Microstructure and Corrosion Behavior of the As-Extruded Mg-4Zn-2Gd-0.5Ca Alloy

doi:10.1007/s40195-019-00984-2

Abstract

Abstract:

In order to study the corrosion resistance of extruded magnesium alloys, the Mg-4Zn-2Gd-0.5Ca alloy was extruded at the speed of 0.01-0.1 mm/s with the temperature of 280-360 °C in present study. Hot extrusion results show that the volume fraction of precipitates (V_pre), V_DRX (the dynamic recrystallization rate) and the average size of DRXed grain (d_DRX) decrease with the decrease in extrusion speed, and the corrosion rate of the alloy also shows a downward trend. On the contrary, the values of V_pre, V_DRX and d_DRX increase with the increase in extrusion temperature, and the corrosion resistance of Mg-4Zn-2Gd-0.5Ca alloy decreases. When the extrusion speed is 0.01 mm/s and the extrusion temperature is 280 °C, the alloy has the best corrosion resistance. The corrosion of extruded Mg-4Zn-2Gd-0.5Ca alloy occurs preferentially on the magnesium matrix around W and I phases in the DRXed zone. With the further corrosion, the corrosion continues to spread along the phase, and the corrosion area gradually increases. Galvanic corrosion plays a leading role in the corrosion process. Moreover, there are a large number of basal plane textures in the unDRXed region, which is conducive to improving the corrosion resistance of magnesium alloys. In addition, the decrease in grain size also makes the corrosion of magnesium alloy more uniform.

Key words: Mg-4Zn-2Gd-0.5Ca alloy, Extrusion, EIS (electrochemical impedance spectroscopy), Weight loss, Corrosion resistance

Hao-Yi Niu, Fang-Fang Cao, Kun-Kun Deng, Kai-Bo Nie, Jin-Wen Kang, Hong-Wei Wang. Microstructure and Corrosion Behavior of the As-Extruded Mg-4Zn-2Gd-0.5Ca Alloy[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(3): 362-374.

Figures/Tables 16

Fig. 1 Schematic for the experimental device in this study

Table 1 Actual chemical compositions of Mg-4Zn-2Gd-0.5Ca alloy (wt%)

Fig. 2 OM of as-extruded Mg-4Zn-2Gd-0.5Ca alloy: a, g 280 °C, 0.1 mm/s, b, h 280 °C, 0.05 mm/s, c, i 280 °C, 0.01 mm/s, d, j 320 °C, 0.01 mm/s, e, k 360 °C, 0.01 mm/s, fVDRX and dDRX of the Mg-4Zn-2Gd-0.5Ca alloy, l XRD pattern of as-extruded Mg-4Zn-2Gd-0.5Ca alloy

Fig. 3 SEM microstructures of as-extruded Mg-4Zn-2Gd-0.5Ca alloy: a, b 280 °C, 0.1 mm/s, c, d 280 °C, 0.05 mm/s, e, f 280 °C, 0.01 mm/s, g, h 320 °C, 0.01 mm/s, i, j 360 °C, 0.01 mm/s

Table 2 EDS analysis results of as-extruded Mg-4Zn-2Gd-0.5Ca alloy (at.%)

Fig. 4 TEM image of as-extruded 360 °C 0.01 mm/s Mg-4Zn-2Gd-0.5Ca alloy, and b the corresponding EDS of the precipitate pointed by the yellow arrow in a

Fig. 5 OCP curves of as-extruded Mg-4Zn-2Gd-0.5Ca alloy immersed in 3.5 wt% NaCl solution for 30 min

Fig. 6 EIS of as-extruded Mg-4Zn-2Gd-0.5Ca alloy immersed in 3.5 wt% NaCl solution for 40 min. a Nyquist diagrams, b Bode plots of frequency versus Z and c Bode plots of frequency versus phase angle, d equivalent circuit of as-extruded Mg-4Zn-2Gd-0.5Ca alloy. Where the Rs represents the solution resistance; CPE represents the constant phase element; Rct represents the charge transfer resistance; Rf represents the film resistance; L represents the inductance; and RL represents the induction resistance

Table 3 EIS data parameters obtained from EIS analysis of as-extruded Mg-4Zn-2Gd-0.5Ca alloy immersed in 3.5 wt% NaCl solution for 40 min

Fig. 7 a Potentiodynamic polarization curves and b 1/Rp of as-extruded Mg-4Zn-2Gd-0.5Ca alloy immersed in 3.5 wt% NaCl solution for 1 h

Table 4 Corrosion potential (Ecorr), cathodic slope (βc) and corrosion current density (icorr) obtained from polarization curves of as-extruded Mg-4Zn-2Gd-0.5Ca alloy immersed in 3.5 wt% NaCl solution for 1 h

Fig. 8 Immersion test result a hydrogen evolution rate plot and b weight loss of as-extruded Mg-4Zn-2Gd-0.5Ca alloy immersed in 3.5 wt% NaCl solution for 148 h

Fig. 9 Section corrosion morphology of as-extruded Mg-4Zn-2Gd-0.5Ca alloy immersed in 3.5 wt% NaCl solution for 148 h: a low magnification; b high magnification

Fig. 10 Curves of corrosion rate of Mg-4Zn-2Gd-0.5Ca alloy under different extrusion conditions with Vpre, VDRX and dDRX

Fig. 11 Corrosion morphologies of as-extruded Mg-4Zn-2Gd-0.5Ca alloy immersed in 3.5 wt% NaCl solution for 40 min: a-c 280 °C, 0.1 mm/s; d-f 280 °C, 0.01 mm/s; g-i 360 °C, 0.01 mm/s

Fig. 12 Schematic illustrations of corrosion mechanism for the as-extruded Mg-4Zn-2Gd-0.5Ca alloy: a less DRXed region; b more DRXed region

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