Preparation of Ti–Zr-Based Conversion Coating on 5052 Aluminum Alloy, and Its Corrosion Resistance and Antifouling Performance

coatings

Article Preparation of Ti–Zr-Based Conversion Coating on 5052 Aluminum Alloy, and Its Corrosion Resistance and Antifouling Performance

Hehong Zhang 1,2, Xiaofeng Zhang 1, Xuhui Zhao 1,*, Yuming Tang 1,* and Yu Zuo 1 1 Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, China; [email protected] (H.Z.); [email protected] (Xi.Z.); [email protected] (Y.Z.) 2 China Special Equipment Inspection and Research Institute, Beijing 100029, China * Correspondence: [email protected] (Xu.Z.); [email protected] (Y.T.); Tel.: +86-10-6443-4908 (Xu.Z.); +86-10-6443-4908 (Y.T.) Received: 14 September 2018; Accepted: 10 November 2018; Published: 12 November 2018

Abstract: A chemical conversion coating on 5052 aluminum alloy was prepared by using K2ZrF6 and K2TiF6 as the main salts, KMnO4 as the oxidant and NaF as the accelerant. The surface morphology, structure and composition were analyzed by SEM, EDS, FT–IR and XPS. The corrosion resistance of the conversion coating was studied by salt water immersion and polarization curve analysis. The influence of fluorosilane (FAS-17) surface modification on its antifouling property was also discussed. The results showed that the prepared conversion coating mainly consisted of AlF3·3H2O, Al2O3, MnO2 and TiO2, and exhibited good corrosion resistance. Its corrosion potential in 3.5 wt % NaCl solution was positively shifted about 590 mV and the corrosion current density was dropped −2 from 1.10 to 0.48 µA cm . By sealing treatment in NiF2 solution, its corrosion resistance was further improved yielding a corrosion current density drop of 0.04 µA cm−2. By fluorosilane (FAS-17) surface modification, the conversion coating became hydrophobic due to low-surface-energy groups such as ◦ CF2 and CF3, and the contact angle reached 136.8 . Moreover, by FAS-17 modification, the corrosion resistance was enhanced significantly and its corrosion rate decreased by about 25 times.

Keywords: conversion coating; aluminum alloy; ﬂuorosilane modiﬁcation; corrosion resistance; antifouling performance

1. Introduction Aluminum alloys are widely used in various ﬁelds such as aerospace, automotive, transportation, petrochemical, construction and electronics industries due to their high strength, good thermal and electrical conductivity, good plasticity and lack of magnetism [1,2]. However, aluminum alloys have a poor corrosion resistance due to their active chemical property. Additionally, aluminum alloys are vulnerable to pollution by organics, so it is necessary to treat the surface of aluminum alloys before application. Common approaches to improve the corrosion resistance of aluminum alloys include anodic oxidation, electroplating, microarc oxidation, chemical conversion coating and other techniques. Conventional chromate conversion coatings exhibit excellent corrosion resistance and “self-healing” function, but are extremely toxic carcinogens and cause serious environmental pollution [3]. The research for chromate conversion coating substitutes has more than 20 years of history, but in the last few years it has become more intense, due to the fact that in 2007 the chromate coatings were banned from European industry except from aeronautics applications that still have a permission to use these treatments. As the ROHS (Restriction of Hazardous Substances), ELV (End-of-Life Vehicle) and WEEE (Waste Electrical and Electronic Equipment) regulations launched,

Coatings 2018, 8, 397; doi:10.3390/coatings8110397 www.mdpi.com/journal/coatings Coatings 2018, 8, 397 2 of 11 hexavalent chromium was strictly limited [4]. The research of non-chromate conversion coatings has become a central issue. Numerous non-chromate conversion coatings were investigated, such as a Mo-based conversion coating [4], Ce-based conversion coatings [2,5,6], Ti-based conversion coating [7], Zr-based conversion coating [8], Mn-based conversion coating [9], phosphate conversion coating [10], V-based conversion coating [11], and Ti–Zr-based conversion coating [12]. Because of their incomparable advantage of being environmentally friendly, these non-chromate conversion coatings could be used as suitable alternatives. The Ti–Zr-based conversion coating has a high corrosion resistance and is already applied in industry. Many researchers have pointed out that Ti–Zr-based conversion coatings are optimal to achieve the same performance as the chromate conversion coatings [12–18]. Liu et al. studied on adhesion strength and corrosion resistance of a Ti–Zr aminotrimethylene phosphonic acid composite conversion coating on 7A52 aluminum alloy [12]. Nordlienet et al. fabricated Ti/Zr oxide film on AA6060 aluminum alloys and studied the mechanism of film formation [13]. Lunder et al. also fabricated a Ti/Zr-based conversion coating on AA6060 aluminum alloys—the results showed that the conversion coating slightly restrained the cathodic activity, but was expected to improve the corrosion resistance of aluminum significantly [14]. Guan et al. developed a novel Ti–Zr-based conversion coating on electrogalvanized steel—the corrosion current density of the substrate decreased by two orders of magnitude due to the formation of the coating, which improved the corrosion resistance significantly [15]. Yi et al. fabricated a golden Ti–Zr-based conversion coating on AA6063 aluminum alloys; the mechanism of film formation was also discussed. Their results found a reduced corrosion current density and a significantly improved corrosion resistance [16]. Coloma et al. developed several Cr-free conversion coatings based on inorganic salts (Zr/Ti/Mn/Mo), which can significantly improve the corrosion resistance of aluminum alloys, although these coatings do not still meet the stringent requirements of the aircraft industry [17]. Zhu et al. developed a Ti/Zr/V conversion coating on the surface of aluminum alloy 6063 (AA6063), and reported that its anticorrosion performance and adhesion properties were superior to that of chromate conversion coating [18]. However, the traditional Ti–Zr-based conversion treatment is mainly aimed at improving the anticorrosion performance and seldom enhances the antifouling performance. In this work, we developed a Ti–Zr-based conversion coating on 5052 aluminum alloys, using fluorosilane (FAS-17) modification as a post-treatment. The results indicate that both the anticorrosion performance and the antifouling performance of aluminum alloys are improved.

2. Materials and Methods 5052 aluminum alloy foils, whose chemical composition is composed of Mg 2.2%–2.8%, Si 0.25%, Cu 0.1%, Zn 0.1%, Mn 0.1%, Fe 0.4%, Cr 0.15%–0.35% and balance of Al, were used as substrate. The samples were first cut to dimensions of 20 mm × 10 mm × 2 mm, abraded with abrasive papers up to 1000 times, then were wiped with acetone, degreased in an alkali solution (NaOH: 40g L−1, −1 −1 −1 ◦ Na2CO3: 20 g L , Na3CH5O7·2H2O: 2.28 g L , SDBS: 1.5 g L ) at 50 C for about 5 min, followed −1 by acid pickling (HNO3: 211 mL L ) at room temperature for about 2 min, and finally rinsed with deionized water. −1 −1 The conversion treatment solution was composed of KMnO4: 2–4 g L , NaF: 0.1 g L ,K2ZrF6: −1 −1 0.25–0.75 g L ,K2TiF6: 1–1.5 g L . The pH was about 2, the treatment temperature was in the range of 70–80 ◦C and the treatment time was about 5 min. The sealing treatment solution was composed of −1 −1 NiSO4·6H2O: 1.2 g L , NaF: 0.6 g L , the treatment time was about 30 min and treatment temperature was room temperature. After the conversion treatment, the samples were treated in boiling water for about 30 min, then immersed in fluorosilane (FAS-17: 2 g, isopropanol: 180 g, H2O: 18 g) at room temperature for about 24 h. The samples were then dried in oven at 120 ◦C for about 30 min. Scanning electron microscopy (SEM, S-4700, Hitachi, Tokyo, Japan) was used to investigate the surface morphology and energy dispersive X-ray spectroscopy (EDS, Isis 300, Oxford Instrument, Oxfordshire, UK) was used to analyze the chemical composition of the coating. The operating potential Coatings 2018, 8, 397 3 of 11 of the field emission source was 20 kV. The samples were coated with gold to preclude the charging effect during measurement. A Fourier-transform infrared spectrometer (FT–IR, Tensor27, Bruker Co., Hamburg, Germany) was used to analyze the atomic groups in the coating, and infrared spectra were recorded in absorbance units in the 400–4000 cm−1 range. The spectra of powder samples were tested with KBr pellet method. The scan number was 30 and the spectral resolution was 16 cm−1. The surface composition of the coating was then measured via X-ray photoelectron spectroscopy (XPS) with a monochromated Al Kα source (1486.6 eV). The operating pressure was less than 8–10 Pa. The measured binding energy (BE) range for the wide scan spectrum was from 0 to 1350 eV in 1 eV steps without sputtering. The binding energy values were calibrated by the C1s peak at 285 eV. The data were fitted by XPSPEAK v4.1 software. The corrosive behavior was studied with a weight loss test and polarization measurement. The tests were both carried out in 3.5 wt % NaCl solution. The weight loss test was carried out at 45 ◦C for 504 h. Specimens for the polarization test were mounted in phenolic resin leaving an exposed area of 1 cm2. Polarization measurement was performed using a CS350 electrochemical workstation. The potential scanning range was −0.3–1.5 V relative to the open circuit potential (OCP) and the scanning rate was 10 mV s−1. The reference electrode was a saturated calomel electrode (SCE), the counter electrode was a Pt electrode and the specimen was a working electrode. The water contact angles were measured by Dataphysics OCA 20 with a distilled water droplet volume of 4 µL. A simulated pollutant test was carried out to analyze the antifouling performance. The pollutant was a mixture of tallow, liquid paraffin, carbon black, and fluorescent substance. The fluorescent substance in residue was observed by a fluorescence microscope (BX53) after the pollutant was removed from the coating. All the reagents used were of analytic grade. The cleaning rate was calculated through Equation (1). Three trials were performed in order to ensure the accuracy of the test. Average cleaning rate was obtained.

Cleaning rate = (m2 − m3)/(m2 − m1) × 100% (1) where m1: mass of samples before pollutant coated; m2: mass of samples after pollutant coated; m3: mass of samples after pollutant removed.

3. Results and Discussion

3.1. Characterizations of the Coating A compact yellow conversion coating was prepared on the 5052 aluminum alloys after the conversion treatment. Figure1 shows the surface morphologies of the coating. A continuous, compact and well-covered coating can be seen by comparing Figure1a,b. The obtained coatings are smooth; however, they became coarse after FAS-17 modification treatment as seen in Figure1c,d. Some micro-protrudes were generated on the coating surface [19]. These micro-protrudes may increase the roughness of the surface and decrease the surface energy, which improves the hydrophobic properties of the coating. All of these characteristics may be related to the antifouling performance of the coating. Figure2 shows the EDS spectra of the coating. The conversion coating contains Al, O, F, Mg, Zr, Ti and Mn as shown in Figure2a. The concentration of Al, Ti and Mn is relatively high, and they may exist in oxide or fluoride states. The concentration of Zr is relatively low, and it is usually covered by other elements [20,21]. After the FAS-17 post treatment, C and Si elements emerged (Figure2b). As is well known, the FAS-17 contains C–F and Si–O atom groups, and the spectrum indicated that a fluorosilane film was produced on the surface of the conversion coating after FAS-17 modification. CoCoatingsatings 20182018,, 88,, x 397 FOR PEER REVIEW 4 4of of 11 11 Coatings 2018, 8, x FOR PEER REVIEW 4 of 11

Figure 1. SEM images of conversion-coated 5052 aluminum alloy: (a) Al substrate; (b) conversion coating;Figure 1. (c ,SEMd) after images FAS- 17 of modification conversion-coatedconversion-coat. ed 5052 aluminum alloy:alloy: ( a) Al substrate; (b) conversionconversion coating; (c,d) after FAS-17FAS-17 modiﬁcation.modification.

Al (b) Al

O Mg F O Zr Mg OMg C F Ti Mn F Zr C Mg Si SiO Ti Mn C Ti Mn F Si C Ti Mn 0.0 1.3 2.6 3.9 5.2 6.5 7.8 9.1 10.4 11.7 13.0 0.0Si 1.3 2.6 3.9 5.2 6.5 7.8 9.1 10.4 11.7 13.0 0.0 1.3 2.6 3.9 Energy/K5.2 6.5 eV7.8 9.1 10.4 11.7 13.0 0.0 1.3 2.6 3.9 Energy/K5.2 6.5 eV7.8 9.1 10.4 11.7 13.0 Energy/K eV Energy/K(a) eV (b) (a) (b) FigureFigure 2. 2. EDSEDS spectra spectra of of conversion conversion-coated-coated 5052 5052 aluminum aluminum alloy alloy:: (a) before (a) before FAS FAS-17-17 modification, modification, (b) after(Figureb) after FAS 2. FAS-17 -EDS17 modification spectra modification. of conversion. -coated 5052 aluminum alloy: (a) before FAS-17 modification, (b) after FAS-17 modification. FTFT–IR–IR was was utilized utilized to to analyze analyze the the compositions compositions of of the the coating. coating. Figure Figure 3 shows the FT–IRFT–IR spectraspectra ofof the theFT conversion conversion–IR was utilized before before andto andanalyze after after FAS the FAS-17- 17compositions modification. modification. of Asthe shown coating. As shown in FigureFigure in Figure33a, shows the3 absorptiona, the the FT absorption–IR aroundspectra −1 3433aroundof the cm conversion− 34331 is assigned cm beforeis to assigned theand absorptions after to FAS the- absorptions17 of modification. a hydroxyl of agroup. As hydroxyl shown The group.characteristicin Figure The 3a, characteristicthe peaks absorption at the range peaks around of at −1 2839the3433 range– cm2978−1 isof cm assigned 2839–2978−1 are assigned to cmthe absorptions are to the assigned absorptions of toa hydroxyl the absorptionsof C– group.H [16, 2The of2] C–H. Thecharacteristic [16 characteristic,22]. The peaks characteristic peak at the at range around peak of −1 −1 1631at2839 around –cm2978−1 is 1631 cm assigned−1 cmare assigned tois the assigned absorptions to the to absorptionsthe of absorptions Al–O. ofThe C of–characterH Al–O. [16,22] Theis. ticThe characteristicpeak characteristic at 1409 cm peak − peak1 is at assigned 1409 at around cm to −1 theis1631 assigned absorptions cm−1 is toassigned the of Mn absorptions to–O. the The absorptions characteristic of Mn–O. of TheAl peak–O. characteristic atThe 1043 character cm−1 peak isis assignedtic at peak 1043 atto cm 1409the absorptions cmis assigned−1 is assigned of to S=O the to −1 inabsorptionsthe surfactant. absorptions of The S=O of characteristicMn in– surfactant.O. The characteristic peak The characteristic at 859 peak cm− 1at is peak1043 assigned atcm 859−1 is to cm assigned the is absorptions assigned to the absorptions to of the Ti– absorptionsO and of S=O the characteristicin surfactant. peak The a characteristict 587 cm−1 is assigned peak at to 859 the cm absorptions−1 is assigned of Al to–F the[23 ]. absorptions A thin fluorosilane of Ti–O film and was the characteristic peak at 587 cm−1 is assigned to the absorptions of Al–F [23]. A thin fluorosilane film was

Coatings 2018, 8, 397 5 of 11 of Ti–O and the characteristic peak at 587 cm−1 is assigned to the absorptions of Al–F [23]. A thin Coatings 2018, 8, x FOR PEER REVIEW 5 of 11 fluorosilaneCoatings 2018, film 8, x FOR was PEER adhered REVIEW to the conversion coating by FAS-17 modification. The spectrum5 of 11 shows some new characteristic peaks located in the range of 1050–1205 cm−1 and 1383–1454 cm−1. adhered to the conversion coating by FAS-17 modification. The spectrum shows some new Theadhered characteristic to the peaks conversion at the range coating of 1050–1205by FAS-17 cm modification.−1 are assigned The to spectrum the absorptions shows of some C–F newin CF characteristic peaks located in the range of 1050–1205 cm−1 and 1383–1454 cm−1. The characteristic 3 characteristic peaks located in the range of 1050–1205 cm−1 and 1383−1 –1454 cm−1. The characteristic and CF2 [24]. The characteristic bands−1 at the range of 1383–1454 cm are assigned to the absorptions peaks at the range of 1050–1205 cm are assigned to the absorptions of C–F in CF3 and CF2 [24]. The ofpeaks CF [ 23at, 25the,26 range]. The of 1050 characteristic–1205 cm−1 peakare assigned at 1204 to cm the−1 absorptionsis assigned of to C the–F in absorptions CF3 and CF2 of [24 CF]. The[25 ]. characteristic3 bands at the range of 1383–1454 cm−1 are assigned to the absorptions of CF3 [23,25,262 ]. characteristic bands at the range of 1383–1454−1 cm−1 are assigned to the absorptions of CF3 [23,25,26]. TheThe characteristic characteristic peaks peak at at 10501204 cm and−1 is 880 assigned cm areto the assigned absorptions to the of CF absorptions2 [25]. The characteristic of Si–O–C and peaks Si–O, The characteristic peak at 1204 cm−1 is assigned to the absorptions of CF2 [25]. The characteristic peaks respectivelyat 1050 and [23 880,24 ].cm−1 are assigned to the absorptions of Si–O–C and Si–O, respectively [23,24]. at 1050 and 880 cm−1 are assigned to the absorptions of Si–O–C and Si–O, respectively [23,24].

a b 880 a 1204 b 880 1204 880 1050 880 2974 1050 1204 1050 2974 1204 3417 1050 after FAS-17 modification after FAS-17 modification 3417 after FAS-17 modification after before FAS-17 FAS-17 modification modification before FAS-17 modification

before FAS-17 modification asorbances/a.u. 2978 2839 asorbances/a.u. before FAS-17 modification

1409 asorbances/a.u. 2921 asorbances/a.u. 1409 2978 2839 1631 859 1409 859 2921 1043 1409 1631 859 587 1043 859 587 1043 587 1043 587 3433

4000 34333500 3000 2500 2000 1500 1000 500 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400

4000 3500 3000 2500 2000 1500 1000 500 1500 1400 1300 1200 1100 1000 900 800-1 700 600 500 400 wavenumber/cm-1 wavenumber/cm -1 wavenumber/cm-1 wavenumber/cm FigureFigure 3. FT–IR 3. FT– spectraIR spectra of of conversion-coated conversion-coated 50525052 aluminumaluminum alloy alloy before before FAS FAS-17-17 modifica modificationtion and and afterFigureafter FAS-17 FAS 3. FT modification:-17–IR modificatio spectra of (nb :conversion )(b is) is (a )(a with) with-coated partial partial 5052 enlargement. enlargement aluminum. alloy before FAS-17 modification and after FAS-17 modification: (b) is (a) with partial enlargement. XPSXPS was was utilized utilized to to characterize characterize the the detailed detailed variationsvariations of of element element information information on on the the conversion conversion coatingcoatingXPS surface. surface.was utilized Figure Figure4 toa showscharacterize4a shows the the overview the overview detailed XPS XPS variations spectra spectra of of of the element the conversion conversion information coating. coating. on AstheAs illustratedconversionillustrated in Figurecoatingin 4Figurea, thesurface. 4a, conversion the Figure conversion 4a coating shows coating is the mainly overviewis mainly composed composedXPS spectra of Al, of ofMn,Al, the Mn, Ti, conversion F,Ti, O F, and O and C.coating. TheC. The peak As peak illustrated of C1sof C1s may inmay Figure be attributed 4a, the conversion to the impurities coating andis mainly surfactant composed, but considering of Al, Mn, its Ti, low F, concentration,O and C. The peakwe omitted of C1s be attributed to the impurities and surfactant, but considering its low concentration, we omitted to mayto perform be attributed a detailed to the analysis. impurities Figure and 4surfactantb–f shows, btheut consideringdetailed peaks its lowof Al2p, concentration, F1s Mn2p, we Ti2p, omitted and perform a detailed analysis. Figure4b–f shows the detailed peaks of Al2p, F1s Mn2p, Ti2p, and O1s toO1s perform spectra, a detailedrespectively. analysis. The Al2p Figure core 4-blevel–f shows spectrum the detailed (Figure 4b)peaks can ofbe Al2p,curve -F1sfitted Mn2p, into three Ti2p, peak and spectra, respectively. The Al2p core-level spectrum (Figure4b) can be curve-fitted into three peak O1scomponents spectra, re withspectively. binding The energies Al2p core at about-level 76.9, spectrum 75.7 and (Figure 74.3 4b)eV, canwhich be curveare assigned-fitted into to AlF three3∙3H peak2O, components with binding energies at about 76.9, 75.7 and 74.3 eV, which are assigned to AlF3·3H2O, componentsAlOx/Al and with Al2O binding3 [12]. The energies F1s spectrum at about (Figure 76.9, 75.7 4c) showsand 74.3 only eV, one which peak are at 686.3eV, assigned corresponding to AlF3∙3H2O, AlOx/Al and Al2O3 [12]. The F1s spectrum (Figure4c) shows only one peak at 686.3eV, corresponding AlOto x the/Al AlFand3 ∙Al3H2O2O3 [12]. [12]. TheThe F1s Mn2p spectrum peaks (Figure (Figure 4c) 4d) shows are composed only one peak of three at 686.3eV, peak structures corresponding with to the AlF ·3H O[12]. The Mn2p peaks (Figure4d) are composed of three peak structures with binding tobinding the AlF3 energies3∙3H2 2O [12].at about The 653.5, Mn2p 642.2 peaks and (Figure 641.3 eV 4d); we are ass composedign these to of thethree MnO peak2 [17 structures–19]. The Ti2p with energiesbindingspectrum at energies about (Figure 653.5, at 4e) about shows 642.2 653.5, andtwo 642.2641.3fitted and eV;peaks we641.3 at assign 464.0 eV; weand these ass 458.2ign to the eVthese corresponding MnO to the2 [17 MnO–19]. 2to [17 The the–19 Ti2pTiO]. The2 [ spectrum16 ,Ti2p27]. (FigurespectrumThe4 detailse) shows (Figure of O1s two 4e) peaks fitted shows (Figure peaks two fiat4f)tted 464.0 reveal peaks and three at 458.2 464.0 peak eV andstructures corresponding 458.2 eVwith corresponding binding to the energies TiO 2to[ 16the at,27 aboutT].iO The2 [ 16532.4 details,27]., of O1sThe531.6 details peaks and 5 (Figureof30.1 O1s eV, peaks4 f)which reveal (Figure are three assigned 4f) peakreveal to structures threethe oxide peaks withofstructures Ti, binding Al and with Mn energies binding, respectively at energies about [16 532.4,].at about 531.6 532.4 and, 530.1531.6 eV, and which 530.1 are eV, assigned which are to assigned the oxides to ofthe Ti, oxide Al ands of Mn,Ti, Al respectively and Mn, respectively [16]. [16]. 5 4.3x103 5x10 (a) (b) 5x105 4.3x103 5 (a) . AlO /Al 4x10 (b) AlF3 3H2O X

O1s Al O 5 3 . AlO /Al2 3 4x10 3.8x10 AlF3 3H2O X 3x105 O1s 3 Al2O3

3.8x10

5 F1s Mn2p 3x10 C1s 2x105

3.3x10

F1s

Intensity(counts/s) Mn2p

5 C1s Intensity(counts/s)

2x10 5 Ti2p

1x10 3.3x103

Intensity(counts/s)

Intensity(counts/s) Al2p 5 Ti2p 1x10 3 0 2.8x10

1,400 1,200 1,000 800 600 400 200 0 80 78 76 74 72 70 Al2p 3 0 B. E. (eV) 2.8x10 B.E.(eV) 1,400 1,200 1,000 800 600 400 200 0 80 78 76 74 72 70 B. E. (eV) Figure 4. Cont. B.E.(eV) FigureFigure 4.4. ContCont..

CoCoatingsatings 20182018,, 88,, x 397 FOR PEER REVIEW 6 6of of 11 11 Coatings 2018, 8, x FOR PEER REVIEW 6 of 11 3.5x104 3.2x104 (c) (d) MnO (2p3/2) 3.5x104 3.2x104 2 Mn O (2p) (c) (d) MnO (2p1/2)MnO (2p3/2) 2 3 2 2 3.3x104 . 3.0x104 AlF3 3H2O MnO (2p1/2) Mn O (2p) 2 2 3 3.3x104 . 3.0x104 AlF3 3H2O

4 4

3.1x10 2.8x10

4 4

3.1x10 2.8x10 Intensity(counts/s)

2.9x104 Intensity(counts/s) 2.6x104 Intensity(counts/s) 2.9x104 Intensity(counts/s) 2.6x104

2.7x104 2.4x104 2.7x106964 694 692 690 688 686 684 682 680 2.4x106604 656 652 648 644 640 636 696 694 692 690 688 686 684 682 680 660 656 652 648 644 640 636 B.E. (eV) B.E. (eV) B.E. (eV) B.E. (eV)

2x104 6x104 2x104(e) 6x104 (f) (e) (f) 4 TiO (Ti2p1/2) 2x10 4 TiO2 (Ti2p1/2) 4 2x10 2 5x104 Al-O 5x10 Al-O 4 TiO (Ti2p3/2) 1x10 4 TiO2 (Ti2p3/2) 1x10 2 4 4x104 4x10 Ti-O

4 Ti-O

1x101x10 4 3x104 Mn-O 4 3x10 Mn-O

1x101x104

Intensity(counts/s)

Intensity(counts/s) Intensity(counts/s) 4 4 2x102x10 1x101x104

4 4 1x101x104 1x101x10 468468 466466 464464 462462 460460 458458 456456 454454 538 536536 534534 532532 530530 528528 526526 B.E.(eV)B.E.(eV) B.E.B.E. (eV) (eV) FigureFigureFigure 4 4.. XPS 4XPS. XPS spectra spectra spectra of of of conversion conversion-coated conversion-coat-coateded 5052 50525052 aluminum aluminum alloy alloy::: ((a (aa)) ) thethe the wide wide wide-scan-scan-scan spectrum, spectrum, spectrum, (b () (b bAl2p)) Al2p Al2p spectrum,spectrum,spectrum, ( (cc) )( F1sc F1s) F1s spectrum, spectrum, spectrum, ( (d ()d) Mn2p) Mn2p Mn2p spectrum, spectrum, spectrum, ( (ee)) Ti2p Ti2p spectrum, and and ( (ff)) O1s O1s spectrum spectrum.spectrum. .

AsAsAs describeddescribed described previously, previously previously thin, ,thin thin fluorosilane fluorosilane fluorosilane films film film weres producedwere produced produced onto the onto onto surface the the of surface surface the conversion of of the the conversioncoatingconversion after coating thecoating FAS-17 after after the modification. the FAS FAS-17-17 modification. modification. The elements TheThe on elementselements the surface onon thethe may surface surface also may may change also also afterchange change this after after step. thisFigurethis step 5step .shows Figure. Figure the 5 shows5 XPS shows spectra the the XP XP ofS S thespectra spectra conversion of of thethe conversionconversion coating after coating FAS-17 afteafte modification.rr FAS FAS-17-17 modification. modification. As illustrated As As illustrated in Figure 5a, the intensity of the F1s peak increased and the Si2s and Si2p spectra emerged. illustratedin Figure5 ina, Figure the intensity 5a, the intensity of the F1sof the peak F1s peak increased increased and and the the Si2s Si2s and and Si2p Si2p spectra spectra emerged.emerged. The C1s spectrum is composed of six fitted peaks with the binding energies at 294.1, 291.8, 286.14, TheThe C1s C1s spectrum spectrum is is composed composed of of six six fitted fitted peaks peaks with with the the binding binding energies energies at at 294.1, 294.1, 291.8, 291.8, 286.14, 286.14, 285.8, 285.2 and 284.8 eV, which are assigned to the CF3, CF2, C–O, CH3, C–C and CH2 groups, 285.8,285.8, 285.2 285.2 and and 284.8 284.8 eV eV,, which are assigned assigned to the the CF CF33, CF CF2,, C C–O,–O, CH3,, C C–C–C and and CH CH22 groups,groups, respectively [19,28,29]. respectivrespectivelyely [19, [19,28,,29]]..

6 2.0x10 3x104 6 (a) F1s 4 2.0x10 3x10 (b) CF (a) F1s 2 6 (b) CF 1.6x10 2 1.6x106 2x104 1.2x106 CH F KLL 2x104 2 1.2x106 CH

2 F KLL C-O

8.0x105

C-O 5 4 CF C-C 8.0x10 O1s C1s 1x10 3 CH

5 Si2s Si2p 4 C-C3 Intensity(counts/s)

Intensity(counts/s) CF 4.0x10 O1s C1s 1x10 3 CH

5 Si2s Si2p 3 Intensity(counts/s)

4.0x10 Intensity(counts/s) 0.0 0 0.0 1400 1200 1000 800 600 400 200 0 296 294 292 290 288 286 284 282 0 1400 1200 1000 800B.E.(eV)600 400 200 0 296 294 292 B.E.(eV)290 288 286 284 282 Figure 5. XPS spectra ofB.E.(eV) conversion-coated 5052 aluminum alloy after FAS-17 B.E.(eV)modification: (a) the wide-scan spectrum, (b) C1s spectrum. FigureFigure 5 5.. XPSXPS spectra spectra of of conversion conversion-coated-coated 5052 5052 aluminum aluminum alloy alloy after after FAS FAS-17-17 modification: modiﬁcation: ( (aa)) the the widewide-scan-scan spectrum, ( b) C1s spectrum spectrum..

Coatings 2018, 8, x FOR PEER REVIEW 7 of 11

The results of EDS, FT–IR and XPS show that the main components are AlF3∙3H2O, Al2O3, MnO2 and TiO2. A thin fluorosilane film formed on the surface of the conversion coating after FAS-17 modification. Coatings 2018, 8, 397 7 of 11 3.2. Corrosion Resistance of Coating Samples

FigureThe results 6 shows of EDS,the polarization FT–IR and XPScurves show of the that substrate the main before components and after are conversio AlF3·3Hn 2treatment.O, Al2O3, TheMnO fitting2 and results TiO2. Aare thin shown fluorosilane in Table 1. film It is formedknown that on the the surfacecorrosion of current the conversion density (I coatingcorr) is one after of theFAS-17 most modification. important parameters to evaluate the corrosion resistance of metal substrate [16]. A smaller Icorr value corresponds to better corrosion resistance. As shown in Figure 6 and Table 1, the Icorr 3.2. Corrosion Resistance of Coating Samples decreased to 0.48 μA cm−2 from 1.10 μA cm−2 after conversion treatment, it decreased to 0.04 μA cm−2 after Figure sealing6 shows treatment, the polarization and it dropped curves to 0.005 of the μA substrate cm−2 by before FAS- and17 modification. after conversion The treatment. corrosion potentialThe fitting ( resultsEcorr) positively are shown shifted in Table from1. It is − known1196 to that −590 the mV corrosion by conversion current density treatment ( Icorr ) and is one FAS of- the17 modification.most important It parameters can be concluded to evaluate that the the corrosion corrosion resistance resistance of metal of substrate Al samples [16 ].was A smaller improvedIcorr significantlyvalue corresponds by conversion to better corrosiontreatment resistance. and FAS-17 As modification. shown in Figure 6 and Table1, the Icorr decreased to 0.48 µAA salt cm water−2 from immersion 1.10 µA cm test−2 wasafter utilized conversion to further treatment, analyze it decreased corrosion to resistance 0.04 µA cmof −the2 after conversion sealing −2 coating.treatment, The and results it dropped indicated to 0.005 thatµA the cm corrosionby FAS-17 resistance modification. of aluminum The corrosion alloy potential substrate ( Ewascorr) significantlypositively shifted improved from −by1196 conversion to −590 treatment. mV by conversion The corrosion treatment rate andof aluminum FAS-17 modification. alloy substrate It can in 3.5%be concluded NaCl solution that the dropped corrosion from resistance 0.240 g∙m of−2 Al·h −1 samples to 0.046 wasg∙m− improved2·h −1, and it significantly further dropped by conversion to 0.0118 gtreatment m−2·h −1 after and FAS FAS-17-17 modification. modification.

0.2 Al substrate Conversion treatment 0.0 Conversion+sealing treatment Conversion+FAS-17 modification -0.2

-0.4

-0.6

E vsSCE/V E -0.8

-1.0

-1.2

-1.4 -10 -8 -6 -4 -2 -2 log(i/A cm ) FigureFigure 6. 6. TheThe polarization polarization curves curves of of 5052 5052 aluminum aluminum alloy alloy samples after different treatment treatments.s.

Table 1. Fitting results of the polarization curves for 5052 aluminum alloy after different treatments. Table 1. Fitting results of the polarization curves for 5052 aluminum alloy after different treatments. −2 Samples Ecorr (mV) Icorr (µA cm ) Samples Ecorr (mV) Icorr (μA cm−2) AlAl substrate substrate −1196−1196 1.101.10 ConversionConversion treatment treatment −605−605 0.480.48 Conversion + sealing treatment −587 0.04 Conversion + sealing treatment −587 0.04 Conversion + FAS-17 modiﬁcation −590 0.005 Conversion + FAS-17 modification −590 0.005

3.3. AntifoulingA salt water Performan immersionce Analysis test was utilized to further analyze corrosion resistance of the conversion coating. The results indicated that the corrosion resistance of aluminum alloy substrate was significantly It has been reported that the surface is usually hydrophobic after FAS-17 modification [29–32]. improved by conversion treatment. The corrosion rate of aluminum alloy substrate in 3.5% NaCl Figure 7 shows the contact angle of conversion coating vs water after FAS-17 modification. It can be solution dropped from 0.240 g·m−2·h−1 to 0.046 g·m−2·h−1, and it further dropped to 0.0118 g·m−2·h−1 seen that the contact angle of conversion coating increased from 84.6° to 136.8°; this implies after FAS-17 modification. hydrophilicity. 3.3. Antifouling Performance Analysis It has been reported that the surface is usually hydrophobic after FAS-17 modification [29–32]. Figure7 shows the contact angle of conversion coating vs water after FAS-17 modification. It can be seen that the contact angle of conversion coating increased from 84.6◦ to 136.8◦; this implies hydrophilicity.

Coatings 2018, 8, 397 8 of 11 CoCoatingsatings 20182018,, 88,, xx FORFOR PEERPEER REVIEWREVIEW 88 ofof 1111

FigureFigure 7.7. TheThe contactcontact angle angle ofof conversion conversion-coatedconversion--coatcoateded 50525052 aluminumaluminum alloy:alloyalloy:: ((aa)) before before FASFAS-17FAS--1717 modiﬁcation,modification,modification, (((bb))) afterafterafter FAS-17FASFAS--1717 modiﬁcation. modificationmodification..

TheThe simulatedsimulated pollutantpollutant testtest waswas carriedcarried outout onon thethe FA FAS-17-modifiedFASS--1717--modifiedmodified coatingcoating inin orderorder toto analyzeanalyze thethe antifoulingantifoulingantifouling performance.performance. Fig FigureFigureure 8 88 shows showsshows the thethe cleaning cleaningcleaning rate raterate of ofof aluminum aluminumaluminum alloy alloyalloy samples samplessamples through throughthrough differentdifferent treatment treatments.treatmentss.. AsAs As shownshown shown inin in FigureFigure Figure 7,7,7 the,the the cleaningcleaning cleaning raterate rate ofof samples ofsamples samples withowitho withoututut anyany anytreatment,treatment, treatment, withwith withpretreatment,pretreatment, pretreatment, andand withwith and conversion withconversion conversion treatmenttreatment treatment areare 91%,91%, are 81%81% 91%, andand 81% 83%83% and,, respectively.respectively. 83%, respectively. SSamplesamples Samples havehave aa havehigherhigher a higher cleaning cleaning cleaning rate rate afterafter rate after FAS FAS-- FAS-171717 modification. modification. modification. Taking Taking Taking the the the corrosion corrosion corrosion resistance resistance resistance and and antifouling antifouling performanceperformance intointo account,account, thethe latterlatter exhibexhibitsexhibitsits better better antifouling antifouling performance. performance. As As ii isss provedproved previously,previously, aa thin thin fluorosilanefluorosilane fluorosilane filmfilm film formed formed on on the the surface surface surface of of thethethe conversionconversionconversion coating. coating. coating. Micro-protrudes Micro Micro--protrudesprotrudes werewere generatedgenerated onon thethe surfacesurface andand somesome atomatom groupsgroupsgroups suchsuchsuch asasas CFCFCF333 andandand CFCFCF222,, whichwhich cancan lowerlower thethe surfacesurfacesurface energy,energenergyy,, were werewere seen; seen;seen; these these these groups groups groups eventually eventually eventually resulted reresultedsulted in a hydrophobic in in a a hydrophobichydrophobic surface. surface. surface. The hydrophobic The The hydrophobic hydrophobic surface improvedsurfacesurface improvedimproved the antifouling thethe antifoulingantifouling performance performanceperformance of the conversion ofof thethe conversionconversion coating. coating.coating.

100100 9494 9191 8181 8383 7575

5050

Cleaningrate/% Cleaningrate/% 2525

00 withoutwithout anyany treatmenttreatment pretreatmentpretreatmentconversionconversion treatmenttreatment FAS-17modificationFAS-17modification FigFigureFigureure 88.8.. TheThe cleaningcleaning raterate ofof 50525052 aluminumaluminum alloyalloy samplessamples underunder differentdifferent treatments.treatmenttreatmentss..

InIn orderordorderer tototo analyze analyzeanalyze the thethe antifouling antifoulingantifouling performance performancperformancee intuitively, intuitively,intuitively, fluorescence fluorescencefluorescence microscopy microscopymicroscopy was waswas used usedused to observetoto observe observe the surface the the surface surface of the of of conversion the the conversion conversion coating. coating. coating. Figure 9 Figure Figureshows the 9 9 shows shows fluorescence the the fluorescence fluorescence images of conversion images images of of coating.conversionconversion Figure coating. coating.9a,b are Figure Figure the fluorescence 9a 9a,,bb are are the imagesthe fluorescence fluorescence of aluminum images images alloy ofsamples of aluminum aluminum after conversion allo alloyy samples samples treatment after after andconversionconversion after FAS-17 treatmenttreatment modification, andand afterafter respectively. FASFAS--1717 modificationmodification It can be,, seenrespectively.respectively. from the ItIt images cancan bebe that seenseen no fromfrom green thethe fluorescence imagesimages thatthat substancenono green green canfluorescence fluorescence be detected substance substance [33], indicating can can be be that detected detected the fluorescence [ [3333],], indicating indicating substance that that comes the the fluorescence fluorescence from coated substance substancepollutant, notcomescomes the fromfrom FAS-17. coatedcoated Figure pollutant,pollutant,9c,d are notnot the thethe fluorescenceFASFAS--17.17. FigureFigure images 9c9c,,dd areare of the coatingthe fluorescencefluorescence after pollutant imaimagesges wasofof coatingcoating removed. afterafter Bypollutantpollutant comparing waswas theremoved.removed. Figure 9 ByByc,d, comparingcomparing we observe thethe that FigureFigure the samples 9c9c,,d,d, wewe after observeobserve FAS-17 thatthat modification thethe samplessamples have afterafter less FASFAS green--1717 fluorescencemodificationmodification substance have have less less greenand green the fluorescence fluorescence brightness substance issubstance relatively and and small. the the brightness brightness These results is is relatively relatively indicate small. thatsmall. there The These isse lessresultsresults pollutant indicateindicate left thatthat on therethere the surfaceisis lessless pollutapolluta of thentnt coating leftleft onon after thethe surfacesurface FAS-17 ofof treatment, thethe coatingcoating corresponding afterafter FASFAS--1717 to treatment,treatment, a better antifoulingcorrespondingcorresponding performance. toto aa betterbetter antifoulingantifouling performance.performance.

Coatings 2018, 8, 397 9 of 11 Coatings 2018, 8, x FOR PEER REVIEW 9 of 11

FigFigureure 9 9.. Fluorescence microscope images images of of the the coating coating before before and and after after the the pollutant pollutant was was removed: removed: before pollutant pollutant coated coated (a (a) )after after convers conversion,ion, (b ()b after) after FAS FAS-17-17 modification; modification; after after pollutant pollutant removed removed (c) before(c) before FAS FAS-17-17 modification, modification, (d) (afterd) after FAS FAS-17-17 modification modification.. 4. Conclusions 4. Conclusions

Conversion coating was prepared on the surface of 5052 aluminum alloys by using K2ZrF6 and Conversion coating was prepared on the surface of 5052 aluminum alloys by using K2ZrF6 and K2TiF6 as the main salts, KMnO4 as the oxidant and NaF as the accelerant. The coating was mainly K2TiF6 as the main salts, KMnO4 as the oxidant and NaF as the accelerant. The coating was mainly composed of AlF3·3H2O, Al2O3, MnO2 and TiO2. composed of AlF3∙3H2O, Al2O3, MnO2 and TiO2. The Ecorr of aluminum alloy samples after conversion treatment and FAS-17 modification The Ecorr of aluminum alloy samples after conversion treatment and FAS-17 modification positively shifted about 600 mV. The Icorr decreased by about three orders of magnitude, and the positively shifted about 600 mV. The Icorr decreased by about three orders of magnitude, and the corrosion resistance significantlysignificantly improved.improved. A thin fluorosilane film formed on the surface of the conversion coating after FAS-17 modification. A thin fluorosilane film formed on the surface of the◦ conversion coating after FAS-17 modification.The surface was The hydrophobic surface was and hydrophobic the contact angleand the reached contact 136.8 angle. The reached antifouling 136.8°. performance The antifouling of the performanceconversion coating of the improvedconversion after coating the FAS-17improved modification, after the FAS corresponding-17 modifica totion a cleaning, corresponding rate of 94%. to a cleaningAuthor Contributions: rate of 94%. Conceptualization, Xi.Z. and Xu.Z.; Methodology, H.Z.; software, H.Z.; Validation, H.Z.; Formal Analysis, H.Z.; Investigation, Xu.Z.; Resources, Y.Z.; Data Curation, Xu.Z.; Writing—Original Draft AuthorPreparation, Contributions: H.Z.; Writing—Review Conceptualization, and Editing, Xi.Z. and Y.T.; X Visualization,u.Z.; Methodology, Xu.Z.; Supervision,H.Z.; software, Xi.Z., H.Z.; Xu.Z., Validation, Y.T. and H Y.Z.;.Z.; FProjectormal Administration,Analysis, H.Z.; Xu.Z.;Investigation, Funding X Acquisition,u.Z.; Resources, Xu.Z. Y and.Z.; Y.Z.Data Curation, Xu.Z.; Writing—Original Draft PFunding:reparation,This H. researchZ.; Writing received—Review no external and Editing, funding. Y.T.; Visualization, Xu.Z.; Supervision, Xi.Z., Xu.Z., Y.T. and YConflicts.Z.; Project of Interest:Administration,The authors Xu.Z declare.; Funding no conflictAcquisition, of interest. Xu.Z. and Y.Z. Funding: This research received no external funding. References Conflicts of Interest: The authors declare no conflict of interest. 1. Wang, Z.X.; Ma, Y.L.; Wu, H.P.; Liao, Y.; Lin, Z.H.; Zhang, H.Y.; Zhang, Y.; Mori, K. Preparation and Properties Referencesof Silane Coating on 5182 Aluminum Alloy. Surf. Technol. 2018, 5, 256–264. (In Chinese) [CrossRef] 2. Arthanari, S.; Shin, S.K. A simple one step cerium conversion coating formation on to magnesium alloy and

1. Wang,electrochemical Z.X.; Ma, corrosion Y.L.; Wu, performance. H.P.; Liao, Y.;Surf. Lin, Coat. Z.H.; Technol. Zhang,2018 H.Y.;, 349 , Zhang, 757–772. Y.; [ CrossRef Mori, K.] Preparation and 3. PropertiesZhong, X.; Wu, of X.S.; Silane Jia, Y.Y.; Coating Liu, Y.L. on Self-repairing 5182 Aluminum Vanadium-Zirconium Alloy. Surf. Composite Technol. Conversion2018, 5, 256 Coating–264, doi:for Aluminum10.16490/j.cnki.issn.1001 Alloys. Appl.-3660.2018.05.039 Surf. Sci. 2013, 280. (In, 489–493. Chinese) [CrossRef ]

2.4. Arthanari,Liang, C.S.; S.; Lv, Shin, Z.F.; Zhu,S.K. A Y.L.; simple Xu, S.A.;one Wang,step cerium H. Protection conversion of Aluminum coating formation Foil AA8021 on to by magnesium Molybdate-based alloy andConversion electrochemical Coatings. Appl. corrosion Surf. Sci. 2014 performance., 280, 497–502. Surf. [CrossRef Coat.] Technol. 2018, 349, 757–772, doi:10.1016/j.surfcoat.2018.06.051.

Coatings 2018, 8, 397 10 of 11

5. Valdez, B.; Kiyota, S.; Stoytcheva, M.; Zlatev, R.; Bastidas, J.M. Cerium-based conversion coatings to improve the corrosion resistance of aluminum alloy 6061–T6. Corros. Sci. 2014, 87, 141–149. [CrossRef] 6. Fahrenholtz, W.G.; O’Keefe, M.J.; Zhou, H.F.; Grant, J.T. Characterization of cerium-based conversion coatings for corrosion protection of aluminum alloys. Surf. Coat. Technol. 2002, 155, 208–213. [CrossRef] 7. Xia, X.F.; Gu, Y.Y.; Xu, S.A. Titanium conversion coatings on the aluminum foil AA 8021 used for lithium–ion battery package. Appl. Surf. Sci. 2017, 419, 447–453. [CrossRef] 8. Cerezo, J.; Vandendael, I.; Posner, R.; de Wit, J.H.W.; Mol, J.M.C.; Terryn, H. The effect of surface pre-conditioning treatments on the local composition of Zr-based conversion coatings formed on aluminium alloys. Appl. Surf. Sci. 2016, 366, 339–347. [CrossRef] 9. Rani, R.U.; Sharma, A.K.; Mayanna, S.M.; Bhojraj, H.; Bhandari, D.R. Black permanganate conversion coatings on aluminium alloys for thermal control of spacecraft. Surf. Eng. 2005, 21, 198–203. [CrossRef] 10. Zhao, M.; Li, J.; He, G.; Xie, H.; Fu, Y. An investigation of the effect of a magnetic ﬁeld on the phosphate conversion coating formed on magnesium alloy. Appl. Surf. Sci. 2013, 282, 499–505. [CrossRef] 11. Ma, Y.B.; Li, N.; Li, D.Y.; Zhang, M.L.; Huang, X.M. Characteristics and corrosion studies of vanadate conversion coating formed on Mg-14wt%Li-1wt%Al-0.1wt%Ce alloy. Appl. Surf. Sci. 2012, 261, 59–67. [CrossRef] 12. Liu, Q.; Cao, X.M.; Du, A.; Ma, RN.; Zhang, X.R.; Shi, T.T.; Fan, Y.Z.; Zhao, X. Investigation on adhesion strength and corrosion resistance of Ti-Zr aminotrimethylene phosphonic acid composite conversion coating on 7A52 aluminum alloy. Appl. Surf. Sci. 2018, 458, 350–359. [CrossRef] 13. Nordlien, J.H.; Walmsley, J.C.; Østerberg, H.; Nisancioglu, K. Formation of a zirconium-titanium based conversion layer on AA 6060aluminum. Surf. Coat. Technol. 2002, 153, 72–78. [CrossRef] 14. Lunder, O.; Simensen, C.; Yu, Y.; Nisancioglu, K. Formation and characterization of Ti–Zr based conversion layers on AA6060 aluminum. Surf. Coat. Technol. 2004, 184, 278–290. [CrossRef] 15. Guan, Y.; Liu, J.G.; Yan, C.W. Novel Ti/Zr Based Non-Chromium Chemical Conversion Coating for the Corrosion Protection of Electrogalvanized Steel. Int. J. Electrochem. Sci. 2011, 6, 4853–4867. 16. Yi, A.H.; Li, W.F.; Du, J.; Mu, S.L. Preparation and Properties of Chrome-free Colored Ti/Zr Based Conversion Coating on Aluminum Alloy. Appl. Surf. Sci. 2012, 258, 5960–5964. [CrossRef] 17. Coloma, P.S.; Izagirre, U.; Belaustegi, Y.; Jorcin, J.B.; Cano, F.J.; Lapeña, N. Chromium free conversion coatings based on inorganic salts (Zr/Ti/Mn/Mo) for aluminum alloys used in aircraft applications. Appl. Surf. Sci. 2015, 354, 24–35. [CrossRef] 18. Zhu, W.; Li, W.F.; Mu, S.L.; Fu, N.Q.; Liao, Z.M. Comparative study on Ti/Zr/V and chromate conversion treated aluminum alloys: Anti-corrosion performance and epoxy coating adhesion properties. Appl. Surf. Sci. 2017, 405, 157–168. [CrossRef]

19. Dong, Y.H.; Li, J.; Zhang, Y.Z.; Hou, Y.L. Self-cleaning Function of TiO2 Nanoparticles Aeronautical Coatings. Chin. J. Rare Met. 2014, 38, 741–748. (In Chinese) [CrossRef] 20. Wu, Y.H.; Li, X.M.; Yang, W.J.; Gao, L. Preparation and Characterization for Permanganate Chemical Conversion Film on LY12 Aluminum Alloy. Surf. Technol. 2011, 40, 81–85. (In Chinese) [CrossRef] 21. Dabala, M.; Brunelli, K.; Napolitani, E.; Magrini, M. Cerium-based Chemical Conversion Coating on AZ63 Magnesium Alloy. Surf. Coat. Technol. 2003, 172, 227–232. [CrossRef] 22. Yu, S.X.; Zhang, R.J.; Tang, Y.F.; Ma, Y.L.; Du, W.C. Composition and Performance of Nanostructured Zirconium Titanium Conversion Coating on Aluminum-Magnesium Alloys. J. Nanomater. 2013, 2013. [CrossRef] 23. Yim, J.H.; Santigo, V.R.; Williams, A.A.; Gougousi, T.; Pappas, D.D.; Hirvonen, J.K. Atmospheric pressure plasma enhanced chemical vapor deposition of hydrophobic coatings using fluorine-based liquid precursors. Surf. Coat. Technol. 2013, 234, 21–32. [CrossRef] 24. Jenog, H.J.; Kim, D.K.; Lee, S.B.; Kwon, S.H.; Kadono, K. Preparation of Water-Repellent Glass by Sol–Gel Process Using Perfluoroalkylsilane and Tetraethoxysilane. J. Colloid Interface Sci. 2001, 235, 130–134. [CrossRef][PubMed] 25. Wang, T.C.; Chang, L.J.; Yang, S.; Jia, Y.; Wong, C.P. Hydrophobic properties of biomorphic carbon surfaces prepared by sintering lotus leaves. Ceram. Int. 2013, 39, 8165–8172. [CrossRef] 26. Kristalyn, C.B.; Watt, S.; Spanninga, S.A.; Barnard, R.A.; Nguyen, K.; Chen, Z. Investigation of sub-monolayer, monolayer, and multilayer self-assembled semifluorinated alkylsilane films. J. Colloid Interface Sci. 2011, 353, 322–330. [CrossRef][PubMed] Coatings 2018, 8, 397 11 of 11

27. Zhang, J.J.; Li, W.F.; Du, J.; Han, D.; Zheng, X.P. Investigation of the Ce-Mn conversion coating on 6063 aluminum alloy. Chin. Sci. Bull. 2010, 17, 1734–1737. (In Chinese) [CrossRef] 28. Yan, L.L.; Wang, K.; Wu, J.S.; Ye, L. Characterization of fluoroalkylsilane monolayer on polystyrene sphere arrays after plasma treatment. Surf. Sci. 2007, 601, 1394–1402. [CrossRef] 29. Saleema, N.; Sarkar, D.K.; Paynter, R.W.; Chen, X.G. Superhydrophobic Aluminum Alloy Surfaces by a Novel One-Step Process. ACS Appl. Mater. Interfaces 2010, 2, 2500–2502. [CrossRef][PubMed] 30. Saleema, N.; Sarkar, D.K.; Paynter, R.W.; Chen, X.G. Chemical Nature of Superhydrophobic Aluminum Alloy Surfaces Produced via a One-Step Process Using Fluoroalkyl-Silane in a Base Medium. ACS Appl. Mater. Interfaces 2011, 3, 4775–4781. [CrossRef][PubMed] 31. Gao, R.; Wang, J.; Zhang, X.F.; Yan, H.J.; Yang, W.L.; Liu, Q.; Zhang, M.L.; Liu, L.H.; Takahashi, K. Fabrication of superhydrophobic magnesium alloy through theoxidation of hydrogen peroxide. Colloid Surf. A 2013, 436, 906–911. [CrossRef] 32. Wankhede, R.G.; Morey, S.; Khanna, A.S.; Birbilis, N. Development of water-repellent organic–inorganic hybrid sol–gel coatings on aluminum using short chain perfluoro polymer emulsion. Appl. Surf. Sci. 2013, 283, 1051–1059. [CrossRef] 33. Dun, Y.C.; Zhao, X.H.; Tang, Y.M.; Dino, S.; Zuo, Y. Microstructure and corrosion resistance of a fluorosilane modified silane-graphene film on 2024 aluminum alloy. Appl. Surf. Sci. 2018, 437, 152–160. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).