Influence of Voltage on the Corrosion and Wear Resistance of Micro-Arc Oxidation Coating on Mg-8Li-2Ca Alloy

doi:10.1007/s40195-018-0845-y

Abstract

Abstract:

Calcium phosphate (CaP) coatings were prepared on Mg-8Li-2Ca magnesium alloy by micro-arc oxidation (MAO) in an alkaline Na₃PO₄-Ca[C₃H₇O₆P] base solution at the different applied voltages. Scanning electron microscope and X-ray diffraction were employed to characterize the microstructure and phase composition of the coatings, respectively. The corrosion resistance of the coatings was assessed by potential dynamic polarization curves, electrochemical impedance spectroscopy and hydrogen evolution experiment in simulated body fluids solution. The friction and wear properties were evaluated by friction and wear testing machine. The results demonstrate that the coating surface is porous and mainly composed of MgO, Ca₅(PO₄)₃(OH) and CaH₂P₂O₅. With the increase in voltage, the corrosion resistance and wear resistance of the MAO coating are both enhanced. The corrosion current density of the MAO coating decreases about two orders of the magnitude compared to the substrate. Additionally, wear and corrosion mechanisms are discussed.

Key words: Magnesium alloy, Micro-arc oxidation, Electrochemical impedance, Wear mechanisms

Bing-Yu Qian, Wei Miao, Min Qiu, Fan Gao, Dong-Hui Hu, Jian-Feng Sun, Rui-Zhi Wu, Boris Krit, Sergey Betsofen. Influence of Voltage on the Corrosion and Wear Resistance of Micro-Arc Oxidation Coating on Mg-8Li-2Ca Alloy[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(2): 194-204.

Figures/Tables 16

Fig. 1 Surface microstructure of MAO coatings on Mg-8Li-2Ca alloy prepared at a 375 V, b 400 V, c 425 V, d 450 V, e 475 V

Fig. 2 Cross-sectional microstructure of MAO coatings on Mg-8Li-2Ca alloy prepared at a 375 V, b 400 V, c 425 V, d 450 V, e 475 V

Fig. 3 Thickness of MAO coatings on Mg-8Li-2Ca alloy prepared at different voltages

Fig. 4 XRD pattern of MAO coatings on Mg-8Li-2Ca alloy prepared at different voltages

Fig. 5 Hydrogen content of MAO coating at different voltages in different immersion time

Fig. 6 Tafel curve of MAO coating at different voltages and matrix

Table 1 Free-corrosion potential and free-corrosion current density of coatings prepared at different voltages and Mg-8Li-2Ca matrix

Parameters	375 V	400 V	425 V	450 V	475 V	Matrix
E_corr (V)	-?0.6471	-?0.6284	-?0.5634	-?0.4079	-?0.3324	-?1.8520
I_corr (μA/cm²)	6.298	4.230	4.375	2.087	1.271	243.8

Fig. 7 EIS spectra of MAO coatings at different voltages in SBF solution a, b magnification of high-frequency region

Fig. 8 Equivalent electric circuits represent a MAO coatings, b Mg-8Li-2Ca matrix

Table 2 Fitting results of EIS plots of the Mg-8Li-2Ca matrix and MAO coatings at different voltages

MAO voltages	R_s (Ω)	R₁ (Ω)	C₁ (F)	R₂ (Ω)	C₂ (F)	W (Ω s^-1/2)
375 V	24.79	245.1	1.333E-7	1464	1.009E-6	2387
400 V	23.74	375.8	1.154E-7	1906	6.415E-7	2480
425 V	27.82	759.9	9.751E-8	2095	5.968E-7	2918
450 V	22.87	701.8	1.091E-7	3456	6.001E-7	3866
475 V	28.91	678.9	5.803E-8	5214	7.448E-7	4268
Matrix	23.56	149	9.635E-7			642.3

Fig. 9 Coefficient of friction of MAO coatings at different voltages

Fig. 10 Abrasion loss of MAO coatings at different voltages

Fig. 11 Wear morphology of MAO coating prepared at 425 V a macromorphology, b high magnification of A region shown in (a), c high magnification of B region shown in (b)

Fig. 12 Corrosion morphology of MAO coating prepared at 425 V after immersion in SBF solution for 160 h a intact coating, b cracks in the corroded coating, c corrosion pit, d high magnification of A region shown in (c)

Fig. 13 XRD of MAO coating prepared at 425 V after immersion in SBF for 160 h

Fig. 14 Schematic representation of the corrosion process of MAO coating immersed in SBF

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