Effect of Zinc Phosphate on the Corrosion Behavior of Waterborne Acrylic Coating/Metal Interface

materials

Article Effect of Zinc Phosphate on the Corrosion Behavior of Waterborne Acrylic Coating/Metal Interface

Hongxia Wan 1, Dongdong Song 1,2,*, Xiaogang Li 1,3,*, Dawei Zhang 1, Jin Gao 1 and Cuiwei Du 1

1 Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China; [email protected] (H.W.); [email protected] (D.Z.); [email protected] (J.G.); [email protected] (C.D.) 2 Aerospace Research Institute of Materials and Processing Technology, Beijing 100076, China 3 Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China * Correspondence: [email protected] (D.S.); [email protected] (X.L.); Tel.: +86-010-68383576 (D.S.); +86-010-62333931 (X.L.)

Academic Editor: Yong-Cheng Lin Received: 22 April 2017; Accepted: 30 May 2017; Published: 14 June 2017

Abstract: Waterborne coating has recently been paid much attention. However, it cannot be used widely due to its performance limitations. Under the speciﬁed conditions of the selected resin, selecting the function pigment is key to improving the anticorrosive properties of the coating. Zinc phosphate is an environmentally protective and efﬁcient anticorrosion pigment. In this work, zinc phosphate was used in modifying waterborne acrylic coatings. Moreover, the disbonding resistance of the coating was studied. Results showed that adding zinc phosphate can effectively inhibit the anode process of metal corrosion and enhance the wet adhesion of the coating, and consequently prevent the horizontal diffusion of the corrosive medium into the coating/metal interface and slow down the disbonding of the coating.

Keywords: waterborne acrylic coating; zinc phosphate; LEIS

1. Introduction Serious environment problems have directed attention toward water-based coatings because of their low volatile organic compounds VOC advantages [1–3]. However, waterborne coatings have performance limitations; moreover, they have been extensively used indoors, where the corrosion environment is below the C3 level [4]. Thus, waterborne coatings need to have improved performance for long-term applications [5–8]. Acrylic coatings have good barrier properties and ageing resistance, which has attracted much attention [9,10]. Disbonding is the first sign of coating/metal interface corrosion [11–14], which is the most common form of failure that limits coatings, especially for long-term applications of waterborne coatings [15]. Hence, the ability to resist disbonding characterizes the coating service life, which is the main purpose for developing long-lasting water-based coatings suited for harsh environments [16]. Traditional coatings would exhibit significant cathodic disbonding under different environments that would affect the coating service life. Adding an electrochemically active antirust pigment is a common method for improving the antirust capacity of the coating [17,18]. Therefore, the effect of an electrochemically active antirust pigment on the cathodic disbonding process when added into a waterborne acrylic coating is significant. Much attention has been given to phosphate antirust pigments for their relative environmental protection [19–23]. Phosphate pigments mainly contain aluminum tripolyphosphate [24] and zinc phosphate [25]. The antirust function and mechanism of zinc phosphate have been previously studied. Bethencourt et al. [26] studied the failure process of a waterborne acrylic zinc phosphate coating using

Materials 2017, 10, 654; doi:10.3390/ma10060654 www.mdpi.com/journal/materials Materials 2017, 10, 654 2 of 13 electrochemical impedance spectroscopy (EIS). The corrosive failure of composite coatings in 3.5% NaCl solution were divided into three stages: the first stage is the water diffusion stage, wherein Materials 2017, 10, 654 2 of 13 the coating impedance decreases; the second stage involves the zinc phosphate coming in contact withcoatings water, whereinin 3.5% NaCl a passivation solution were rust divided resistance into three effect stages: reaches the a first certain stage concentration is the water diffusion after water absorption;stage, wherein and the the third coating stage isimpedance the start ofdecreases; the shielding the second decline stage after involves the zinc the phosphate zinc phosphate is exhausted. Yaweicoming ShaoMaterials etin al. contact2017 [,27 10, ,65428 with ] studied water, the wherein corrosion a passivation inhibition rust of zincresistance phosphate effect inreaches an oil-based 2 aof 13certain epoxy coatingconcentration using EIS after and water electrochemical absorption; and noise, the andthird showedstage is the that start zinc of the phosphate shielding providesdecline after a recovery the zinc phosphatecoatings in 3.5%is exhausted. NaCl solution Yawei were Shaodivided et intoal. three[27,28] stages: studied the first the stage corrosion is the water inhibition diffusion of zinc effect for coatingstage, wherein defects. the Currently, coating impedance researchers decreases; [29] foundthe second that stage when involves the corrosive the zinc mediumphosphate permeates phosphate in an oil-based epoxy coating using EIS and electrochemical noise, and showed that zinc the oil-basedcoming epoxy in coating/metalcontact with water, interface, wherein zinc a passivation phosphate rust partly resistance dissolves effect andreaches forms a certain metal substrate phosphate provides a recovery effect for coating defects. Currently, researchers [29] found that when concentration after water absorption; and the third stage is the start of the shielding decline after the phosphate and stabilizes the protective film, which contains γ-Fe2O3, α-FeOOH, γ-FeOOH, and iron the corrosivezinc phosphate medium is permeatesexhausted. Yaweithe oil-based Shao et epoxyal. [27,28] coating/metal studied the interface,corrosion zincinhibition phosphate of zinc partly phosphate.dissolvesphosphate Moreover, and forms in an theoil-based metal protective substrate epoxy coating film phosphate containsusing EIS and and carboxyl st electrochemicalabilizes and the hydroxylprotective noise, and groups film,showed which that that zinc contains enhance the combinationγ-Fe2Ophosphate3, α of-FeOOH, coating provides γ-FeOOH, and a recovery metal. and effect Someiron for phosphate. coating researchers defects. Moreover, [Currently,30] have the researchers proposedprotective [29] film that found contains zinc that phosphatewhen carboxyl can formand stable hydroxylthe iron corrosive phosphates groups medium that permeates andenhance block thethe medium combinationoil-based diffusionepoxy of coating/metal coating into and the interface,metal. channels Some zinc and researchersphosphate slow down partly [30] corrosion.have However,proposeddissolves the that effects andzinc offorms phosphate zinc metal phosphate substratecan form activephosphate stable fillersiron and phosphates onstabilizes the waterborne the and protective block medium acrylic film, which cathodicdiffusion contains into disbonding the channelsγ-Fe 2andO3, α slow-FeOOH, down γ-FeOOH, corrosion. and ironHowever, phosphate. the Moreover, effects of the zinc protective phosphate film contains active carboxylfillers on the process, especiallyand hydroxyl involving groups that the enhance interface the combination corrosion mechanismof coating and ofmetal. cathodic Some researchers and anodic [30] processes, have have waterborne acrylic cathodic disbonding process, especially involving the interface corrosion not been comprehensivelyproposed that zinc phosphate studied. can form stable iron phosphates and block medium diffusion into the mechanism of cathodic and anodic processes, have not been comprehensively studied. In thischannels paper, and we slow studied down thecorrosion. coating/metal However, the interface effects of corrosionzinc phosphate process active of fillers waterborne on the acrylic In this paper, we studied the coating/metal interface corrosion process of waterborne acrylic coatings withwaterborne zinc phosphate acrylic cathodic using disbonding EIS and localizedprocess, especially EIS (LEIS). involving Furthermore, the interface we studiedcorrosion the effects coatingsmechanism with zinc of cathodicphosphate and using anodic EIS processes, and localize have notd EIS been (LEIS). comprehensively Furthermore, studied. we studied the effects of activeof active pigments Inpigments this paper, on on coating/metal wecoating/metal studied the coating/metal interfaceinterface co corrosion rrosioninterface andcorrosion and disbonding disbonding process ofprocesses waterborne processes in waterborneacrylic in waterborne acrylicacrylic coatings.coatings coatings. with zinc phosphate using EIS and localized EIS (LEIS). Furthermore, we studied the effects of active pigments on coating/metal interface corrosion and disbonding processes in waterborne 2. Results2. Resultsacrylic coatings.

2. Results 2.1. The2.1. CoatingThe Coating Adhesion Adhesion Test Test FigureFigure2.1.1 showsThe 1 Coating shows the Adhesion the adhesion adhesion Test values values of of 0% 0% and and 8%8% zinc phosphate phosphate coatings. coatings. The The adhesion adhesion values values werewere the samethe sameFigure in the in 1 showsthe 0% 0% and the and adhesion 8% 8% zinc zinc values phosphate phosphate of 0% and coatings.co 8%atings. zinc phosphateAdding Adding zinc coatings. zinc phosphate phosphate The adhesion did didnot values affect not affect the the initialinitial coatingwere coating mechanicalthe same mechanical in the performance. 0% performance.and 8% zinc The phosphate The coating coating coatings. fractures fractures Adding after zincafter the phosphate the pull-off pull-off did experiments notexperiments affect the are are shown in Figureshown2initial. in Figure coating 2. mechanical performance. The coating fractures after the pull-off experiments are shown in Figure 2.

FigureFigure 1. The 1. adhesionThe adhesion values values ofof 0% an andd 8% 8% zinc zinc phosphate phosphate coatings. coatings. Figure 1. The adhesion values of 0% and 8% zinc phosphate coatings.

Figure 2. The fractures after the pull-off experiments in (a1,a2) 0% and (b1,b2) 8% zinc phosphate coatings.

Figure 2. The fractures after the pull-off experiments in (a1,a2) 0% and (b1,b2) 8% zinc phosphate Figure 2. The fractures after the pull-off experiments in (a1,a2) 0% and (b1,b2) 8% zinc coatings. phosphate coatings.

Materials 2017, 10, 654 3 of 13

FigureMaterials2 shows 2017, 10, the654 adhesive fracture in the samples without and with 8% zinc phosphate3 of 13 coating. The fracture mode for the 8% zinc phosphate coating after the pull-off experiment, which is the mixture of the coatingFigure body 2 andshows adhesive the adhesive interlayer fracture fractures, in the samples showed without strong and adhesion. with 8% Thiszinc resultphosphate is the same coating.Materials 2017 The, 10 fracture, 654 mode for the 8% zinc phosphate coating after the pull-off experiment, which3 of 13is with the sample without zinc phosphate coating, only the ratio of the coating body and adhesive the mixture of the coating body and adhesive interlayer fractures, showed strong adhesion. This interlayerresult fracturesFigure is the same2 isshows different. with the the adhesive sample without fracture zinc in phosphatethe samples coating, without only and the with ratio 8%of the zinc coating phosphate body andcoating. adhesive The fracture interlayer mode fractures for the is 8% different. zinc phosphat e coating after the pull-off experiment, which is 2.2. Thethe Coating mixture Morphology of the coating body and adhesive interlayer fractures, showed strong adhesion. This 2.2.result The is Coatingthe same Morphology with the sample without zinc phosphate coating, only the ratio of the coating body Figureand 3adhesive shows interlayer the scanning fractures electron is different. microscope (SEM) section of the 8% zinc phosphate coating. Figure3a showsFigure the 3 shows secondary the scanning electron electron morphology microscope and (SEM) Figure section3b shows of the the8% scatteringzinc phosphate electronic morphology.coating.2.2. The The CoatingFigure zinc Morphology3a phosphate shows the coatingsecondary has electron a homogeneous morphology dispersionand Figure and3b shows combined the scattering with the resin electronic morphology. The zinc phosphate coating has a homogeneous dispersion and combined tightly.with Moreover, Figurethe resin 3 adding tightly.shows zinctheMoreover, scanning phosphate adding electron didzinc notmicroscopephosphate show defects.did (SEM) not showsection defects. of the 8% zinc phosphate coating. Figure 3a shows the secondary electron morphology and Figure 3b shows the scattering electronic morphology. The zinc phosphate coating has a homogeneous dispersion and combined with the resin tightly. Moreover, adding zinc phosphate did not show defects.

Figure 3. The section morphology of the 8% zinc phosphate coating: (a) secondary electron morphology Figure 3. The section morphology of the 8% zinc phosphate coating: (a) secondary electron morphology and (b) back scattering morphology. and (b) back scattering morphology. Figure 3. 4 The shows section the morphology macromorphology of the 8% zinc of thephosphate coating coating: without (a) secondary and with electron 8% zinc morphology phosphate in and (b) back scattering morphology. Figure3.5%4 NaCl shows solution the macromorphology after various immersion of the days. coating At the withoutbeginning, and the with 0% and 8% 8% zinc zinc phosphate phosphate in 3.5% coating morphologies were the same. However, after two days of immersion, the 0% zinc phosphate NaCl solutioncoatingFigure aftergenerated 4 shows various obvious the immersion macromorphology corrosion. days. The Atcoatingof the coatbeginning,generateding without serious the and 0% corrosion with and 8% afterzinc phosphatefour phosphate days ofin coating morphologiesimmersion,3.5% NaCl were solution and the the same. defectiveafter various However, coating immersion bubbled after twodays. after days Ateigh thet ofdays. beginning, immersion, In the 8%the zinc 0% the andphosphate 0% 8% zinc zinc phosphatecoating, phosphate rust coating generatedoccurredcoating obvious morphologies after corrosion.four days were of Theimmersion the same. coating However, and generated increase afterd after two serious dayseight corrosion ofdays, immersion, but no after bubbling the four0% zinc was days phosphate observed. of immersion, and thecoating defective generated coating obvious bubbled corrosion. after eightThe coating days. ge Innerated the 8% serious zinc phosphate corrosion after coating, four days rust of occurred immersion, and the defective coatinga bubbled0% Zn3(PO after4)2 eight days. b 8% In Zn the3(PO 8%4 )zinc2 phosphate coating, rust after fouroccurred days ofafter immersion four days of and immersion increased and afterincrease eightd after days, eight but days, no but bubbling no bubbling was was observed. observed.

a 0% Zn3(PO4)2 b 8% Zn3(PO4)2 0 day

0 day

1 day

2 days

Figure 4. Cont. Materials 2017, 10, 654 4 of 13

Materials 2017, 10, 654 4 of 13 Materials 2017, 10, 654 4 of 13

4 days 4 days

8 days

Figure 4. The macromorphology of (a) 0% and (b) 8% zinc phosphate coating scratches with different Figure 4. The macromorphology of (a) 0% and (b) 8% zinc phosphate coating scratches with different immersion days. immersionFigure days.4. The macromorphology of (a) 0% and (b) 8% zinc phosphate coating scratches with different immersionThe microstructures days. after immersion are shown in Figure 5. The coating defects without zinc Thephosphate microstructures bubbled in after3.5% NaCl immersion solution after are eight shown days, in and Figure that with5. The8% zinc coating phosphate defects did not without bubble.The microstructuresThese results indicated after immersion that the coating are shown with zincin Figure phosphate 5. The can coating improve defects the disbonding without zinc zinc phosphate bubbled in 3.5% NaCl solution after eight days, and that with 8% zinc phosphate phosphateresistance. bubbled in 3.5% NaCl solution after eight days, and that with 8% zinc phosphate did not did not bubble. These results indicated that the coating with zinc phosphate can improve the bubble. These results indicated that the coating with zinc phosphate can improve the disbonding disbondingresistance. resistance.

Figure 5. Microstructures of coating with (a) 0% zinc phosphate and (b) 8% zinc phosphate immersed for eight days in 3.5% NaCl solution.

2.3.Figure The Electrochemical 5. Microstructures Behavi ofor coating of Zinc with Phosphate (a) 0% zincCoating phosphate and (b) 8% zinc phosphate immersed Figure 5. Microstructures of coating with (a) 0% zinc phosphate and (b) 8% zinc phosphate immersed for eight days in 3.5% NaCl solution. for2.3.1. eight Macroscopic days in 3.5% Electrochemical NaCl solution. Behavior 2.3. TheFigure Electrochemical 6 shows the Behavi EIS resultsor of Zinc of coatingsPhosphate immers Coatinged in 3.5% NaCl solution. The drop in the low 2.3. Thefrequency Electrochemical impedance Behavior value ofof Zincthe 0% Phosphate zinc phosphate Coating coating indicates the fast corrosion along the 2.3.1.coating/metal Macroscopic interface. Electrochemical The high-frequency Behavior impedance value of 0% zinc phosphate coating was 2.3.1. Macroscopicreduced after Electrochemical two days of immers Behaviorion, indicating that the corrosive medium in the coating/metal Figure 6 shows the EIS results of coatings immersed in 3.5% NaCl solution. The drop in the low interface was diffused and that disbonding of the coating occurred. However, for the coating with frequency impedance value of the 0% zinc phosphate coating indicates the fast corrosion along the Figure8% zinc6 shows phosphate, the EISthe low-frequency results of coatings impedance immersed value declined in 3.5% within NaCl one solution.day of immersion, The drop but in the low frequencycoating/metalthe value impedance remained interface. unchanged valueThe high-frequency of theafter 0% one zinc day. impedance phosphate The low-frequency value coating of impedance0% indicates zinc phosphate value the fast was corrosioncoating fitted in was along the coating/metalreducedFigure after7a. At two interface.the beginning,days of The immers the high-frequency low-frequencyion, indicating impedance impedance that the values corrosive value of ofthe medium 0%0% zincand 8%in phosphate thezinc coating/metalphosphate coating was interface was diffused and that disbonding of the coating occurred. However, for the coating with reducedcoatings after twowere days assessed, of immersion, and that of indicatingthe 8% zinc that phosphate the corrosive coating remained medium after in the one coating/metal day of 8% zinc phosphate, the low-frequency impedance value declined within one day of immersion, but interfaceimmersion. was diffused As immersion and that time disbonding increased, of the the low-frequency coating occurred. impedance However, value of for the the 8% coating zinc with thephosphate value remained coating was unchanged an order ofafter magnitude one day. higher The than low-frequency the value of theimpedance 0% zinc phosphate value was coating. fitted in 8% zinc phosphate, the low-frequency impedance value declined within one day of immersion, but FigureTo study 7a. At the the zinc beginning, phosphate the coating, low-frequency the equivalent impedance circuit was values fitted of in the Figure 0% and7b. 8% zinc phosphate the valuecoatings remained were assessed, unchanged and that after of one the day.8% zinc The phosphate low-frequency coating impedance remained after value one was day ﬁtted of in Figureimmersion.7a. At the As beginning, immersion the time low-frequency increased, the impedance low-frequency values impedance of the 0% value and 8%of the zinc 8% phosphate zinc coatingsphosphate were assessed, coating was and an that order of theof magnitude 8% zinc phosphate higher than coating the value remained of the 0% after zinc phosphate one day of coating. immersion. As immersionTo study the time zinc increased, phosphate the coating, low-frequency the equivalent impedance circuit was value fitted of in the Figure 8% 7b. zinc phosphate coating was an order of magnitude higher than the value of the 0% zinc phosphate coating. To study the zinc phosphate coating, the equivalent circuit was ﬁtted in Figure7b. Materials 2017, 10, 654 5 of 13 Materials 2017, 10, 654 5 of 13 Materials 2017, 10, 654 5 of 13

Figure 6. The electrochemical impedance spectroscopy (EIS) of (a1-Nquist, a2-Bote) 0% and (b1-Nquist, FigureFigure 6. 6.The The electrochemical electrochemical impedance impedance spectroscopy spectroscopy (EIS)(EIS) ofof (a1-Nquist-Nquist, ,aa2-Bote2-Bote) 0%) 0% and and (b (1b-Nquist1-Nquist, , b2-Bote) 8% zinc phosphate coatings. b2-Boteb2-Bote) 8%) 8% zinc zinc phosphate phosphate coatings. coatings.

Figure 7. (a) Low-frequency impedance value and (b) equivalent circuit of EIS of the 0% and 8% zinc Figure 7. (a) Low-frequency impedance value and (b) equivalent circuit of EIS of the 0% and 8% zinc Figurephosphate 7. (a) coatings. Low-frequency impedance value and (b) equivalent circuit of EIS of the 0% and 8% zinc phosphatephosphate coatings. coatings. Figure 7b shows the equivalent circuit to fit the impedance spectrum. In Figure 7b, Rs is the Figure 7b shows the equivalent circuit to fit the impedance spectrum. In Figure 7b, Rs is the solution resistance, Qc is the coating capacitance, and Rc is the coating resistance. Also, Qdl is the solutionFigure resistance,7b shows theQc is equivalent the coating circuit capacitance, to fit the and impedance Rc is the coating spectrum. resistance. In Figure Also,7b, Q Rdl iss isthe the capacitance of the double layer between the coating/metal interfaces that can characterize the coating solutioncapacitance resistance, of the double Qc is the layer coating between capacitance, the coating/metal and Rc interfacesis the coating that can resistance. characterize Also, the Qcoatingdl is the disbonding condition. Rct is the charge transfer resistance. In general, a high Rct reflects low corrosion capacitancedisbonding of condition. the double Rct layer is the between charge transfer the coating/metal resistance. In interfaces general, a that high can Rctcharacterize reflects low corrosion the coating because the exchange current is directly associated with the electrochemical process of corrosion; disbondingbecause the condition. exchange R ctcurrentis the chargeis directly transfer associat resistance.ed with Inthe general, electrochemical a high R ctprocessreflects of low corrosion; corrosion therefore, Rct can show the degree of corrosion of the coating. The values of Qdl and Rct are shown in becausetherefore, the R exchangect can show current the degree is directly of corrosion associated of the withcoating. the The electrochemical values of Qdl and process Rct are of shown corrosion; in Figure 8. As immersion time increased, the Qdl values of 0% and 8% zinc phosphate coatings increased therefore,Figure 8. R Asct can immersion show the time degree increased, of corrosion the Qdl values of the of coating. 0% and The 8% zinc values phosphate of Qdl and coatings Rct are increased shown in and the Rct of both decreased, thereby showing serious corrosion between coating/metal interfaces. Figureand 8the. As R immersionct of both decreased, time increased, thereby the showing Q values serious of 0%corrosion and 8% between zinc phosphate coating/metal coatings interfaces. increased However, the Qdl value of the 8% zinc phosphatedl coating was significantly lower than that of the 0% andHowever, the R of the both Qdl decreased,value of the thereby8% zinc showingphosphate serious coating corrosion was significantly between lower coating/metal than that of interfaces. the 0% zinc phosphatect coating, and its Rct value was significantly higher than the Qdl value of 0% zinc zinc phosphate coating, and its Rct value was significantly higher than the Qdl value of 0% zinc However, the Qdl value of the 8% zinc phosphate coating was significantly lower than that of the 0% zinc phosphate coating, and its Rct value was significantly higher than the Qdl value of 0% zinc

Materials 2017, 10, 654 6 of 13 Materials 2017, 10, 654 6 of 13 phosphate coating. Therefore, Therefore, the the added added zinc zinc phos phosphatephate into into the the coating coating slowed slowed down down the the corrosion corrosion reactionreactionMaterials at at the the2017 coating, 10, 654 defect. 6 of 13

phosphate coating. Therefore, the added zinc phosphate into the coating slowed down the corrosion reaction at the coating defect.

Figure 8. Qdl andand R Rctct valuesvalues of of 0% 0% and and 8% 8% zinc zinc phosphate phosphate coatings. coatings.

Figure 8. Qdl and Rct values of 0% and 8% zinc phosphate coatings. 2.3.2.2.3.2. Microscopic Microscopic Electrochemical Electrochemical Behavior 2.3.2. Microscopic Electrochemical Behavior FigureFigure 99 showsshows thethe pointpoint measurementsmeasurements ofof thethe locallocal alternating current impedance of the (a) 0% Figure 9 shows the point measurements of the local alternating current impedance of the (a) 0% andand (b) (b) 8% 8% zinc zinc phosphate phosphate coating coating distances distances from from the the scratch scratch defect defect at at 1600 1600 µmµm in in 3.5% 3.5% NaCl NaCl solution. solution. and (b) 8% zinc phosphate coating distances from the scratch defect at 1600 µm in 3.5% NaCl solution. After thethe immersion,immersion, the the LEIS LEIS value value of theof 0%the zinc0% phosphatezinc phosphate coating coating decreased decreased in 3.5% in NaCl, 3.5% whereas NaCl, whereasAfter that the of immersion, the 8% zinc the phosphate LEIS value coating of the 0%remained zinc phosphate relatively coating stable decreasedafter immersion in 3.5% forNaCl, 6 h. The that ofwhereas the 8% that zinc of phosphatethe 8% zinc phosphate coating remained coating remained relatively relatively stable afterstable immersion after immersion for 6 for h. The6 h. The solution solution penetrated the coating without zinc phosphate and corrosion occurred under the coating. penetratedsolution the penetrated coating the without coating zinc without phosphate zinc phos andphate corrosion and corrosion occurred occurred under under the coating. the coating. However, However,the coatingHowever, the with coatingthe 8%coating zinc with with phosphate 8% 8% zinc zinc phosphate resistedphosphate the resiresi mediumsted thethe permeabilitymedium medium permeabi permeabi of thelity coating.lity of the of coating.the coating.

Figure 9. The point measurements of localized EIS (LEIS) of 0% (a 1-Nquist, a2-Bote) and 8% Figure 9. The point measurements of localized EIS (LEIS) of 0% (a 1-Nquist, a2-Bote) and 8% (b1-Nquist, b2-Bote) zinc phosphate coatings immersed in 3.5% NaCl solution. (b1-Nquist, b2-Bote) zinc phosphate coatings immersed in 3.5% NaCl solution. Figure 9. The point measurements of localized EIS (LEIS) of 0% (a 1-Nquist, a2-Bote) and 8%

(b1-Nquist, b2-Bote) zinc phosphate coatings immersed in 3.5% NaCl solution.

Materials 2017, 10, 654 7 of 13 Materials 2017, 10, 654 7 of 13

FigureFigure 1010 showsshows thethe mapmap measurementmeasurement ofof LEISLEIS ofof thethe 0%0% zinczinc phosphatephosphate coatingcoating immersedimmersed inin 3.5%3.5% NaClNaCl solution.solution. At thethe initialinitial stagestage ofof immersion,immersion, thethe impedanceimpedance valuevalue ofof thethe coatingcoating defectdefect inin contactcontact withwith the the electrolyte electrolyte solution solution was was signiﬁcantly significantly below below the impedance the impedance value invalue the intactin the coating. intact Ascoating. immersion As immersion time increased, time increased, the whole the area whole of impedance area of impedance was reduced. was Thereduced. impedance The impedance modulus valuemodulus of the value coating of the defect coating and defect around and dropped around dropped rapidly afterrapidly immersion after immersion for 6 h. Thefor 6 coatingh. The coating defect wasdefect corroded was corroded severely severely and the and area the around area thearound coating the defectcoating was defect exfoliated. was ex Thefoliated. solution The mediumsolution penetratedmedium penetrated the bottom the of bottom the coating, of the followed coating, byfollowed corrosion. by corrosion.

(a) (b)

FigureFigure 10.10. TheThe mapmap measurementsmeasurements ofof LEISLEIS ofof thethe 0%0% zinczinc phosphatephosphate coatingcoating immersedimmersed inin 3.5%3.5% NaClNaCl solution:solution: ((aa)) 3D3D ofof 00 h;h; ((bb)) 2D2D ofof 00 h;h; ((cc)) 2D2D ofof 33 h;h; ((dd)) 2D2D ofof 66 h.h.

Figure 11 shows the map measurements of LEIS of the 8% zinc phosphate coating immersed in Figure 11 shows the map measurements of LEIS of the 8% zinc phosphate coating immersed in 3.5% NaCl solution. At the initial stage of immersion, the impedance value of the coating defect was 3.5% NaCl solution. At the initial stage of immersion, the impedance value of the coating defect was significantly below the impedance value in the intact coating, which was the same as the 0% zinc signiﬁcantly below the impedance value in the intact coating, which was the same as the 0% zinc phosphate coating. As immersion time increased, the impedance of the coating defect did not change phosphate coating. As immersion time increased, the impedance of the coating defect did not change and the coating defect around it slightly increased, which was completely different from the 0% zinc and the coating defect around it slightly increased, which was completely different from the 0% zinc phosphate coating. The added zinc phosphate coating prevented the diffusion of corrosive medium phosphate coating. The added zinc phosphate coating prevented the diffusion of corrosive medium in in the lateral of coating/metal interfaces and slowed down the corrosion. the lateral of coating/metal interfaces and slowed down the corrosion. After the immersion test in 3.5% NaCl solution, Raman spectroscopy of rust and corrosion After the immersion test in 3.5% NaCl solution, Raman spectroscopy of rust and corrosion product product in the defect of the 8% zinc phosphate coating were measured. As shown in Figure 12, the in the defect of the 8% zinc phosphate coating were measured. As shown in Figure 12, the corrosion corrosion product was made up by the composition of γ-Fe2O3, α-FeOOH, γ-FeOOH, and iron product was made up by the composition of γ-Fe O , α-FeOOH, γ-FeOOH, and iron phosphate. phosphate. 2 3

Materials 2017, 10, 654 8 of 13 Materials 2017, 10, 654 8 of 13 Materials 2017, 10, 654 8 of 13

(a) (b) (a) (b)

(c) (d) (c) (d) FigureFigure 11. The 11. The map map measurements measurements of of LEIS LEIS of of thethe 8%8% zinc phosphate phosphate coating coating immersed immersed in 3.5% in 3.5% NaCl NaCl Figuresolution: 11. The ( amap) 3D ofmeasurements 0 h; (b) 2D of 0 of h; LEIS(c) 2D of of the 3 h; 8% (d) zinc2D of phosphate 6 h. coating immersed in 3.5% NaCl solution:solution: ( (aa)) 3D 3D of of 0 0 h; h; ( (bb)) 2D 2D of of 0 0 h; h; ( (cc)) 2D 2D of of 3 3 h; h; ( (dd)) 2D 2D of of 6 6 h. h.

Figure 12. Raman analyses of rust and corrosion product in the defect of the 8% zinc phosphate coating.

3. Discussion FigureFigure 12. 12. RamanRaman analyses analyses of of rust rust and and corrosion corrosion product product in in the the defect defect of of the the 8% 8% zinc zinc phosphate phosphate coating. coating. In comparing the EIS results of the 0% and 8% zinc phosphate coatings (Figure 6) at an 85 ± 5 µm 3.3. Discussion Discussiondefects on the coatings, the metals were shown to corrode at the defect areas. At the beginning of immersion, the low-frequency impedance value decreased in both, but as immersion time increased, InInthe comparing comparing low-frequency the impedanceEIS results value of the of 0% the and8% zi 8%nc phosphate zinczinc phosphatephosphate coating coatingscoatings remained (Figure(Figure stable 6at 6)) 10 at at5 Ohm·cm anan 8585 ±± 25,5 µmµm defectsdefectswhich on on the thewas coatings, higher than the the that metals of the were 0% zinc shown phosphate to corrode (Figure at 7a). the Moreover, defect areas. the R ctAt At value the ofbeginning the 8% of of immersion,immersion,zinc phosphate the the low-frequency low-frequency coating was significantlyimpedance impedance higher value value thandecreased decreased that of thein in both,0% both, zinc but but phos as as phateimmersion immersion coating time time(Figure increased, increased, 8a). thethe low-frequency low-frequencyThe added zinc impedance phosphate value in the ofcoating the 8% slowed zinczinc phosphatedown the corrosion coating reaction remainedremained at stablethestable coating atat 1010 defects.55 Ohm·cmOhm· cm22, , whichThe was Q dlhigher value thanof the that 8% ofzinc the phosphate 0% zinc phosphatecoating was (Figure significantly 7a). Moreover,lower than thethat R ofct valuethe 0% of zinc the 8% which was higher than that of the 0% zinc phosphate (Figure7a). Moreover, the R ct value of the 8% phosphate coating (Figure 8b), showing that the added zinc phosphate slowed down the coating zinczinc phosphate phosphate coating coating was was significantly signiﬁcantly higher higher than than that that of of the the 0% 0% zinc zinc phos phosphatephate coating coating (Figure (Figure 8a).8a). disbonding. The reason for this may be that, when the corrosive medium permeated the coating/metal The added zinc zinc phosphate phosphate in in the the coating coating slowed slowed down down the the corrosion corrosion reaction reaction at at the the coating coating defects. defects.

The Qdl value of the 8% zinc phosphate coating was significantly lower than that of the 0% zinc The Qdl value of the 8% zinc phosphate coating was signiﬁcantly lower than that of the 0% zinc phosphate coating coating (Figure (Figure 88b),b), showingshowing thatthat thethe addedadded zinczinc phosphatephosphate slowedslowed downdown thethe coatingcoating disbonding. The reason for this may be that, when the corrosive medium permeated the coating/metal

Materials 2017, 10, 654 9 of 13 Materials 2017, 10, 654 9 of 13 interfaces, the zinc phosphate was partly dissolved and produced metal substrate phosphate. Zinc phosphate with the composition of γ-Fe2O3, α-FeOOH, γ-FeOOH, and iron phosphate (Figure 12) can disbonding. The reason for this may be that, when the corrosive medium permeated the coating/metal serve as the protective film. The protective film contained carboxyl and hydroxyl groups, which can interfaces, the zinc phosphate was partly dissolved and produced metal substrate phosphate. Zinc strengthen the combination of the coating and substrate [29]. phosphate with the composition of γ-Fe2O3, α-FeOOH, γ-FeOOH, and iron phosphate (Figure 12) can 2 serve as the protective film. The protectiveFe film contained Fe 2e carboxyl and hydroxyl groups, which(1) can strengthen the combination of the coating and substrate [29].  O22 2H O 4e 4OH (2) → 2+ + − Fe Fe 2e 2 (1) Zn PO 2H O 4OH  3Zn OH  2HPO (3) 3422 2− − 4 O2 + 2H2O + 4e → 4OH (2) 22  Fe HPO44− FePO H e 2− (4) Zn3(PO4)2 + 2H2O + 4OH → 3Zn(OH)2 + 2HPO4 (3) In comparing the LEIS of the 20%+ and 8%2 zinc− phosphate coatings,+ − the low-frequency impedance Fe + HPO4 → FePO4 + H + e (4) value of the 8% zinc phosphate coating defect declined slowly and the 0% zinc phosphate coating defectIn declined comparing visibly the LEIS (Figure of the 9) 0%. These and 8% results zinc phosphateshow that coatings,the water the absorption low-frequency of the impedance 8% zinc valuephosphate of the 8%coating zinc phosphatewas decreased coating significantly. defect declined When slowly the and corrosion the 0% zinc medium phosphate penetrated coating defect the declinedcoating/metal visibly interfaces, (Figure9 part). These of the results zinc phosphate show that dissolved the water and absorption reacted with of the Fe 8% to zincform phosphate stable iron coatingphosphates was that decreased can block significantly. the medium Whendiffusion the [30]. corrosion medium penetrated the coating/metal interfaces,The impedance part of the values zinc phosphate of the 8% dissolved zinc phosph and reactedate coating with increased Fe to form as stable the ironimmersion phosphates time thatincreased can block (Figure the medium11), because diffusion the chemical [30]. activation of zinc phosphate enhanced the wet coating adhesion.The impedance The spread values of the of corrosive the 8% zinc medium phosphate in the coating coating/metal increased interface as the immersion was prevented time increased and the (Figurecoating 11defect), because corrosion the reaction chemical and activation disbonding of zinc were phosphate slowed. enhanced the wet coating adhesion. The spreadThe failure of the mechanisms corrosive mediumof the 0% in and the 8% coating/metal zinc phosphate interface coatings was are prevented shown in Figure and the 13. coating When defectthe 0% corrosion zinc phosphate reaction coating and disbonding with the weredefect slowed. (Figure 13a) was exposed to corrosive media, the corrosionThe failurereaction mechanisms occurred rapidly of the on 0% the and coatin 8%g/metal zinc phosphate interface and coatings grew aresuddenly. shown Furthermore, in Figure 13. Whenthe coating the 0% disbanded zinc phosphate from coatingthe defect. with However, the defect the (Figure coating 13a) defect was exposed with zinc to corrosivephosphate media, has two the corrosionaspects of reactionprotection occurred (Figure rapidly 13b). First, on the when coating/metal the corrosion interface medium and grewpermeates suddenly. the coating/metal Furthermore, theinterfaces, coating the disbanded zinc phosphate from the and defect. Fe generates However, stable the iron coating phosphates defect and with inhibits zinc phosphate the anodic has process two aspectsof corrosion. of protection Second, (Figurezinc phosphate 13b). First, can when combine the corrosion with the mediumcarboxyl permeatesand hydroxyl the coating/metalgroups of the interfaces,coating, through the zinc which phosphate a stable and protective Fe generates film stablewas formed iron phosphates by the zinc and phosphate, inhibits thecoating anodic and process metal ofsubstrate. corrosion. All Second, of these zinc reactions phosphate can caninhibit combine the ca withthode the reaction carboxyl from and damaging hydroxyl groupsthe coating, of the prevent coating, throughthe corrosion which medium a stable protectivefrom a horizontal film was formeddiffusion, by theand zinc slow phosphate, down the coating coating and defect metal corrosion substrate. Alland of disbonding. these reactions can inhibit the cathode reaction from damaging the coating, prevent the corrosion medium from a horizontal diffusion, and slow down the coating defect corrosion and disbonding.

Figure 13. SchematicSchematic of of the the failure mechanis mechanismm of styrene-acrylic with 0% ( a) and 8% (b) zinc phosphate.

Materials 2017, 10, 654 10 of 13 Materials 2017, 10, 654 10 of 13

4. Materials and Methods

4.1. Sample Preparation This work used waterborne acrylic resin made in a chemical plant in Beijing, and the zinc phosphate was 800 mesh produced in Guangxi Research Institute of Chemical Industry. A total of 8 wt.%wt.% of of zinc zinc phosphate phosphate composite composite coating coating was preparedwas prepared using high-speedusing high-speed shearing shearing mixing. Themixing. zinc phosphateThe zinc phosphate was dried forwas 15 dried min atfor 50 15◦C, min and at then 50 the°C, film-forming and then the agent, film-forming wetting dispersant, agent, wetting flash rustdispersant, agent, zincflash phosphate, rust agent, andzinc a phosphate, certain amount and a of certain water amount were added of water successively. were added The successively. above fillers wereThe above added fillers and thenwere stirred added usingand then a glass stirred bar using stirrer. a Afterglass addingbar stirrer. the After above adding fillers, the solutionabove fillers, was manuallythe solution stirred was usingmanually a glass stirred rod forusing approximately a glass rod 10for minapproximately and then dispersed 10 min and using then a high-speed dispersed shearingusing a high-speed dispersion shearing machine dispersion (America Redmachine Devil (A 5400,merica Trenton, Red Devil NJ, 5400, USA) Trenton, for 30 min NJ, toUSA) obtain for component30 min to obtain A. Acrylic component resin andA. Acrylic a defoaming resin and agent a defoaming were mixed agent manually were mixed using manually a glass rod using and a thenglass withrod aand magnetic then with stirrer a magnetic at 360 r/min stirrer for at 30 360 min r/min to obtain for 30 component min to obtain B. Finally, component component B. Finally, A was mixedcomponent with componentA was mixed B usingwith component a glass rod forB using effective a glass distribution, rod for effective followed distribution, by magnetic followed stirring by at 180magnetic r/min stirring for 30 min at 180 to obtainr/min for the 30 coating, min to asobtain shown the in coating, the Figure as shown 14. in the Figure 14.

Figure 14. Flowchart to illustrate the processprocess of preparing the coating.

The metal substrate surface was burnished with a no. 240 sandpaper, cleanedcleaned with acetone, and then dried.dried. TheThe aboveabove coating coating was was brushed brushed onto onto the th metale metal substrate substrate surface. surface. The The coating coating thickness thickness for thefor the morphology morphology observation, observation, adhesion adhesion test, test, and and EIS EIS was was 85 85± ±5 5µ µmm;; whereas, whereas,that that forfor thethe LEISLEIS was 20 ± 22 µm.µm. Both Both were were cured cured for 15 days at room temperature. The length and width of the coating defects for the EIS test were 10 mm and 120 µm,µm, respectively, and those for the LEIS were 1 mm and 120 µµm,m, respectively,respectively, andand werewere implementedimplemented byby aa tooltool knifeknife made made of of LB30N LB30N steel. steel.

4.2. Test MethodsMethods According toto ISO4624 ISO4624 standard standard [31 ],[31], PosiTestdrawing PosiTestdrawing was usedwas toused determine to determine the adhesion the adhesion strength ofstrength the organic of the coating.organic coating. The 20-mm The diameter20-mm diamet dollieser weredollies degreased were degreased using acetone using acetone and then and glued then toglued the surfaceto the surface of the coatedof the platescoated using plates a two-componentusing a two-component epoxy-based epoxy-based cyano-acrylate cyano-acrylate adhesive. Afteradhesive. adhesive After curing,adhesive a testingcuring, apparatus a testing wasapparatus attached was to attached the loading to the ﬁxture loading and ﬁ wasxture strained and was at 5strained mm/min at 5 until mm/min the coating until the material coating had mate detachedrial had from detached the substrate. from the substrate. The micromorphology of the coated section sampled was performed using using SEM SEM (QUANTA (QUANTA 250, FEI, Hillsboro, OR, USA). The macromorphologymacromorphology and micromorphologymicromorphology of the coating defects were obtained using a digital camera and stereomicroscope.

Materials 2017, 10, 654 11 of 13 Materials 2017, 10, 654 11 of 13

Raman spectra from 100 cm−1 to 1000 cm−1 were used to analyze the composition of the corrosion productsRaman of the spectra defect from of 100the cm8%− zinc1 to 1000phosphate cm−1 werecoating. used It towas analyze measured the composition by the Renishaw of the corrosionDXP780 Ramanproducts (Renishaw, of the defect London, of the UK). 8% zinc phosphate coating. It was measured by the Renishaw DXP780 RamanThe (Renishaw, macroscopic London, electrochemical UK). studies on the coating specimens were conducted using a PARSTAT-2273The macroscopic electrochemical electrochemical workstation studies (Princet on theon coatingInstruments, specimens Trenton, were NJ, conducted USA) with using an electrolytica PARSTAT-2273 cell device electrochemical as shown workstationin Figure 15. (Princeton A three-electrode Instruments, cell Trenton, system NJ,wasUSA) used within this an experiment.electrolytic cell The device system as was shown assembled in Figure with 15. Athe three-electrode coating sample cell as systemthe working was used electrode, in this a experiment. saturated calomelThe system electrode was assembled (SCE) (Rex, with theShanghai, coating sampleChina) as as the the working reference electrode, electrode, a saturated and platinum calomel electrode as the auxiliary(SCE) (Rex, electrode. Shanghai, The China) EIS asmeasurements the reference were electrode, taken and in platinuma 3.14 cm as2 area the auxiliaryfor the coating electrode. samples The EIS in 3.5%measurements NaCl solution were in takenthe frequency in a 3.14 range cm2 areaof 100 for kHz–0.01 the coating Hz and samples a disturbance in 3.5% NaClalternating solution potential in the voltagefrequency of range20 mV. of 100The kHz–0.01 fitting of Hz EIS and data a disturbance was conducted alternating using potential ZSimpWin3.2. voltage ofAll 20 EIS mV.The tests ﬁttingwere conductedof EIS data at was 22 conducted°C. using ZSimpWin3.2. All EIS tests were conducted at 22 ◦C.

FigureFigure 15. 15. MacroscopicMacroscopic electrochemical electrochemical test test devices. devices.

The LEIS studies on coating specimens were conducted using LEIS components in a M370 micro The LEIS studies on coating specimens were conducted using LEIS components in a M370 micro electrochemical workstation (Bio-Logic, Seyssinet-Pariset, France) The LEIS probe made from a electrochemical workstation (Bio-Logic, Seyssinet-Pariset, France) The LEIS probe made from a 100-µm 100-µm diameter platinum wire was used as an auxiliary electrode, the coating sample was the diameter platinum wire was used as an auxiliary electrode, the coating sample was the working working electrode, and an SCE was used as the reference electrode. The distance between the probe electrode, and an SCE was used as the reference electrode. The distance between the probe and and the sample was approximately 100 µm and the disturbance alternating potential voltage was the sample was approximately 100 µm and the disturbance alternating potential voltage was 10 mV. 10 mV. To ensure the stability of the measurement system, the working electrode was placed in the To ensure the stability of the measurement system, the working electrode was placed in the solution for solution for 20 min until the fluctuation was less than 10 mV. Thereafter, the potential was set as a 20 min until the fluctuation was less than 10 mV. Thereafter, the potential was set as a fixed potential to fixed potential to ensure that the measurement was under open circuit potential. The LEIS used two ensure that the measurement was under open circuit potential. The LEIS used two kinds of test modes, kinds of test modes, as described below: as described below: Point measurement mode: measurement frequency range was 10,000–0.05 Hz; distance from Point measurement mode: measurement frequency range was 10,000–0.05 Hz; distance from defects was approximately 1600 µm. defects was approximately 1600 µm. Map scanning mode: testing scope was 3100 × 1000 µm with a step length of 100 µm; according Map scanning mode: testing scope was 3100 × 1000 µm with a step length of 100 µm; according to previous studies [32], 5 kHz was selected to characterize the coating disbonding. to previous studies [32], 5 kHz was selected to characterize the coating disbonding. 5. Conclusions 5. Conclusions Zinc phosphate particles were dispersed evenly in the coating and combined closely with the Zinc phosphate particles were dispersed evenly in the coating and combined closely with the waterborne acrylic resin. No defects in the coating were found, and the initial adhesion of the coatings waterborne acrylic resin. No defects in the coating were found, and the initial adhesion of the coatings with and without zinc phosphate remained. These results showed that zinc phosphate and with and without zinc phosphate remained. These results showed that zinc phosphate and waterborne waterborne acrylic resin have good compatibility. The experimental results of EIS revealed that acrylic resin have good compatibility. The experimental results of EIS revealed that adding zinc adding zinc phosphate did not change the electrochemical process of the coating failure but phosphate did not change the electrochemical process of the coating failure but effectively slowed effectively slowed the corrosion degree of the coating defects. The anode and cathode electrochemical the corrosion degree of the coating defects. The anode and cathode electrochemical processes were processes were characterized via LEIS. Adding zinc phosphate inhibited the anode process of the characterized via LEIS. Adding zinc phosphate inhibited the anode process of the bare metal in the bare metal in the defect and slowed the corrosion reaction of the anodic metal. Simultaneously, zinc defect and slowed the corrosion reaction of the anodic metal. Simultaneously, zinc phosphate enhanced phosphate enhanced the wet adhesion of coating, and was beneficial for preventing the lateral the wet adhesion of coating, and was beneficial for preventing the lateral diffusion of the corrosive diffusion of the corrosive medium into the coating/metal interface. Moreover, it slowed down the

Materials 2017, 10, 654 12 of 13 medium into the coating/metal interface. Moreover, it slowed down the disbonding of the intact coating around the defect. The results showed that adding zinc phosphate to the waterborne acrylic coating can highly improve corrosion resistance.

Acknowledgments: The authors wish to acknowledgement the financial support from National Basic Research Program of China (973 Program) (No. 2014CB643300) and National Natural Science Foundation of China (Grant Nos. 51371036). Author Contributions: Dongdong Song and Xiaogang Li conceived and designed the experiments; Hongxia Wan and Dawei Zhang performed the experiments; Dongdong Song and Hongxia Wan analyzed the data; Jin Gao and Cuiwei Du contributed reagents/materials/analysis tools; Hongxia Wan wrote the paper. Conflicts of Interest: We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

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