Corrosion Resistance and Durability of Superhydrophobic Coating on AZ31 Mg Alloy via One-Step Electrodeposition

doi:10.1007/s40195-020-01168-z

Abstract

Abstract:

To enhance durability and adhesion of superhydrophobic surface, an integrated superhydrophobic calcium myristate (Ca[CH₃(CH₂)₁₂COO]₂) coating with excellent corrosion resistance was fabricated on AZ31 magnesium (Mg) alloy via one-step electrodeposition process. Field-emission scanning electron microscopy, Fourier transform infrared spectrometry and X-ray photoelectron spectroscopy as well as X-ray diffraction were employed to investigate the surface characteristics (morphology, composition and structure) of the coatings. Hydrophobicity of the coating was evaluated by means of contact and sliding angles. Additionally, potentiodynamic polarization, electrochemical impedance spectroscopy and hydrogen evolution tests were conducted to characterize the corrosion resistance. Results indicated that the coating exhibited super-hydrophobicity with large static water contact angle (CA) and small sliding angle of 155.2° ± 1.5° and 6.0° ± 0.5°, respectively, owing to spherical rough structure and low surface energy (7.01 mJ m ^-2). The average hydrogen evolution rate (HER_a) and corrosion current density (i_corr) of the coated sample were 5.3 μL cm ^-2 h ^-1 and 5.60 × 10 ^-9 A cm ^-2, about one and four orders of magnitude lower than that of AZ31 substrate, respectively, implying the excellent corrosion resistance. The CA of the coating remained 155.6° ± 0.9° after soaking for 13 days, showing the super-hydrophobicity and stability of the coating. Simultaneously, the large critical load (5004 mN) for the coating designated the outstanding adhesion to the substrate by nano-scratch test.

Key words: Magnesium alloy, Electrodeposition, Durability, Super-hydrophobicity, Adhesion, Corrosion resistance

Zheng-Zheng Yin, Zhao-Qi Zhang, Xiu-Juan Tian, Zhen-Lin Wang, Rong-Chang Zeng. Corrosion Resistance and Durability of Superhydrophobic Coating on AZ31 Mg Alloy via One-Step Electrodeposition[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(1): 25-38.

Figures/Tables 17

Table 1 Surface energy parameters (γ) of three liquids phase (mJ m-2) [38]

Liquids	$\gamma_{L}$	$gamma_{L}^{LW}$	$\gamma_{L}^{ + }$	$\gamma_{L}^{ - }$
Diiodomethane	50.8	50.8	0	0
Glycol	48.0	29.0	1.92	47.0
Water	72.8	21.8	25.5	25.5

Fig. 1 a SEM image and b its magnified one of the prepared superhydrophobic coating; c EDS results of selected points in a; d cross-sectional morphology, e-h corresponding elements mapping images

Fig. 2 Water contact angles of a AZ31 alloy and b coating surface; c sliding angles for coated samples

Fig. 3 XPS results of a C 1s, b O 1s and c Ca 2p

Fig. 4 a FTIR spectra and b XRD results of the AZ31 substrate, myristic acid (MA) and coated samples as well as immersed AZ31 (I-AZ31) and coating (I-coating)

Table 2 Volume fractions (vol%) of each phase in XRD patterns

Sample	Mg	Mg(OH)₂	CM
AZ31	100	-	-
I-AZ31	27.72	72.28	-
Coating	71.05	-	28.95
I-coating	73.37	0.58	26.05

Fig. 5 Electrochemical results of a Nyquist, b Bode plots of Zmod, c Bode plots of phase angle and corresponding EC models of the d AZ31 substrate, e coated sample; f potentiodynamic polarization curves of the AZ31 substrate and coating in 3.5% NaCl

Table 3 Equivalent circuit fitting results of the EIS date

Sample	R_s (Ω cm²)	CPE_f (Ω^-1 sⁿ cm^-2)	n	R_f (Ω cm²)	CPE₁ (Ω^-1 sⁿ cm^-2)	n	R_ct (Ω cm²)	R_L (Ω cm²)	L (H cm²)
AZ31	22.03	-	-	-	1.53 × 10^-5	0.91	1.41 × 10²	58.28	67.30
Coating	1.03 × 10³	1.90 × 10^-10	0.98	1.72 × 10⁵	7.57 × 10^-8	0.53	1.49 × 10⁵	1.21 × 10^-2	6.70 × 10⁶

Fig. 6 Hydrogen evolution results of a pH value, b hydrogen evolution volume (HEV), c average hydrogen evolution rate (HERa), d instantaneous hydrogen evolution rate (HERi) in 3.5% NaCl solutionFull size image

Table 4 Corrosion resistance of superhydrophobic coatings (icorr, A cm-2) prepared by two-step method (the best results among different parameters)

Method/coating type	Substrate	Middle layer	Superhydrophobic layer	References
MAO + electrodeposition	3.55 × 10^-5	8.36 × 10^-6	7.68 × 10^-8	[47]
MAO + soak	4.21 × 10^-4	1.13 × 10^-6	2.35 × 10^-7	[48]
MAO + soak (multiple-cycle assembly)	2.1 × 10^-5	8.2 × 10^-7	3.5 × 10^-8	[49]
Hydrothermal (Mg(OH)₂) + soak	1.62 × 10^-5	-	1.72 × 10^-7	[50]
LDH + electrodeposition	9 × 10^-5	4 × 10^-5	4 × 10^-6	[51]
Electrodeposition (CeO₂) + soak	4.71 × 10^-4	5.43 × 10^-5	1.14 × 10^-6	[52]

Fig. 7 Schematic illustration of the coating forming process

Fig. 8 a Nanoscratch test of the coating; b water static contact angles, c-f mean values of surface roughness of superhydrophobic coating under different immersion time of 0 days, 1 days, 7 days and 13 days in 3.5% NaCl solution

Fig. 9 a SEM morphology, b EDS composition of the coating after 13 days immersion in 3.5% NaCl solution

Fig. 10 Electrochemical results of a Nyquist, b Bode plots of Zmod, c Bode plots of phase angle of the coating after immersion in 3.5% NaCl solution for a few days

Table 5 Equivalent circuit fitting results of the EIS date after immersion

Immersion time (days)	R_s (Ω cm²)	CPE_f (Ω^-1 sⁿ cm^-2)	n	R_f (Ω cm²)	CPE₁ (Ω^-1 sⁿ cm^-2)	n	R_ct (Ω cm²)	R_L (Ω cm²)	L (H cm²)	\|Z\| (Ω cm²)
1	1.74 × 10³	2.93 × 10^-10	0.85	1.12 × 10⁴	1.06 × 10^-5	0.54	3.82 × 10⁴	5.51 × 10⁴	8.99 × 10⁵	3.81 × 10⁴
3	1.67 × 10³	8.93 × 10^-9	0.77	1.21 × 10⁴	8.40 × 10^-6	0.57	2.30 × 10⁴	3.33 × 10^-2	2.20 × 10⁶	3.50 × 10⁴
5	1.55 × 10³	1.78 × 10^-9	0.87	4.13 × 10³	2.90 × 10^-5	0.45	1.20 × 10⁴	-	-	1.65 × 10⁴
7	3.60 × 10²	2.05 × 10^-6	0.57	3.04 × 10³	2.83 × 10^-5	0.80	5.76 × 10³	-	-	9.70 × 10³

Fig. 11 Schematic illustration of corrosion resistance of the superhydrophobic coating

Fig. 12 Contact angles of the CM coating under different liquid phases: a water, b glycol, c diiodomethane

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