Microstructure and Corrosion Resistance of Plasma Electrolytic Oxidized Recycled Mg Alloy

doi:10.1007/s40195-021-01324-z

Abstract

Abstract:

The wide application of Mg is hindered by its low corrosion resistance, especially for recycled Mg. In this work, a novel plasma electrolytic oxidation (PEO) process was conducted on a recycled Mg alloy with a high content of Al (~ 2.3 wt%), which results in a polycrystalline PEO coating. The PEO coating displays a three-layered structure with a uniform elemental distribution. Unlike the outer layer decorated by channels of micrometre scales, the middle layer mainly contains submicron-sized pores associated with plasma discharges, whereas the fine pores originated from oxygen evolution were detected in the inner layer. The electrochemical measurements, hydrogen evolution tests and salt spray tests were conducted, indicating the superior corrosion resistance of PEO-coated recycled Mg alloy. Hence, the purposed PEO method could dramatically enhance the corrosion resistance of recycled Mg, which may broaden its application in the future.

Key words: Recycled Mg, Plasma electrolytic oxidation, Corrosion resistance, Layered structure

You Lv, Chao Zhang, Yupeng Zhang, Qishi Wang, Xinxin Zhang, Zehua Dong. Microstructure and Corrosion Resistance of Plasma Electrolytic Oxidized Recycled Mg Alloy[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(6): 961-974.

Figures/Tables 16

Table 1 Chemical composition of the recycled Mg alloy (wt%)

Mg	Al	Mn	Zn
96.83	2.38	0.16	0.63

Fig. 1 a SEM micrograph of the recycled Mg alloy surface; b grain orientation distribution in inverse pole figure colouring of the recycled Mg alloy; c a bright field TEM micrograph of the recycled Mg alloy

Fig. 2 ltage-time response during the PEO process of the recycled Mg alloy

Fig. 3 Surface morphology of the PEO coating on the recycled Mg alloy surface: a general view; b framed area of a; c framed area of b; d a high resolution SEM of the coating; e surface topography; f WCA

Fig. 4 Cross-sectional view of the PEO coating on the recycled Mg alloy: a general view; b framed area of a

Fig. 5 Elemental profiles of the PEO coating on the recycled Mg alloy surface

Table 2 Chemical composition of the PEO coating on the recycled Mg alloy (wt%)

Mg	Al	Si	O
54.2	0.9	15.5	29.4

Fig. 6 XRD and XPS analysis of the PEO-coated recycled Mg alloy: a XRD pattern; b XPS wide scan; c-e high-resolution XPS spectra of Mg 1s, Si 2p and O 1s

Fig. 7 TEM analysis of the PEO coating on the recycled Mg alloy: a general view; b framed area of a at an increased magnification; c HAADF micrograph of the interfacial area in the coating; d a lattice image of the coating; e)elemental profiles of the coating

Fig. 8 Potentiodynamic polarization curves of both PEO-coated and bare recycled Mg alloys in a 3.5 wt% NaCl solution

Table 3 Tafel slope (βa), corrosion current densities (icorr) and corrosion potentials (Ecorr) derived from the potentiodynamic polarization curves

Samples	β_a (mV)	i_corr (A/cm²)	E_corr (V)
Bare recycled Mg	33.39	6.72 × 10^-4	-1.53
PEO-coated recycled Mg	28.45	5.02 × 10^-7	-1.45

Fig. 9 a Hydrogen evolution during the immersion in a 3.5 wt% NaCl solution; SEM micrographs of b, c bare recycled Mg substrate after immersion for 2 h and d, e PEO-coated recycled Mg after immersion for 24 h

Fig. 10 Electrochemical impedance spectra of both PEO-coated and bare recycled Mg substrates in a 3.5 wt% NaCl solution: a, b PEO-coated recycled Mg alloy; c, d bare recycled Mg alloy; e-g EC models

Table 4 Fitting results of EIS for PEO-coated recycled Mg alloy

Immersion time	CPE_coat-Q (F/cm²)	CPE_coat-n	R_coat (Ω/cm²)	CPE_dl-Q (F/cm²)	CPE_dl-n	R_ct (Ω/cm²)
1 h	3.071 × 10^-8	0.841	5452	1.855 × 10^-6	0.660	1.036 × 10⁵
24 h	1.572 × 10^-8	0.991	2947	6.071 × 10^-6	0.528	5.524 × 10⁴
46 h	-	-	-	3.412 × 10^-6	0.730	2.482 × 10⁴
48 h	-	-	-	2.004 × 10^-6	0.868	1.276 × 10⁴
50 h	-	-	-	1.783 × 10^-6	0.950	8.442 × 10³

Table 5 Fitting results of EIS for bare recycled Mg alloy

Immersion time	CPE_dl-Q (F/cm²)	CPE_dl-n	R_ct (Ω/cm²)
1 h	7.634 × 10^-6	0.995	145
2 h	3.421 × 10^-5	0.967	121
5 h	3.568 × 10^-4	0.943	98

Fig. 11 Surface appearances after salt spray tests for various periods

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