Comparative Study on Corrosion Behavior and Mechanism of As-Cast Mg-Zn-Y and Mg-Zn-Gd Alloys

doi:10.1007/s40195-022-01455-x

Abstract

Abstract:

Rare earth (RE) elements have large solid solubility in magnesium and are widely used to regulate the microstructure and property of advanced magnesium alloys. However, different kinds of RE elements have different effects on microstructure and property of the alloy. In this study, a Mg-Zn-Y alloy and a Mg-Zn-Gd alloy with alloying elements of the same atomic percentage were designed to clarify the effect of yttrium (Y) and gadolinium (Gd) on the corrosion behavior of as-cast MgZn₂Y_2.66 and MgZn₂Gd_2.66 alloys. The results show that the MgZn₂Y_2.66 alloy is mainly composed of α-Mg phase and long period stacking ordered (LPSO) phase, while MgZn₂Gd_2.66 alloy is mainly composed of α-Mg phase and (Mg, Gd)₃Zn phase (W phase). Generally speaking, the corrosion phenomena of the two alloys in 3.5 wt% NaCl solution are similar. In the early stages of exposure, the alloys underwent uniform corrosion at a relatively low corrosion rate. With prolonged exposure, localized corrosion became dominated and the corrosion rate was greatly increased. However, the corrosion rate of the MgZn₂Y_2.66 alloy, in terms of the corrosion current density, is about one order of magnitude lower than that of the MgZn₂Gd_2.66 alloy. The high corrosion resistance of the MgZn₂Y_2.66 alloy is mainly attributed to the presence of LPSO phase in form of continuous networks and the relatively high corrosion resistance of the corrosion product layer on the alloy.

Key words: Magnesium alloys, Rare earth, Long period stacking ordered (LPSO) phase, Microstructure, Corrosion

Guoqiang Xi, Xuhan Zhao, Yanlong Ma, Yu Mou, Ju Xiong, Kai Ma, Jingfeng Wang. Comparative Study on Corrosion Behavior and Mechanism of As-Cast Mg-Zn-Y and Mg-Zn-Gd Alloys[J]. Acta Metallurgica Sinica (English Letters), 2023, 36(2): 310-322.

Figures/Tables 16

Table 1 Chemical composition of the alloys

Alloy	Mg (wt%)	Gd (wt%)	Y (wt%)	Zn (wt%)
MgZn₂Y_2.66	Bal	-	7.43	4.20
MgZn₂Gd_2.66	Bal	12.40	-	3.28

Fig. 1 Backscattered SEM images of as-cast MgZn2Y2.66 alloy a, b and MgZn2Gd2.66 alloy c, d

Table 2 EDS elemental analysis of the phase marked in Fig. 1

Position	Element (at.%)
	Mg	Gd	Y	Zn
A	87.2	-	7.9	4.8
B	98.5	-	0.9	0.4
C	90.4	5.0	-	4.4
D	99.1	0.3	-	0.5

Fig. 2 XRD patterns of alloy: a MgZn2Y2.66, b MgZn2Gd2.66

Fig. 3 Macroscopic corrosion morphologies of the two alloys in 3.5 wt% NaCl solution: a MgZn2Y2.66 alloy, b MgZn2Gd2.66 alloy

Fig. 4 a Average hydrogen evolution of studied alloys in 3.5 wt% NaCl solution, b the enlargement of the red rectangle region in a; and c mass loss rate of studied alloys in 3.5 wt% NaCl solution

Fig. 5 a Open circuit potential and b potentiodynamic polarization curves of the two alloys in 3.5 wt% NaCl solution

Table 3 Corrosion potentials and corrosion current densities of the two alloys obtained by Tafel fitting of the potentiodynamic polarization curves

Alloy	Corrosion current density (A/cm²)	Corrosion potential (V)
MgZn₂Y_2.66	1.02 × 10^-5	− 1.579
MgZn₂Gd_2.66	3.24 × 10^-4	− 1.551

Fig. 6 Electrochemical impedance spectra (EIS) of the two alloys after immersion in 3.5 wt% NaCl solution for 5 min: a Nyquist diagram, b Bode diagram

Fig. 7 Equivalent circuit used to fit the EIS shown in Fig. 6

Table 4 Parameters obtained by fitting the EIS shown in Fig. 6 using the equivalent circuit as shown in Fig. 7

Alloy	R_s (Ω cm²)	R_L (Ω·cm²)	L (H·cm²)	Q_f (Ω⁻¹ cm⁻² Sⁿ)	N_f	R_f (Ω·cm²)	Q_dl (Ω⁻¹ cm⁻² Sⁿ)	N_dl	R_ct (Ω·cm²)
MgZn₂Y_2.66	9.132	1339	2513	3.66 × 10^-3	0.92	192	1.46 × 10^-5	0.91	562.3
MgZn₂Gd_2.66	8.68	452.2	62.25	2.50 × 10^-5	0.91	57.63	1.10 × 10^-7	0.86	75.32

Fig. 8 XPS survey spectra of the corrosion products: a MgZn2Y2.66, b MgZn2Gd2.66. High-resolution XPS for c Mg 1s and e Y 3d of MgZn2Y2.66, d Mg 1s and f Gd 4d of MgZn2Gd2.66

Fig. 9 Microscopic corrosion morphologies of the two alloys after immersion in 3.5 wt% NaCl solution: a, c MgZn2Y2.66 for 300 min, b, d MgZn2Gd2.66 for 170 min

Fig. 10 Cross-sectional morphologies of the alloys after immersion in 3.5 wt% NaCl solution for 5 min: a MgZn2Y2.66, b MgZn2Gd2.66

Fig. 11 a Low magnification cross-sectional morphology of the MgZn2Y2.66 alloys after immersion in 3.5 wt% NaCl solution for 5 h; b-d the high-magnification micrographs of blue, orange and green rectangle regions in a, respectively

Fig. 12 Low magnification a and high magnification b cross-sectional morphologies of the MgZn2Gd2.66 alloys after immersion in 3.5 wt% NaCl solution for 3 h; c the high-magnification micrograph of blue rectangle region in a, d the high-magnification micrograph of orange rectangle region in b

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