Corrosion resistance of self-cleaning silane/polypropylene composite coatings on magnesium alloy AZ31

doi:10.1016/j.jmst.2019.08.056

Abstract

Abstract:

Magnesium (Mg) and its alloys have been widely used in a variety of industrial fields, however, the high corrosion rate and surface contamination restrict their applications. In this study, a corrosion-resistant polymer coating with self-cleaning properties on Mg alloy AZ31 was successfully fabricated via a pretreatment of amino-silane (poly(3-aminopropyl)trimethoxysilane, PAPTMS) and subsequently covered with a polypropylene (PP) film. Surface morphology and chemical compositions were examined using field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and Fourier transform infrared spectrophotometry (FT-IR) and X-ray photoelectron spectroscopy (XPS) as well. Contact angle was measured to determine hydrophobicity of the composite coatings. Water permeability of the coatings was evaluated through electrochemical impedance spectroscopy (EIS). Corrosion resistance of the coating was investigated using electrochemical and hydrogen evolution tests. Results indicated that PAPTMS/PP coatings possessed a micrometer-scaled porous spherical microstructure, and super-hydrophobicity with high water contact angle (162 ± 3.4°) and low sliding angle (5 ± 0.6°) due to the low surface energy (10.38 mJ/m²). Moreover, the coating exhibited a smaller water diffusion coefficient (8.12 × 10^-10 cm²/s) and water uptake volume fraction (24.5 %), demonstrating low water permeability and good physical barrier performance. As a result, the corrosion current density of PAPTMS/PP coating exhibited approximately three orders of magnitude lower than that of the AZ31 substrate, suggesting excellent corrosion resistance. Finally, corrosion-resistant mechanism of the hybrid coating was proposed.

Key words: Magnesium alloy, Polymer coating, Hydrophobicity, Corrosion, Water permeability

Zhao-Qi Zhang, Rong-Chang Zeng, Cun-Guo Lin, Li Wang, Xiao-Bo Chen, Dong-Chu Chen. Corrosion resistance of self-cleaning silane/polypropylene composite coatings on magnesium alloy AZ31[J]. J. Mater. Sci. Technol., 2020, 41: 43-55.

Figures/Tables 19

Fig. 1. Schematic illustration of PAPTMS/PP coating.

Fig. 2. FE-SEM images of (a-b) the PAPTMS coatings, (c) elemental compositions of samples in wt%, and (d-f) the PAPTMS/PP coatings.

Fig. 3. Cross-sectional morphologies of (a) PAPTMS/PP coating; the EDS mapping of (b) Mg, (c) Si, (d) Al, (e) C and (f) O.

Fig. 4. FT-IR spectra (a) and XPS spectra of PAPTMS/PP composite coating: (b) survey of elements, high resolutions of (c) C 1s, (d) O 1s, (e) Si 2p and (f) N 1s.

Fig. 5. Contact angle (CA) measured using (a-1) water, (a-2) glycol and (a-3) diiodomethane as liquid phase; (b) sliding angle (SA).

Table 1 Surface energy (γ) parameters of three liquids phase, mJ/m2.

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

Fig. 6. Self-cleaning tests of (a, b) the PAPTMS/PP coatings; Optical photographs (c, d) of the spherical droplets on the surface (front and vertical view).

Fig. 7. Stability tests of water jet hit the PAPTMS/PP coating at different time: (a) and (b).

Fig. 8. Scratch tests (a) and polarization curves (b) of samples.

Table 2 Electrochemical data of the polarization curves.

Samples	E_corr (V/SCE)	I_corr (A·cm^-2)	β_a (mV·dec^-1)	-β_c (mV·dec^-1)	P_i, (mm·year^-1)
AZ31	-1.51	4.96 × 10^-5	50.62	147.76	1.14
AZ31/PAPTMS	-1.45	1.95 × 10^-6	37.29	140.71	4.46 × 10^-2
AZ31/PAPTMS/PP	-1.38	9.08 × 10^-8	20.36	106.92	2.07 × 10^-3

Fig. 9. (a-c) Nyquist plots, (d) Bode plots of |Z| vs. Frequency and (e) Bode plots of phase angle vs. frequency; equivalent circuits of (f) the AZ31 substrate, (g) the PAPTMS coatings and (h) the PAPTMS/PP coatings.

Table 3 Electrochemical data for EIS curves obtained by equivalent circuit fitting.

Sample	R_s (Ω·cm²)	CPE₁ (Ω^-1·cm^-2·sⁿ)	n₁	R₁ (Ω·cm²)	CPE₂ (Ω^-1·cm^-2·sⁿ)	n₂	R₂ (Ω·cm²)	CPE₃ (Ω^-1·cm^-2·sⁿ)	n₃	R_ct (Ω·cm²)	R_L (Ω·cm²)	L (H·cm^-2)
AZ31	19.80	1.85 × 10^-5	0.92	-	-	-	-	-	-	190.9	312.9	81.2
AZ31/PAPTMS	19.82	2.44 × 10^-6	0.80	1.03 ×10²	3.56 × 10^-5	0.80	-	-	-	7578	-	-
AZ31/PAPTMS/PP	1554	1.25 × 10^-7	0.59	2.11 × 10⁴	4.02 × 10^-7	0.56	2.79 × 10⁵	3.65 × 10^-6	0.69	2.80 × 10⁵	-	-

Fig. 10. Hydrogen evolution tests of (a) hydrogen evolution volume and (b) hydrogen evolution rates for (a) the AZ31 substrates, (b) the PAPTMS coatings and (c) the PAPTMS/PP composite coatings in 3.5 wt% NaCl solution for 250 h of immersion.

Fig. 11. Digital camera photographs and FE-SEM images of (a) and (d) the AZ31 subtrate, (b) and (e) the PAPTMS coating, (c) and (f) the PAPTMS/PP coating immersed in 3.5 wt% NaCl solution for 250 h; (g) corresponding elemental analysis of samples.

Fig. 12. FT-IR spectra (a) and XRD patterns (b) of samples immersed in 3.5 wt% NaCl solution after 250 h of immersion.

Fig. 13. Cc- t1/2 curve of PAPTMS/PP coating/AZ31 system: (a) complete immersion stage and (b) linear part.

Table 4 PH of the samples after immersion time of 1, 100 and 250 h, mm·year-1.

Samples	1 h	100 h	250 h
AZ31	21.88	2.55	11.71
AZ31/PAPTMS	1.37	1.48	0.82
AZ31/PAPTMS/PP	0.91	0.21	0.14

Table 5 Corrosion resistance of polymer coatings on Mg alloys in NaCl solution.

Coatings	Substrate	Thickness (μm)	Electrolyte (wt%)	I_corr (A/cm²)		E_corr (V/SCE)		Immersion (h) time (day)	Refs.
Coatings	Substrate	Thickness (μm)	Electrolyte (wt%)	Substrate	Coating	Substrate	Coating	Immersion (h) time (day)	Refs.
PEO/ SPTFE	MA8	20 + 1～1.5	3.0	5.3 × 10^-5	3.1 × 10^-9	-1.56	-1.27	-	[53]
Mg(OH)₂/SA	AZ31	23.5 + 11	3.5	3.2 × 10^-5	1.7 × 10^-7	-1.54	-1.23	120	[20]
PEO/PANI + SAE	AZ91D	10 + 20	3.5	4.2 × 10^-2	2.0 × 10^-4	-1.47	-1.34	-	[54]
HZC/FAS	AZ31	120	3.5	2.6 × 10^-5	2.6 × 10^-8	-1.54	-0.55	-	[55]
MAO/PMTMS	AZ31	2.8 + 13.6	3.5	1.4 × 10^-5	2.8 × 10^-8	-1.51	-1.41	248	[7]
MA	Mg-Mn-Ce	-	3.5	2.1 × 10^-5	1.4 × 10^-7	-1.59	-1.48	-	[56]
Ni-Co/SA	AZ91D	8.5	3.5	1.5 × 10^-5	9.2 × 10^-9	-1.59	-0.17	-	[57]
Mg(OH)₂/PMTMS/CeO₂	AZ31	8.04	3.5	1.5 × 10^-5	2.4 × 10^-8	-1.44	-1.67	192	[2]
PDMS/SiO₂	AZ31	-	3.5	6.9 × 10^-5	1.2 × 10^-6	-1.45	-1.11	-	[58]
PAPTMS/PP	AZ31	10.5 + 49.5	3.5	5.1 × 10^-5	9.0 × 10^-8	-1.51	-1.38	250	Present work

Fig. 14. Schematic diagram of the corrosion mechanisms of the PAPTMS/PP coatings.

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