Coupling a titanium dioxide based heterostructure photoanode with electroless-deposited nickel-phosphorus alloy coating on magnesium alloy for enhanced corrosion protection

doi:10.1016/j.jmst.2022.02.049

Abstract

Abstract:

The utilization of photoelectrochemical cathodic protection (PECCP) enables an indirect corrosion protection of metals with low self-corrosion potential by introducing a metallic nickel interlayer. However, the ability to enhance the PECCP efficiency remains challenging because of the inherent property of the semiconductor. Herein, this ability is demonstrated by coupling a covalent organic framework (TpBD) decorated TiO₂ photoanode (TiO₂/TpBD) with nickel coating on magnesium alloy for an effective corrosion protection. The composite photoanode showed direct PECCP for the nickel interlayer and indirect corrosion protection of the magnesium alloy. The composite structure of the nanotube array and the covalent organic framework for the photoanode were confirmed by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The enhanced photoelectrochemical conversion capability and PECCP performance of the nickel-coated Mg alloy were evidenced by the results from electrochemical and photoelectrochemical measurements including Mott-Schottky curves, photoinduced potential variations, and electrochemical impedance spectroscopy (EIS). Lastly, a corrosion protection mechanism is proposed, where the enhanced PECCP efficiency is attributed to the formation of a direct Z-scheme heterojunction, which is substantiated by the results from valence band (VB) XPS and electron spin resonance characterizations.

Key words: Magnesium alloy, Corrosion, Nickel coating, Electroless, Photoelectrochemical cathodic protection

Yue Liu, Feng Peng, Guang-Ling Yang, Zhi-Hui Xie, Wenxin Dai, Yuejun Ouyang, Liang Wu, Chuan-Jian Zhong. Coupling a titanium dioxide based heterostructure photoanode with electroless-deposited nickel-phosphorus alloy coating on magnesium alloy for enhanced corrosion protection[J]. J. Mater. Sci. Technol., 2022, 126: 252-265.

Figures/Tables 16

Fig. 1. Schematic diagram for preparation of the TiO2/TpBD composite photoanode.

Fig. 2. SEM and TEM images, and EDS spectra of the fabricated semiconductors. The surface morphologies of (a) TiO2 and (b) TiO2/TpBD photoanodes, and (d) TpBD powder. (c) TEM image of single TiO2 nanotube after deposition of TpBD semiconductor. (e) and (f) the corresponding EDS spectra of the square areas (pink dashed squares) in (a) and (b), respectively. Inset at the bottom left corner in (a) is the cross-sectional SEM image of TiO2 nanotubes. Insets at the top right corner in (a), (b), and (d) are the digital pictures of the corresponding samples.

Table 1. Chemical compositions of the TiO2 and TiO2/TpBD based on EDS spectra (at.%).

Empty Cell	Ti	O	C	N
TiO₂	34.04	65.96	NA	NA
TiO₂/TpBD-1	15.19	35.85	48.96	0
TiO₂/TpBD-2	NA	33.35	53.16	13.49

Fig. 3. (a) HRTEM image and (b) corresponding SAED pattern of TiO2/TpBD. (c)-(e) Magnified lattice fringe images of the areas marked with square ①, ②, and ③ in (a), respectively.

Fig. 4. (a) Powder XRD pattern of the TpBD powder. Inset at the upper right corner is the small-angle XRD pattern of the corresponding sample. (b) FTIR spectra of the TiO2, TpBD, and TiO2/TpBD.

Fig. 5. XPS spectra of the TiO2/TpBD composite. The (a) survey, high-resolution of (b) C 1s, (c) N 1s, and (d) O 1s spectra.

Fig. 6. Photoelectrochemical performance of the TiO2, TpBD, and TiO2/TpBD photoelectrodes. (a) UV-vis spectra and (b) plots of (αhv)2 versus hv for the TiO2 and TpBD photoelectrodes. (c) Mott-Schottky curves of the TiO2. (d) VB XPS spectrum of TpBD powder. (e) Mott-Schottky curve of the TiO2/TpBD. (f) The photoinduced variation of the current density over potential (j - E) curves under periodically switching on and off the visible light in 0.1 mol L−1 Na2SO4 solutions. (g) Enlarged j - E curve for the pure TiO2 photoelectrode. (h) Nyquist diagram and corresponding EC model for pure TiO2 and TiO2/TpBD photoelectrodes in solutions containing 0.1 mol L−1 Na2S and 0.2 mol L−1 NaOH.

Table 2. The fitted electrochemical parameters based on EIS data of TiO2 and TiO2/TpBD photoanodes under visible light.

Samples	R_s/Ω cm²	CPE₁/S sⁿ cm⁻²	R₁/Ω cm²	CPE_dl/S sⁿ cm⁻²	R_ct/Ω cm²	χ²
TiO₂	3.10	6.64 × 10⁻⁴	4.75 × 10²	1.22 × 10⁻⁴	6.38 × 10³	2.53 × 10⁻⁴
TiO₂/TpBD	2.87	1.74 × 10⁻²	1.8	1.29 × 10⁻³	885	2.84 × 10⁻³

Fig. 7. Time-dependent profiles of the photoinduced (a) current densities and (b) OCP of the uncoupled Mg/Ni electrode and the Mg/Ni working electrode coupled with TiO2/TpBD photoanode or TiO2 photoanode under intermittent visible light illumination (λ > 420 nm). (c) The long-term time-dependent OCP response of the Mg/Ni electrode coupled with TiO2/TpBD photoanode under visible light irradiation. (d) OCP - t curve of the Mg/Ni electrode (electroless Ni plating time is purposefully decreased to 20 min) with and without coupling with TiO2/TpBD photoanode under visible light irradiation. The insets in (d) are the digital photos of the Mg/Ni electrodes (①) uncoupled with photoanode after 108 min of exposure to NaCl solutions, and coupled with photoanode after (②) 108 min and (③) 150 min of exposure to NaCl solutions.

Fig. 8. EIS of the Mg/Ni electrode with and without coupling with TiO2/TpBD photoanode in a 3.5 wt.% NaCl solution under dark state and visible light irradiation. (a) Nyquist diagram with an inset showing the EC diagram for fitting. (b) An enlarged view of the orange rectangle area in (a). Bode diagrams of (c) modulus versus frequency and (d) phase versus frequency.

Table 3. Fitted EIS parameters for the Mg/Ni electrodes under different conditions in the NaCl solutions.

Samples	R_s/Ω cm²	CPE₁/S sⁿ cm⁻²	R₁/Ω cm²	CPE_dl/S sⁿ cm⁻²	R_ct/Ω cm²	χ²
Mg/Ni	4.90	4.83 × 10⁻⁵	1.20 × 10³	2.53 × 10⁻⁷	2.15 × 10⁴	1.95 × 10⁻⁴
Mg/Ni- TiO₂/TpBD-dark	7.52	2.26 × 10⁻⁵	1.15 × 10³	2.26 × 10⁻³	2.81 × 10³	8.52 × 10⁻⁵
Mg/Ni- TiO₂/TpBD-light	5.62	9.90 × 10⁻⁵	1.19 × 10³	3.87 × 10⁻²	171.80	2.08 × 10⁻⁵

Fig. 9. (a) Potentiodynamic polarization curves of the Mg alloy, single Mg/Ni electrode, and Mg/Ni electrodes coupled with TiO2/TpBD photoanode in 3.5 wt.% NaCl corrosive mediums in the dark and under visible light illumination. (b) The corresponding Ecorr and jcorr for the samples under different conditions based on the polarization curves.

Table 4. Electrochemical parameters based on the potentiodynamic polarization curves.

Samples	E_corr/ V (vs. SCE)	j_corr/ μA cm⁻²	β_c/ mV dec⁻¹
Mg	−1.50 ± 0.038	7.08 ± 1.14	−490
Mg/Ni	−0.33 ± 0.015	3.43 ± 0.75	−347
Mg/Ni-TiO₂/TpBD-dark	−0.84 ± 0.038	16.4 ± 1.05	−197
Mg/Ni-TiO₂/TpBD-light	−1.12 ± 0.049	69.6 ± 1.72	−190

Fig. 10. High-resolution XPS spectra of (a) C 1s, (b) N 1s, and (c) O 1s for the pure TpBD powder.

Fig. 11. ESR spectra of pure TiO2, TpBD, and TiO2/TpBD composite semiconductors for (a) DMPO-·OH in water and (b) DMPO-·O2− in methanol under visible light illumination for 5 min. DMPO: 5, 5-dimethyl-1-pyrroline N-oxide.

Fig. 12. A schematic illustration of the charge-carrier migration pathways of (a) type-II and (b) direct Z-scheme heterojunctions for the TiO2/TpBD composite photoanode, along with the transfer of accumulated photogenerated electrons from the CB of the TpBD semiconductor to the nickel-coated magnesium alloy to induce PECCP performance.

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