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Failure Mechanisms of the Coating/Metal Interface in Waterborne Coatings: The Effect of Bonding Hongxia Wan 1 , Dongdong Song 1,2, *, Xiaogang Li 1,3, *, Dawei Zhang 1 , Jin Gao 1 and Cuiwei Du 1 1

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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.) Aerospace Research Institute of Materials and Processing Technology, Beijing 100076, China Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China Correspondence: [email protected] (D.S.); [email protected] (X.L.); Tel.: +86-10-6838-3576 (D.S.); +86-10-6233-3931 (X.L.)

Academic Editor: Jérôme Chevalier Received: 27 February 2017; Accepted: 6 April 2017; Published: 9 April 2017

Abstract: Waterborne coating is the most popular type of coating, and improving its performance is a key point of research. Cathodic delamination is one of the major modes of failure for organic coatings. It refers to the weakening or loss of adhesion between the coating and substrate. Physical and chemical characteristics of coatings have been studied via scanning electron microscopy (SEM), atomic force microscopy (AFM), contact angle measurements, Fourier transform infrared spectroscopy (FTIR), and secondary ion mass spectrometry (SIMS). Early heterogeneous swelling at the metal-coating interface in non-defective coated metals was elucidated using frequency-dependent alternating-current scanning electrochemical microscopy. Two types of coatings (styrene-acrylic coating and terpolymer coating) were compared. The effects of thickness, surface roughness, and chemical bonding on cathodic delamination were investigated. Keywords: waterborne coating; scanning electrochemical microscopy (SECM); delamination

1. Introduction Organic coating is the most effective and economical method to protect metallic materials from corrosion [1–6]. The protective coating can isolate the metal substrate from corrosive media [7], provide electrochemical protection (inhibition [8] and cathodic protection [9]), and exhibit an adhesive function [10]. However, organic coatings degrade under aggressive environments because of underfilm corrosion, which can shorten the service life of coatings [11–15]. Degradation is accelerated by scratches or wear, allowing the diffusion of aggressive media to the interface between the coating and metal. Studies [10,16] have shown that the main reason for the anti-corrosion role of coatings is not physical isolation, but the adhesion of coatings is fundamental to its protective effects. The process and mechanism of coating swelling has been investigated by numerous studies. Stratmann [17–19] studied, in situ, the delamination characterization on coating defects and obtained the kinetics and the electrochemical model of the delamination using the Scanning Kelvinprobe (SKP) method, a mechanical test with a homemade device and infrared analysis. McMurray et al. [20–22] used the SKP method to study the effect of an inhibitor on corrosion in the coating/metal interface; results showed that the inhibitor in the coating can slow down oxygen reduction and underfilm corrosion, thereby relieving delamination in the coating. Santana et al. [23] studied the early specific effect of chloride ions on heterogeneous swelling at the metal-polymer interface using frequency-dependent Materials 2017, 10, 397; doi:10.3390/ma10040397

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alternating current-scanning electrochemical microscopy (AC-SECM). Souto reported changes in coatings induced by chloride ions in situ during immersion [24–26]. The majority of research focused on the relationship between aggressive environment and coating failure. However, studies on the Materials 2017, 10, 397 2 of 15 effect of the adhesion mechanism on coating swelling are extremely rare. When corrosive have direct accessaggressive to metal substrate, corrosion electrochemical reactions research focused onmedia the relationship between environment and coating failure. However, willstudies take place the metal-coating in the presence of water then delamination occurs. on theateffect of the adhesioninterface mechanism on coating swelling are and extremely rare. media have requires direct access tobonding metal substrate, corrosion electrochemical Hence, When a goodcorrosive protective coating tight between the coating and metalreactions substrate to willthe take place at theofmetal-coating the mechanical presence of water then delamination resist penetration water to the interface interface.inThe bond and plays a dominant roleoccurs. in coating Hence, a good protective coating requires tight bonding between the coating and metal substrate to adhesion [27–30]. It is often used in engineering practices to enlarge the surface roughness of metals the penetration of water theimproving interface. The mechanical bond plays a dominant in coating for resist increasing the contact area to and adhesion between the coating androle substrate metal. adhesion [27–30]. It is often used in engineering practices to enlarge the surface roughness of metals However, given that physical adsorption is the essence of the mechanical bond, this type of bond does for increasing the contact area and improving adhesion between the coating and substrate metal. not provide sufficient protection for long-term wet adhesion. Therefore, researchers have investigated However, given that physical adsorption is the essence of the mechanical bond, this type of bond various methods to enhance the wet adhesion of coatings. Chemical bonding is much stronger than does not provide sufficient protection for long-term wet adhesion. Therefore, researchers have physical adsorption, and the former can effectively impede lateral diffusion of water on the interface investigated various methods to enhance the wet adhesion of coatings. Chemical bonding is much between thethan coating and metal [31,32]. Chemical bonding can maintain thelateral wet adhesion corrosion stronger physical adsorption, and the former can effectively impede diffusionand of water protection of coatings for a long time [10,33,34]. on the interface between the coating and metal [31,32]. Chemical bonding can maintain the wet In this study, the swelling behaviors of twofor waterborne coatings were evaluated via SECM adhesion and corrosion protection of coatings a long timeacrylic [10,33,34]. and theIn effects of adhesion mechanisms on the coating failure were discussed. this study, the swelling behaviors of two waterborne acrylic coatings were evaluated via SECM and the effects of adhesion mechanisms on the coating failure were discussed.

2. Results and Discussion

2. Results and Discussion

2.1. Characteristics of the Coating Structure 2.1. of the Coating Structure ToCharacteristics analyze the characteristics of two resins after curing, the surface and cross-sectional shape of

the twoTo resins were by scanning electron (SEM)and andcross-sectional atomic force shape microscopy analyze theobserved characteristics of two resins aftermicroscopy curing, the surface of the two resins were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The contact angles of coatings were determined to analyze the hydrophilic character of the (AFM).Infrared The contact angles of coatings were determined analyze the hydrophilic character of the the coatings. spectroscopy and secondary ion mass to spectroscopy were combined to analyze coatings. Infrared spectroscopy and secondary ion mass spectroscopy were combined to analyze the bonding between coating and metal. bonding and metal. Figure between 1 showscoating the SEM micrograph of the cross-section of the two coatings. The styrene-acrylic Figure 1 shows the SEM micrograph the cross-section of theproperties. two coatings. The styrene-acrylic coating is formed by styrene-acrylic latex, of which has high barrier The terpolymer coating, coating is formed by styrene-acrylic latex, which has high barrier properties. The terpolymer coating, which contains acrylic acid (CH2 =CH–COOH), vinyl chloride (CHCl=CH2 ), and 1,1-dichloroethylene which contains acrylic acid (CH2=CH–COOH), vinyl chloride (CHCl=CH2), and 1,1-dichloroethylene (CH2 =CCl2 ), has excellent adhesion performance. Both of them are single components and can be (CH2=CCl2), has excellent adhesion performance. Both of them are single components and can be cured at normal atmospheric temperature. The interface of both coatings was intact without obvious cured at normal atmospheric temperature. The interface of both coatings was intact without obvious defects, and thethe bonding and metal metalwas wasclose. close.Wrinkles Wrinkles caused friction defects, and bondingbetween betweenthe the coating coating and caused by by friction werewere observed on the cross-section of the styrene-acrylic coating. Under the same conditions, the cutting observed on the cross-section of the styrene-acrylic coating. Under the same conditions, the cutting surface of the terpolymer coating was relatively smooth. Thus, the hardness of the terpolymer coating surface of the terpolymer coating was relatively smooth. Thus, the hardness of the terpolymer coating waswas softer than that ofofthe [35]. softer than that thestyrene-acrylic styrene-acrylic coating coating [35].

Figure 1. Section microstructure of the styrene-acrylic (a) and terpolymer (b) coatings.

Figure 1. Section microstructure of the styrene-acrylic (a) and terpolymer (b) coatings.

Figure 2 shows the AFM images of the two coatings before immersion. Within the scanning Figure shows the the AFM images ofofthe two coatings immersion. Within the range (10 2 μm × 10 μm), morphology both coatings wasbefore relatively flat and composed of ascanning large number of small whose diameter of was about 100 nm.was Thisrelatively phenomenon was due to film-of a range (10 µm × 10particles µm), the morphology both coatings flat and composed formation [36,37].

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large number of small particles whose diameter was about 100 nm. This phenomenon was due to film-formation [36,37]. Materials 2017, 10, 397 3 of 15

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Figure 2. Initial microstructure of styrene-acrylic (a) and terpolymer (b) coatings.

Figure 2. Initial microstructure of styrene-acrylic (a) and terpolymer (b) coatings. Upon comparing the morphology of the two coatings, the styrene-acrylic coating was smoother thancomparing the terpolymer In addition, thetwo terpolymer coating revealed a largercoating size andwas higher Upon the2.coating. morphology of the coatings, the terpolymer styrene-acrylic smoother Figure Initial microstructure of styrene-acrylic (a) and (b) coatings. roughness than the styrene-acrylic coating. than the terpolymer coating. In addition, the terpolymer coating revealed a larger size and higher Figurecomparing 3 illustrates the contact angle testtwo results of the the twostyrene-acrylic coating surfaces. The contact angles Upon the morphology of the coatings, coating was smoother roughness than the styrene-acrylic coating. of thethe styrene-acrylic coating In and terpolymer coating werecoating 76° ± 1.68° and 50° ± 0.29°, respectively. than terpolymer coating. addition, the terpolymer revealed a larger size and higher Figure 3 illustrates the contact angle test results of the two coating surfaces. The contact angles The different contact angles of the coating. two acrylic coatings indicated their difference in wettability. The roughness than the styrene-acrylic ◦ , respectively. of the styrene-acrylic coatingdemonstrated and terpolymer coating 76◦ ± 1.68◦ and 50◦ ± 0.29 styrene-acrylic coating relatively highofwere hydrophobicity. contrast, terpolymer Figure 3 illustrates the contact angle test results the two coating By surfaces. Thethe contact angles The different contact angles of and themay two coatings indicated their in wettability. coating was hydrophilic, which be acrylic due to aggregation of the hydrophilic group on of the styrene-acrylic coating terpolymer coating were 76° ± 1.68° and 50° ±difference 0.29°,surfactant respectively. the coating surface [38]. The styrene-acrylic coatingangles demonstrated relatively high hydrophobicity. By contrast, the terpolymer The different contact of the two acrylic coatings indicated their difference in wettability. The coating demonstrated relatively high hydrophobicity. By contrast, the terpolymer coating styrene-acrylic was hydrophilic, which may be due to aggregation of the hydrophilic group surfactant on the hydrophilic, which may be due to aggregation of the hydrophilic group surfactant on coating coating surfacewas [38]. the coating surface [38].

Figure 3. Contact angle of styrene-acrylic and terpolymer coating surfaces.

Figure 4 shows the results of infrared analysis of the two resins and coatings in carbon steel substrate. When the coating was thin (less than 4 μm), infrared waves could traverse through it and 3. Contact angle styrene-acrylic and coating surfaces. FigureFigure 3. Contact angle ofofstyrene-acrylic andterpolymer terpolymer coating surfaces. reflect the composition of the coating/metal interface. For the terpolymer resin, a new peak formed in theFigure vicinity of 1740the cm–1 after curing on theanalysis surfaceof of the carbon which was due in to the carbonyl 4 shows results of infrared twosteel, resins and coatings carbon steel Figure 4 shows the results of infrared analysis the two resins and coatings carbon produced When by thethe reaction resin’s carboxyl groups and the matrix metalthrough [39].inFor the steel substrate. coatingbetween was thinthe (less than 4 μm),of infrared waves could traverse it and styrene-acrylic resin, no change in the original position was observed, except the peak intensity substrate. When the coating wascoating/metal thin (less than 4 µm), infrared waves could through it reflect the composition of the interface. For the terpolymer resin, a newtraverse peak formed increased. To further the infrared theinterface. terpolymer coating on the carbon steelcarbonyl surface and reflect composition thecuring coating/metal the terpolymer a new peak in thethe vicinity of 1740 verify cm–1ofafter onresults, the surface of carbon For steel, which was due to resin, the was analyzed by secondary ion–1massthe spectrometry (SIMS). produced by the reaction between resin’s carboxyl groups and the matrix metal [39]. For the

formed in the vicinity of 1740 cm after curing on the surface of carbon steel, which was due to the styrene-acrylic resin, change in the original position carboxyl was observed, except peak intensity carbonyl produced by theno reaction between the resin’s groups andthethe matrix metal [39]. increased. To further verify the infrared results, the terpolymer coating on the carbon steel surface For the styrene-acrylic resin, no change in the original position was observed, except the peak intensity was analyzed by secondary ion mass spectrometry (SIMS). increased. To further verify the infrared results, the terpolymer coating on the carbon steel surface was analyzed by secondary ion mass spectrometry (SIMS).

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Figure 4. Infrared results of styrene-acrylic (a) and terpolymer (b) coatings.

Figure 4. Infrared results of styrene-acrylic (a) and terpolymer (b) coatings. In SIMS, ionized particles are ejected from the surface by the bombardment of a primary ion +, F−, OFigure +) results Infrared of styrene-acrylic (a) and coatings.Both beam 2+, O−,4.and Csejected and then separated according their(b)masses. atoms and In SIMS,(Ar ionized particles are from the surface by terpolymer thetobombardment of a primary ion beam molecules can be ionized. Thus, details about the chemical state of atoms of the surface, such as + − − + + (Ar , F , O2In, SIMS, O , and Cs ) and then separated according to their masses. Both atoms and molecules ionized particles are ejected fromresult the surface the bombardment primarysteel ion bonding, are obtained. Figure 5 shows the SIMS for the by terpolymer coating onof thea carbon can be ionized. about chemical state according of atoms to of their the surface, suchatoms as bonding, are +Thus, −, Odetails +) the beam (Ar , F 2+, O−, and Cs and then separated masses. Both and surface. The peak at 100 nm means that COOFe bonding existed on the coating/metal interface. This obtained. Figure 5 shows SIMS resultabout for terpolymer the carboncoating steelassurface. molecules can be ionized. Thus, the chemical statecoating of atomson of the surface, such finding conformed to thethe results ofdetails infrared (IR) the in which the surface between the terpolymer bonding, are Figure 5 shows thebond. SIMSHowever, result foron thebonding terpolymer coating oninterface. the the carbon steel finding The peak atthe 100 nmobtained. means that COOFe bonding existed the coating/metal This and metal substrate exhibited a stable no occurred between styrenesurface. Theresults peak 100 nm means that existed on the coating/metal interface. Thisand the acrylic andatthe metal substrate. conformed tocoating the of infrared (IR) inCOOFe whichbonding the surface between the terpolymer coating finding conformed to the results of infrared (IR) in which the surface between the terpolymer coating metal substrate exhibited a stable bond. However, no bonding occurred between the styrene-acrylic and the metal substrate exhibited a stable bond. However, no bonding occurred between the styrenecoating acrylic and the metal substrate. coating and the metal substrate.

Figure 5. SIMS result of the interface between the terpolymer coating and metal.

2.2. Adhesion Force between the Metal and the Two Coatings Figure 5. SIMS result of the interface between the terpolymer coating and metal.

Figure 5.the SIMS of the interface betweena the terpolymer coatingtests andinmetal. To evaluate wet result adhesive force of the coatings, series of wet adhesion the different immersion circles was performed. Figure 6 shows the wet adhesive force under different test cycles. 2.2. Adhesion Force between the Metal and the Two Coatings The initial adhesive force of the styrene-acrylic coating is about 5 MPa which is almost twice that of 2.2. AdhesionToForce between theadhesive Metal and the Two Coatingsa series of wet adhesion tests in the different evaluatecoating. the wetThe force ofcoating the coatings, the terpolymer terpolymer is too soft to withstand the pulling force [27,28]. immersion circles was performed. Figure 6the shows the wet force different ToHowever, evaluatewith the wet adhesive force of the coatings, a adhesive series wet under adhesion teststest incycles. the different increasing immersion time, adhesive force of theof styrene-acrylic coating decreases The initial adhesive force of the styrene-acrylic coating is about 5 MPa which is almost twice that of rapidly, and the is only 20% of the6initial amount afteradhesive eight daysforce of immersion. By contrast, immersion circles wasvalue performed. Figure shows the wet under different test cycles. theterpolymer terpolymercoating coating. The terpolymer coating is too soft to withstand thereliable pulling force [27,28]. the maintains its state. The terpolymer coating has a more bonding in the The initial adhesive force of theimmersion styrene-acrylic is about 5theMPa which is almost twice that of the However, withinterface increasing time, thecoating adhesive styrene-acrylic coating decreases coating/metal than the styrene-acrylic coating force whenofthe defect exists. This result can be terpolymer coating. The terpolymer coating is too soft to withstand the pulling force [27,28]. However, rapidly, and therobust value is only 20% offorce the initial after eight days of By contrast, attributed to the wet adhesive whichamount can effectively suppress theimmersion. lateral diffusion of the with increasing immersion time, the adhesive force of the styrene-acrylic coating decreases rapidly, and the terpolymer coating maintains its state. The terpolymer coating has a more reliable bonding in the corrosive ions to the coating/metal interface. coating/metal interface than the styrene-acrylic coating when the defect exists. This result can be the value is only 20% of the initial amount after eight days of immersion. By contrast, the terpolymer to its thestate. robustThe wet adhesive forcecoating which can effectively the lateral in diffusion of the coating attributed maintains terpolymer has a more suppress reliable bonding the coating/metal corrosive ions to the coating/metal interface.

interface than the styrene-acrylic coating when the defect exists. This result can be attributed to the robust wet adhesive force which can effectively suppress the lateral diffusion of the corrosive ions to the coating/metal interface.

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Figure 6. Wet adhesive force with the immersion time.

Figure 6. Wet adhesive force with the immersion time. 2.3. Microbubbling Process of the Coating Interface

2.3. Microbubbling Process of the to Coating SECM was conducted test theInterface micro bubbling process on the coating interface (all X and Y coordinates are in micrometers). According to Equationprocess (1), a stable current interface was obtained SECM was conducted to test the micro bubbling on probe the coating (allinX and Y 3.5% NaCl solution for a probe potential of −0.7 V (SCE). When the probe approached the sample, the coordinates are in micrometers). According to Equation (1), a stable probe current was obtained in current presented the morphology of the surface of the sample. Therefore, the current could be used 3.5% NaCl solution for probe potential of −0.7surface V (SCE). When the probe Figure 6. of Wet force with the immersion time. approached the sample, the to characterize theamorphology theadhesive sample [24]. current presented morphology of between the surface of the sample. current be used to During the analysis, the distance the probe and sample Therefore, was fixed at the 40 μm and thecould test area 2. Process 2.3. Microbubbling of the Coating Interface was 0.25 To observe thethe changes in the sample in situ during immersion, the current signal of characterize themm morphology of sample surface [24]. the sample surface repeatedly tested within a certain time (4–24was h). fixed at 40 µm and the test area During thewas distance thebubbling probe and sample SECManalysis, was conducted to testbetween the micro process on the coating interface (all X and Y Figure 7 shows the current changes on the surface of the s-1 coating sample immersed in 3.5% 2 wascoordinates 0.25 mm .are To in observe the changes in thetosample in situ during immersion, the current signal micrometers). According Equation (1), a stable probe current was obtained in of NaCl solution. The coating’s morphology was relatively flat with featureless points, and the range of the3.5% sample surface was repeatedly tested within a certain time (4–24 h). NaCl solution for a probe potential −0.7immersion V (SCE). for When theByprobe approached the sample, variation of the current was 0.5 × 10−9 Aof after 0.5 h. increasing the immersion time, the Figure 7 shows themorphology current changes the portion surface the s-1 coating sample immersed in 3.5% current the of The the on surface of the sample. Therefore, the current could be used the presented coating’s morphology changed. central ofof the coating gradually changed from dark to characterize the morphology of increased the sample surface NaCl solution. The coating’s morphology was relatively blue to red, and the current also to 1.7 × 10−9[24]. A. flat with featureless points, and the range of −9 A after During the immersion probe and sample fixed at 40 μmthe andimmersion the test areatime, variation of theanalysis, current the wasdistance 0.5 × 10between for 0.5was h. By increasing 2 was 0.25 mm . To observe the changes in the sample in situ during immersion, the current signal the coating’s morphology changed. The central portion of the coating gradually changed fromofdark − 9 the sample surface was repeatedly tested within a certain time (4–24 h). blue to red, and the current also increased to 1.7 × 10 A. Figure 7 shows the currentchanges changeson onthe the surface surface of of the in in 3.5% Figure 8 shows the current the s-1 s-2 coating coatingsample sampleimmersed immersed a 3.5% NaCl solution. The coating’s morphology was relatively flat with featureless points, and the range of NaCl solution. At the start of immersion, the coating’s morphology was relatively flat. By increasing variation of the current was 0.5 × 10−9 A after immersion for 0.5 h. By increasing the immersion time, the immersion time, the coating’s morphology changed. The SECM map of the coating transformed, the coating’s morphology changed. The central portion of the coating gradually changed from dark and the current also increased to 4 × 10−9 A, which −9 was higher than the current of s-1.

blue to red, and the current also increased to 1.7 × 10 A.

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Figure 7. Current changes on the surface of the s-1 coating sample immersed in 3.5% NaCl solution. (a) 0.5 h, (b) 1 h, (c) 2 h and (d) 5 h.

Figure 8 shows the current changes on the surface of the s-2 coating sample immersed in a 3.5% NaCl solution. At the start of immersion, the coating’s morphology was relatively flat. By increasing the immersion time, the coating’s morphology changed. The SECM map of the coating transformed, and the current also increased to 4 × 10−9 A, which was higher than the current of s-1. Upon comparing the surface changes in the s-1 and s-2 coating samples immersed in 3.5% NaCl (d) and the occurrence of solution, we found that(c)the surface roughness of the substrate decreased, microbubble declined. the between theimmersed styrene-acrylic and metal Figure 7. formation Current changes on theThus, surface of bonding in 3.5% coating NaCl solution. Figure 7. Current changes on the surface of the the s-1 s-1coating coatingsample sample immersed in 3.5% NaCl solution. weakened when surface (a) 0.5 h, (b) 1 the h, (c)substrate 2 h and (d) 5 h. roughness decreased. This result indicated that the bonding (a) 0.5 h, (b) 1 h, (c) 2 h and (d) 5 h. between the styrene-acrylic coating and metal was based on physical interactions. Figure 8 shows the current changes on the surface of the s-2 coating sample immersed in a 3.5% NaCl solution. At the start of immersion, the coating’s morphology was relatively flat. By increasing the immersion time, the coating’s morphology changed. The SECM map of the coating transformed, and the current also increased to 4 × 10−9 A, which was higher than the current of s-1. Upon comparing the surface changes in the s-1 and s-2 coating samples immersed in 3.5% NaCl solution, we found that the surface roughness of the substrate decreased, and the occurrence of microbubble formation declined. Thus, the bonding between the styrene-acrylic coating and metal weakened when the substrate surface roughness decreased. This result indicated that the bonding between the styrene-acrylic coating and metal was based on physical interactions.

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Figure 8. Current changes on the surface of the s-2 coating sample immersed in 3.5% NaCl solution.

Figure 8. Current changes on the surface of the s-2 coating sample immersed in 3.5% NaCl solution. (a) 0.5 h, (b) 1 h, (c) 2 h and (d) 4 h. (a) 0.5 h, (b) 1 h, (c) 2 h and (d) 4 h.

Upon comparing the surface changes in the s-1 and s-2 coating samples immersed in 3.5% NaCl solution, we found that the surface roughness of the substrate decreased, and the occurrence of microbubble formation declined. Thus, the bonding between the styrene-acrylic coating and metal weakened when the substrate surface roughness decreased. This result indicated that the bonding (c) coating and metal was based on physical interactions. (d) between the styrene-acrylic Figure 9 shows the current changes on the surface of the t-1 coating sample immersed in 3.5% NaCl solution. The morphology of the coating was relatively flat with featureless points, and the range of variation in the current value was 0.3 × 10−9 A after immersion for 0.5 h. The coating’s morphology

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Materials 2017, 10, 397 Figure 9 shows the current changes on the surface of the t-1 coating sample immersed in 3.5%7 of 15 Materials 2017, 10, 397 7 of the 15 NaCl solution. The morphology of the coating was relatively flat with featureless points, and range of variation in the current value was 0.3 × 10−9 A after immersion for 0.5 h. The coating’s did not change after immersion h. The coating’s morphology was the in same Figure significantly 9 shows the current changes on for the 10 surface of the t-1 coating sample immersed 3.5%as the morphology did not change significantly after immersion−for 10 h. The coating’s morphology was the 9 A. flat with featureless points, and the NaCl solution. The morphology of the coating was×relatively initial morphology, and the current remained at 0.3 10 same as the initial morphology, and the current remained at 0.3 × 10−9 A. range of variation in the current value was 0.3 × 10−9 A after immersion for 0.5 h. The coating’s morphology did not change significantly after immersion for 10 h. The coating’s morphology was the same as the initial morphology, and the current remained at 0.3 × 10−9 A.

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Figure 9. Current changes on the surface of t-1 coating sample immersed in 3.5% NaCl solution.

Figure 9. Current changes (c) on the surface of t-1 coating sample immersed (d) in 3.5% NaCl solution. (a) 0.5 h, (b) 2 h, (c) 5 h and (d) 10 h. (a) 0.5 h, (b) 2 h, (c) 5 h and (d) 10 h. Figure 9. Current changes on the surface of t-1 coating sample immersed in 3.5% NaCl solution.

Figure 10(b) shows changes on the surface of the t-2 coating sample immersed in 3.5% (a) 0.5 h, 2 h, (c)the 5 hcurrent and (d) 10 h. NaCl solution. Prior immersion, the coating’s morphology relatively flat with featureless Figure 10 shows thetocurrent changes on the surface of the was t-2 coating sample immersed in 3.5% Figure 10 shows the0.5 current changes the surface the t-2The coating sample immersed in 3.5% points. The Prior current × 10−9the A coating’s afteron immersion forof0.5 h. coating’s did not NaCl solution. to was immersion, morphology was relatively flatmorphology with featureless points. NaCl solution. Priorafter to−9immersion, the coating’s morphology was relatively flatsame withas featureless change significantly immersion for 8 h. The coating’s morphology was the the initial The current was 0.5 × 10 A after immersion for 0.5 h. The coating’s morphology did not change points. The current 0.5 ×remained 10−9 A after immersion The coating’s morphology did not morphology, thewas current at 0.5 × 10−9 A. for 0.5 h.was significantly afterand immersion for 8 h. The coating’s morphology the same as the initial morphology, change significantly after immersion for 8 h. The coating’s morphology was the same as the initial −9 A. and the current remained at 0.5 × 10 morphology, and the current remained at 0.5 × 10−9 A.

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Figure 10. Current changes on the surface of the t-2 coating sample immersed in 3.5% NaCl solution.

Figure 10. Current changes on the surface of the t-2 coating sample immersed (d) in 3.5% NaCl solution. (a) 0.5 h, (b) 2 h, (c) 4 (c) h and (d) 8 h. (a) 0.5 h, (b) 2 h, (c) 4 h and (d) 8 h. Figure 10. Current changes on the surface of the t-2 coating sample immersed in 3.5% NaCl solution.

Upon comparing the surface change of the t-1 and t-2 coating samples immersed in 3.5% NaCl (a) 0.5 h, (b) 2 h, (c) 4 h and (d) 8 h. solution, it can be found that chemical bonding major role between theimmersed terpolymerincoating Upon comparing the surface change of the played t-1 anda t-2 coating samples 3.5% NaCl and metal substrate, and it was less affected by changes in surface roughness of the substrate. With solution, it can be found that chemical bonding played a major role between the terpolymer coating Upon comparing the surface change of the t-1 and t-2 coating samples immersed in 3.5% NaCl respect to physical bonding, the samples were resistant to water and other media damage to the and metal substrate, and it was less affected by changes in surface roughness of the substrate. solution, it can be found that chemical bonding played a major role between the terpolymer coating With coating adhesion. andto metal substrate, and it was affected by changes in to surface roughness of the substrate. Withto the respect physical bonding, the less samples were resistant water and other media damage Figure 11 illustrates the current changes on resistant the surface of the and s-3 coating sample immersed in respect to physical bonding, the samples were to water other media damage to the coating adhesion. 3.5% NaCl solution. After immersion for 13 h, the coating’s morphology was relatively flat with coating 11 adhesion. Figure illustrates the current changes on−9the surface of after the s-3 coating sample immersed in 3.5% featureless and thethe current waschanges 0.8 × 10on A. However, 24 h, the coating’s Figurepoints, 11 illustrates current the surface of theimmersion s-3 coatingfor sample immersed in NaClmorphology solution. After immersion for 13 h, the coating’s morphology was relatively flat with featureless significantly andfor the13current the upper and lowerwas portions of theflat coating 3.5% NaCl solution. Afterchanged, immersion h, the of coating’s morphology relatively with points, and the current was 0.8 × 10−9 that A. However, afteroccurred immersion for 24 h, the coating’s morphology were altered. This finding indicated microbubbles in the coating. −9 featureless points, and the current was 0.8 × 10 A. However, after immersion for 24 h, the coating’s

significantly changed, and the current of the upper and lower portions of the coating were altered. morphology significantly changed, and the current of the upper and lower portions of the coating This finding indicated that microbubbles in the coating. were altered. This finding indicated thatoccurred microbubbles occurred in the coating.

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Figure 11. Current changes on the surface of the s-3 coating sample immersed in 3.5% NaCl solution. (d) (a) 0.5 h, (b) 1 h, (c) 13 h(c) and (d) 24 h. Figure 11. Current changes on the surface of the s-3 coating sample immersed in 3.5% NaCl solution.

Figure 11. Current changes on the surface of the s-3 coating sample immersed in 3.5% NaCl solution. (a) 0.5 h, (b) 1 h, (c) 13 h and (d) 24 h. (a) 0.5 h, (b) 1 h, (c) 13 h and (d) 24 h.

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Figure 12 shows the current changes on the surface of the t-3 coating sample immersed in 3.5% Figure 12 shows the current changes on the surface of the t-3 coating sample immersed in 3.5% NaCl solution. After immersion for 24 h, the coating’s morphology was relatively flat with featureless NaCl solution. After immersion for 24 h, the−coating’s morphology was relatively flat with featureless 9 points, and and the the current remained atat0.5 points, current remained 0.5×× 10 10−9 A.A.

(a)

(b)

(c)

(d)

Figure 12. Current changes on the surface of the t-3 coating sample immersed in 3.5% NaCl solution.

Figure 12. Current changes on the surface of the t-3 coating sample immersed in 3.5% NaCl solution. (a) 0.5 h, (b) 1 h, (c) 13 h and (d) 24 h. (a) 0.5 h, (b) 1 h, (c) 13 h and (d) 24 h.

2.4. Failure Mechanism of the Styrene-Acrylic and Terpolymer Coatings

2.4. Failure Mechanism of the Styrene-Acrylic and Terpolymer Coatings

Mechanical fitting is the most effective method for improving coating/metal adhesion. Before coating on thefitting metalissubstrate, roughening (such as sandblasting) adhesion. on the metal Mechanical the mostsurface effective methodtreatment for improving coating/metal Before substrate’s can increasesurface the porosity of the surface so that (such paint can penetrate into the coating on the surface metal substrate, roughening treatment as sandblasting) onpores. the metal The coating cancan relyincrease on co-anchor, hooks, of staples, and other forms of roots fixed on the substrate’s surface the porosity the surface so that paint can penetrate intometal the pores. substrate surface and firmly attached after curing, thereby enhancing the combined coating/metals. The coating can rely on co-anchor, hooks, staples, and other forms of roots fixed on the metal substrate Such treatment result in physical bonding and improves the adhesion of the coating. The results of surface and firmly attached after curing, thereby enhancing the combined coating/metals. Such SECM analysis of styrene-acrylic showed that the coating on the 240# sandpaper-treated substrate treatment result in physical bonding and improves the adhesion of the coating. The results of SECM exhibited microbubbles later than the coating on 2000# sandpaper-treated substrate. analysis of styrene-acrylic showed the coating on theThese 240# sandpaper-treated substrate Defects in the service life of that a coating are inevitable. defects will provide the channelexhibited for microbubbles later than coating onexpose 2000#the sandpaper-treated corrosive media, suchthe as water, and metal substrate tosubstrate. corrosive media. When the metal Defects in the service life of coating arecathode inevitable. These will the channel substrate begins to corrode, thea anode and area will be defects generated in provide these exposed areas for randomly. Metal will at theexpose anode zone and the pH will reduce. Oxygen will beWhen reduced corrosive media, such asdissolve water, and the metal substrate to corrosive media. theatmetal the cathode zone and the pH will rise, which will promote coating delamination. The anode and areas substrate begins to corrode, the anode and cathode area will be generated in these exposed cathode will separate because of aggravating corrosion, and delamination is extended [40]. Therefore randomly. Metal will dissolve at the anode zone and the pH will reduce. Oxygen will be reduced at the afterzone damage at the interface between the coating and the surface cathode and occurs, the pHthe willactivities rise, which will promote coating delamination. Thesteel anode and are cathode important for the mechanism of cathodic delamination. The interactions take place on top of a thin will separate because of aggravating corrosion, and delamination is extended [40]. Therefore after layer of ferrous oxide because steel surfaces prepared by abrasive blasting are oxidized damage occurs, the activities at the interface between the coating and the steel surface are important for instantaneously upon contact with the atmosphere [41]. Good barrier effects require robust adhesion the mechanism cathodic interactions takebeplace on topattack of a thin layeratofthe ferrous between the of coating and delamination. metal substrate. The If good adhesion can established, by water

oxide because steel surfaces prepared by abrasive blasting are oxidized instantaneously upon contact with the atmosphere [41]. Good barrier effects require robust adhesion between the coating and metal substrate. If good adhesion can be established, attack by water at the interface can be prevented [10,33].

necessary between the coating and metal. The results of infrared (Figure 4) and SIMS (Figure 5) analysis reveal COOFe bonding in the interface between the terpolymer coating and metal surface. The bonding energy of COOFe is stronger than hydrogen bonding of water and metal [39], so it can terminate the layers of water molecules formed on the surface of metal and improve the wetting Materials 2017, 10, 397 10 of 15 adhesion of the coating. Interface combination is mainly for physical adsorption (Figure 13a). Improving the metal surface roughness and increasing the contact area are effective ways to improve adhesion, but such As a consequence, tight bonding of a covalent or ionic character is necessary between the coating and methods for improving wet coating adhesion are limited. Although interface combination could metal. The results of infrared (Figure 4) and SIMS (Figure 5) analysis reveal COOFe bonding in the directly increase the surface contact area between the coating and metal substrate, it failed to improve interface between the terpolymer coating and metal surface. The bonding energy of COOFe is stronger the water medium diffusion resistance in the coating/metal interface. The presented method only than hydrogen bonding of water and metal [39], so it can terminate the layers of water molecules extended the lateral diffusion channel distance of the water medium and prolonged the time for formed on the surface of metal and improve the wetting adhesion of the coating. microbubble formation, but the effect was limited (Figures 7 and 8). Given that the mechanism of Interface combination is mainly for physical adsorption (Figure 13a). Improving the metal surface interface combination is chemical bonding (Figure 13b), metal surface roughness is not the main roughness and increasing the contact area are effective ways to improve adhesion, but such methods for factor influencing the stability of interface bonding. COOFe bonding can effectively prevent the improving wet coating adhesion are limited. Although interface combination could directly increase lateral diffusion of the water medium in the metal/coating interface, so it can prolong the time of the surface contact area between the coating and metal substrate, it failed to improve the water medium microbubble formation [42–45]. Under different roughness values, the terpolymer coating showed diffusion resistance in the coating/metal interface. The presented method only extended the lateral good wet adhesion (Figures 9 and 10). Compared with the styrene-acrylic coating, the terpolymer diffusion channel distance of the water medium and prolonged the time for microbubble formation, coating exhibited better wet adhesion in different surfaces. The coating/metal interface of the but the effect was limited (Figures 7 and 8). Given that the mechanism of interface combination terpolymer coating was mainly combined with chemical bonds and minimally affected by the is chemical bonding (Figure 13b), metal surface roughness is not the main factor influencing the roughness of the metal surface because the bonding energy of COOFe is stronger than the hydrogen stability of interface bonding. COOFe bonding can effectively prevent the lateral diffusion of the water bond between water and metal. The metal substrate’s surface roughness and interface bonding ways medium in the metal/coating interface, so it can prolong the time of microbubble formation [42–45]. are critical factors that affect the stability of the combination of the coating/metal interface, but the Under different roughness values, the terpolymer coating showed good wet adhesion (Figures 9 influence of the substrate surface roughness on wet adhesion is limited, and bonding is crucial for and 10). Compared with the styrene-acrylic coating, the terpolymer coating exhibited better wet wet adhesion. adhesion in different surfaces. The coating/metal interface of the terpolymer coating was mainly Upon comparing the surface changes in the s-1 and s-3 coating samples immersed in 3.5% NaCl combined with chemical bonds and minimally affected by the roughness of the metal surface because solution (Figures 7 and 11), water permeation was delayed with increasing coating thickness. Thus, the bonding energy of COOFe is stronger than the hydrogen bond between water and metal. The metal the change in the styrene-acrylic coating occurred later. However, when corrosive media passed substrate’s surface roughness and interface bonding ways are critical factors that affect the stability of through the coating and entered the coating/metal interface, the coating/metal interface failed to resist the combination of the coating/metal interface, but the influence of the substrate surface roughness on microbubble formation. wet adhesion is limited, and bonding is crucial for wet adhesion.

Figure 13. Schematic Schematicof of failure mechanism of the styrene-acrylic (a) and (b) terpolymer (b) Figure 13. thethe failure mechanism of the styrene-acrylic (a) and terpolymer coating/metal coating/metal interface. interface.

AFM observations revealed that the microstructure of the styrene-acrylic coating was denser Upon comparing the surface changes in the s-1 and s-3 coating samples immersed in 3.5% NaCl than that of the terpolymer coating (Figure 1). Simultaneously, the contact angle test showed that the solution (Figures 7 and 11), water permeation was delayed with increasing coating thickness. Thus, terpolymer coating exhibited certain hydrophobicity, which was disadvantageous for the penetration the change in the styrene-acrylic coating occurred later. However, when corrosive media passed resistance of the coating. These findings indirectly illustrate that the penetration resistance of the through the coating and entered the coating/metal interface, the coating/metal interface failed to resist microbubble formation. AFM observations revealed that the microstructure of the styrene-acrylic coating was denser than that of the terpolymer coating (Figure 1). Simultaneously, the contact angle test showed that the terpolymer coating exhibited certain hydrophobicity, which was disadvantageous for the penetration resistance of the coating. These findings indirectly illustrate that the penetration resistance of the

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styrene-acrylic coating was stronger than that of the terpolymer coating. When the surface changes in s-3 and t-3 were compared (Figures 11 and 12), we noted that the increased thickness of the coating extended the longitudinal diffusion time of the water medium and delayed the time for microbubble formation. However, the interface stability of the terpolymer coating was still superior to that of the styrene-acrylic coating. Therefore, the permeability resistance of the coating did not affect the stability of the coating/metal interface. The bonding of the coating/metal interface is a key factor affecting the stability of the coating/metal interface. Effective chemical bonds in the coating/metal interface are essential to resist coating damage and improve the coating’s service life. 3. Materials and Methods 3.1. Materials In this paper, we used two water-based coatings, namely, styrene-acrylic coating and terpolymer coating consisting of acrylic acid (CH2 =CH–COOH), vinyl chloride (CHCl=CH2 ), and 1,1-dichloroethylene (CH2 =CCl2 ). 3.2. Sample Preparation The samples for SECM were explained in Table 1. Q235 carbon steel, which was used as a metallic substrate, was polished by 240# sandpaper to remove the surface oxide layer. Some samples were polished successively by 240#, 400#, and 2000# to achieve different surface roughness. After polishing, the metal surface was carefully washed in ethanol and acetone and then dried prior to the coating process. The water-based acrylic acid coating was painted on the metallic surface with a brush, and the metal was cured at room temperature for 15 days. The thicknesses of the dry film were controlled at 10 and 20 µm. The measuring device (QNix4500, Automation Dr. Nix GmbH & Co.KG, Cologne, Germany, 0–50 µm ≤ ±1 µm) was used to control the thicknesses of the dry film, and both of the coatings have good liquidity and leveling. Table 1. Designation, film thickness, and surface roughness of the tested coating systems. Number

Coating Species

Coating Thickness (µm)

Surface Roughness

s-1 s-2 s-3 t-1 t-2 t-3

styrene-acrylic styrene-acrylic styrene-acrylic terpolymer terpolymer terpolymer

10 10 20 10 10 20

240# (Ra0.673) 2000# (Ra0.088) 240# (Ra0.673) 240# (Ra0.673) 2000# (Ra0.088) 240# (Ra0.673)

3.3. Characterization of the Coating Properties The coating properties were characterized by observing the morphology and measuring the contact angle, and analyzing the chemical changes via FTIR (PerkinElmer, Waltham, MA, USA). Moreover, SIMS (ION-TOF, Munster, Germany) was conducted to analyze the chemical composition of the coating/metal interface. SECM (Bio-Logic, Seyssinet-Pariset, France) was performed to measure the surface current. The current distribution on the surface of the coating was measured with different times and the change in the current distribution was obtained to characterize the development of micro-bubbles. 3.3.1. Morphology The section and surface morphologies of the coatings were observed via scanning electron microscopy (SEM, QUANTA 250, FEI, Hillsboro, OR, USA) and atomic force microscopy (AFM MultiModeTM Nanoscope V, Bruker, Madison, WI, USA).

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3.3.2. Contact Angle The contact angles of the coatings were measured using a DataPhysics contact angle measuring system (OCA 20, DataPhysics, Stuttgart, Germany). The droplet volume was 5 µL. 3.3.3. FTIR Analysis Chemical changes in coatings were monitored by Attenuated Total Reflectance (ATR) FTIR analysis using a PerkinElmer Frontier spectrometer in the range of 4000–650 cm−1 with 16 scans and a resolution of 4 cm−1 . Changes between the resin and coating were tested to study the bond between the coating and metal. The samples were made by the spreader (OSP-04, OSP, Aichi Prefecture, Japan). The thicknesses of the dry film were less than 2 µm. 3.3.4. SIMS An ION-TOF GmbH TOF-SIMS 5 TOF ion mass spectrometry system (ION-TOF) was used to analyze the chemical composition of the coating/metal interface. 3.3.5. Adhesion Testing The sample size was 50 mm × 150 mm and the thickness of the coating was 85 ± 3 µm. A 20 mm width defect in the center of the sample was produced by an art knife, and the metallic substrates were exposed to air. In this study, the scratch depths of all metallic substrate were the same. The samples were immersed in 3.5 wt% NaCl solution (pH 7) at 30 ◦ C with different immersion times. Subsequently, the samples were taken out and placed in 50% humidity and 25 ◦ C for 2 h. A pillar was then bonded to the sample surface, and the distance between the center of the pillar and the defect was 25 mm. Prior to the test, the sample was placed in 50% humidity and 25 ◦ C for 24 h to ensure a tight bond between the pillar and the sample. The wet adhesion test was performed by the PosiTest AT Pull-Off Adhesion Tester (DeFelsko, New York, NY, USA). The diameter of the pillar was 20 mm. 3.3.6. SECM Measurements SECM measurements were carried out over a large surface area (e.g., 0.25 mm2 ) as a function of immersion time. SECM scans were acquired by rastering over the sample surface in steps of 50 µm in the Z-direction, 500 µm in the Y-direction, and 500 µm in the X-direction. All of the experiments were conducted at room temperature (25 ◦ C) in a naturally-aerated cell consisting of 3.5% NaCl solution. The oxygen dissolved in the electrolytic phase was employed as redox mediator for SECM imaging: O2 + 2H2 O + 4e– ↔ 4OH–

(1)

4. Conclusions Characteristics of the physical structure of coatings were studied by SEM and AFM. FTIR and SIMS were conducted to characterize the bonding between the terpolymer coating and metal substrate. SECM was used as a tool for the accelerated investigation of coating samples by measuring topographic changes in the exposed surface as a function of elapsed time. The effects of thickness, surface roughness, and chemical bonding on the rate of cathodic delamination were investigated to gain further insight into the detailed mechanism of cathodic delamination and help optimize the coating formulation against cathodic delamination. Increasing the surface roughness and thickness could help adhesion, but the effect was limited. COOFe bonding is an efficient technique to improve wetting adhesion and prevent attack by water at the interface. It can also promote anticorrosion. 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 No. 51371036).

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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; and 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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