Robust Slippery Coating with Superior Corrosion Resistance and Anti

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Robust Slippery Coating with Superior Corrosion Resistance and Anti-Icing Performance for AZ31B Mg Alloy Protection J. L. Zhang, Changdong Gu, and Jiangping Tu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00972 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017

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ACS Applied Materials & Interfaces

Robust Slippery Coating with Superior Corrosion Resistance and Anti-Icing Performance for AZ31B Mg Alloy Protection

Jialei Zhanga,b, Changdong Gu*a,b, Jiangping Tua,b a.

School of Materials Science and Engineering, State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China b.

Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Hangzhou 310027, China

* Corresponding author. E-mail addresses: [email protected]; [email protected]; (C.D. Gu), Tel./fax: +86 571 87952573.

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ABSTRACT Biomimetic slippery liquid-infused porous surfaces (SLIPSs) are developed as a potential alternative to superhydrophobic surfaces (SHSs) to resolve the issues of poor durability in corrosion protection and susceptibility to frosting. Herein, we fabricated a double-layered SLIPS coating on the AZ31 Mg alloy for corrosion protection and anti-icing application. The porous top layer was infused by lubricant and the compact under layer was utilized as a corrosion barrier. The water-repellent SLIPS coating exhibits a small sliding angle and durable corrosion resistance compared with the SHS coating. Moreover, the SLIPS coating also delivers durable anti-icing performance for the Mg alloy substrate, which is obviously superior to the SHS coating. Multiple barriers in the SLIPS coating including the infused water-repellent lubricant, the SAMs coated porous top layer, and the compact layered double hydroxide/carbonate composite under layer, are suggested as being responsible for the enhanced corrosion resistance and anti-icing performance. The robust double-layered SLIPS coating should be of great importance to expand the potential applications of light metals and their alloys.

KEYWORDS: SLIPS; Mg alloy; corrosion resistance; anti-icing; durability

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1. INTRODUCTION Light metals and their alloys have a potential prospect in the progress of sustainable development which requires enhanced energy saving and environmental protection as well as further reduced cost of equipment. Magnesium alloys, regarded as the lightest structural alloying materials, have been extensively applied in automotive, aerospace, defence, biomedical, sporting and electronic goods sectors due to their superior specific strength, castability, ductility, weldability and machinability.1 In particular, the use of low-density Mg alloys improves fuel efficiency through weight reduction, which further reduces harmful gas emissions.2 However, the most pernicious issue of Mg alloys for industrial application is their poor corrosion resistance. Unlike Al or Ti, natural corrosion does not produce a uniform and large-area passive layer with desirable micro- or nano-scale structures on Mg alloys.3 Worse still, Mg alloys have no effective sacrificial coatings similar to those of zinc on steel surfaces due to the high reactivity of Mg. In the past decades, researchers have made a lot of efforts in fabricating effective anti-corrosion coatings on Mg alloys including metallic/alloy coatings, composite coatings, chemical conversion coatings, organic coatings, etc.4, 5, 6, 7, 8 On the other hand, surface icing of light engineering materials applied in aircrafts, wind turbines and power lines can cause very serious problems.9 Protective coatings with effective corrosion resistance and anti-icing performance are necessary on Mg alloys.10 Meanwhile, it remains great challenges to develop novel multifunctional coatings that meet different requirements in a single system. With natural evolution, organisms have developed special structures, patterns or textures with complex and spontaneous multifunctionality.11 Inspired by lotus leaves and water 3



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strider legs, superhydrophobic surfaces (SHSs) have been deeply investigated and widely applied on Mg alloys owing to their multifunctional characters in water repellency, self-cleaning/anti-fouling, anti-icing and corrosion resistance.12,

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Conventionally,

artificial SHSs on engineering materials are consisted of hierarchical porous structures with low surface energy.14, 15 They can meet the wide requirements in general materials. For instance, SHSs can isolate substrates from corrosive liquids to retard the corrosion by the self-assembled monolayers (SAMs) and trapped air pockets. As well, SHSs possess small areas of contact interface with ice, which reduces the ice adhesion strength.16, 17 Nevertheless, this kind of barrier is impermanent and non-wetting surfaces degrade as the SAMs collapsed at some local contact area. It has been a significant strategy to improve the durability of SHSs.18 Recent research also pointed out that SHSs are generally susceptible to frosting, especially under high-humidity environments.19, 20, 21, 22 It has been evidenced that superhydrophobicity is actually not a perfect strategy for neither long-term corrosion resistance,23 nor anti-icing performance.24 Investigations are hence focused on searching for approaches to obtain robust corrosion resistant and anti-icing coatings for Mg alloy protections. Slippery liquid-infused porous surfaces (SLIPSs) have been recently attracted owing to their excellent liquid repellency,25, performance,32,

33, 34, 35

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anti-fouling property,29,

30, 31

anti-icing

self-repairing ability,36 bacteriostasis,37 as well as corrosion

resistance.38, 39 The rational design of artificial SLIPSs is inspired by natural Nepenthes pitcher plants, which is on the basis of the superior liquid adsorption capacity of superwetting surfaces and the strategy that using one liquid to repel another immiscible 4


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liquid.27, 40, 41 Although droplet motion on a SLIPS is typically slower than that on a SHS, the stability of the lubricant film under high droplet impact pressures and the high humidity tolerance make the SLIPS a promising alternative to the SHS in some scenarios.42, 43 To the best of our knowledge, such biomimetic SLIPSs with multifunctionality have not been fabricated on Mg alloys, especially for corrosion protection and anti-icing application. Our previous work proposed a smart coating on Mg alloy with self-healing effect stimulated by the collapse of SHS to further improve the corrosion resistance.44 In the present work, we make a new strategy to prepare a robust corrosion resistant and anti-icing SLIPS coating on the Mg alloy substrate, which works prior to the SHS. A composite coating with a double-layered structure was synthesized on the AZ31 Mg alloy. Such a double-layered structure with a porous top layer and a compact under layer has been reported by one-step hydrothermal routes with various formulas, especially in preparing hydrotalcite or hydrotalcite-like coatings on Mg alloys.3, 45, 46 The porous top layer can be modified by low-surface-energy material to fabricate a SHS coating and the compact under layer can be utilized as a corrosion barrier to protect the active Mg alloy. Furthermore, liquids can be locked in the hierarchical porous structure due to capillary action.36 SHS and SLIPS were obtained by surface modification and lubricant infusion, respectively. The corrosion resistance and anti-icing performance, as well as their durability, of the SHS and SLIPS coatings were compared and evaluated. Multiple protection mechanism was further proposed for the as-prepared multifunctional SLIPS coating on the AZ31 Mg alloy.

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2. EXPERIMENTAL SECTION Hydrothermal preparation of the composite coating. An economical and convenient hydrothermal treatment was carried out to produce a barrier layer on the AZ31B Mg alloy. The hydrothermal solvent was prepared by dissolving 0.6 g ammonium carbonate (AR, 99.6%) into 100 ml de-ionized water and stirring at room temperature. AZ31B Mg alloy sheets (25 mm × 15 mm × 1 mm) were used as substrates which were previously abraded by SiC papers up to 1000 grit and subsequently washed with acetone in an ultrasonic bath. Afterwards, the pre-treated Mg alloy sheets were vertically placed in a Teflon-lined autoclave containing 45 ml (NH4)2CO3 solution and maintained at 100 °C for 24 h. Finally, the specimens were rinsed with de-ionized water and dried in air at 60 °C. Superhydrophobic modification and lubricant infusion. SHS was obtained by a conventional surface modification procedure. The hydrothermally prepared coatings were immersed in an ethanol solution of 0.5 wt.% 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PTES, Amethyst®, 97%) for 2 h at room temperature, and then dried in an oven for 1 h at 120 °C. Approximately 20 µl perfluoropolyether (PFPE, Fomblin® YR-1800) liquid was dropped onto the superhydrophobic coating to obtain SLIPS. The liquid gradually spread onto the whole surface. A simple gas blowing can facilitate the very slow spreading process. After fully infused, the specimen was then vertically placed at laboratory environment for about 6 h to drain off the excess liquid. Characterization. Crystalline structure was analyzed by X-ray diffraction (XRD, XPert Pro-MPD with CuKα radiation, λ=0.15406 nm). Microstructure was observed by a transmission electron microscopy (TEM, FEI Tecnai G2F20 S-Twin). Surface morphology 6


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and chemical composition were examined by a field emission scanning electron microscope (FE-SEM, Hitachi SU-70) equipped with energy dispersive X-ray spectrometer (EDS). Surface chemical state was detected by X-ray photoelectron spectroscopy (XPS, AXIS UTLTRADLD) used AlKα (monochromatic) radiation with E=1486.6 eV. The XPS data was analyzed through peak fitting (XPSPEAK Version 4.1) and all the core-level spectra were referred to C 1s peak at 284.6 eV. Water contact angle (CA) and sliding angle (SA) measurements were performed by a contact angle meter (SL200B, Solon Tech.) based on a sessile drop measuring method with a water droplet volume of 5 µl or 50µl. The average traveling speeds of 50 µl water droplets on the tilted SLIPS coating (~10°) at different temperatures were measured to show the water repellence. Corrosion resistance was evaluated in a 3.5 wt.% NaCl aqueous solution at room temperature, including electrochemical impedance spectroscopic (EIS) tests, polarization potentiodynamic polarization measurements and long-term immersion experiments. EIS tests were conducted at open circuit potential (OCP) in the frequency from 100 kHz to 10 mHz with a sinusoidal signal perturbation of 10 mV. Corrosion potential (Ecorr) and corrosion current density (icorr) were obtained by Tafel extrapolation method from the polarization curves obtained at a scan rate of 1 mV s-1. Platinum plate and Ag/AgCl electrode were used as counter electrode and reference electrode, respectively. OCPs were stabilized by immersing the samples for about 20 min. During the long-term immersion, specimens were taken out and the morphologies as well as the wettability of the surfaces were characterized. Anti-icing performance was evaluated by a visual and qualitative method where specimens were placed sideways to the wall of a petri dish 7



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containing water and then frozen in a freezing chamber. Specimens were compulsively drawn out from ice by external mechanical forces. As well, the water CA and SA values were measured every time of drawing to test the durability. Ice adhesion strength was repeatedly determined by a homemade apparatus (see Figure S1), which is set up according to the previous work.47 Condensation and frosting processes were observed through an optical microscope (M Plan Apo 10X, Mitutoyo Corporation) on a cooling stage connected with a cryostat. The environment temperature and air humidity were -20±2 °C and 65±5%, respectively. All the measurements were repeated for at least three times to ensure the reproducibility. 3. RESULTS AND DISCUSSION Figure 1 displays the XRD patterns of the hydrothermally prepared coating on the AZ31 Mg alloy. Distinct and sharp diffraction peaks corresponding to Mg5(CO3)4(OH)2∙4H2O (JCPDS 25-0513) are detected, which implies the crystalline magnesium hydroxy carbonate with a high degree of crystallinity. A specific diffraction peak at 2θ position of ~11.4° can be assigned to the typical layered double hydroxides (LDH) phase, most likely the reported phase of Mg6Al2(OH)18∙4H2O (JCPDS 38-0478). The crystalline plane spacing corresponding to the characteristic LDH reflection of (003) is calculated to be about 7.74 Å. The other characteristic reflection of (006) cannot be easily identified due to the overlap with that of magnesium hydroxy carbonate. Therefore, the hydrothermally prepared coating on the AZ31 Mg alloy is composed of the mixture of Mg-Al LDH and Mg carbonate phases (denoted as LDH/carbonate). TEM observations (see Figure S2) further confirm the crystalline structure. Clear lattice fringes assigned to Mg5(CO3)4(OH)2 can be 8


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found in most of the debris scraped from the composite coating. Lattice fringes corresponding to LDH can be hardly found, which may be due to the little content and instability of LDH single layers under the high-power electron beam. The surface and cross-section morphologies of the LDH/carbonate composite coating are shown in Figure 2. Significant double-layered structure with each layer of ~8 µm in thickness was found in the one-step hydrothermally prepared coating. When cutting the coated substrate, a top layer was separated from the substrate owing to a low adhesion. The detached top layer and under layer exhibits entirely different morphology, which was obviously observed under SEM. The top layer is constructed by many vertically cross-linked sheets forming interconnected porous arrays. Each single sheet possesses an ultra thin thickness of ~100 nm, which agrees with the TEM observations (see Figure S2). In the case of low intensity of LDH diffraction peaks in the present composite coating, the self-assembled nano-sheets should be mainly consisted of Mg5(CO3)4(OH)2∙4H2O.48 By contrast, the under layer is relatively more compact, which is tightly adhered to the substrate. Rough surface with uneven textures can be found for the under layer by magnification. Such a porous top layer with interconnected cavities serves as micro- and nano-containers for the following water-immiscible lubricant infusion. Furthermore, the compact under layer performs as a barrier for protecting the Mg alloy substrate from corrosive medium. Thus, a layered architecture is designed on Mg alloy to provide multiple corrosion barriers, which includes the infused lubricant, the SAMs coated porous top layer and the compact LDH/carbonate composite under layer, as demonstrated in Figure 2e. The EDS 9



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results (see Figure S3) suggest that the LDH/carbonate coating is primarily composed of C, O, Mg and a trace amount of Al at the surface. All the elements are evenly distributed on the surface indicating a uniform conversion layer with complex structures on the AZ31 Mg alloy. Surface chemical states were analyzed in order to further understand the differences between the top layer and under layer of the LDH/carbonate coating. Here, the top layer was carefully removed to obtain the under layer for XPS detections. Figure 3 reveals the C 1s, O 1s, Mg 1s and Al 2p XPS core-level spectra corresponding to the two layers in the LDH/carbonate coating. The peak at 284.6 eV corresponding to C-C is used for the calibration of binding energy (BE) scale. Another peak centered at 289.3-289.6 eV can be assigned to CO32-,49 which confirms the constituent of Mg5(CO3)4(OH)2∙4H2O in the coating. It is found that the BE of CO32- in the under layer shifts towards the lower BE direction compared with that in the top layer. It might result from the lower BE of CO32(289.2 eV) in the intercalation of LDH than that in the magnesium carbonate compounds.50 The peak corresponding to CO32- can be further fitted by two independent peaks (not demonstrated here) including one for magnesium carbonate at BE of 289.6 eV and another for intercalated carbonate ion at BE of 289.2 eV. Moreover, the intensity of peak corresponding to CO32- decreases for the under layer. As a consequence, we can draw the conclusion that the top layer is rich in Mg5(CO3)4(OH)2·4H2O while the under layer has more content of Mg6Al2(OH)18·4H2O (i.e. Mg-Al LDH intercalated by CO32-). An undefined peak at about 285.7 eV may be correlated with C-OR from the adsorbed impurities. The O 1s spectrum can be fitted by three peaks at BEs of 529.3, 530.8 and 10


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531.8 eV corresponding to O2-, OH- and CO32-, respectively.50 An increased peak area of OH- is obtained for the under layer, which is ascribed to the relatively high content of Mg-Al LDH. Correspondingly, the broad Mg 1s spectrum is perfectly fitted by three peaks at BEs of 1303.3, 1303.9 and 1304.7 eV assigned to Mg2+ in its hydroxides, oxides and carbonates, respectively.49, 50 Similarly, the increased contribution of Mghyd2+ peak agrees well with the different components in the two layers. The Al 2p peak for the top layer is inconspicuous, which implies a very low content of Mg-Al LDH. Whereas, the Al 2p spectrum in the under layer is well fitted by one peak at BE of 73.4 eV, which is considered to be Al3+ in the Mg-Al LDH. Consequently, the as-prepared composite coating is composed of a carbonate top layer and a LDH/carbonate composite under layer. It is probably ascribed to the surface reaction of the specimen with CO2 or CO32- during the hydrothermal process. Moreover, the XPS core-level spectra of the PTES modified coating are also analyzed (see Figure S4). The peaks located at BEs of 284.6, 285.7, 289.6, 291.5 and 293.7 eV are corresponding to CH2-CH2, C-OR, CO32-, CF2-CF2 and CF3-CF2 groups, respectively.51 The only peak located at BE of 688.9 eV is assigned to C-F species. It implies that the PTES coating is successfully self-assembled as SAMs on the outermost surface after immersion in ethanol solution of PTES and subsequent heat treatment.. Figure 4 exhibits the optical photographs of 5 µl water droplets on the as-prepared LDH/carbonate, PTES-modified SHS and PFPE-infused SLIPS coatings. It can be seen that the water droplet quickly spreads and permeates through the LDH/carbonate surface, thus giving a CA value of approximate 0. The superhydrophilicity of the LDH/carbonate coating is caused by the porous structure of the top layer. After PTES modification, SAMs 11



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covers the porous surface and the water droplet can stand on it with a CA value of about 163°. The superhydrophilicity of the SHS coating results from the air pockets trapped in the cavities. Water CA value declines to about 122° after PFPE infusion. The infused water-immiscible lubricant occupies the cavities leaving no space for air. Therefore, the contact interface is actually a composite solid-lubricant-water interface for a water droplet on the SLIPS coating.35, 52 Both the SHS and SLIPS coated Mg alloys exhibit excellent water repellency (see Figure S5). The water droplet (5 µl) can gradually slide off the SLIPS coated Mg alloy as the substrate is slightly tilted by ~3° (see Supporting Information, Video S1). From the video screenshot given in Figure 4d, the advancing (θA) and receding (θR) CAs are determined to be 122±2° and 110±2°, respectively. It indicates a little higher CA hysteresis of ~12° for the present SLIPS coating compared to the previously reported work (usually

Robust Slippery Coating with Superior Corrosion Resistance and Anti

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