Microstructure and Corrosion Properties of Duplex-Structured Extruded Mg-6Li-4Zn-xMn Alloys

doi:10.1007/s40195-022-01377-8

Abstract

Abstract:

The extruded Mg-6Li-4Zn-xMn (x = 0, 0.4, 0.8, 1.2 wt%) alloys were prepared, and the microstructure of the test alloys was investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy. The corrosion properties were determined by electrochemical measurements and immersion measurements in 3.5% NaCl solution. The results indicate that the extruded Mg-6Li-4Zn-xMn alloys are mainly composed of α-Mg phase, β-Li phase, Mn precipitates and some intermetallic compounds (MgLi₂Zn). With the addition of Mn, stable corrosion products were formed on the surface of the test alloy, which can effectively inhibit further corrosion progress and improve the corrosion resistance. Mg-6Li-4Zn-1.2Mn alloy exhibits the best corrosion resistance, attributed to grain refinement, the improvement of the stability of corrosion product film and uniform distribution of fine second phases.

Key words: Mg-Li alloys, Microstructure, Extruded, Corrosion resistance property

Minmin Li, Zhe Qin, Yan Yang, Xiaoming Xiong, Gang Zhou, Xiaofei Cui, Bin Jiang, Xiaodong Peng, Fusheng Pan. Microstructure and Corrosion Properties of Duplex-Structured Extruded Mg-6Li-4Zn-xMn Alloys[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(5): 867-878.

Figures/Tables 18

Table 1 Chemical composition of Mg-Li-Zn-xMn alloys

Sample	Designed composition	Actual composition
1	Mg-6Li-4Zn	Mg-6.9Li-4Zn
2	Mg-6Li-4Zn-0.4Mn	Mg-6Li-3.8Zn-0.4Mn
3	Mg-6Li-4Zn-0.8Mn	Mg-5.9Li-3.7Zn-0.8Mn
4	Mg-6Li-4Zn-1.2Mn	Mg-5.7Li-3.3Zn-1.3Mn

Fig. 1 Optical microstructures of as-extruded Mg-6Li-4Zn-xMn alloys (perpendicular to the ED): a x?=?0; b x?=?0.4; c x?=?0.8; d x?=?1.2

Fig. 2 Optical microstructures of as-extruded Mg-6Li-4Zn-xMn alloys (parallel to the ED): a x?=?0; b x?=?0.4; c x?=?0.8; d x?=?1.2

Fig. 3 XRD patterns of Mg-6Li-4Zn-xMn alloys: a x?=?0; b x?=?0.4; c x?=?0.8; d x?=?1.2

Fig. 4 SEM micrographs of as-extruded Mg-6Li-4Zn-xMn alloys (perpendicular to the ED): a x?=?0; b x?=?0.4; c x?=?0.8; d x?=?1.2

Table 2 EDS results of as-extruded Mg-6Li-4Zn-xMn alloys at different positions

Position	Alloy elements (wt%)
	Mg	Zn	Mn
A	70	30	0
B	70.2	29.7	0.1
C	30.6	1.6	67.8
D	35.9	63.1	1
E	65.8	33.9	0.3
F	54.1	44.8	1.1
G	11.4	1.4	87.2
H	54.3	45	0.7

Fig. 5 a Bright-field TEM image of as-extruded Mg-6Li-4Zn-1.2Mn alloy with EDS of points A and B; b partial enlargement of image (a); c high-resolution TEM image of Mn precipitates

Fig. 6 EIS patterns of as-extruded Mg-6Li-4Zn-xMn alloys immersed in 3.5 wt% NaCl solution: a Nyquist plot; b Bode plots; c phase angle plots

Fig. 7 Fitted equivalent circuits of a Mg-6Li-4Zn and Mg-6Li-4Zn-0.4Mn; b Mg-6Li-4Zn-0.8Mn; c Mg-6Li-4Zn-1.2Mn

Table 3 Fitted results of the EIS tests of the experimental alloys using equivalent circuits (a) Mg-6Li-4Zn, (b) Mg-6Li-4Zn-0.4Mn; (c) Mg-6Li-4Zn-0.8Mn; (d) Mg-6Li-4Zn-1.2Mn

Alloys	R_S (Ω)	R (Ω cm²)	CPE₁ (10^-6 sⁿ Ω ^-1 cm ^-2)	n	R_f (Ω cm²)	C_d1 (μF cm ^-2)	R_L	L
A	17.27	129.9	14.19	0.93	543.2	2247.1	-	-
B	15.23	95	13.22	0.93	436.2	2761.9	-	-
C	16.81	-	25.89	0.88	382	-	-	-
D	18.35	-	12.31	0.91	792.1	0.00649	4324	1862

Fig. 8 Polarization curves of as-extruded Mg-6Li-4Zn-xMn alloys immersed in 3.5 wt% NaCl solution

Table 4 Corrosion potential (Ecorr), cathodic slope (βc) and corrosion current density (icorr) of samples obtained from potentiodynamic polarization test

Alloys	E_corr (V/SCE)	β_c (mV/dec)	i_corr (μA/cm²)
Mg-6Li-4Zn	-1.5087	-180.5	47.3
Mg-6Li-4Zn-0.4Mn	-1.5072	-179.5	63.1
Mg-6Li-4Zn-0.8Mn	-1.5330	-230.8	79.4
Mg-6Li-4Zn-1.2Mn	-1.4436	-310.3	25.1

Fig. 9 Weight loss rates of as-extruded Mg-6Li-4Zn-xMn alloys immersed in 3.5 wt% NaCl solution

Fig. 10 Hydrogen evolution rates of as-extruded Mg-6Li-4Zn-xMn alloys immersed in 3.5 wt% NaCl solution

Fig. 11 Morphologies of as-extruded Mg-6Li-4Zn-xMn alloys after immersion in 3.5 wt% NaCl solution for 72 h: a x?=?0; b x?=?0.4; c x?=?0.8; d x?=?1.2

Table 5 EDS analysis results at different positions

Position	Alloy elements (wt%)
	Mg	Zn	Mn
A	90.5	9.5	0
B	66.4	33.6	0
C	63.4	35.4	1.2
D	65.6	33.8	0.6

Fig. 12 SEM of corrosion products of Mg-6Li-4Zn-xMn alloys in 3.5 wt% NaCl solution for 1 h: a x?=?0; b x?=?0.4; c x?=?0.8; d x?=?1.2

Table 6 EDS analysis results at different positions as shown in Fig. 12

Position	Alloy elements (at.%)
	Mg	O	Zn	Mn
A	71	27.9	1.1	0
B	36.8	63	0.2	0
C	34.7	65.2	0.1	0
D	75.8	15.8	7.8	0.6
E	40.8	58.8	0.4	0
F	78.2	20.3	1.1	0.4

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