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A detailed molecular dynamics simulation and experimental investigation on the interfacial bonding mechanism of an epoxy adhesive on carbon steel sheets decorated with a novel cerium-lanthanum nanofilm Ghasem Bahlakeh, and Bahram Ramezanzadeh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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ACS Applied Materials & Interfaces

A detailed molecular dynamics simulation and experimental investigation on the interfacial bonding mechanism of an epoxy adhesive on carbon steel sheets decorated with a novel cerium-lanthanum nanofilm Ghasem Bahlakeh a*1, Bahram Ramezanzadehb a

Department of Engineering and Technology, Golestan University, Aliabad Katool, Iran

b

Department of Surface Coatings and Corrosion, Institute for Color Science and Technology, P.O. Box 16765-

654, Tehran, Iran

Abstract The influences of steel surface treatment by a novel cerium-lanthanum (Ce-La) nanofilm on the adhesion mechanism of an epoxy adhesive were studied through experimental and modeling approaches. The surface morphology and microstructure of the film deposited were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The surface free energy and work of adhesion values were evaluated through contact angle analysis. Also, the interfacial adhesion strength between the epoxy adhesive and steel surface together with failure forms were examined through pull-off test, in dry and wet conditions, and Fourier transform infrared (FT-IR) spectroscopy. The results obtained from experiments revealed that depositing a Ce-La nanofilm on the steel surface increased its roughness and surface free energy, and strengthened the epoxy coating adhesion. It was also observed that the epoxy adhesion on the Ce-La treated steel was stronger as compared with Cetreated surface.

To whom correspondence should be addressed: Ghasem Bahlakeh: Tel.: +981734266235; [email protected], [email protected] 1

*Dr.

Fax,

ACS Paragon Plus Environment

+981734266235;

e-mail,


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Furthermore, the adhesion extent and surface bonding mechanism of aminoamide-cross-linked epoxy resin were computationally modeled applying atomistic molecular dynamics (MD) and electronic density functional theory (DFT) methods. The modeling results evidenced that epoxy resin adhered more strongly to conversion layer (represented by CeO2 and La2O3) compared with untreated steel surface (i.e., pure and oxidized iron). The epoxy binding onto CeO2 (111), La2O3 (001), Fe2O3 (110), and Fe3O4 (100) almost occurred via electrostatic interactions, while its adhesion mechanism over FeO (100) and Fe (110) surfaces was based on van der Waals forces. The computations also demonstrated that the epoxy adsorption energy lowered in wet environments caused by solvent affinity towards epoxy and surface, but rate of reduction was smaller over CeO2 and La2O3 as compared with iron oxides. These modeling outcomes in line with our experiments proposed the superior epoxy adhesion on modified steel sheets. Keywords: Surface treatment, Interfacial adhesion, Cerium-lanthanum, Epoxy coating, Molecular dynamics (MD), Density functional theory (DFT)

1. Introduction Epoxy based organic adhesives have been used in large amounts as an effective corrosion protective primer of steel substrates1-3. The high cross-linking density and good adhesion to the metals have made it as a popular protective primer4-6. It has been largely reported that the durability of epoxy bonding to steel determines the coating service life and protection degree. Epoxy can be categorized as a polar coating with many oxygen containing groups i.e., epoxide and hydroxyl 7. So the adhesion forces between the epoxy and steel surface are mostly in the form of secondary bonds including hydrogen and van der Waals bondings, which are not strong adhesion bonds compared with primary covalent bonding 8-9.


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The secondary interfacial bonds are not stable and can be easily deteriorated in the presence of water molecules. The epoxy/steel adhesion strongly depends on the physics and chemistry of the interface 10-12. Roughening the interface is a promising approach to enhance the adhesion bonds through mechanical interlocking. Another approach is to enhance the wettability of substrate by increasing the surface free energy. Strong adhesion bonds can be created between the polymer and steel surface through engineering the interface by deposition of thin inorganic films

13-16

. The weakly interacted naturally produced iron oxides can be removed and a new

oxide film with stronger interactions with steel surface and higher surface free energy can be deposited. In this regard different types of conversion films have been used and studied. Chromates and phosphates are the most popular types of these coatings that their applications have been restricted in recent years due to the environmental regulations and their toxic nature17-18. Recently, the environmentally friendly alternatives i.e., sol-gel organic-inorganic coatings, zirconium and conversion coatings based on rare earth elements have attracted large amount of the researchers' attentions19-25. Amongst different kinds of conversion coatings the cerium based coatings are utilized in a great number of applications. It has been reported that depositing cerium conversion film on different metals provides good corrosion resistance and adhesion to the polymer coatings

23, 26-27

. However, there has been no work addressed the

mechanism of adhesion between the epoxy and steel treated by this kind of coating. Conventional techniques such as contact angle and pull-off can be applied to assess the interfacial adhesion characteristics (e.g., work and strength of adhesion), but these methods could not give us molecular-level views concerning the adhesion mechanism. Modeling approaches based on first-principle electronic-structure quantum mechanics (QM) and all-atom molecular dynamics (MD) have been found as highly valuable tools to unravel the interaction mechanism behind the polymeric coating adhesion to metal substrate 28. These approaches operating at electronic/atomic scales deliver a qualitative/quantitative view



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regarding the interfacial adhesion and surface bonding features of coating films. It has been proven that using QM techniques based on density functional theory (DFT) it is possible to derive fundamental details about the coating electronic features governing its adhesion manner with regards to metal/metal oxide substrate. On the other hand, classical atomistic MD simulations involving force fields (FFs) could be applied to model the coating/metal interface zone at more realistic length scales, and thereby reliably mimic the situations relevant for experiments 28-29. In literature, these modeling methods have been recently utilized to get a microscopic understanding of coating molecule interactions with inorganic surfaces

30

. Semoto et al.

31

studied the epoxy-aluminum oxide surface interactions and concluded that the hydroxyl functionality in epoxy resin backbone is the main adhesion sites. Ogata and Takahashi probed the interfacial forces of epoxy and aluminum oxide under different degrees of hydration 32, and found that the adhesion level considerably reduced by increasing moisture content. They have also reported that the interaction between epoxide oxygen and surface Al atoms was the main force for epoxy adhesion. In another study, Lee et al. 33 examined the adhesion mechanism of epoxy onto pure iron surface (Fe (100)) by using periodic DFT calculations and observed that epoxy resin adhered to iron substrate with a flat-lying orientation. In a recent study, we have extensively investigated the interactions of epoxy coating with steel substrate applying DFT calculations and MD simulations 28. That modeling study revealed the effectiveness of epoxy materials to form coating films over carbon steel surfaces. In the present study, it is intended to assess the effects of Ce-La film upon the interfacial adhesion characteristics of the epoxy coating onto surface-modified steel substrate using combined experimental and modeling methods. To this purpose, the steel surface was treated by a composite Ce-La nanofilm and characterized by SEM/EDS, XPS AFM, FT-IR and contact angle analyses. The epoxy adhesion was then analyzed through pull-off test. In the second part,


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the modeling approaches involving atomistic MD simulations and electronic DFT computations were applied to explore the surface boding and adhesion of epoxy onto untreated and surface-treated carbon steel at microscopic levels. The computational MD and DFT studies were executed in both dry and wet environments. MD simulations were performed to investigate the adsorption behavior and interfacial interactions responsible for aminoamidecrosslinked epoxy resin adhesion to untreated/treated steel sheets. These atomistic simulations were specifically performed to scrutinize the epoxy adhesion in humid situations. Additionally, DFT-based studies were conducted to explore the electronic features (e.g., frontier molecular orbitals and point charges, Fukui indices) affecting the extent of adhesion. 2. Experimental procedures 2.1. Raw materials The steel sheets with chemical composition of 0.04 wt.% Al, 0.05 wt.% P, 0.05 wt.% S, 0.19 wt.% C, 0.32 wt.% Mn, 0.34 wt.% Si and 99.01 wt.% Fe were prepared from Foolad Mobarakeh Co. (Iran). Hydrogen peroxide (H2O2), lanthanum nitrate (La(NO3)3.6H2O), cerium nitrate (Ce(NO3).5H2O), HCl (37%) and NaOH were purchased from Merck Co. (Germany). Epoxy resin, Araldite GZ 7071X 75, and polyamide hardener, CRAYAMID 115, were obtained from Saman (Iran) and Arkema Co., respectively. 2.2. Samples preparations The steel sheets were modified by Ce (1 g/l Ce(NO3).5H2O, 0.6 g/l H2O2 and 6 ml/l HCl 37%), and Ce-La (1 g/l Ce(NO3).5H2O, 0.5 g/l La(NO3)3.6H2O, 0.6 g/l H2O2 and 6 ml/l HCl 37%) solutions. The temperature, pH and immersion time during surface treatment procedure were 45 °C, 2.5 and 5 min, respectively. Prior surface modification process the steel sheets were abraded by emery papers of 600, 800 and 1200 grades, followed by acetone degreasing. Then, the epoxy/polyamide (70:30 w/w) coating was applied on the untreated and surface treated



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samples. Finally, they were cured in an oven at 100 °C for 3 h. The dry film thickness was 70±5 µm. 2.3. Techniques The surface morphology and microstructure of the samples treated by Ce and Ce-La were analyzed by SEM/EDS model Phenom ProX and AFM model Dualscope DS 95-200, DME, Denmark. The surface free energy and work of adhesion were calculated through measuring the static contact angle by an OCA 15 plus type system. The adhesion strength of epoxy coated samples was analyzed by Posi test-pull off adhesion tester, DEFELSKO. X-ray photoelectron spectroscopy (XPS) model Specs EA 10 Plus with radiation source of Al Kα at pressure of 109 mbar was employed to characterize the chemical structure of the Ce and Ce-La coatings. The interactions between epoxy/steel and epoxy/Ce-La-steel were evaluated by a FT-IR diffuse reflectance spectroscopy model Brucker IFS66 with an MCT detector, using a resolution of 4 cm-1 and an acquisition of 100 scans. 3. Modeling details 3.1. Structural optimization of crosslinked epoxy resin To model the corrosion-protective film formation capacity of organic coating based on poly (diglycidyl ether of bisphenol A, DGEBA) epoxy resin over untreated/treated carbon steel substrates, a crosslinked epoxy resin molecule was built to be applied as an adsorbate. Crosslinking of epoxy resin was done by aminoamide curing agent as used in experiments. Figure 1 displays the chemical structure for aminoamide-crosslinked DGEBA epoxy resin. Figure 1 The potential energy of this crosslinked DGEBA epoxy resin was minimized by means of electronic-structure calculations, i.e., quantum mechanics (QM) approaches. Such an electronic-scale QM study was particularly conducted on constructed crosslinked epoxy resin to elucidate fundamental electronic/atomic views regarding the adhesion features of coating


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molecule above steel layers. For this purpose, the aminoamide-crosslinked DGEBA epoxy resin was subjected to sequential geometry optimizations to obtain a global minimum energy epoxy resin. First, the energy of crosslinked epoxy resin was minimized based on Hartree-Fock (HF) theory with basis sets of 6-31G(d,p) 34. Afterwards, the energy minimization process was continued using density functional theory (DFT) techniques

35

. The DFT studies were

performed with B3LYP hybrid exchange-correlation functional initially with 6-31G(d,p) basis set and then by larger 6-311G(d,p) basis set 36. These first-principle theoretical investigations were done both in vacuum (i.e., gas phase or dry conditions) and in solution (i.e., liquid phase or humid conditions) without applying any constraints. Within QM calculations carried out in liquid phase, the theory of self-consistent reaction field (SCRF) based on Tomasi’s polarized continuum model (PCM) 37 was employed to describe the effects linked to solvent (water) using the integral equation formalism

38

. In this theory, the solvent molecules are modeled as a

continuum of uniform dielectric constant, while the solute molecule is positioned inside a cavity defined within the solvent phase. Finally, the energy-minimized crosslinked epoxy resin attained from DFT modeling at B3LYP/6-311G(d,p) level was used for subsequent examinations of electronic properties including frontier molecular orbitals, fukui indices (FI), and partial atomic charges. The point charges distributed on atoms were determined by using ChelpG method 39. The global minimum energy geometry of crosslinked epoxy resin was subsequently employed to assess the epoxy-solvent interactions with the use of QM computations. For this purpose, single water molecule was firstly laid in vicinity of each functionality of epoxy resin (i.e., ether, hydroxyl, epoxide, amine and amide) illustrated in Figure 1. The electronic energy of epoxywater models was then minimized stepwise through the aforementioned QM approaches. The energy-minimized models elucidated from DFT computation at B3LYP/6-311G(d,p) theory level was utilized to determine the magnitude of solvent interaction with different epoxy



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functional moieties. Furthermore, in order to analyze the effects associated with basis set superposition error (BSSE) on the solvent interactions with epoxy resin fragments, counterpoise (CP) scheme developed by Boys and Bernardi was adopted 40. These ab initio QM studies were done by making use of Gaussian 09 suite of programs 41. 3.2. Building metallic slab models After constructing and optimizing the epoxy resin adsorbate, the untreated/treated carbon steel substrates were prepared for use in classical MD simulations. The unmodified steel substrate was taken into considered by means of pure iron (Fe) and its oxidized forms (i.e., ferrous oxide (FeO), ferric oxide (Fe2O3, hematite), and ferrous ferric oxide (Fe3O4, magnetite)). In order to simulate the adsorption behavior of aminoamide-crosslinked epoxy resin with these iron/iron oxides, the crystallographic planes Fe (110), FeO (100), Fe2O3 (110) and Fe3O4 (100) were adopted for MD simulations in line with previous theoretical studies

28, 42

. These iron oxide

surfaces with the least lattice mismatch 43 were also studied in our recent effort 28. In the case of treated carbon steel samples, the cerium dioxide (CeO2, ceria) and lanthanum oxide (La2O3, lanthana) metals were selected for simulating the adsorption of crosslinked epoxy coating molecule onto Ce-/La-modified steel sheets. Herein, for MD simulations of interactions between epoxy resin and conversion layer the crystalline facets (111) in CeO2 and (001) in La2O3 were considered. It has been observed that the CeO2 (111) facet is the most thermodynamically stable crystallographic plane, and for this reason it has been extensively studied for probing the interactions of ceria crystals with various adsorbing molecules

44

. In

addition, according to literature, the (001) face in La2O3 is the most energetically favorable one 45

, and thus the dissociative adsorption of small molecules (e.g., O2 and CH4) has been

evaluated over this lanthana surface applying QM methods 46. To prepare slab models for these iron/iron oxides and cerium/lanthanum oxides representing the unmodified and surface-modified steel sheets, respectively, the corresponding Fe, FeO,


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Fe2O3, Fe3O4, CeO2, and La2O3 unit cells were cleaved along desired planes by Materials Studio software (version 6) 47. Upon cleaving, the thickness for all crystalline surfaces was set to be approximately 1.5 nm, which is higher than the cutoff distance used for non-boned interactions. Subsequently, to generate surfaces with sufficient area and to realistically simulate the coating/substrate interface, all surfaces were periodically replicated in their directions (i.e., x and y axes). A vacuum space with thickness 3 nm was then placed above all surfaces. The size of prepared slabs designed for MD simulations was approximately 3.5×3.5×4.4 (nm)3 for Fe (110), Fe3O4 (100) and La2O3 (001), and 3.3×3.3×4.3 (nm)3 for FeO (100), 3.7×3.2×4.4 (nm)3 for Fe2O3 (110), and 3.4×3.4×4.3 (nm)3 for CeO2 (111) substrates. 3.3. Molecular dynamics simulations Similar to QM computations, MD simulation studies of crosslinked epoxy resin binding behavior over carbon steel adsorbents were executed in both dry and humid environments. For in vacuum simulations, the DFT-optimized geometry of crosslinked DGEBA epoxy was used as starting coating molecule and put above all metallic slabs such that its initial distance from the outermost atoms in all slabs was relatively large. It is better to notice that no water molecules were introduced into the slabs, as the simulations are done in dry environments. A rather parallel orientation was selected for initial alignment of inserted coating adsorbate relative to all studied metallic adsorbents so as to obtain equilibrated adsorbate independent of its starting orientation. To simulate the crosslinked epoxy resin adsorption in wet environments, 600 water (H2O) molecules were inserted into the final coating/substrate snapshots achieved from MD simulations in dry situations. This was carried out by Layer builder module implemented in Materials Studio software 47. The added water molecules were considered as two solvent layers. A lower solvent layer in which water molecules are close to metal/metal oxide slabs dissolved the crosslinked epoxy resin. The number of water molecules in this layer is nearly 300, and all



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of them were allowed to move freely. The second solvent layer, denoted as upper layer, consists of only solvent molecules. In this pure solvent layer, position of all H2O molecules was fixed to serve as a rigid wall to the lower layer. Before MD simulations, all prepared slab models for aminoamide-crosslinked bisphenol A epoxy resin over conversion layer (CeO2 (111) and La2O3 (001)) and untreated steel layer (represented by Fe (110), FeO (100), Fe2O3 (110), and Fe3O4 (100)) were minimized for at least 500 steps with Smart minimizer, as available in Materials Studio software

47

. The resulting

energy-minimized systems were simulated in NVT ensemble for 250 ps at 298 K. All potential energy parameters required for inter-/intra-molecular interactions between crosslinked epoxy resin and metal/metal oxides were taken from COMPASS force field

48

, except the partial

charges of epoxy which were determined using ChelpG method within QM computations. Atom-based cutoff and Ewald methods were utilized to model the van der Waals and columbic interactions, respectively. The integration algorithm based on velocity Verlet was used for solving the Newton’s equation of motion with a time step of 1 fs (10-15 s)

49

. The Andersen

thermostat was used to monitor the operational temperature at 298 K. The position of all substrate atoms (Ce, La, Fe, and O) was kept frozen within gas-phase and liquid-phase MD simulations. 4. Results and discussion 4.1. Surface studies It is known that the surface morphology and topography influence the epoxy bonding to the steel surface. So, the surface morphology of the steel sheets treated by Ce and Ce-La nanofilms was studied by SEM/EDS, XPS and AFM analyses. As can be seen in Figure 2 the surface of steel was uniformly covered with combination of cerium and lanthanum oxides. The film deposited on the Ce treated sample includes some large agglomerated particles but in the case of Ce-La treated sample no agglomeration can be seen. Results show that addition of La to the


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Ce solution decreased the particle size (less than 100 nm) and resulted in a crack free and dense Ce-La film deposition on the steel surface. The EDS results (Table 1) confirmed the presence of Fe, O, Ce, Na and C on the Ce treated sample and La on the Ce-La treated sample. The lower Fe and higher Ce together with La detected on the Ce-La treated sample than the one treated by Ce clarifies that the film is more uniform and dense. It has been shown in the previous reports that the Ce and La can be existed on the surface mainly in the form of CeO2 and La2O3 21, 26, 50

. Table 1

The chemical composition of the film deposited on the surface of the sample treated by Ce-La was also studied by XPS analysis. Different forms of Ce and La oxides/hydroxides can be studied in this way. According to Figure 3a the overall XPS survey includes O 1s, Ce 3d, Fe 2p3/2 and La 3d. In order to determine the presence of various components in the deposited film the Ce 3d and O 1s peaks were deconvoluted using a peak fitting process (Figure 3b and c). In this way the O 1s spectrum was separated into six peaks. Figure 2 Figure 3 As can be seen, the O 1s is deconvoluted into six peaks at binding energies of 534, 530.16 and 529.7 eV, corresponding to the presence of CeO2 and Ce2O3 oxides 23, 51, and at 533.26, 532.31 and 531.18 eV which are attributed to the La(OH)3 and La2O3 compounds. These results confirm that the cerium oxides exist in both trivalent and tetravalent states. The Ce 3d3 spectra is deconvoluted into four peaks at binding energies of 897.5, 901.56, 905.1 and 908.45 eV, verifying that the cerium is available in two oxidation states of Ce3+ and Ce4+. These observations suggest that combination of cerium and lanthanum oxides/hydroxides deposited on the steel surface. It can be seen that the intensity of the peaks related to Ce4+ is greater than Ce3+, pointing out the fact that the cerium is mostly in the form of CeO2.



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The film formation process includes different steps as shown by Eqs.1 to 9. It seems that in the presence of La the competition between the Ce and La cations to be deposited on the steel sheets results in the decrease of particle size of CeO2 and La2O3 oxides. Results show that addition of La could mostly affect the surface morphology. Fe  Fe2+ + 2e

(on micro anode)

Eq.1

2H+ +1/2O2+ 2e- H2O

(on micro cathode)

Eq.2

H2O2 + 2e→2OH-

(near the steel surface)

Eq.3

H2O + 1/2O2+ 2e→2OH-

(on micro cathode)

Eq.4

Ce3+ + 3OH-Ce(OH)3

Eq.5

La3+ + 3OH-La(OH)3

Eq.6

2Ce3+ + 2OH- + H2O2 →2Ce(OH)22+

Eq.7

Ce(OH)22+ + 2OH- → CeO2 + 2H2O

Eq.8

2La(OH)3 → La2O3 + 3H2O

Eq.9

In Figure 4 it can be seen that the surface modification by Ce and Ce-La resulted in the increase of surface roughness. The increase of surface roughness was most pronounced when modified by Ce-La film. The AFM micrographs clearly show uniform distribution of the nanometric CeLa particles on the surface but in the case of Ce treated sample some agglomerations can be seen. These results are in complete agreement with SEM results. One effective parameter affecting the adhesion between the epoxy and steel substrate would be the surface roughness. The increase of surface roughness results in the increase of surface area and therefore the number of adhesion sites and enhancement of adhesion through mechanical interlocking of the coating. Figure 4 The influence of Ce and Ce-La deposits on the surface free energy and work of adhesion of steel was evaluated through contact angle measurements. The contact angle of water was


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measured and then the surface free energy (γsv) and work of adhesion (WA) parameters were calculated through Neumann’s (Eq.10) and Young’s (Eq.11) equations 52: WA = 2(γlv.γsv)1/2exp[-β(γlv-γsv)2]

Eq.10

WA = γlv(1+cosθ)

Eq.11

In Eq.10 and 11, θ, γlv and γsv are the contact angle of water, surface tension of water and the surface free energy of the substrate. Also β is 0.0001247 ± 0.000010 (mJ/m2)2. According to Figure 5 the surface treatment of steel resulted in the increase of γsv and WA and decrease of contact angle. The highest γsv and WA were obtained in the case of Ce-La treated sample. This implies the fact that the wettability of the steel sheets can be noticeably improved through surface modification by Ce-La film. The oxides deposition enhances the hydrophilicity of surface, providing nanometric roughness and therefore increases the surface free energy. These are key factors that can affect the adhesion between epoxy and steel. Figure 5 4.2. Adhesion measurements Pull-off is a popular technique for adhesion measurement of organic coatings. So the adhesion strength between the steel sheets without and with treatment and epoxy coating was measured by pull-off test in the absence and presence of water. Figure 5 shows the pull-off strengths of the samples before (dry adhesion) and after exposure to water (3.5 wt.% NaCl solution for 30 days). According to Figure 5 the coating failure after pull-off test of the untreated sample is in complete adhesive form but in the case of Ce-La treaded sample the cohesive failure occurred. The increase of adhesion strength of the epoxy coating on the surface treated sample was observed. Moreover, the highest adhesion strength and the lowest adhesion decline after exposure to corrosive electrolyte were obtained for the Ce-La treated sample. These results all confirm significant adhesion promotion at the interface of epoxy coating and steel substrate when treated by Ce-La. Also, the adhesion forces produced are stronger and more stable that



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those created between the untreated steel and epoxy film. It seems that the pull-off results are in good agreement with AFM and contact angle test results. The adhesion forces between the steel sheet and epoxy film are mostly in the form of secondary bonds i.e., hydrogen bonding and van der Waals forces. However, the adhesion forces between the epoxy coating and steel surface treated by Ce-La would be in the form of primary chemical bonds. Not only the increase of wettability but also the nanometric roughness produced by Ce-La deposit could enhance the adhesion strength at the steel/epoxy interface. The water molecules diffusion into the epoxy film/steel interface deteriorates the polar bonds i.e., hydrogen bonding, leading to the loss of adhesion. However, the adhesion bonds failure in the case of chemical bonding between the epoxy and Ce-La treated sample would not be significant. The chemical bonds are stable against water and cannot be easily deteriorated. It has been shown that addition of La to the Ce solution could enhance the bonding between the epoxy and steel. This can be related to both effects of La on the physics and chemistry of steel. 4.2.1. FT-IR analysis The chemical structure of epoxy coating and interface of epoxy/steel was examined by a FTIR diffuse reflectance spectroscopy. From Figure 6 the IR spectrum of epoxy coating displays a strong broad band in the 3600–3200 cm-1 region assigned to O–H stretching vibrations. The –C–N at 1109 cm-1, the ether C–O–R at 1025 cm-1, the aromatic –C-H and –C=C– vibrations at around 3030 and 1605 cm-1 are other characteristic peaks available in the IR spectrum of epoxy coating 53-54. Observation of O-H and C-N groups reveals the epoxy group reaction with aminoamide cross-linker. As can be seen there is no difference between the FT-IR spectra of the epoxy coating at the surface and interface of epoxy/steel, depicting the same chemical structures at these regions. However, for the epoxy coating applied on the Ce-La treated sample a broad doublet peak can be observed in the wavenumber range of 3200–3340 cm-1, which is due to the –NH2 vibration absorption of amine compounds, and at 3300-3650 cm-1 which is


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attributed to the O–H stretching vibration 53. Not only the intensity of O–H stretching vibration increased on the Ce-La treated sample but also this peak showed a bathochromic shift resulted from the interaction of epoxy and Ce-La oxides. The absorption peak at 3150 to 3700 cm-1 was deconvoluted into four peaks. For the epoxy coating applied on the untreated steel the absorption peak related to N-H vibration appeared at 3198 cm-1 but for the coating applied on the Ce-La treated samples the peak is observed at 3247 cm-1. In addition to the bathochromic shift of N-H vibration on the Ce-La treated sample the intensity of this peak significantly increased. This may imply the presence of more unreacted -NH2 groups in aminoamide crosslinker at the interface of epoxy/Ce-La than the coating applied on the untreated substrate. Also, three peaks related to -OH groups are noted in the deconvoluted spectra of both samples. However, the shift of -OH vibration peaks to higher wavenumbers is obvious for the deconvoluted spectra. Appearance of intensive peaks related to N-H vibration and O–H stretching vibration reveals the physical/chemical interactions between the Ce-La film and epoxy coating at the epoxy/Ce-La-steel interface. The Ce-La oxide film provides many oxygenrich sites with high capability of creating hydrogen bonding with polar groups of epoxy coating i.e., epoxide and hydroxyl groups. The intensive bathochromic shift of O–H stretching vibration may be due to the chemical interactions between the Ce-La and epoxy coating. In fact, at the interface, the amine groups of aminoamide cross-linker and epoxide groups of epoxy resin can interact with Ce-La film through both hydrogen bonding and chemical interactions5457

. Figure 6

4.3. Modeling results Computational DFT calculations and MD simulations were performed over experiments to gain basic electronic-/atomic-scale insights concerning the affinity of epoxy resin molecules to adsorb and form corrosion-resistant films onto unmodified/modified carbon steel sheets.



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Figure 7 gives the initial and corresponding final structures for aminoamide-crosslinked epoxy resin molecule over CeO2 (111) and La2O3 (001) conversion layers. The visualized structures clearly show that the distance of epoxy resin with respect to both CeO2 (111) and La2O3 (001) slabs in final snapshots is lower as compared with initial ones. Such an observation clarifies the propensity of crosslinked epoxy resin to adsorb onto conversion layers (or cerium/lanthanum-covered steel sheets). When MD simulations were continued for longer times, the epoxy resin stayed as adsorbed signifying the strong and stabilized coating molecule adhesion to conversion layers. From the side view of final structure it can be seen that the molecular skeleton in adsorbed epoxy is parallel to both surfaces, which maximizes the surface coverage ability of coating molecule. Also, a closer inspection of side view of final structure indicates that the crosslinked epoxy resin adhered to ceria with its heteroatoms (i.e., oxygen and nitrogen) pointed towards the surface, while in the epoxy localized near the lanthana slab these atoms pointed upwards the surface due to the oxygen termination of the uppermost atomic layer in La2O3 (001). To further examine the adsorbed geometry of epoxy resin above conversion coating layers, the top view of the final structures are presented. From this view, it can be observed that the aromatic benzene rings in bisphenol A moieties of crosslinked epoxy took parallel orientation against metallic surfaces, especially the CeO2 (111) substrate. The epoxy resin bonding to conversion layers based on CeO2 (111) and La2O3 (001) was quantitatively studied by computing the adsorption energy term, ΔEads, mathematically expressed according to following equation: ΔEads = Etotal – (Eepoxy + Esubstrate). In this formula, Etotal is the potential energy for entire system (i.e., epoxy plus metal oxide substrate), and Eepoxy and Esubstrate represent the potential energy of the isolated epoxy resin and the metal oxide substrate, respectively. The quantity ΔEads was particularly analyzed to get a quantitative outlook of the interfacial bonding properties of aminoamide-crosslinked epoxy resin. The ΔEads values for epoxy resin adsorption were computed as -850.1 and -341.6 kcal/mol over CeO2


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(111) and La2O3 (001) adsorbents, respectively. The calculated negative ΔEads values quantitatively verify the attachment of organic aminoamide-crosslinked epoxy resin coating to ceria-/lanthana-treated steel sheets, which is in agreement with our experiments. The greater adsorption energy found over ceria surface reflects the stronger binding of crosslinked epoxy resin to surface of CeO2 (111), which is on account of stronger interfacial interactions of epoxy resin with surface atoms in ceria substrate. Figure 7 To compare the binding level of epoxy resin over conversion layer with unmodified steel substrates, MD simulations were also performed for iron and iron oxides representing steel sheets. Figure 8 presents the pictures for initial together with final (side and top view) structures for epoxy resin coating molecule interacting with pure Fe (110), and iron oxides FeO (100), Fe2O3 (110), and Fe3O4 (100). The significantly lower epoxy-iron/iron oxides distance in final snapshots in comparison with initial prepared slabs points out the fact that crosslinked epoxy resin adsorbate moved towards the pure iron and its oxidized adsorbents due to the presence of interfacial adhesion forces. As illustrated, the crosslinked epoxy resin bound to pure iron surface with completely parallel orientation, while coating adhesion to highest layer of Fe2O3 and Fe3O4 occurred mostly through O and N atoms as these heteroatoms pointed towards the surface Fe cations. Furthermore, based on the top view of the attached epoxy configuration, it is noted that all benzene cycles in epoxy aligned parallel with respect to plane (110) in Fe and Fe2O3 crystals. As a result, the rather parallel alignment when adhered to FeO and Fe3O4 substrates implies the weaker interfacial interactions above these two oxidized iron surfaces. The estimated adsorption energies for epoxy resin equilibrated adjacent to pure and oxidized iron substrates amount to -428.1, -117.7, -561.8, and -401.8 kcal/mol for Fe (110), FeO (100), Fe2O3 (110), and Fe3O4 (100), respectively. When these ΔEads values are compared with those



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for conversion coating layers, it is found that ΔEads for epoxy-CeO2 (111) complex is considerably higher than those for epoxy-pure/oxidized Fe complexes, meaning the stronger epoxy resin adhesion to Ce-based conversion layer as compared with unmodified steel substrates, which agrees with our experimentally-observed higher work of adhesion of epoxy coating on Ce-treated steel (Figure 5). Furthermore, the highest work of epoxy adhesion over Ce-/La-treated sample is rationalized by its increased surface roughness revealed from AFM results shown in Figure 4. It is believed that a surface of higher roughness provides more adhesion regions for coating interactions. Thus, in the case of Ce-/La-modified steel with greater surface roughness compared to Ce-modified sample, increased adhesion sites (i.e., Ce cations) are available for epoxy interactions which promote the binding of coating. On the other hand, additional interfacial sites in the form of La cations are present at steel surface modified by Ce-La nanofilm relative to sample treated only by Ce layer. This further assists the coating adhesion as cross-linked epoxy resin adsorbed onto La2O3 surface with ΔEads value (-341.6 kcal/mol) comparable to those on iron/iron oxides. In order to examine the epoxy resin adhesion mechanism onto metallic substrates, the adsorption energies based on non-bonded interactions, that is, van der Waals (vdW) and electrostatic, were assessed for final snapshots from MD simulations, and the results are listed in Table 2. On the basis of computed electrostatic and vdW adsorption energies, it can be found that epoxy resin adhered to CeO2 (111), La2O3 (001), Fe2O3 (110) and Fe3O4 (100) surfaces almost through electrostatic interactions. These interactions mainly occur between charged heteroatoms in polar epoxy resin (i.e., O and N) and surface metal cations, as these atoms pointed towards the uppermost layer in metal oxide substrates. On the other hand, in iron oxide surface of FeO (100) the adsorption energy relevant for vdW interaction is twice that of electrostatic one, implying the major role of vdW forces in epoxy adhesion to this oxide surface. In case of Fe (110) surface, the adsorption happened only through vdW interactions,


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which is attributed to the fact that the iron atoms in pure iron crystal carry no partial charges 58 and thus no interfacial electrostatic interactions present between epoxy coating and pure Fe substrate. Figure 8 Table 2 From the final snapshots resulted from MD simulations of epoxy resin attached to untreated/treated steel surfaces, it was found that the epoxy benzene rings aligned parallel to metallic surface, especially over CeO2 (111), Fe2O3 (110) and Fe (110), and the O and N atoms pointed towards the surface. From an electronic point of view, such an adsorption configuration of corrosion-protective molecules originates from the electronic donor-acceptor (or acid-base) and/or electrostatic interactions

28-29, 42, 59

. Previous studies revealed that corrosion inhibitor

molecules adsorbed to Fe substrate through the donor-acceptor interactions

42, 59-60

. It is well

documented that in a donor-acceptor interaction, the active electron-containing sites in adsorbing molecule supply their electrons to unfilled orbitals in surface metal atoms, and thereby form a coordination bond

61

. Therefore, the Lewis bases in crosslinked epoxy resin

(i.e., polar O and N atoms as well as bisphenol A rings) participate in donor-acceptor interactions with Lewis acids in substrate (i.e., Ce, La, and Fe atoms)

28

. Accordingly, the

intensity of epoxy binding via such interactions greatly relies on the capacity of its adhesion sites to give electrons to metal atoms containing empty orbitals, which in turn is dependent on the frontier molecular orbitals, namely highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). The HOMO represents the sites which have the greatest electron donation capability, while the LUMO is indicative of atoms which accept electrons from filled orbitals in surface atoms. Figure 9 demonstrates the optimized crosslinked epoxy resin and the plots of HOMO as well as LUMO extracted from gas-/liquid-phase DFT calculations at level of B3LYP/6-311G(d,p). It is seen that in liquid-phase optimized structure



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the epoxy monomers are more separated from each other compared to gas-phase, which is assigned to solvation of hydrophilic groups. It is obvious from graphical HOMO and LUMO results that the HOMO emerged on two benzene rings in one bisphenol A fragment and its nearest ether and epoxide O atoms, while LUMO distributed over aromatic rings in the other bisphenol A moiety. This observation proposes that the bisphenol A fragments as an adsorption site contribute in crosslinked epoxy resin binding by donating their π electrons. As a result, the observed preferred flat alignment of adsorbed coating (Figures 7 and 8), in particular over CeO2, Fe, and Fe2O3 slabs, is rationalized by the presence of these donor-acceptor interactions. The electrostatic and Lewis acid-base interactions of O and N atoms are correlated with average electron distribution (or point charge) on these polar atoms. The panels (c) and (c′) in Figure 9 provides the partial charges for O and N atoms determined based on ChelpG method. It is obvious that in crosslinked epoxy resin optimized in gas and liquid phases both atoms especially those in aminoamide curing agent appeared as sites with highly negative charges. Hence, these negatively-charged heteroatoms as other active sites are capable of taking part in acid-base/electrostatic interactions with oxidized surfaces. In case of pure metal substrate, the coating adhesion takes place only through acid-base adhesion mechanism. Existence of such interfacial forces caused these charged atoms to point towards surface cations in most of the studied metal oxides. Figure 9 To further study the local reactive regions in crosslinked epoxy resin, and their donor-acceptor interactions with metal surfaces, the Fukui indices (FI) were investigated 62. The Fukui function ( f (r ) ) is determined from the first derivative of electron density (  (r ) ) with regards to the number of electrons ( N ) at constant potential ( (r ) ):

  (r )  f (r )     N  ( r )

Eq.12


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By using finite difference (FD) approximation and Hirshfeld population analysis 63, the atomcondensed Fukui functions related to electrophilic ( f



(r ) ) and nucleophilic ( f  (r ) ) centers

were estimated using following equations:

f  (r )  N 1 (r )  N (r )

Eq.13

f  (r )  N (r )  N 1 (r )

Eq.14

In these equations,

N (r ) corresponds to the electronic density of neutral species, and

N 1 (r ) and N 1 (r ) are the electronic densities of anionic and cationic species, respectively. Fukui functions were computed by means of DMol3 code, which is based on DFT calculations. The generalized-gradient approximation (GGA) using Perdew-Burke-Ernzerhof (PBE) scheme

64

was applied to calculate electronic exchange-correlation term with double

numeric polarization (DNP) basis set. Figure S1 of the Supporting Information presents the graphical results for condensed Fukui functions of crosslinked epoxy resin in both gas and aqueous phases. As displayed, the Fukui f



function appeared over aromatic rings in one

bisphenol A fragment, its nearest ether oxygen atoms and aminoamide nitrogen atoms. This observation further confirms the fact that these sites behave as reactive electrophilic centers and thus lose their electrons in donor-acceptor interactions with surface metal cations, in agreement with distributions of frontier molecular orbital and point charge within considered epoxy resin. On the other side, the local reactive zones for nucleophilic behavior (i.e., f



)

located on two benzene rings forming the other bisphenol A fragment, which is in close consistent with the LUMO region. Figure 10 Experiments suggested that the solvent exist at coating/metal interface leads to coating disbondment due to reduced binding between coating film and metallic layer. To mimic these experimental situations, and to shed light on solvent behavior at interface, MD simulations



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were also performed by considering water molecules. The last snapshots for dissolved crosslinked epoxy resin over both untreated and surface-modified steel sheets are visualized in Figure 10. It is clear that the crosslinked epoxy resin attached to metal/metal oxide surfaces even in humid environments, which again emphasizes the strong epoxy affinity to adhere to carbon steel substrates. On the basis of these ultimate structures, it is also noted that in addition to epoxy resin molecule the water solvents also adhered to all surfaces with distinct orientations and finally established thin solvent films inhibiting the direct contact of epoxy coating molecule with uppermost surface atoms. The crosslinked epoxy resin adhesion to investigated crystalline materials under humid conditions was quantified in terms of adsorption energy, ΔEads. The computed ΔEads data for single epoxy coating molecule dissolved by water molecules are depicted in Figure 11. Additionally, in order to compare these ΔEads values with those obtained in the absence of solvent molecules, the aforementioned adsorption energies related to dry medium are also plotted in this figure. It is apparent that the adsorption energies followed almost the same trend as observed in dry environments, revealing the fact the strongest crosslinked epoxy resin binding took place onto CeO2 conversion layer, which is in consistent with the experimental finding of higher corrosion-inhibitive performance of conversion layer as compared with bare steel sheets even in humid conditions. Furthermore, the energies for crosslinked epoxy resin adsorption over pure iron and all oxidized slabs are noted to decline when metallic surfaces are subjected to solvent molecules. Such a theoretical observation, which supports the results from our experiments, means the weakened coating attachment to both bare and surface-modified steel sheets in wet situations. Furthermore, analysis of interfacial vdW and electrostatic adsorption energies collected in Table S1 of the Supporting Information demonstrates that even in the presence of water molecules, the epoxy resin adhesion over CeO2 (111), La2O3 (001), Fe2O3 (110) and Fe3O4 (100) substrates almost takes place via the electrostatic interactions,


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while the vdW interactions plays a central role in epoxy-FeO (100) binding. Also, as tabulated, crosslinked epoxy resin bound to pure iron surface only through the interfacial vdW interactions. The weakened affinity between crosslinked epoxy coating and metallic adsorbents happened in the presence of solvent is rationalized by solvent interactions with coating molecule as well as metal/metal oxide adsorbents. On the one hand, the interactions of solvent molecules with adhesion sites of coating molecules lead to solvent gathering around these active sites, which weakens their interfacial interactions with surface atoms. On the other hand, the tendency of solvent molecules to interact with outermost atoms in substrate brings about formation of a solvent film near the surface, which further decreases the coating-substrate binding extent. To get a deeper insight into these solvent interactions responsible for reduced ΔEads values, the visualized snapshots (e.g., Fe (110) and FeO (100) in Figure 10) were further inspected focusing on the interface of solvent/epoxy/substrate. As illustrated in Figure 12, solvent (H2O) molecules above both Fe and FeO substrates participated in hydrogen bonds with functional groups in aminoamide-crosslinked epoxy resin, e.g., epoxide and ether O atoms, and curing agent O and N atoms. Meanwhile, water molecules were able to strongly bind to FeO slab through hydrogen bonds built with surface O atoms. It is noteworthy that the least rate of reduction in epoxy resin adsorption energy above pure iron surface (~ 5%) is attributed to solvent contribution in hydrogen bonding interactions with only epoxy molecule. For this reason, almost no solvent film formed at epoxy/Fe interface and epoxy molecule preserved its equilibrium configuration as found in dry conditions. Interestingly, amongst the oxide substrates the least reduction rate in ΔEads for dissolved epoxy resin took place over CeO2 and La2O3 conversion layers (~ 16%). Such a modeling outcome once again highlights the strengthened adhesion of epoxy-based adhesives on Ce-/La-covered steel sheets even in wet environments. This in turn emphasizes the superior performance of epoxy resin when applied



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as a corrosion-resistant coating over modified steel as compared to unmodified one. Due to the central role played by interfacial solvent, the solvent-epoxy interaction characteristics were further examined in more detail from an electronic perspective. To this purpose, a single H2O molecule was positioned adjacent to crosslinked epoxy functionalities namely epoxide, ether, hydroxyl, amine and amide, as these groups participated in H-bonds with solvent molecules (Figure 12). The water-epoxy was gradually subjected to geometry optimization carried out using: (1) HF/6-31G(d,p), (2) DFT at B3LYP/6-31G(d,p), and (3) DFT at B3LYP/6-311G(d,p). The interaction energy (ΔE) between this H2O and epoxy was calculated by the equation ΔE = Eepoxy/water – (Eepoxy

+

Ewater). Figure S2 (Supporting

Information) depicts the final equilibrium geometry of water-epoxy derived from DFT computations. From this figure it is apparent that water molecule interacted with all epoxy active moieties by establishing intermolecular H-bonds. It can be seen that solvent H2O molecule behaved as both H-bond donor and acceptor. The respective uncorrected (ΔEelectronic) and counterpoise-corrected (ΔEBSSE) interaction energies are collected in Table S2 (Supporting Information). The calculated negative ΔE values quantitatively affirm the water affinity to solvate the epoxy active centers. The higher ΔE data of -8.2408 and -8.4232 kcal/mol is ascribed to the shorter H-bond lengths equivalent to the stronger H-bond interactions. Hence, it is deduced that appearance of solvent hydrogen bonds at the interface weakens the potential electrostatic and Lewis acid-base interactions between epoxy coating and metal/metal oxide substrate, which is accompanied by reduced coating adhesion to untreated/treated carbon steel sheets. Figure 11 Figure 12 5. Conclusion


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The adhesion properties of epoxy adhesive applied on the surface-modified steel samples were studied from experimental and modeling perspectives. The steel sheets treated by Ce and composite Ce-La films enhanced the surface free energy, work of adhesion and roughness. The adhesion improvement was most significant for epoxy coating employed on Ce-La modified steel sheets. Alongside experiments, computational MD simulations and DFT calculations were used to explore the interactions and mechanism behind the crosslinked epoxy resin adhesion to untreated/treated steel substrates under dry and wet conditions. The modeling results showed that aminoamide-crosslinked epoxy adsorbed to steel surfaces via interfacial electrostatic and vdW interactions. The epoxy resin adhesion weakened in the order of CeO2 > Fe2O3 > Fe > Fe3O4 > La2O3 > FeO in gas phase, and CeO2 > Fe > Fe2O3 > Fe3O4 > La2O3 > FeO in solution phase, reflecting the stronger epoxy bonding to Ce-/La-covered steel sheet in both dry and wet conditions in agreement with experiments. The computations also clarified that the weakened coating-metal binding was due to solvent interactions with both epoxy resin and surface atoms. Acknowledgment The authors gratefully thank the use of School of Computer Science, Institute for Research in Fundamental Science (IPM) as the computations were done there. Supporting Information The liquid-phase interaction energies including total, vdW, and electrostatic for coating adhesion onto metallic surfaces, different variants of electronic energy for water-epoxy resin interaction, computed atom-centered Fukui indices of optimized cross-linked epoxy resin, and DFT optimized solvent-coating complex.

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21. Zhang, S.-h.; Kong, G.; Lu, J.-t.; Che, C.-s.; Liu, L.-y., Growth Behavior of Lanthanum Conversion Coating on Hot-Dip Galvanized Steel. Surf. Coat. Technol. 2014, 259, 654-659. 22. Mahidashti, Z.; Shahrabi, T.; Ramezanzadeh, B., A New Strategy for Improvement of the Corrosion Resistance of a Green Cerium Conversion Coating through Thermal Treatment Procedure before and after Application of Epoxy Coating. Appl. Surf. Sci. 2016, 390, 623-632. 23. Ramezanzadeh, B.; Rostami, M., The Effect of Cerium-Based Conversion Treatment on the Cathodic Delamination and Corrosion Protection Performance of Carbon Steel-FusionBonded Epoxy Coating Systems. Appl. Surf. Sci. 2017, 392, 1004-1016. 24. Vakili, H.; Ramezanzadeh, B.; Amini, R., The Corrosion Performance and Adhesion Properties of the Epoxy Coating Applied on the Steel Substrates Treated by Cerium-Based Conversion Coatings. Corros. Sci. 2015, 94, 466-475. 25. Jamali, S. S.; Moulton, S. E.; Tallman, D. E.; Forsyth, M.; Weber, J.; Mirabedini, A.; Wallace, G. G., Corrosion Protection Afforded by Praseodymium Conversion Film on Mg Alloy Aznd in Simulated Biological Fluid Studied by Scanning Electrochemical Microscopy. J. Electroanal. Chem. 2015, 739, 211-217. 26. Lei, L.; Shi, J.; Wang, X.; Liu, D.; Xu, H., Microstructure and Electrochemical Behavior of Cerium Conversion Coating Modified with Silane Agent on Magnesium Substrates. Appl. Surf. Sci. 2016, 376, 161-171. 27. Ramezanzadeh, B.; Vakili, H.; Amini, R., Improved Performance of Cerium Conversion Coatings on Steel with Zinc Phosphate Post-Treatment. J. Ind. Eng. Chem. 2015, 30, 225-233. 28. Bahlakeh, G.; Ghaffari, M.; Saeb, M. R.; Ramezanzadeh, B.; De Proft, F.; Terryn, H., A Close-up of the Effect of Iron Oxide Type on the Interfacial Interaction between Epoxy and Carbon Steel: Combined Molecular Dynamics Simulations and Quantum Mechanics. J. Phys. Chem. C 2016, 120 (20), 11014-11026. 29. Khaled, K. F.; Amin, M., Electrochemical and Molecular Dynamics Simulation Studies on the Corrosion Inhibition of Aluminum in Molar Hydrochloric Acid Using Some Imidazole Derivatives. J Appl Electrochem 2009, 39 (12), 2553-2568. 30. Suérez, J.; Miguel, S.; Pinilla, P.; López, F., Molecular Dynamics Simulation of Polymer–Metal Bonds. J. Adhes. Sci. Technol. 2008, 22 (13), 1387-1400. 31. Semoto, T.; Tsuji, Y.; Yoshizawa, K., Molecular Understanding of the Adhesive Force between a Metal Oxide Surface and an Epoxy Resin. J. Phys. Chem. C 2011, 115 (23), 1170111708. 32. Ogata, S.; Takahashi, Y., Moisture-Induced Reduction of Adhesion Strength between Surface Oxidized Al and Epoxy Resin: Dynamics Simulation with Electronic Structure Calculation. J. Phys. Chem. C 2016, 120 (25), 13630-13637. 33. Lee, J. H.; Kang, S. G.; Choe, Y.; Lee, S. G., Mechanism of Adhesion of the Diglycidyl Ether of Bisphenol a (Dgeba) to the Fe (100) Surface. Compos. Sci. Technol. 2016, 126, 9-16. 34. Hariharan, P. C.; Pople, J. A., The Influence of Polarization Functions on Molecular Orbital Hydrogenation Energies. Theoret. Chim. Acta 1973, 28 (3), 213-222. 35. Kohn, W.; Sham, L. J., Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140 (4A), A1133-A1138. 36. Becke, A. D., Density‐Functional Thermochemistry. Iii. The Role of Exact Exchange. J Chem Phys 1993, 98 (7), 5648-5652. 37. Tomasi, J.; Mennucci, B.; Cammi, R., Quantum Mechanical Continuum Solvation Models. Chem. Rev. 2005, 105 (8), 2999-3094. 38. Mennucci, B.; Cances, E.; Tomasi, J., Evaluation of Solvent Effects in Isotropic and Anisotropic Dielectrics and in Ionic Solutions with a Unified Integral Equation Method: Theoretical Bases, Computational Implementation, and Numerical Applications. J Phys Chem B 1997, 101 (49), 10506-10517.



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39. Breneman, C. M.; Wiberg, K. B., Determining Atom-Centered Monopoles from Molecular Electrostatic Potentials. The Need for High Sampling Density in Formamide Conformational Analysis. J. Comput. Chem. 1990, 11 (3), 361-373. 40. Boys, S. F.; Bernardi, F., The Calculation of Small Molecular Interactions by the Differences of Separate Total Energies. Some Procedures with Reduced Errors. Mol Phys 1970, 19 (4), 553-566. 41. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J., Gaussian 09, Revision D.01. 2009. 42. Verma, C.; Olasunkanmi, L. O.; Ebenso, E. E.; Quraishi, M. A.; Obot, I. B., Adsorption Behavior of Glucosamine-Based, Pyrimidine-Fused Heterocycles as Green Corrosion Inhibitors for Mild Steel: Experimental and Theoretical Studies. J. Phys. Chem. C 2016, 120 (21), 11598-11611. 43. Feng, L.; Yang, H.; Wang, F., Experimental and Theoretical Studies for Corrosion Inhibition of Carbon Steel by Imidazoline Derivative in 5% Nacl Saturated Ca(Oh)2 Solution. Electrochim. Acta 2011, 58, 427-436. 44. Beste, A.; Overbury, S. H., Pathways for Ethanol Dehydrogenation and Dehydration Catalyzed by Ceria (111) and (100) Surfaces. J. Phys. Chem. C 2015, 119 (5), 2447-2455. 45. Manoilova, O. V.; Podkolzin, S. G.; Tope, B.; Lercher, J.; Stangland, E. E.; Goupil, J.M.; Weckhuysen, B. M., Surface Acidity and Basicity of La2o3, Laocl, and Lacl3 Characterized by Ir Spectroscopy, Tpd, and Dft Calculations. J Phys Chem B 2004, 108 (40), 15770-15781. 46. Palmer, M. S.; Neurock, M.; Olken, M. M., Periodic Density Functional Theory Study of the Dissociative Adsorption of Molecular Oxygen over La2o3. J Phys Chem B 2002, 106 (25), 6543-6547. 47. Accelrys Software Inc., S. D., 2009. 48. Sun, H., Compass: An Ab Initio Force-Field Optimized for Condensed-Phase Applications Overview with Details on Alkane and Benzene Compounds. J Phys Chem B 1998, 102 (38), 7338-7364. 49. Swope, W. C.; Andersen, H. C.; Berens, P. H.; Wilson, K. R., A Computer Simulation Method for the Calculation of Equilibrium Constants for the Formation of Physical Clusters of Molecules: Application to Small Water Clusters. J Chem Phys 1982, 76 (1), 637-649. 50. Zeng, R.-c.; Yan, H.; Zhang, F.; Huang, Y.-d.; Wang, Z.-l.; Li, S.-q.; Han, E.-h., Corrosion Resistance of Cerium-Doped Zinc Calcium Phosphate Chemical Conversion Coatings on Az31 Magnesium Alloy. Trans. Nonferrous Met. Soc. China 2016, 26 (2), 472483. 51. Xu, J.; Xin, S.; Han, P.; Ma, R.; Li, M., Cerium Chemical Conversion Coatings for Corrosion Protection of Stainless Steels in Hot Seawater Environments. Mater. Corros. 2013, 64 (7), 619-624. 52. Li, D.; Neumann, A., A Reformulation of the Equation of State for Interfacial Tensions. J. Colloid Interface Sci. 1990, 137 (1), 304-307.


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53. Nikolic, G.; Zlatkovic, S.; Cakic, M.; Cakic, S.; Lacnjevac, C.; Rajic, Z., Fast Fourier Transform Ir Characterization of Epoxy Gy Systems Crosslinked with Aliphatic and Cycloaliphatic Eh Polyamine Adducts. Sensors 2010, 10 (1), 684-696. 54. Billaud, C.; Vandeuren, M.; Legras, R.; Carlier, V., Quantitative Analysis of Epoxy Resin Cure Reaction: A Study by near-Infrared Spectroscopy. Appl. Spectrosc. 2002, 56 (11), 1413-1421. 55. Bistac, S.; Vallat, M.; Schultz, J., Investigation of Chemical Interactions at the Steel/Polymer Interface by Ft-Ir Diffuse Reflectance Spectroscopy. Appl. Spectrosc. 1997, 51 (12), 1823-1825. 56. Gaillard, F.; Romand, M.; Verchere, D.; Hocquaux, H., Zinc Coated Steel/Epoxy Adhesive Systems: Investigation of the Interfacial Zone by Ftir Spectroscopy. The Journal of Adhesion 1994, 46 (1-4), 227-241. 57. Öhman, M.; Persson, D., Atr‐Ftir Kretschmann Spectroscopy for Interfacial Studies of a Hidden Aluminum Surface Coated with a Silane Film and Epoxy I. Characterization by Irras and Atr‐Ftir. Surf. Interface Anal. 2012, 44 (2), 133-143. 58. Ta, T. D.; Tieu, A. K.; Zhu, H.; Kosasih, P. B., Adsorption of Normal-Alkanes on Fe (110), Feo (110), and Fe2o3 (0001): Influence of Iron Oxide Surfaces. J. Phys. Chem. C 2015. 59. Olasunkanmi, L. O.; Obot, I. B.; Kabanda, M. M.; Ebenso, E. E., Some Quinoxalin-6Yl Derivatives as Corrosion Inhibitors for Mild Steel in Hydrochloric Acid: Experimental and Theoretical Studies. J. Phys. Chem. C 2015, 119 (28), 16004-16019. 60. Fu, J.; Zang, H.; Wang, Y.; Li, S.; Chen, T.; Liu, X., Experimental and Theoretical Study on the Inhibition Performances of Quinoxaline and Its Derivatives for the Corrosion of Mild Steel in Hydrochloric Acid. Ind. Eng. Chem. Res. 2012, 51 (18), 6377-6386. 61. Chrétien, S.; Metiu, H., Acid–Base Interaction and Its Role in Alkane Dissociative Chemisorption on Oxide Surfaces. J. Phys. Chem. C 2014, 118 (47), 27336-27342. 62. Yang, W.; Mortier, W. J., The Use of Global and Local Molecular Parameters for the Analysis of the Gas-Phase Basicity of Amines. Journal of the American Chemical Society 1986, 108 (19), 5708-5711. 63. Hirshfeld, F. L., Bonded-Atom Fragments for Describing Molecular Charge Densities. Theoret. Chim. Acta 1977, 44 (2), 129-138. 64. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Physical review letters 1996, 77 (18), 3865.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CH2

Amine

N

H H 2C O

OH

O

C

O

Ether

CH2

O

Me

Hydroxyl

N

Amide

Me

CH2 CH

CH2

Page 30 of 42

Epoxide group

H

C

CH2 H 2C

Curing agent Me CH2

N

CH

H

O CH2

CH2

OH

O

C

O

Me

Figure 1-Chemical structure of aminoamide-cross-linked DGEBA epoxy resin. The functional groups were encircled in red line.


Page 31 of 42

(a1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 (b1) 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a2)

(b2)

Figure 2- SEM micrographs at two magnifications prepared from the steel samples treated by (a1 and a2) Ce-La and (b1 and b2) Ce conversion coatings.



Page 32 of 42


O 1s

C 1s

Intensity (arbitrary unit)

Intensity (arbirtrary unit)

Intensity (arbitrary unit)

Intensity (arbirtrary unit)

C 1s

Intensity

O 1s

110000 8000 2 3 4 8000 5 6000 6 7 8 6000 9 4000 10 114000 12 13 14 2000 152000 16 4800 17 850 855 860 18 0 0 19 0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200 20 21 Binding energy (eV) Binding energy (eV) 22 Data Ce 3d3 23 Ce 3d3 Fit 24 Ce3d3 25 Ce 4+ Ce3d3 26 Ce3d3 Ce4+ 27 Ce3+ Ce3d3 28 Ce3d3 Ce3d Ce4+ 3 Ce3d3 29 4+ 3+ Ce Ce 30 Ce4+ 31 32 33 Ce3d3 34 35 36 37 895 900 905 910 897 902 907 38 Binding Energy (eV) Binding Energy (eV) 39 40 O 1s Data 41 42 Fit 43 44 45 La(OH)3 La2O3 46 47 La2O3 48 49 50 51 Ce4+ 3+ Ce4+ Ce 52 53 54 55 528 530 532 534 536 56 Binding Energy (eV) 57 Figure 3Overall XPS survey and high resolution O 1s and Ce 3d3 spectra of the film 58 59 deposited on the steel sheets treated by Ce and Ce-La oxides. 60

Page 33 of 42

(a1)

(a1)

(a2)

(a2)

175 150

Roughness (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Sy

(a3)

Sz

125 100 75 50 25 0 Untreated sample

Ce treated

Ce-La treated

Figure 4- AFM micrographs from the surface of steel samples treated by (a1) Ce-La and (a2) Ce conversion coatings; (a3) roughness parameters obtained from AFM results



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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150 Contact angle (deg) 125

Surface free energy (mJ/m2) Work of adhesion (mJ/m2)

100

75

50

25

0 Untreated sample

Ce treated

Ce-La treated

Cohesive failure

Adhesive failure

Untreated sample

Adhesive failure

Ce treated sample

Ce-La treated sample

Dry adhesion: 4.2 MPa



Wet adhesion: 2.1 MPa



Figure 5- The results obtained from contact angle test including contact sample, surface free energy and work of adhesion for the samples without and with Ce and Ce-La treatments; pulloff test results of the epoxy coatings applied on differently treated samples.


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0.5

C-H stretching

Epoxy/Steel interface Epoxy/Air interface

(a1) 0.4

O-H stretching

0.3 0.2 0.1 0 3900

3400

2900

2400 1900 Wavenumber (cm-1)

0.6

(a2) 0.5 0.4

O-H stretching

1400

900

400

Epoxy/Ce-La-Steel interface Epoxy/Air interface

0.4 C-H stretching

0.3 N-H stretching Bathochromic shift

0.2

O-H stretching

0.3

0.1

0.2

0 1800

1600

1400

0.1 0 3900 0.6

3400

2900

2400 1900 Wavenumber (cm-1) 0.4

Fit 3274 cm-1 (N-H vibration) 3420 cm-1 (O-H vibration) 3577 cm-1 (O-H vibration) 3514 cm-1 (O-H vibration)

0.5

(b1)

1400

900

400

Fit

(b2)

3198 cm-1 (N-H vibration) 3308 cm-1 (O-H vibration) 0.3

3410 cm-1 (O-H vibration) 3521 cm-1 (O-H vibration)

0.4

Absorbance

Absorbance

Absorbance

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60.3%

0.3

0.2

49.9% 36.5%

0.2 0.1 0.1 15.8%

0 3700

3600

10.4%

3500

13.5%

3400

3300

11.6%

3200

0 3100 3700

3600

3500

2.1%

3400

3300

Wavenumber (cm-1)

Wavenumber (cm-1)

Figure 6- FT-IR spectroscopy results obtained from (a1) the surface of epoxy and interface of epoxy/steel, (a2) the interface of epoxy/Ce-La-Steel, and deconvoluted peaks at 3100-3700 cm1

for (b1) epoxy/Ce-La-steel interface and (b2) epoxy/steel interface.


3200

3100


Final structure (side view)

Final structure (top view)

CeO2 (111)

Initial structure

La2O3 (001)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 42

Figure 7- The initial and final (side and top view) snapshot of cross-linked epoxy resin over conversion coating layers of CeO2 (111) and La2O3 (001).


FeO (100) Fe2O3 (110) Fe3O4 (100)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fe (110)

Page 37 of 42

Figure 8- The initial and final (side and top view) snapshot of cross-linked epoxy resin over iron oxides of Fe (110), FeO (100), Fe2O3 (110) and Fe3O4 (100).



Optimized geometry

HOMO

LUMO

a′)

a″)

b)

b′)

b″)

Gas phase

a)

Liquid phase

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 42

Partial charge c′)

c)

-0.55 -0.64 -0.63

-0.75

-0.54

-0.69

-0.69 -0.75

-0.55

-0.55 -0.56 -0.54

-0.54

-0.70

-0.56

-0.77 -0.69

-0.62

-0.54

-0.69 -0.77

-0.55

-0.56 -0.55

Figure 9- The B3LYP/6-311G(d,p) optimized geometry (a,b), HOMO (a′,b′) and LUMO (a″,b″) of cross-linked epoxy resin in gas and liquid phases; The calculated partial atomic charges of O and N atoms in cross-linked epoxy resin in gas (c) and liquid (c′) phases using ChelpG method.


Page 39 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CeO2

La2O3

Fe

3

FeO

Fe2O3

Fe3O4

Figure 10-The side view of final snapshot of dissolved cross-linked epoxy resin over CeO2 (111), La2O3 (001), Fe (110), FeO (100), Fe2O3 (110), and Fe3O4 (100) substrates. The crosslinked epoxy resin molecule and metal oxide substrates are shown in Ball & Stick and CPK styles, respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 42

Figure 11-Comparison of the computed adsorption energy of cross-linked epoxy resinoverCeO2 (111), La2O3 (001), Fe (110), FeO (100), Fe2O3 (110), and Fe3O4 (100) substrates in dry and wet environments.


Page 41 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fe (110) solvent

FeO (100) Figure 12-The interfacial interactions of solvent molecules with cross-linked epoxy resin and surface metal atoms. The H-bonds are shown as dashed line, and their length is in angstrom (Å).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents Graphic


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A Detailed Molecular Dynamics Simulation and ... - ACS Publications

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