Elsevier

Chemical Engineering Journal

Volume 402, 15 December 2020, 126116
Chemical Engineering Journal

A self-adjusting PTFE/TiO2 hydrophobic double-layer coating for corrosion resistance and electrical insulation

https://doi.org/10.1016/j.cej.2020.126116Get rights and content

Highlights

  • A novel self-adjusting PTFE/TiO2 hydrophobic double-layer coating is designed.

  • The coating is prepared by thermal field/pressure field enhanced plasma electrolytic oxidation.

  • The compact double-layer coating endows titanium alloy excellent insulating protection.

  • The hydrophobic coating shows superior corrosion resistance and long-term chemical stability.

Abstract

The self-adjusting polytetrafluoroethylene (PTFE)/TiO2 organic-inorganic double-layer coatings are designed on titanium alloy using thermal field and gradient pressure field enhanced plasma electrolytic oxidation (TGEPEO), which leads to the in-situ incorporation of PTFE nano-particles into the bottom TiO2 ceramic coating. The PTFE top layer with adjustable thickness is strongly bonded on a TiO2 bottom porous layer by subsequent deposition, chemical bonding and crosslinking curing of PTFE nanoparticles during TGEPEO process. Simultaneously, the double-layer coating possesses promising self-cleaning function due to a special hydrophobic surface with a water contact angle of ≈138.0 ± 4.4° caused by self-adjusted surface micro-nano structure. Moreover, the PTFE/TiO2 double-layer coating exhibits excellent corrosion resistance and high electrical insulation. More importantly, the microstructure of the double-layer coating underwent self-adjustment in long-term immersion, caused by the accumulation-redeposition of corrosive ions and the closure of the reticular microchannels at the damaged regions, thus enhancing long-term chemical stability.

Introduction

As one of the most capable eco-materials, titanium and its alloys have been widely used in various fields, especially in the aerospace, seawater desalination, coastal power station and electronics industries, owing to their high specific strength, high toughness and excellent dimensional stability [1], [2], [3], [4], [5], [6]. In terms of ocean temperature differential power, titanium alloys have great application potentials in seawater pipeline systems, seawater circulation pumps, valves and housings et al [4]. In this case, the high insulation to shield strong electrical interference, hydrophobicity to inhibit scaling and high corrosion resistance are highly required to tolerate the special serving environments. Hence, to develop functional coatings with high protective performance on titanium alloys seem to keep an ever increasing requirement.

These functional coatings such as tribological properties [7], [8], [9], corrosion resistance [10], [11], [12], insulating property [13], hydrophobicity [14], [15] and heat dispersion [14], [16], [17] have been extensively explored. For instance, Jiang Xu et al. [11] fabricated a ZrCN coated Ti alloy using double cathode glow discharge technique to improve the corrosion resistance and surface electrical conductivity. M. Montazeri et al. [12] reported a ceramic coatings on titanium by plasma electrolytic oxidation (PEO) technique in different electrolytes to discuss the growth characteristics and corrosion behavior. Besides, S.V. Gnedenkov et al. [18] prepared a composite fluoropolymer coating on titanium alloy and investigated hydrophobic properties and corrosion resistance. Meanwhile, there are also many studies involving protective coatings to improve the friction and wear behaviour of titanium alloys [7], [8], [19], [20]. Moreover, some hydrophobic modification methods by low surface energy materials have been widely used to construct hydrophobic surfaces, with the purpose to improve the wetting behavior, corrosion resistance, self-cleaning, etc [21], [22], [23], [24]. Specifically, for ocean temperature differential power components of titanium or titanium-based composites, such as heat exchanger tubes, insulating, antifouling, anticorrosion and antiscaling components etc., in these regards, some efforts also have been devoted to the functional exploration [25], [26], [27]. For example, V.S. Rudneva et al. [28] fabricated composite coating on titanium alloys that it has hydrophobicity with 105° and good corrosion resistance. S.L. Sinebryukhov et al. [29] reported an organic–inorganic composite coating prepared by two-step method, the hydrophobic surface (contact angle ≈ 131°) improves corrosion resistance in seawater and shows self-lubricating and antiscaling property. Although those methods are safe and environmentally friendly, the accompanying electrical insulation is usually ignored with few published literatures, as well as complex preparation process and poor durability restrict their widespread application.

Additionally, among the various composite coating materials ever reported, in spite of the numerous efforts to improve wear-resistance and corrosion resistance of titanium and its alloys, most of them focused on the separate property of the specific protective coating, which is hard to fulfill the ever-growing demands like ocean temperature differential power. The multifunctional coating with combined properties of hydrophobicity, corrosion resistance and electrical insulation is highly needed, while it remains a formidable challenge to develop an easy and scale fabrication method.

Herein, we proposed a novel technology that can prepare organic–inorganic double-layer coatings in one step on titanium alloys surface, which was called thermal field and gradient pressure field enhanced plasma electrolytic oxidation (TGEPEO). Consequently, the PTFE/TiO2 double-layer coatings were one-step in-situ fabricated on titanium by TGEPEO process under different PTFE volume fraction in the electrolyte, for the purpose of improving the combined strong hydrophobicity, anti-corrosion, long-term chemical stability and electrical insulation. A composition-structure–property paradigm was proposed based on the chemical, electrochemical and thermal foundations. Simultaneously, the surface micro-nano morphology and wettability of the double-layer coatings with various volume fraction of PTFE were comparatively investigated. The corrosion resistance of the coated samples was evaluated by the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The chemical stability after long-term immersion of the PTFE/TiO2 double-layer coating, as well as the electrical insulation were experimentally demonstrated. Our work showcases an effective and versatile one-step TGEPEO fabrication of multifunctional coating on titanium alloys with potential application in ocean temperature differential power. It also demonstrates rational design as an approach for the practical synthesis of multifunctional coating on various light metals.

Section snippets

Materials

The commercial Ti-6.5Al-2Zr-1Mo-1V plates in dimensions of 25 × 25 × 1.5 mm and Φ30 mm × 1.5 mm were used as substrate. The samples were ground with 600# and 800# grades silicon carbide papers, respectively, to remove the contaminants and native oxide layer for obtaining relatively smooth surface. Then the samples were ultrasonically cleaned with acetone followed by distilled water.

Fabrication of organic–inorganic double-layer coatings

Organic-inorganic double-layer coatings were fabricated by one-step TGEPEO technique in the electrolyte, which was

Current density-time responses

The current density-time (A∙cm−2-min) responses during TGEPEO process with the different volume fraction of PTFE additions are shown in Fig. 1. The addition of PTFE causes interesting changes of current density, which can be ascertained on the basis of the slope of the current density and the current density fluctuations. As shown in I and II of Fig. 1, the current density rise rate of P0.08 and P0.12 is lower, compared with the basic electrolyte without PTFE during the growth process. It can

Conclusion

In summary, the self-adjusting PTFE/TiO2 double-layer coatings with hydrophobicity, corrosion resistance, long-term chemical stability and high insulation were synthesized successfully by novel TGEPEO technique. Different volume fractions of added PTFE causes interesting changes in current density with different ways of rising and falling. The surface roughness self-adjustment of organic–inorganic double-layer coating enables to form a special micro-nano structure for hydrophobic surface with

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The partial supports from the NSFC grant nos. 51571077 and 51621091, National Basic Science Research Program (2012CB933900), Aviation Science Foundation of China (NO. 20163877014) and the Fundamental Research Funds for the Central Universities (HIT. BRETIII.201202) are gratefully acknowledged.

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