Corrosion resistance and superhydrophobicity of one-step polypropylene coating on anodized AZ31 Mg alloy

Zho-Qi Zhng ,Li Wng ,Mi-Qi Zng ,Rong-Chng Zng,c,* ,Cun-Guo Lin,** ,Zhn-Lin Wng ,Dong-Chu Chn,Qing Zhng

aCorrosion Laboratory for Light Metals,College of Material Science and Engineering,Shandong University of Science and Technology,Qingdao 266590,China

b State Key Laboratory for Marine Corrosion and Protection,Luoyang Ship Material Research Institute,Qingdao 266101,China

c School of Materials Science and Engineering,Zhengzhou University,Zhengzhou 450002,China

d College of Materials Science and Engineering,Chongqing University of Technology,Chongqing 400065,China

e School of Materials Science and Energy Engineering,Foshan University,Foshan 528000,China

Abstract Superhydrophobic coatings have been considerably used in corrosion and its protection of metallic Mg.And the comprehensive performance(hydrophobicity,bonding strength,and corrosion resistance,etc.) of the top coating may be highly dependent on the physical and chemical properties of the primer or under coat.Herein,an integrated superhydrophobic polypropylene (PP) coating was fabricated on the micro-arc oxidized Mg substrate via one-step dipping.Surface morphologies and chemical compositions of the composite coating were examined through Fourier transform infrared spectroscopy (FT-IR),X-ray diffraction (XRD),and field-emissio scanning electron microscopy (FESEM) together with X-ray photoelectron spectroscopy (XPS).The surface wettability of the coating was determined by contact angle and sliding angle.The corrosion-resistant performance was evaluated via electrochemical and immersion measurements.The results showed that the hybrid coating possessed micron-scaled granular structure on the surface with a high water contact angle of 167.2±0.8° and a low water sliding angle of 2.7±0.5°.The corrosion resistance of superhydrophobic coating was obviously enhanced with a low corrosion current density of 8.76×10-9 A/cm2,and the coating still maintained integrity after 248h of immersion in 3.5wt% NaCl aqueous solution.The MAO coating provides better adhesion of PP to the surface.Hence,the superhydrophobic coating exhibited superior bonding strength,corrosion resistance and durability.

Keywords: Magnesium alloy;Corrosion resistance;Superhydrophobicity;Bonding strength;Polymer coating.

1.Introduction

Today,magnesium (Mg) alloys have been broadly used in various industrial applications (automobiles,aerospace,and military equipment,etc.) because of their light weight,high specifi strength and recycling use [1-5].Nevertheless,the high chemical activity of Mg alloys implies that they are easily prone to attack in aqueous solution [6,7].Hence,multifarious surface treatments have been exploited to decrease the corrosion rate of Mg alloys,including micro-arc oxidation coating (MAO) [8-12],chemical conversion coating [13-15],and layer-by-layer (LBL) coating [16,17] together with polymeric coating [18,19].

Among all of the surface modificatio available for Mg and its alloys,MAO coating stands out because of its good wear resistance,high hardness and corrosion resistance [20].However,MAO coating possesses many micro-pores and throughpores or cracks on the surface which can become a transport channel and thus promote the ingress of aggressive ions and water molecules [21].Simultaneously,the micro pores on the surface of MAO coating also provide mechanical interlocking sites,which can prominently enhance the adhesion of organic or polymeric top coating to its substrate.Therefore,a postsealing treatment is of necessity to delay the pre-failure and extend the in-service life of MAO coating.Our group used zinc stearate (ZnSA) to seal the as-obtained MAO coating on the bare AZ31 Mg sheet by electrodeposition [22];as a result,the MAO-ZnSA composite coating exhibited outstanding long-term corrosion-resistant performance.

Polymeric coatings have been widely applied in corrosion protection of metals [2].Commercial polymeric coatings include polyethylene (PE) [23],polypropylene (PP) [24],and polyurethane(PU)[25]and so on.As regards these polymeric coatings,PP coating plays a vital role in the improvement of corrosion-resistant performance for metal since it is a common inexpensive polymer material with excellent water-proof resistance.Nevertheless,PP fil is not easy to keep stable on the surface for its weak adherence.Therefore,it is of great significanc to fin an effective way to enhance the adherence between the outer PP coating and its substrate.Wu et al.[26] prepared a polar maleic anhydride grafted polypropylene(PP-g-MAH) coating on the pretreated substrate by using a phase-separation process;the fabricated PP-g-MAH coating can substantially enhance the adherence of the PP coating synthesized by the same process.Furthermore,our previous studies also indicate that a thin PP film created on the silane or Mg(OH)2-pretreated Mg alloy substrate through addition of small amount of PP-g-MAH,exhibits an enhanced adhesion strength and corrosion resistance [2,27].However,the bonding strength of silane or Mg(OH)2/PP hybrid coatings is relatively weak;thereby,it is urgent to apply a novel undercoat to improve the bonding strength of the PP coating.

So far,superhydrophobic coatings have become one of the main focus for enhancing the corrosion resistance of Mg alloys [28].Usually,a superhydrophobic coating possesses both a rough microstructure and a lower surface energy,and thus results in superhydrophobic performance [29,30].The superhydrophobic surface substantially prolongs the use cycles of Mg alloy since it greatly reduces the contact area between aqueous solution and the coating,and thus restrains the ingress of corrosive medium.Ishizaki et al.[31]used the fluo roalkylsilane(FAS)to modify the prepared cerium oxide coating with nanostructure on the Mg substrate;and the obtained superhydrophobic coating shows excellent corrosion resistance and durability.Zhang et al.[32]fabricated a MAO coating with a robust structure on the Mg-Li-Ca alloy and subsequently modifie by stearic acid (SA,CH3(CH2)16COOH).The synthesized superhydrophobic MAO-SA coating provides a prolonged corrosion protection.However,there still exist some basic problems to be solved.For one thing,the unstable microstructure of superhydrophobic coating is easily destroyed,leading to the disappearance of hydrophobic characteristics of the composite coating [33].For another,many superhydrophobic coatings indwell low adhesion strength to the Mg substrate and are likely detached from the substrate[18].These disadvantages limit the widespread use of superhydrophobic surfaces.Therefore,screening an ideal inner or base coating may be capable of enhancing the overall performance (i.e.bonding strength and corrosion resistance) of the top PP coating.

The purpose of the study is to fabricate a porous MAO coating on AZ31 Mg alloy,subsequently to obtain a micronscaled granular PP coating through one-step dipping.The adhesion,hydrophobicity and corrosion resistance of the hybrid coating are evaluated.A comparison is made among different base coatings.Finally,the super-hydrophobicity and corrosion mechanisms of the coating are proposed.

2.Experiments

2.1.Materials

The Mg alloys AZ31 (as-extruded) were bought from Shandong Yinguang Yuyuan Light Metal Precision Molding Co.,Ltd.(China).Potassium fluorid (KF),sodium hydroxide (NaOH),and polypropylene (PP) together with sodium silicate (Na2SiO3) were supplied by Shandong Xiya Chemical Reagent Co.,Ltd.,China.The AZ31 Mg alloy ingots were cut into 20mm×20mm×5mm,which were successively ground with sandpaper (up to 1500 #).Subsequently,the pre-treated substrate was rinsed with distilled water and alcohol,and dried in an oven (100 °C).

2.2.Preparation of coating

The polished AZ31 sheet was anodized through a microarc oxidation(MAO)device.The equipment was composed of a power supply unit,a stainless-steel plate (cathode) and a cooling system.The MAO coating was performed at a constant voltage of 300V at 350Hz for 3min using an AC power supply with a duty cycle of 30%;and the electrolyte consisted of NaOH (8g/L),NaSiO3(10g/L) and KF (5g/L).Then,the fabricated MAO coating was rinsed with distilled water to remove the residual electrolyte.A mixed solution was prepared by adding PP (1.5g) and PP-g-MAH (0.3g) into xylene (100mL) at 130 °C;the MAO specimen was immersed in the mixed solution for 3min,and the PP layer could be obtained on the MAO surface through dip-coating method.The composite coating was designated as MAO-PP coating.The preparation process of MAO-PP coating is shown in Fig.1.

2.3.Surface characterization

Fig.1.Flow diagram of preparation process for superhydrophobic MAO-PP coating on AZ31 Mg alloy.

The microstructure of the specimens was observed via a fiel emission scanning electron microscope (FE-SEM,Hitachi S-4800);the surface chemistry of the coating was analyzed through an X-ray photoelectron spectroscopy (XPS,EscaLab 250Xi,USA) with a monochromatic focused Al K X-ray source (1486.6eV);the phase structure of sample was investigated by means of an X-ray diffractometry instrument(XRD,XRD-7000LX,Shimadzu,Japan) with a Cu target(λ=0.154nm) at a fi ed scan rate (0.02 s-1) in the 2θrange from 10° to 75°.Fourier transform infrared spectra (FT-IR,VER-TEX 70,Germany),operated in a wave-number range of 450-3950 cm-1,was applied to characterize the surface chemical bonds.The CA and SA of MAO-PP coating were observed by virtue of a contact angle goniometer (Sigma700,Finland),and the bulk of individual water droplet was fi ed at 30μL.Moreover,the CA was assessed on three different positions for each sample surface.

2.4.Corrosion resistance of coating

2.4.1.Electrochemical tests

Electrochemical measurement was us ed to evaluate the corrosion resistance of sample through an electrochemical analyzer (PAR Model 2273,Princeton);a three-electrode system wherein the sample itself as the working electrode (1.0 cm2),a platinum sheet (counter electrode),and a saturated calomel electrode (SCE,reference electrode).The samples were immersed in the NaCl solution for 10min,allowing the system to be stabilized [14].The electrochemical impedance spectroscopy (EIS) were carried out in the determined frequency range of 105to 10-2Hz [34].Potentiodynamic polarization curves (Tafel) were measured in a range of -2.0 and -1.0V/SCE at a fi ed scan rate (2.0 mV·s-1).The electrochemical parameters (corrosion current density (icorr),corrosion potential (Ecorr),and Tafel slopes) were obtained by using the Tafel extrapolation method;and the corrosion rate,Pi(mm·year-1),could be calculated via theicorr(mA·cm-2)according to the following equation [35]:

Moreover,polarization resistance (Rp) could be obtained through the Stern-Geary formula [36]:whereβaandβcrepresented the Tafel slopes of the anode and cathode,respectively.

2.4.2.Immersion test

The immersion test also was used to determine the corrosion resistance of samples [37].The samples were soaked in beaker containing 3.5wt% NaCl solution (25 °C).Subsequently,the beaker connected to an inverted funnel and a burette.Then,the hydrogen evolution volume(HEV)of specimens could be obtained by intermittently recording the values of solution level for 248h.The experiment was in triplicate.Moreover,the hydrogen evolution rate (HER),VH(mL·cm-2h-1),could be expressed as:

whereVis the produced volume of hydrogen,sandtrepresent the exposed area and soaked time of specimen,respectively.Moreover,the HER could be changed into the corrosion rate,PH(mm·year-1) [2]:

2.5.Stability test of coating

The chemical stability of MAO-PP coating was checked by the droplets with different pH values (2,4,6,8,10);and the change in CA values was observed.The NaOH and HCl solutions were used to adjust the pH value of water solution.Moreover,the physical stability was evaluated through using water jet impact on the MAO-PP surface [38].In generally,the superhydrophobic surface can easily lost its hydrophobic property because high-speed jetted fl ws can damage the surface microstructure and remove the trapped air pockets from the rough cavities.Two cases can occur after water fl ws impinging on the surface;either it wet the superhydrophobic surface presenting the destruction of the structure or bounce on the surface confirmin the stability of the fil [39].A water jet was produced by applying pressure to the syringe(the needle diameter～0.5mm) to repeatedly hit the MAO-PP coating from a height of～5cm for 10s [39,40].

Fig.2.SEM morphologies of (a-c) MAO and (d-f) MAO-PP coatings,and 3D surface morphology of (g) MAO-PP coating.

2.6.Scratch test of coating

The bonding strength between AZ31 Mg substrate and its coatings was evaluated through an MML Nano-test system with Rockwell diamond probe (tip diameter～25mm)[34].The scratch test was measured with a constant scan rate(i.e.2mm/min) via increasing linearly load to 20N,until the length of scratches reached 2mm.

3.Results

3.1.Surface characterization

Fig.2 displays the SEM micrographs of the samples.For MAO coating (Fig.2a-c),distinctive porous structure could be observed,which was attributed to the high-voltage spark discharge channel formed by the coating rapidly cooled in the electrolyte [41].The EDS data indicated that the main elements were Mg and O (Table 1:Spectrum 1 and 2),which corresponded to the presence of MgO.As shown in Fig.2d-f,the MAO-PP coating,with a micron-scaled granular structure,was mainly composed of C element with a high concentration of>95 at.% (Table 1:Spectrum 3 and 4),designating the full coverage of PP on MAO coating.This result was further confirme by 3D surface morphology of MAO-PP coating as shown in Fig.2g,a uniform and compact surface could be observed.The surface roughness of MAO-PP coating was 4.21±0.28μm,revealing the MAO-PP hybrid fil with relatively high surface roughness on AZ31 Mg substrate.

Table 1 Elemental compositions for MAO and MAO-PP coating in wt.(at.)%.

Table 2 Electrochemical data of the polarization curves of the samples.

The thickness of the MAO and PP coatings was 4.5 and 22.3μm,respectively (Fig.3a).Moreover,the distribution of Mg,O and C further confirme that the MAO-PP coating was prepared on the Mg substrate (Fig.3b-d).Interestingly,the interface boundary between the MAO coating and PP one was not apparent,suggesting that PP layer was sealed into the micro pores/cracks of MAO coating.

Fig.3.Cross-sectional SEM morphology of (a) the MAO-PP coating and its corresponding EDS mapping:(b) Mg,(c) O,and (d) C.

Fig.4.FT-IR spectra (a) and XRD patterns (b) of the bare Mg alloys,the MAO coatings,and the MAO-PP coatings.

The peak at 3375 cm-1for MAO coating was ascribed to the -OH stretch (Fig.4a).The absorption peak at about 450 cm-1indicated the presence of MgO,corresponding to the MAO coating itself.The adsorption band at 3433 cm-1was assigned to -OH stretch [42];the bands at 2958,2879,and 2846 cm-1could be attributed to C-H,revealing the presence of PP.The signal at 1835 cm-1manifested the existence of (C=O)2[43],which could be ascribed to the PP-g-MAH(anhydride structures).The peak at 1750 cm-1was assigned to C=O,which was related to carboxy group.The obvious peak at 1630 cm-1was because of carboxylate complex (Mg-O-C=O) [42].The XRD patterns showed that the AZ31 Mg substrate mainly consisted ofα-Mg matrix (Fig.4b),which could be attributed to the substrate itself.The characteristic peaks of the MgO and Mg2SiO4phases were observed,indicating the successful formation of the MAO coating on the bare AZ31 substrate.In the MAO-PP coating case,the peaks from 12° to 25° belonged to PP phase [24],revealing that the PP layer was fabricated on the surface of MAO coating.

The XPS survey and high resolution spectra of C,O and Mg (Fig.5a-d) were applied to confir the results of FTIR spectra and XRD patterns.It is clear that the elements of C,O and Mg could be observed (Fig.5a);the high resolution spectrum of C1s (Fig.5b) was disintegrated into four peaks:C-C/C-H (284.6eV),C-C(=O)-O (285.07eV),C(=O)-O-C (286.63eV),and C(=O)-OH (288.75eV) [44],separately.The C-C(=O)-O and C(=O)-O-C could be attributed to the C(=O)-O-C(=O)structures(PP-g-MAH).With respect to O 1s spectra (Fig.5c),the peak consisted of three contribution at 530.84,531.65,and 532.3eV,which corresponded to C(=O)-OH,C(=O)-OH,and C(=O)-O-C(=O),respectively.The appearance of C(=O)-OH was the result of hydrolyzed C(=O)-O-C(=O) groups (PP-g-MAH) [2].The high resolution spectrum of Mg 1s showed the peaks centered at 1303.9eV,which could be assigned to MgO (Fig.5d) [45].These results confirme a successful preparation of MAO-PP fil on the Mg substrate

Fig.5.XPS spectra of MAO-PP coating:(c) survey,high resolution spectra of (d) C1s,(e) O1s,and (f) Mg 1s.

The water contact angle (CA) and sliding angle (SA) of samples are shown in Fig.6.The AZ31 substrate exhibited a CA of 98.5±1.5° (Fig.6a-1).After MAO treatment,the CA of the sample decreased to 45.6±2.7° (Fig.6a-2),which could be attributed to the hydrophilic MgO and porous structures of MAO coating.Interestingly,the MAO-PP coating designated a higher CA of 167.2±0.8° (Fig.6a-3) because of the synergistic effect of the rougher microstructure and the lower surface energy (SFE) of the PP coating (Fig.2g).Fig.6b depicts that the MAO-PP hybrid coating had a lower SA of 2.7±0.5°,revealing the MAO-PP coating possessing excellent superhydrophobicity.Moreover,the change in CA for MAO-PP coating at different times is discerned,as shown in Fig.6c.The droplet could keep a spherical shape on MAOPP coating for 2.5h or more.Nevertheless,the contact angle gradually decreased with increasing time.The reason for this was ascribed to that the volume of droplets reduced gradually due to evaporation,while the contact area between droplets and coating surface did not change markedly.

Fig.6.Contact angle for (a-1) the AZ31 Mg sheet,(a-2) the MAO coating,and (a-3) the MAO-PP coating;sliding angle for (b) the MAO-PP coating;water contact angle for MAO-PP coating at different time:(c-1) 0h,(c-2) 0.5h,(c-3) 1h,(c-4) 1.5h,(c-5) 2h,and (c-6) 2.5h.

Fig.7.Potentiodynamic polarization curves for the bare AZ31 substrate,the MAO,and the MAO-PP coated samples.

3.2.Corrosion resistance of the coating

3.2.1.Electrochemical test

Fig.7 shows the potentiodynamic polarization curves of all specimens,and the corresponding parameters are listed in Table 2.The MAO-PP coating exhibited a lowest value oficorr(7.21×10-9A·cm-2) compared with the AZ31 substrate (7.79×10-5A·cm-2) and MAO coating (9.70×10-8A·cm-2),confirmin the best corrosion resistance of the MAO-PP coating.Moreover,the highestRpindicates the best corrosion-resistant performance [36].It could be observed that the values ofRpfor samples showed an increasing trend:AZ31 Mg substrate (7.01×102Ω·cm2)

The Nyquist plots of all samples are shown in Fig.8a-c.The semicircle diameter of MAO-PP coating was larger than bare AZ31 Mg substrate and MAO coating.In Bode plots(Fig.8d),the |Z| values of samples in the low frequency(10 mHz) followed this decreasing order:MAO-PP coating(7116.79 kΩ·cm2)

Fig.8f-h illustrates equivalent circuits (ECs) models of samples.In the ECs,the charge transfer resistance and solution resistance were represented byRctandRs;the pitting corrosion was denoted byRLwith an inductanceL;the coating resistance was indicated byRxwith a constant phase element (CPE) or layer capacitance (Cx).In the ECs of AZ31 Mg sheet (Fig.8f),a capacitance loop,at the high frequency,was represented byRctandCPE1,which was related to the loose corrosion products;the inductive loop at the low frequency was described byRct andL,suggesting the beginning of pitting corrosion.In the MAO coating case(Fig.8g),R1described the coating resistance.Unlike MAO coating,the ECs for the MAO-PP coating consisted ofR1andR2,which represented the outer PP coating and inner MAO coating,respectively (Fig.8h).Moreover,a largerRctvalue represents a better corrosion resistance [36].As shown in Table 3,the MAO-PP coating (8750.00 kΩ·cm2) showed the largestRctvalues compared with AZ31 Mg substrate(93.10Ω·cm2) and MAO coating (949.00 kΩ·cm2),revealing that the corrosion-resistant performance of MAO-PP coating was effectively enhanced.

Fig.8.(a-c) Nyquist plots (d) Bode plots and (e) phase angle of the samples.Equivalent circuits of the (f) AZ31 Mg substrate and the (g) MAO and (h)MAO-PP coatings.

Fig.9.(a) HEV and (b) HER as functions of the soaking time in 3.5wt% NaCl solution.

3.2.2.Immersion tests

Fig.9 displays the hydrogen evolution tests of the specimens soaked in a corrosive solution (3.5wt% NaCl) for 248h.The hydrogen evolution volume (HEV) of the MAOPP coating was about 0.29±0.10 mL·cm-2(Fig.9a),which showed the lowest HEV compared with the MAO coating(0.80±0.06 mL·cm-2) and the bare AZ31 Mg substrate(4.22±0.48 mL·cm-2).In addition,the hydrogen evolution rates (HER) of samples were in this descending order:AZ31 Mg substrate (17.03±1.96 μL·cm-2·h-1)>MAO coating(3.25±0.24 μL·cm-2·h-1)>MAO-PP coating (1.18±0.42 μL·cm-2·h-1) (Fig.9b).These results proclaimed that the MAO-PP coating possessed a superior corrosion resistance.

The corrosion morphologies and corresponded components analysis after an immersion of 248h are shown in Fig.10.A lot of corrosion products was observed on the bare AZ31 Mg substrate(Fig.10a and d)and the main elements were Mg and O (Fig.10g),its corresponded to the formed Mg(OH)2.Unlike Mg substrate,the MAO coating only suffered from slight corrosion damage (Fig.10b and e),indicating that the MAO film in a manner,improved the corrosion resistance of AZ31 Mg substrate [46].For MAO-PP coating (Fig.10c and f),it is obviously that the outer PP fil kept complete morphology and the main element was C (80.00±14.17wt%),revealing the superior corrosion-resistant performance of MAO-PP coating.In the FT-IR spectra (Fig.10h),the peak intensity of Mg(OH)2for samples could be ranked in an increasing order as follows:the MAO-PP coating

3.3.Stability test of coating

The chemical stability of MAO-PP coating was checked in Fig.11.The CA of different pH solutions on the MAOPP coating were basically in the range of 158-167°,which demonstrated a superior chemical stability (Fig.11a).It can be seen from Fig.11b that MAO-PP superhydrophobic surface still possessed good superhydrophobic performance after 5 d These outcomes demonstrated that MAO-PP coating possessed a better chemical stability.

Fig.10.Optical photographs (a-c) and SEM images (d-f) of the AZ31 substrate (a,d),the MAO coating (b,e),and the MAO-PP coating (c,f);(g) elemental compositions,(h) FT-IR spectrum,and (i) XRD pattern of all sample soaked in corrosive solution for 248h.

Fig.11.The CA of different pH solution (a) on the MAO-PP coating,and CA of MAO-PP coating exposed to air for different times.

Except for excellent chemical stability,the MAO-PP coating also needs superior physical stability.Fig.12 shows the process of the MAO-PP composite coating hit by the water jet with increasing impact time.It is clear that the water jet was continuously rebounded on the superhydrophobic surface in the striking stage (Fig.12),revealing that MAO-PP coating had good physical stability.

3.4.Scratch test

Fig.13a shows the adhesion bonding between the AZ31 Mg substrate and its coating through a scratch test.The values of critical loads were about 4393 mN for the MAO coating and 2358 mN for the MAO-PP coating,respectively(Fig.13).The adhesion strength of the MAO-PP coating was much higher than that of the MAO-ZnSA coating (440.61 mN) [22] and the EDTA-Mg(OH)2coating (1785 mN) [47].Moreover,the bonding strength of MAO-PP hybrid coating was 0.7 times higher than that of the PAPTMS-PP coating(1441 mN) and 0.8 times higher than that of the Mg(OH)2-PP coating (1323 mN) [2,27].The better bonding strength of the MAO-PP coating was due to the fact that the microcracks and micro-pore of MAO coating could improve the mechanical lock force of outer PP layer [32].

4.Discussion

4.1.Comparison of corrosion rates between MAO-top coatings and bottom-PP coatings on Mg alloys

Fig.12.Physical stability tests by the water fl w impact on the MAO-PP coating with increased time:(a) 1,(b) 5,(c) 10s.

Fig.13.Scratch test (a) for the MAO and MAO-PP coating;(b) a comparison of bonding strength with previously prepared coatings on the Mg alloy [2,27].

These reported MAO-top hybrid coatings and PAPTMSand Mg(OH)2-PP coatings(Fig.14)exhibited encouraging results in terms of corrosion-resistant performance;and a comparison of corrosion rates of the present work with other hybrid coatings was discussed and summarized.Piwas obtained byicorrdetermined through the from Tafel extrapolation(Eq.(1)),and the corrosion rates (Pi) of all hybrid coatings are shown in Fig.14a.It is clear that the MAO-PP coating(1.65×10-4mm·year-1)exhibited the lowestPicompared to the PAPTMS-PP coating(2.91×10-3mm·year-1),the MAOSA coating (1.73×10-3mm·year-1),and the MAO-PMTMS coating (6.54×10-4mm·year-1).ThePiof MAO-PP coating was slightly higher than that of Mg(OH)2-PP coating(7.13×10-5mm·year-1) [2,21,27,32].PHdesignated the dissolution rate of Mg alloys as a function of soaking time.The PHfor hybrid coatings was ranked in decreasing order as follows:the PAPTMS-PP coating (0.31 mm·year-1)>the MAO-SA coating (0.14 mm·year-1)>the MAO-PMTMS coating (0.08 mm·year-1)>the MAO-PP coating (0.06 mm·year-1)>the Mg(OH)2-PP coating (0.01 mm·year-1).Although the Mg(OH)2-PP hybrid coating exhibited superior corrosion-resistant performance,the preparation process was time-consuming [2].According to the comparison in Fig.14,the PP layer prepared by simple one-step dipping on the MAO coated substrate in the present work had both superior performance in decreasing the preparation time and improving the corrosion resistance.Note that,there are some differences betweenPi(Table 2) and PH(Table 4),which were interpreted as the negative difference effect [48]

Fig.14.Comparison of corrosion rates of the coating in this study with MAO-top coatings [21,32] and PAPTMS-and Mg(OH)2-PP coatings [2,27]:(a) Pi and (b) PH.

Table 4 PH of the specimen after soaking of different time (1,100,and 248h),mm·year-1.

4.2.Formation mechanism of the coating

The preparation diagram of MAO-PP coating is shown in Fig.1.The MAO coating mainly consists of Mg,O,and Si,which corresponded to the MgO and Mg2SiO4.Then,the PP coating was fabricated on the as-obtained MAO coating through adding the tiny amounts of compatibilizer (PP-g-MAH).The MAH(-COO-OC)groups could be hydrolyzed to carboxyl (-COOH) groups when it contacted with the water molecules (Eq.(5)) [49].The -COOH groups reacted with MgO to form Mg-O-C=O carboxylate complex (Eq.(6))[50].

The PP-g-MAH chains were prepared on the MAO coating by the chemical interactions;the PP chains was successfully modifie on the surface because of the entanglement between the chains of PP and PP-g-MAH (Fig.1).In addition,the rough and porous surface of MAO also provided numerous mechanical interlocking sites [51],which was beneficia for improving the adhesion of the PP layer.

4.3.Superhydrophobic mechanism of the coating

Fig.1 displays the hydrophobic mechanism of MAO-PP coating.The porous MAO coating was entirely covered with the microstructural MAO-PP coating.A lot of air could be captured by granular structures which fille the voids around the microstructure,resulting in a smaller solid-liquid contact area on the superhydrophobic coating according to the Cassie-Baxter equation [52]:

whereθandθSrepresented the water CA of the microstructural and smooth PP coating,separately.fwas the fractions of the solid surface and air at the interface.For the superhydrophobic coating,theθandθSwere about 167.2° and 105°[53],respectively.Hence,theffor the MAO-PP hydrophobic coating was around 3.35%.That is to say,the contact area between the water droplets and the air was 96.65%.However,the Cassie-Baxter model is only suitable for ideal condition.Due to the defects or inhomogeneity of the microstructure on the superhydrophobic surface,as well as various slight disturbances to the droplets,it is easy to make the droplets wet the air pocket around microstructure and cause the Wenzel state in the local area of the Cassie-Baxter state on the actual superhydrophobic surface[54].When the droplets exhibit a mixing state,the contact area between droplets and superhydrophobic surface is larger than that calculated value obtained using the Cassie ideal model (3.35%).Even so,a lower contact area reveals that the MAO-PP coating markedly prevents the ingress of the aggressive media.

Fig.15 depicts the hydrophobicity of PAPTMS-,Mg(OH)2-and MAO-PP composite coatings.Obviously,the MAO-PP coating exhibited the lowest SA (2.7±0.5°) and the highest CA (167.2±0.8°) compared with the PAPTMS-PP coating (SA,5.0±0.6°;CA,162.3±3.4°) and the Mg(OH)2-PP coating (SA,4.0±0.6°;CA,165.5±3.6°).The results were because the inner rough MAO coating increased the roughness of whole outer PP coating to a certain degree.Similar outcomes have been reported in other literatures [55].

Fig.15.Comparison of hydrophobicity of PAPTMS-,Mg(OH)2-and MAOPP composite coatings [2,27].

4.4.Corrosion mechanism of coating

These above results indicated that the MAO-PP superhydrophobic coating possesses a better corrosion-resistant performance in the corrosive solution(3.5wt%NaCl).The corrosion mechanism of MAO-PP coating is illustrated in Fig.16.Firstly,the micro-scale granular structures of outer PP coating captured air layers could well insulate the contact of the corrosive liquid with the substrate (Fig.16a) [56].However,the air layer gradually disappeared with the increased immersion time,leading to the increase in contact area between corrosion media and PP coating.The uniform and compact microstructure surface of MAO-PP coating further hindered the penetration of aggressive solution.Then,the corrosive solution slowly penetrated into the coating since PP coating got deteriorated by the formation of micro-cracks during immersion (Fig.10f) [24].The MAO coating,namely,a ceramic oxide MgO,was slowly hydrated as the corrosive solution contacted the porous coating.The chemical reaction is as follows [57]:

The formed corrosion products (Mg(OH)2) could seal the micro-cracks/pores of MAO coating (Fig.16b).Nevertheless,the aggressiveions derived from chloride-containing solution and CO2molecules came from air could dissolve the generated Mg(OH)2according to the following chemical reactions [58]:

Finally,the Al-Mn phase,an intermetallic compound,acted as the initiators of the appearance of pitting corrosion on the AZ31 Mg substrate.Once the corrosive salt solution penetrated into the Mg substrate through the porous MAO coating,resulting in the occurrence of electrochemical corrosion on the Mg substrate.The electrochemical reactions as below [59-61]:

Soon afterwards,the generated corrosion products,consisted of Mg(OH)2and MgCO3,led to the detachment of PP coating from the Mg substrate (Fig.16c).Unfortunately,peeling off of PP coating promoted the corrosion behavior again.

Fig.16.Schematic diagram of the corrosion mechanism of the MAO-PP coating.

5.Conclusion

In this study,an integrated polypropylene (PP) fil was successfully synthesized on the micro arc oxidation (MAO)pretreated AZ31 Mg substrate by simple one-step dipping.The hydrophobicity,bonding strength,and stability as well as corrosion characteristic of composite coating were investigated and summarized.

1.The MAO-PP coating exhibited the superior superhydrophobicity of a high contact angle of 167.2±0.8°and a low sliding angle of 2.7±0.5°.Results was because the outer PP fil had both the micron scale granular structures and the low surface energy.In addition,the inner MAO coating further increased the roughness of outer PP coating and,thus,leaded to the more superhydrophobic surface.

2.The MAO-PP hybrid coating showed a strong bonding strength(2358 mN),which was due to the entanglement between PP chains and PP-g-MAH ones after the surface preparation of PP-g-MAH by chemical interaction;moreover,the porous structures of bottom MAO coating further enhanced the bonding strength due to the increased physical interlocking sites.

3.The MAO-PP hybrid coating showed the slowest corrosion rate ofPi(1.65×10-4mm·year-1) and PH(0.06 mm·year-1) compared to the bare AZ31 Mg sheet and MAO coated sample,confirmin an excellent corrosion resistance of MAO-PP coating.

4.Stability tests demonstrated that the MAO-PP superhydrophobic coating possessed superior stability.

Declaration of Competing Interest

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

Acknowledgments

This work was supported by the National Natural Science Foundation of China (51571134) and the SDUST Research Fund (2014TDJH104).