0.1M NaCl, the α-Mg matrix at the periphery of solute-rich SFs was preferentially dissolved. In addition, re-deposition of Zn and Gd was observed upon the α-Mg matrix, accompanying matrix dissolution. Such findings elucidate the relative inertness of solute-rich SFs at this near atomic length scale for the first time, revealing both the nanoscale localized corrosion morphology and unambiguously revealing re-deposition of alloying elements at this scale. Distortion and disintegration of solute-rich SFs at corrosion sites indicate an evolution of stress during the α-Mg matrix dissolution and Mg oxide/hydroxide formation. Localized corrosion morphology on the micrometer scale revealed that the α-Mg matrix surrounding coarser eutectic phase particles was protected as a result of localized alkalinity by the eutectic phase.

Corrosion behavior and mechanical degradation of as‐extruded Mg–Gd–Zn–Zr alloys for orthopedic application

“…More importantly, the parallel lamellae were several nanometers wide with streaking lines between diffraction spots along the c ‐axis, but no periodic extra spots, indicating the formation of Gd and Zn atom‐rich SFs in the basal plane (Jiao et al, ; Xu et al, ; Yamasaki, Sasaki, Nishijima, Hiraga, & Kawamura, ). SFs were also detected in the as‐cast GZ31K alloy (Zhang et al, ).…”

“…However, these precipitates are ternary Mg–Gd–Zn phase and quaternary Mg–Gd–Zn–Zr phase according to EDS analysis, while those in GZ60K are binary Mg–Gd phase. It was reported that the corrosion potential of Gd was a little higher and that of Zn was much nobler than Mg–3Gd–1Zn–0.4Zr alloy (Zhang et al, ), implying that the corrosion potential difference between matrix and ternary Mg–Gd–Zn precipitates is higher than that between matrix and binary Mg–Gd precipitates. Furthermore, Zr‐rich needle‐like precipitates in Mg alloys have been confirmed to own much nobler corrosion potential than α‐Mg matrix (Coy, Viejo, Skeldon, & Thompson, ).…”

“…It has been reported that a high density of SFs in Mg–Ho–Zn (Zhang, Zhang, et al, ) and Mg–Er–Zn (Zhang, Xu, et al, ) alloys led to lower corrosion rates as compared to Mg–Ho and Mg–Er alloys without SFs. Our previous work found that SFs rich in Gd and Zn were more corrosion resistant than the matrix of as‐cast Mg–3Gd–1Zn–0.4Zr alloy based on quasi in situ TEM analysis and electrochemical measurements because the corrosion potential of Gd and Zn is nobler than the α‐Mg matrix (Zhang et al, ). The corrosion rates of the GZ60K and GZ61K alloys are relatively low in SBF calculated after 168 hr immersion test (0.45 ± 0.09 and 0.34 ± 0.13 mm/year), which can meet the corrosion rate criteria of biodegradable Mg alloys (<0.5 mm/year) proposed by Erinc et al ().…”

“…Higher Zn concentration in the GZ61K alloys is responsible for the finer grain sizes and precipitates in this work, which are considered the main factors impacting corrosion resistance. First, higher Zn concentration in matrix grains for the as‐extruded GZ61K alloy will lead to better corrosion resistance of the matrix due to higher corrosion potential of Zn (Zhang et al, ). Moreover, compared with GZ60K, the GZ61K alloy had finer α‐Mg grains and more dispersive nanoscale precipitates.…”

The Effect of Rod-Shaped Long-Period Stacking Ordered Phases Evolution on Corrosion Behavior of Mg95.33Zn2Y2.67 Alloy

“…For the post-extrusion heat-treated alloy, the LPSO phases are of different lengths without orientation distribution, as shown in Figure 2d. It was proposed by [28,29] that the LPSO phases were the effective cathode phases in the Mg–Zn–Y alloy, and corrosion generally occurred in the magnesium matrix around the LPSO phases. In this study, since LPSO phases in the post-extrusion heat-treated alloys are of regular and uniform rod shape, corrosion initiated at the adjacent region of the LPSO phases.…”