Mg(OH)2 Film on Micro-arc Oxidation Ceramic Coating of AZ31 Magnesium Alloy: Preparation and Corrosion Resistance

doi:10.15541/jim20190426

Abstract

Abstract:

The micro-arc oxidation (MAO) ceramic coating on AZ31 magnesium alloy was treated by hydrothermal treatment in NaOH solution to study the effect of the concentration of NaOH solution on the microstructure and corrosion resistance of the obtained Mg(OH)₂/MAO composite coating, the formation mechanism and the corrosion mechanism of the hydrothermal film was annalysed at the same time. The results showed that MgO in the surface of MAO ceramic layer was partially dissolved during hydrothermal reaction. And the released Mg ²⁺ was combined with OH ^- in hydrothermal solution to form Mg(OH)₂ nanosheets which deposited on the surface of ceramic layer and inner wall of the pores. With increasing of NaOH concentration of the hydrothermal solution, more Mg(OH)₂ formed during hydrothermal reaction and sealed the inherent defects such as pores and cracks in the MAO ceramic layer, which improved the compactness of the coating. In addition, the electrochemical test results demonstrated that the Mg(OH)₂/MAO composite coating prepared by MAO and hydrothermal treatment achieved better corrosion resistance than the single MAO ceramic layer, and the corrosion resistance of Mg(OH)₂/MAO composite coating increased with the increasing concentration of the hydrothermal solution. And the immersion test showed that Mg(OH)₂/MAO composite coating could provide long-term corrosion protection for magnesium alloy.

Key words: magnesium alloy, micro-arc oxidation, hydrothermal treatment, composite coating, corrosion resistance

CLC Number:

TG174
TG179

WANG Zhihu,ZHANG Jumei,BAI Lijing,ZHANG Guojun. Mg(OH)₂ Film on Micro-arc Oxidation Ceramic Coating of AZ31 Magnesium Alloy: Preparation and Corrosion Resistance[J]. Journal of Inorganic Materials, 2020, 35(6): 709-716.

Figures/Tables 10

Fig. 1 XRD patterns of AZ31 substrate, MAO coating and HT/MAO composite coating

Fig. 2 Surface morphologies of MAO coating and HT/MAO composite coating (a) MAO; (b) HT-0/MAO; (c) HT-0.2/MAO; (d) HT-0.5/MAO; (e) HT-2.0/MAO; (f) HT-2.0/MAO in high magnification

Fig. 3 EDS spectra and element contents of (a) MAO and (b) HT-2.0/MAO

Fig. 4 Static contact angles of MAO coating and HT/MAO composite coating (a)MAO; (b) HT-0/MAO; (c)HT-0.2/MAO; (d)HT-0.5/MAO; (e)HT-2.0/MAO

Fig. 5 Potentiodynamic polarization curves of AZ31 substrate, MAO, HT-0.2/MAO and HT-2.0/MAO

Table 1 Fitting parameters derived from the potentiodynamic polarization curves

Samples	E_corr/V	I_corr/(μA·cm^-2)	β_a/V	β_c/V	R_P/(kΩ·cm²)	C_R/(mm·a^-1)
Bare AZ31	-1.460	22.34	0.284	0.395	3.211	510.5×10^-3
MAO	-1.383	0.0530	1.340	0.209	1.481×10³	1.211×10^-3
HT-0.2/MAO	-1.376	0.0345	0.938	0.409	3.585×10³	0.788×10^-3
HT-2.0/MAO	-1.350	0.0187	1.226	0.208	4.129×10³	0.427×10^-3

Fig. 6 Nyquist and Bode plots of bare AZ31 and coated samples as well as equivalent circuit (a) Nyquist plots of bare AZ31; (b) Nyquist plots of coated samples; (c) Bode plots of all samples; (d-e) physical model and equivalent circuit for coated samples

Fig. 7 Rout, Rinn and Rtotal values of three coatings

Fig. 8 XRD patterns of MAO and HT-0.2/MAO before and after immersion in 3.5wt% NaCl solution

Fig. 9 Surface morphologies of MAO and HT-0.2 /MAO after immersion in 3.5wt% NaCl solution for 14 d (a-b) MAO; (c-d) HT-0.2/MAO

References 29

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Mg(OH)₂ Film on Micro-arc Oxidation Ceramic Coating of AZ31 Magnesium Alloy: Preparation and Corrosion Resistance

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