Corrosion resistance of a superhydrophobic micro-arc oxidation coating on Mg-4Li-1Ca alloy

doi:10.1016/j.jmst.2017.10.010

Abstract

Abstract:

A micro-arc oxidation (MAO)/zinc stearate (ZnSA) composite coating was fabricated via MAO processing and subsequent sealing with electrodeposition of a superhydrophobic ZnSA. The surface morphologies, chemical composition and corrosion resistance of the coatings were investigated using field-emission scanning electron microscopy, Fourier transform infrared, X-ray diffraction and electrochemical and hydrogen evolution measurements. Results indicated that the MAO coating was efficiently sealed by the following superhydrophobic ZnSA coating. The MAO/ZnSA composite coating significantly enhanced the corrosion resistance of Mg alloy Mg-4Li-1Ca due to its superhydrophobic function. Additionally, corrosion mechanism was suggested and discussed for the composite coating.

Key words: Magnesium alloys, Micro-arc oxidation, Superhydrophobic coating, Corrosion

Cui Lan-Yue, Liu Han-Peng, Zhang Wen-Le, Han Zhuang-Zhuang, Deng Mei-Xu, Zeng Rong-Chang, Li Shuo-Qi, Wang Zhen-Lin. Corrosion resistance of a superhydrophobic micro-arc oxidation coating on Mg-4Li-1Ca alloy[J]. J. Mater. Sci. Technol., 2017, 33(11): 1263-1271.

Figures/Tables 16

Fig. 1. Schematic illustration of the electrodeposition process.

Fig. 2. Macro-photographs, SEM images and corresponding EDS spectra of the (a-d) Mg-4Li-1Ca substrate, (e-h) MAO and (i-l) MAO/ZnSA coating.

Fig. 4 Cross-sectional SEM images of the (a) MAO/ZnSA coating and its corresponding (b) EDS mapping of elemental (b) Si and (c) Zn.

Fig. 4. FTIR spectra of the MAO and MAO/ZnSA coatings.

Fig. 5. XRD patterns of the samples before and after the immersion: (a and d) Mg-4Li-1Ca substrate, (b and e) MAO and (c and f) MAO/ZnSA coating.

Fig. 6. (a) Relationships among depth, load and sliding displacement of the MAO (curve 2) and MAO/ZnSA (curve 3) coating, scratch track views of the nanoscratchs made on the (b) MAO and (c) MAO/ZnSA coatings.

Fig. 7. CA for the (a) Mg-4Li-1Ca substrate, (b) MAO and (c) MAO/ZnSA coating.

Fig. 8. Polarization curves of the (a) Mg-4Li-1Ca substrate, (b) MAO and (c) MAO/ZnSA coating in 3.5% NaCl solution.

Table 1 Parameters of potentiodynamic polarization curves for Mg-4Li-1Ca, MAO and MAO/ZnSA coated samples.

Sample	E_corr (V/SCE)	E_b (V/SCE)	i_corr (A cm^-2)	β_a (mV dec^-1)	-β_c (mV dec^-1)	R_p (Ω cm²)
Mg-4Li-1Ca substrate	-1.51	-	3.55 × 10^-5	58.43	146.17	5.11 × 10⁵
MAO coating	-1.78	-1.59	8.36 × 10^-6	90.74	198.79	3.24 × 10⁶
MAO/ZnSA coating	-1.65	-1.45	7.68 × 10^-8	68.89	126.22	2.52 × 10⁸

Fig. 9. EIS and the fitted results for the (I) Mg-4Li-1Ca substrate, (II) MAO coating, (III) MAO/ZnSA coating: (a) Nyquist plots and (b and c) enlarged Nyquist plots, (d) Bode plots of |Z| vs. frequency, (e) Bode plots of phase angle vs. frequency in 3.5 wt% NaCl solution; equivalent circuits of the (f) Mg-4Li-1Ca substrate (g) MAO and MAO/ZnSA coating.

Table 2 Electrochemical data obtained via equivalent circuit fitting of the EIS curves.

Sample	R_s (Ω cm²)	CPE₁ (Ω^-1 sⁿ cm^-2)	n₁	R_f (Ω cm²)	C (F cm^-2)	R_ct (Ω cm²)	CPE₁ (Ω^-1 sⁿ cm^-2)	n₂	R_L (Ω cm²)	L (H cm^-2)	χ²
Mg-4Li-1Ca substrate	18.00	1.99 × 10^-5	0.91	5.61 × 10²	3.36 × 10^-3	1.96 × 10²	-		8.98 × 10²	9.13 × 10³	1.31 × 10^-4
MAO coating	23.66	5.88 × 10^-5	0.32	6.05 × 10²	1.39 × 10^-7	2.70 × 10⁴	1.44 × 10^-6	0.83	2.71 × 10⁴	1.40 × 10⁵	4.28 × 10^-4
MAO/ZnSA coating	83.06	1.60 × 10^-6	0.45	9.91 × 10³	1.52 × 10^-9	4.48 × 10⁵	8.67 × 10^-8	0.75	6.94 × 10⁵	5.64 × 10⁶	4.93 × 10^-4

Fig. 10. HERs as a function of immersion time for the (a) Mg-4Li-1Ca substrate, (b) MAO and (c) MAO/ZnSA coating in 3.5 wt% NaCl for 85 h.

Fig. 11. CA as a function of immersion time for the MAO/ZnSA coating in 3.5 wt% NaCl solution for 85 h.

Fig. 12. Macro-photographs, SEM images and corresponding EDS spectra of the (a-d) Mg-4Li-1Ca substrate, (e-h) MAO and (i-l) MAO/ZnSA coating in 3.5 wt% NaCl solution after an 85-h immersion.

Fig. 13. FTIR spectra of the Mg-4Li-1Ca substrate, MAO and MAO/ZnSA coating in 3.5 wt% NaCl solution after 85-h immersion.

Fig. 14. Schematic representation of the corrosion mechanism of MAO/ZnSA coating.

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