Assessing the Effect of CeO2 Nanoparticles as Corrosion Inhibitor in Hybrid Biobased Coatings - PDF

polymers Article Assessing the Effect of CeO2 Nanoparticles as Corrosion Inhibitor in Hybrid Biobased Waterborne Acrylic Direct to Metal Coating Binders Edurne González 1 , Robin Stuhr 1 , Jesús Manuel Vega 2 , Eva García-Lecina 2 , Hans-Jürgen Grande 2,3 , Jose Ramon Leiza 1 and María Paulis 1, *...

polymers Article Assessing the Effect of CeO2 Nanoparticles as Corrosion Inhibitor in Hybrid Biobased Waterborne Acrylic Direct to Metal Coating Binders Edurne González 1 , Robin Stuhr 1 , Jesús Manuel Vega 2 , Eva García-Lecina 2 , Hans-Jürgen Grande 2,3 , Jose Ramon Leiza 1 and María Paulis 1, * 1 POLYMAT, Applied Chemistry Department, Faculty of Chemistry, University of the Basque Country (UPV/EHU), 20018 Donostia-San Sebastián, Spain; [email protected] (E.G.); [email protected] (R.S.); [email protected] (J.R.L.) 2 CIDETEC, Basque Research and Technology Alliance (BRTA), Paseo Miramón 196, 20014 Donostia-San Sebastián, Spain; [email protected] (J.M.V.); [email protected] (E.G.-L.); [email protected] (H.-J.G.) 3 POLYMAT, Polymers and Advanced Materials: Physics, Chemistry and Technology Department, Faculty of Chemistry, University of the Basque Country (UPV/EHU), 20018 Donostia-San Sebastián, Spain * Correspondence: [email protected] Abstract: CeO2 nanoparticles were incorporated in waterborne binders containing high biobased content (up to 70%) in order to analyze the anticorrosion performance for direct to metal coatings. Biobased binders were synthesized by batch miniemulsion polymerization of 2-octyl acrylate and



 isobornyl methacrylate monomers using a phosphate polymerizable surfactant (Sipomer PAM200) that lead to the formation of phosphate functionalized latexes. Upon the direct application of such Citation: González, E.; Stuhr, R.; binders on steel, the functionalized polymer particles were able to interact with steel, creating a Vega, J.M.; García-Lecina, E.; Grande, thin phosphatization layer between the metal and the polymer and avoiding flash rust. The in situ H.-J.; Leiza, J.R.; Paulis, M. Assessing the Effect of CeO2 Nanoparticles as incorporation of the CeO2 nanoparticles during the polymerization process led to their homogeneous Corrosion Inhibitor in Hybrid distribution in the final polymer film, which produced outstanding anticorrosion performance Biobased Waterborne Acrylic Direct according to the Electrochemical Impedance Spectroscopy measurements. In fact, steel substrates to Metal Coating Binders. Polymers coated with the hybrid polymer film (30–40 µm thick) showed high barrier corrosion resistance after 2021, 13, 848. https://doi.org/ 41 days (~1000 h) of immersion in NaCl water solution and active inhibition capabilities thanks to 10.3390/polym13060848 the presence of the CeO2 nanoparticles. This work opens the door to the fabrication of sustainable hybrid anticorrosion waterborne coatings. Academic Editor: Andrzej Rybak Keywords: waterborne binder; anticorrosion; biobased acrylic binder; CeO2 /acrylic hybrid; CeO2 Received: 17 February 2021 nanoparticles; EIS Accepted: 7 March 2021 Published: 10 March 2021 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional affil- Nowadays, mild steel is one of the most important materials in construction, industry iations. and transportation because of its versatility and good mechanical properties. Neverthe- less, the main drawback of steel is its susceptibility to deterioration by corrosion, which causes dramatic economic losses (3.4% of the global GDP in 2013 according to NACE ). Therefore, the development of a successful protective organic coating is still an important Copyright: © 2021 by the authors. scientific challenge. Licensee MDPI, Basel, Switzerland. An efficient anticorrosion coating must offer good barrier properties, in order to avoid This article is an open access article the contact of the steel with water and oxygen (i.e., hindering their permeability). Such distributed under the terms and barrier capabilities are mainly provided by the polymer matrix in organic coatings, where conditions of the Creative Commons solvent-based polymers are the most popular among the commercial ones. However, Attribution (CC BY) license (https:// due to the more and more demanding environmental regulations on the emission of creativecommons.org/licenses/by/ volatile organic compounds (VOC), the coating market is moving towards the use of 4.0/). waterborne coatings. Polymers 2021, 13, 848. https://doi.org/10.3390/polym13060848 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 848 2 of 13 Waterborne coatings are based on polymer latexes, and even if they are an excellent environmentally friendly alternative to solvent-based coatings, their main drawback is the inherent higher hydrophilicity of the formed films due to the presence of surfactants and salts (needed for the synthesis of the latex). Films cast from waterborne latexes have shown to present higher permeability to water than the ones cast from solvent-based systems [3–5]. This is detrimental to achieve a good anticorrosion protection. Water permeability can be reduced by the use of polymerizable surfactants (also called surfmers). This type of surfactant is chemically bonded to the polymer particle; and therefore, their migration during the film formation is restricted, avoiding the formation of hydrophilic pockets in the film and improving its water resistance [6–8]. Chimenti et al. created a steel protective coating based on an acrylic latex (made of a copolymer of methyl methacrylate (MMA) and butyl acrylate (BA)) stabilized by a phosphate functionalized polymerizable surfactant (Sipomer-PAM200). This coating showed a lower water sensitivity compared to one stabilized with a conventional anionic surfactant; additionally, the phosphate groups of the polymerizable surfactant were able to react with the metal surface, forming an iron phosphate layer at the substrate/coating interface that provided a great corrosion resistance even in harsh conditions. In later works, the barrier properties of the phosphate functional- ized acrylic latexes were improved either by introducing crystalline nanodomains or incorporating 30% of perfluorooctyl acrylate (POA). Nevertheless, an important challenge while designing an environmentally friendly waterborne coating is the replacement of oil-based monomers by biobased ones to reduce the overall carbon footprint of the final product, while maintaining or improving its performance. In fact, the market demand of biobased paints and coatings has constantly increased in the last years [12,13]. The use of different types of biobased monomers to produce waterborne coatings has been extensively reviewed in the literature [14,15]. Even if several works have been published on the use of biobased monomers in emulsion and miniemulsion polymerization, few of them have used commercially available monomers, which makes the industrial implementation of the process difficult. To this end, one of the objectives of this work has been to use commercially available high biobased content monomers, namely 2-octyl acrylate (2-OA) and isobornyl methacrylate (IBOMA), in order to produce a biobased acrylic binder. 2-OA is a monomer derived from castor oil that has a biocontent of 73%. IBOMA comes from pine resin and has a biocontent of 71%. Both monomers have been previously used for the fabrication of biobased Pressure Sensitive Adhesives (PSA) [16–19] and coatings , producing polymers with superior hydrophobic character than conventional MMA/BA copolymers. Apart from the barrier properties, anticorrosion properties of coatings can also be improved by adding corrosion inhibitors. Most chemical inhibitors reduce the rate of corrosion forming a passive adsorption layer on the metal surface. Chromate based inhibitors incorporated in coatings are known to be the most efficient anticorrosive method for a large range of metals and alloys, reducing both the anodic and cathodic reactions that result in corrosion and metal loss. However, hexavalent chromium was banned due to its high toxicity; thus, alternative and non-toxic chemical inhibitors are needed in order to replace these highly efficient chromate-based compounds. In the last decades, new inorganic and organic inhibitors have been investigated. Either anodic or cathodic inhibition mechanism can be found in corrosion inhibitors [23,24]. In the case of inorganic ones, ion-exchange pigments (e.g., cation-exchange [25–27] or anion-exchange ones [28,29]) and nanoparticles (e.g., cerium oxide (CeO2 ) [30–33], silica (SiO2 ) and zinc oxide (ZnO) [35,36]) have shown promising results as corrosion inhibitors. In the case of the cerium compounds, the inhibiting effect of cerium salts has been outlined by various authors [37–40], and it is under debate if the nanoparticles as such provide inhibiting effect. In any case, it seems that the more homogeneously the nanoparticles are distributed in the polymeric film, the better the anticorrosion performance [36,41,42]. In this work, and for the first time, a high biobased content waterborne anticor- rosion binder containing a phosphatizing agent and CeO2 nanoparticles as inhibitor Polymers 2021, 13, 848 3 of 13 have been successfully synthesized and assessed for the production of direct to metal sustainable coatings. 2. Materials and Methods 2.1. Materials IBOMA (Evonik, Essen, Germany) and 2-OA (Arkema, Colombes, France) monomers were used as supplied. The thermal initiator azobisisobutyronitrile (AIBN, Sigma-Aldrich, Madrid, Spain) and the polymerizable surfactant Sipomer® PAM200 (Solvay) were used as received. Octadecyl acrylate (Sigma-Aldrich, Madrid, Spain) was used as co-stabilizer during the miniemulsion polymerization. A solution of CeO2 nanoparticles (NANO BYK 3812) was kindly supplied by ALTANA (Wesel, Germany). In order to obtain the pure nanoparticles, the solvent was evaporated in an oven at 60 ◦ C for 48 h. The resulting crystals were grinded before their use. Distilled water (MilliQ quality) was used in all reactions. Sodium bicarbonate (Sigma-Aldrich, Madrid, Spain) and ammonium hydroxide solution (25%, Sigma-Aldrich, Madrid, Spain) were used to adjust pH values. Steel substrates (medium carbon steel with 0.5% of C) were purchased from URDURI ACEROS. UniClean 251 (Atotech, Erandio, Spain) was used as a degreasing agent for the steel substrates. HCl 1 M solution (Sigma-Aldrich, Madrid, Spain) was used in the cleaning treatment of the steel substrates. High purity NaCl (Corrosalt, Ascott-Analytical, Tamworth, UK) was used for the preparation of a 3.5 wt% solution for the corrosion test. 2.2. Synthesis and Characterization of Latexes Two different latexes (without and with 1 wbm % of CeO2 , named Bioacrylic and CeO2 -Bioacrylic, respectively) were synthesized by batch miniemulsion polymerization. The used recipe is shown in Table 1. For the miniemulsion preparation, the oil phase was prepared by mixing the main monomers (IBOMA/2-OA), the monomeric costabilizer (octadecyl acrylate) and the dried cerium dioxide (CeO2 ) powder. This mixture was stirred magnetically for 5 min. The aqueous phase was obtained by dissolving the Sipomer® PAM200 in water and adjusting the pH to 7 adding ammonium hydroxide dropwise. Both phases were mixed for 5 min under magnetic agitation and then sonified for 20 min using a Branson 450 w. During sonication, the flask was immersed in an ice bath to avoid overheating. The miniemulsion was later charged into a 0.5 L glass jacketed reactor fitted with a reflux condenser, a sampling device, a N2 inlet and a stirrer rotating at 150 rpm. The temperature was controlled by an automatic control system (Camile TG, CRW Automation Solutions, Austin, USA). After reaching the desired temperature (70 ◦ C), a shot of AIBN initiator was added. The reaction was carried out for three hours. Table 1. Formulation used for the miniemulsion polymerization. The target solids content was 40 wt%. Component wt% IBOMA 16.6 2-OA 23.4 Organic phase Octadecyl acrylate * 4 CeO2 * 0–1 AIBN * 1 Water 60 Water phase Sipomer® PAM200 * 2 * Weight % with respect to the total weight of monomers (wbm %). Conversion of the final latexes was measured gravimetrically. Dynamic Light Scatter- ing (DLS, Zetasizer Nano ZS, Malvern Instruments, Malvern, UK) was used to measure the z-average diameter of the miniemulsion droplets and final polymer particles. The morphology of the final latex particles and of the cryosectioned films was analyzed by Transmission Electron Microscopy (TEM) in a TECNAI G2 20 TWIN (FEI) operating at an Polymers 2021, 13, 848 4 of 13 accelerating voltage of 200 kV in a bright field image mode. Polymer particles and films were observed without any staining. 2.3. Film Formation and Properties For water uptake experiments, the films were formed by casting the latexes (around 1.5 g) onto round silicone molds and drying them at 24 ± 2 ◦ C and 50 ± 5% relative humidity during 6 days until a constant weight was achieved. These films were carefully peeled from the silicone mold and immersed in distilled water during fourteen days. Films were withdrawn from the water container at 24 h intervals, they were smoothly dried and quickly weighed. For the water static contact angle and EIS measurements, 90 µm wet thick films were cast onto steel plates. Steel substrates were degreased with UniClean 251 solution at 70 ◦ C in a shaking bath for 5 min, followed by 1 min pickling in HCl solution (1:1). Then, the waterborne latexes were uniformly applied on the steel substrates using a quadruple film applicator (Khushbooscientific). Films were dried at a relative humidity of 60% and a temperature of 23 ◦ C for 24 h using a humidity chamber (ESPEC SH-641 benchtop type). The contact angle of the film–air interface was measured in a Contact Angle System OCA (Dataphysics, Filderstadt, Germany) equipment, taking an average value from 20 measurements. A potentiostat (brand BIO-LOGIC, model VMP3, Seyssinet-Pariset, France) was used to evaluate the corrosion behaviour of the systems by electrochemical measurements: open circuit potential (OCP) and EIS. The following three electrodes configuration was used: Ag/AgCl saturated with KCl as reference electrode, platinum mesh as a counter electrode and coated steel specimens as working electrode. OCP and EIS tests were conducted in 3.5 wt% NaCl solution at room temperature at least by triplicate, using an area of 1 cm2. Although OCP was monitored continuously with time, it was interrupted to carry out EIS measurements (once per hour). The frequency range was from 100 kHz to 10 mHz, obtaining 10 points per decade. Frequency scans were carried out by applying ± 10 mV sinusoidal wave perturbation versus OCP. 3. Results and Discussion 3.1. Latex and Film Characterization In this work, high biobased content latexes were produced using 2-OA and IBOMA as monomers. 2-OA is a soft monomer whereas IBOMA is a hard one; their homopolymers have a Tg of −44 ◦ C and 150 ◦ C, respectively. For coating formulations, the Tg of the used polymer should be below the application temperature (normally room temperature) in order to form a coherent film, but at the same time it should be high enough to produce good mechanical properties and avoid problems such as dirt pick up and blocking. The Tg of the polymers used for coatings is usually around 10–15 ◦ C. Therefore, the 2-OA/IBOMA ratio used in this work was 58.5/41.5 wt% in order to obtain a copolymer with a Tg of 10 ◦ C. Badia et al. synthesized partially biodegradable waterborne coatings with a biocontent ranging from 30 to 65% using Ecomer® , an allyl polyglucoside maleic acid ester functional monomer, in combination with 2-OA, IBOMA and butyl acrylate (because Ecomer® is commercialized as a solution in butyl acrylate). To the best of our knowledge, this is the first time that 2-OA and IBOMA are used as sole monomers in a coating formulation. Table 2 shows the characterization of the synthesized latexes. There was no significant difference between the average size of the initial miniemulsion droplet and the final latex polymer particle, indicating that there was no secondary nucleation nor coagulation during the polymerization. Total conversion was achieved in both cases. Whereas no important coagulation was observed in the sample with no CeO2 , less than 5% coagulum was obtained in the sample with 1 wbm % of CeO2 , which can be attributed to a certain incompatibility between the CeO2 nanoparticles and the Sipomer PAM200, as observed previously with ZnO by Chimenti et al..  during the polymerization. Total conversion was achieved in both cases. Whereas no im- portant coagulation was observed in the sample with no CeO2, less than 5% coagulum Polymers 2021, 13, 848 5 of 13 was obtained in the sample with 1 wbm % of CeO2, which can be attributed to a certain incompatibility between the CeO2 nanoparticles and the Sipomer PAM200, as observed previously with ZnO by Chimenti et al.. Table 2. Characterization of synthesized latexes. Both latexes have a pH value of 7. Table 2. Characterization of synthesized latexes. Both latexes have a pH value of 7. CeO Droplet Diameter Particle Diameter Latex CeO22 Droplet Diameter Particle Diameter Latex (wbm %) (nm) (nm) (wbm %) (nm) (nm) Bioacrylic - 204 ± 5 200 ± 2 Bioacrylic CeO2 -Bioacrylic -1 204 ±5 218 ± 5 200 ±2 230 ± 2 CeO2-Bioacrylic 1 218 ± 5 230 ± 2 Figure 11 shows Figure shows the the TEM TEM micrographs micrographs of of the the water water dispersed dispersed polymer polymer particles particlescon- con- taining 1 wbm % of CeO 2 nanoparticles (a) and the cryosections of the film produced taining 1 wbm % of CeO2 nanoparticles (a) and the cryosections of the film produced by by casting such latex (b). casting such latex (b). (a) (b) Figure 1. TEM micrographs of the hybrid CeO2-Bioacrylic latex polymer particles (a) and hybrid CeO2-Bioacrylic film (b). Figure 1. TEM micrographs of the hybrid CeO2 -Bioacrylic latex polymer particles (a) and hybrid CeO2 -Bioacrylic film (b). Figure 1a,b show that the individual CeO2 nanoparticles did not aggregate during the polymerization but migrated to the surface of the polymer particles as in Pickering stabilized latexes. The lack of aggregation of the individual CeO2 nanoparticles was also proved by XRD of the hybrid CeO2 -Bioacrylic films, by which an average CeO2 nanoparticle size of 6.8 nm, close to the original CeO2 size , was obtained by the use of the Scherrer equation (see Figure S1 in Supplementary Material). This is surprising because the CeO2 the polymerization but migrated to the surface of the polymer particles as in Pickering stabilized latexes. The lack of aggregation of the individual CeO2 nanoparticles was also proved by XRD of the hybrid CeO2-Bioacrylic films, by which an average CeO2 nanopar- ticle size2021, Polymers of13,6.8 848 nm, close to the original CeO2 size , was obtained by the use of the 6 of 13 Scherrer equation (see Figure S1 in Supplementary Material). This is surprising because the CeO2 nanoparticles do disperse well in the monomer mixture (see Figure 2a with the transparent dispersion of CeO nanoparticles do disperse well in the monomer mixture (see Figure 2a with the transparent 2 in 2-OA/IBOMA) as they do in a mixture of MMA/BA/AA dispersion of CeO2 in 2-OA/IBOMA) as they do in a mixture of MMA/BA/AA. Note. Note that if acrylic acid was used with 2-OA and IBOMA, the CeO2 nanoparticles that if acrylic acid was used with 2-OA and IBOMA, the CeO2 nanoparticles agglomerated agglomerated (Figure 2b).(Figure 2b). (a) (b) Figure 2. 1 wbm% Figure 2. 1 wbm% of CeO2 nanoparticles of CeO2 nanoparticles dispersed dispersed in in 2-OA/IBOMA (a)2-OA/IBOMA (a) and in 2-OA/IBOMA/AA (b). and in 2-OA/IBOMA/AA (b). The morphology of the hybrid polymer particles and the film shown in Figure 1 were not expected because for a similar formulation with oil-based monomers (e.g., The morphology of the hybrid polymer MMA/BA/AA), and theparticles same CeOand the film (although 2 nanoparticles shown in Figure with the use1of were a conventional anionic surfactant, Dowfax 2A1), a single not expected because for a similar formulation with oil-based monomers (e.g., nanoparticle aggregate per polymer particle was found at the edge of the polymer particles [45–48]. A detailed monitoring of the evolution MMA/BA/AA), and the same CeO2 nanoparticles (although with the use of a conventional of the morphology by cryo-TEM demonstrated that the CeO2 nanoparticles were initially anionic surfactant, Dowfaxwell 2A1), a single dispersed nanoparticle inside the MMA/BA/AA aggregate per droplets, polymer but particleproceeded, as polymerization was they found at the edge of the polymer particles [45–48]. A detailed monitoring of the evolution aggregated due to the incompatibility between the formed copolymer and the modified of the morphology by cryo-TEM CeO2 nanoparticle demonstrated surface, that leadingthetoCeO the formation of a single larger CeO2 nanoparticle 2 nanoparticles were initially aggregate. well dispersed inside the MMA/BA/AA Therefore, thedroplets, interaction ofbuttheas polymerization phosphate groups of theproceeded, Sipomer PAM200 theyand the sur- aggregated due to the incompatibility between face of the organically modifiedtheCeOformed copolymer 2 nanoparticles (whichand was notthedisclosed modified by ALTANA), CeO2 nanoparticle surface, leading to the formation of a single larger CeO2 nanoparticle interface made the CeO 2 nanoparticles to migrate to the monomer droplet/aqueous phase (polymer particle/aqueous phase after polymerization). This surfactant–nanoparticle in- aggregate. teraction could be the reason for the decreased stabilization capability of the surfactant, Therefore, the interaction causing ofthe the phosphate formation of somegroups coagulumof the Sipomer in this PAM200 polymerization. and the Anyway, good quality surface of the organically modified CeO2 nanoparticles (which was not disclosed clear films were obtained after casting Bioacrylic and CeO 2 -Bioacrylic by AL-in silicone latexes TANA), made Polymers 2021, 13, x molds, even if the the CeO2 nanoparticles hybrid onetohad to migrate thea monomer slight yellowish color due to the presence droplet/aqueous phase7 of 14of CeO2 nanoparticles (Figure 3). interface (polymer particle/aqueous phase after polymerization). This surfactant–nano- particle interaction could be the reason for the decreased stabilization capability of the surfactant, causing the formation of some(a) coagulum in this polymerization. (b) Anyway, good quality clear films were obtained after casting Bioacrylic and CeO2-Bioacrylic latexes in silicone molds, even if the hybrid one had a slight yellowish color due to the presence of CeO2 nanoparticles (Figure 3). Figure 3. Free Figure filmsfilms 3. Free formed at 24 at formed ± 224 2 ◦C °C±and 50and ± 5%50relative humidity ± 5% relative from (a) from humidity Bioacrylic latex andlatex and (a) Bioacrylic (b) hybrid CeO 2-Bioacrylic latex. (b) hybrid CeO2 -Bioacrylic latex. Regarding the water sensitivity of the films, Figure 4 shows the results of the water uptake experiments. The final values as well as the water contact angle measurements are shown in Table 3. Polymers 2021, 13, 848 7 of 13 Figure 3. Free films formed at 24 ± 2 °C and 50 ± 5% relative humidity from (a) Bioacrylic latex and (b) hybrid CeO2-Bioacrylic latex. Regardingthe Regarding thewater watersensitivity sensitivityofofthe thefilms, films,Figure Figure44shows showsthe theresults resultsof ofthe thewater water uptakeexperiments. uptake experiments.The Thefinal finalvalues valuesasaswell wellasasthe thewater watercontact contactangle anglemeasurements measurementsare are shown in Table shown in Table 3.3. Wateruptake Figure4.4.Water Figure uptakeexperiment experimentresults resultsof ofneat neatpolymer polymerfilm filmand andthe thehybrid hybridone. one. Watersensitivity Table3.3.Water Table sensitivityof ofthe theneat neatpolymer polymerfilm film and and the the hybrid hybrid one. one. Film Contact Angle ◦ Film Contact Angle to to Water(°) Water ( ) Water Water Uptake Uptake after1414Days after Days (%) (%) Bioacrylic Bioacrylic ± 3± 3 92 92 7.07.0± ± 0.30.3 CeO CeO 2 -Bioacrylic 2-Bioacrylic ± 1± 1 92 92 8.38.3± ± 0.20.2 Hydrophobic Hydrophobicfilmsfilmswere obtained, were providing obtained, providing lowlow water uptake water values uptake and high values and con- high tact angleangle contact compared comparedto values reported to values in the reported in literature for acrylic the literature latexes for acrylic stabilized latexes by stabilized polymerizable surfactant by polymerizable (18% surfactant of water (18% uptake of water uptake in in 1414days daysand and75°75◦contact contactangle anglefor foran an MMA/BAacrylic MMA/BA acrylicbinder binder[7,10]). [7,10]).The Thehydrophobicity hydrophobicityofofthesethesefilms filmsisisdue duetotothe theuse useof of polymericsurfactants polymeric surfactantsandandthe thechemical chemicalnature natureofofthe thecopolymer. copolymer. The Theaddition addition of ofthe theCeO CeO22 nanoparticles(1(1wbm%) nanoparticles wbm%)did didnot nothave haveaasignificant significantdetrimental detrimentaleffect effecton onthe thehydrophobic hydrophobic propertiesof properties ofthe thefilms. films. 3.2.Anticorrosion 3.2. AnticorrosionProperties Properties In order to study the effect of the CeO2 nanoparticles, intact films (both neat and hybrid) with similar thickness (30–40 µm) were evaluated by EIS. Figure 5 shows the impedance diagrams after different exposure time (1 h and after 41 days, respectively). A capacitive behavior can be observed for both films at the beginning of the exposure (1 h), as an indication of the good barrier capabilities. Indeed, a single time constant is observed for both films, having an impedance modulus (|Z|) at low frequency (0.01 Hz) in the range 109 –1010 Ωcm2 (|Z|0.01Hz ). It indicates an excellent barrier protection compared to acrylic waterborne coating without and with a topcoat or to epoxy coatings formulated with nano-CeO2 during exposure to NaCl electrolytes [41,51], where coatings were showing a much lower impedance value at shorter exposure time. However, this excellent barrier protection was only maintained after 41 days of exposure for the hybrid film. In fact, after that time, the Bioacrylic film decreased its impedance |Z|0.01Hz in more than one order of magnitude, reaching a value around 108 Ωcm2. The better performance of the hybrid coating can be justified by the physical blocking effect to the electrolyte diffusion thanks to the CeO2 nanoparticles [31,52], taking into account their stability in neutral to basic aqueous environemnts. Therefore, the long-term durability shown by the hybrid film can be attributed to the inhibition effect of CeO2 nanoparticles located in the surface of the polymer particles.  of magnitude, reaching a value around 108 Ωcm2. The better performance of the hybrid coating can be justified by the physical blocking effect to the electrolyte diffusion thanks to the CeO2 nanoparticles [31,52], taking into account their stability in neutral to basic aqueous environemnts. Therefore, the long-term durability shown by the hybrid film can be attributed to the inhibition effect of CeO2 nanoparticles located in the surface of the Polymers 2021, 13, 848 polymer particles. 8 of 13 11 10 9 8 log Z (Ω·cm2) 7 Bare steel (1 h) 6 Bioacrylic (1 h) 5 CeO2-Bioacrylic (1 h) Bioacrylic (41 days) 4 CeO2-Bioacrylic (41 days) 3 2 1 -2 -1 0 1 2 3 4 5 log Freq (Hz) Figure 5. Bode plot of intact binders after 1 h and 41 days of exposure to 3.5 wt% NaCl solution. Figure 5. Bode plot of intact binders after 1 h and 41 days of exposure to 3.5 wt% NaCl solution. In order to confirm such hypothesis, an artificial defect has been created by laser In order to confirm such hypothesis, an artificial defect has been created by laser on on both films. The aim was to reach the metal/film interface in order to explore the both films. The aim was to reach the metal/film interface in order to explore the inhibition inhibition capabilities of CeO2 capabilities nanoparticlesof CeO. 2 nanoparticles Just immediately after. Just immediately provoking after provoking the the defect, films defect,tofilms were exposed were 3.5 wt% exposed NaCl to 3.5 In electrolyte. wt% NaCl to addition electrolyte. In additionper the EIS measurement to the hour,EIS measurement OCP was per hour, OCP monitored was with timemonitored (Figure 6).with time(after Initially (Figure 10 h),6).a Initially potential(after value 10 h), a potential value around −0.50/−0.55 V was−obtained around 0.50/−0.55 V was for both obtained systems. for bothvalue This potential systems. Thisofpotential is typical unprotected value is typical of unprotected low carbon steel , low carbon and it steel confirms , that theand it confirms artificial defect wasthatable the to artificial reach thedefect inter-was able to reach face. A monotonous the interface.increase of the potential A monotonous increasewasofobserved for thewas the potential Bioacrylic observed filmfor along the Bioacrylic film the entire period along theofentire exposure (except period of for a random exposure increase (except forupa to −0.36V at random 65 h), reaching increase up to −0.36V at 65 h), a value of −0.43 V after reaching 100 h. a value of In contrast, −0.43 the CeO V after -Bioacrylic 100 2h. film showed In contrast, the CeOan exponential 2 -Bioacrylic film showed an increaseexponential of the potential from −0.52/−0.51 V (11/12 h) up to −0.02 increase of the potential from −0.52/−0.51 V (11/12 V (94 h), which is in h) up to −0.02 V (94 h), which is in agreement with the potential trend observed for a waterborne acrylic coating doped with CeO2 nanoparticles acting as corrosion inhibitor. The quasi steady-state behavior (from 25 to 94 h) around −0.1/0.02V was also observed on steel protected with metallic coatings (nickel nanocomposite) having CeO2 nanoparticles grafted with ferrocene, where the formation of a passive layer provided less negative (i.e., anodic) OCP values over a large period of time (30 days). Finally, an abrupt drop of the potential suddenly occurred, reaching a similar potential value to the one obtained at beginning of the test (−0.55 V) after 100 h. Apparently, the completely different behavior shown by both films, in terms of OCP, might be attributed to the role of CeO2 as a corrosion inhibitor. Therefore, a tailored analysis of the EIS diagrams was done for the different times of interest (i.e., before and after the variation of the potential values in Figure 6). Figure 7 shows the EIS diagrams for the CeO2 -Bioacrylic film with an artificial defect after 1, 11, 12, 13, 94 and 95 h of exposure, respectively. If the impedance modulus is compared at low frequency (|Z|0.01Hz ) for each time, it can be observed that |Z|0.01Hz was 6 × 105 Ωcm2 after 1 h of exposure. It was decreasing slightly to around 105 Ωcm2 after 11 h, indicating that the corrosion process was taking place up to then. However, this trend was drastically changed when a sudden increase of |Z|0.01Hz occurred at 12 h—impedance reaching 106 Ωcm2 and 6 × 106 Ωcm2 at 12 and 13 h, respectively. A steady state value was maintained above 5 × 106 Ωcm2 from 13 h until 94 h of exposure, thanks to the corrosion inhibition capabilities of CeO2 nanoparticles. This is in agreement with the delay of the corrosion onset shown on coatings formulated with CeO2 nanoparticles having an artificial defect: the corrosion activity is limited to the vicinity of the defective area according to the results obtained by local- ized electrochemical techniques. Finally, the impedance dropped down again to the  agreement with the potential trend observed for a waterborne acrylic coating doped with CeO2 nanoparticles acting as corrosion inhibitor. The quasi steady-state behavior (from 25 to 94 h) around −0.1/0.02V was also observed on steel protected with metallic coatings (nickel nanocomposite) having CeO2 nanoparticles grafted with ferrocene, where Polymers 2021, 13, 848 the formation of a passive layer provided less negative (i.e., anodic) OCP values over a 9 of 13 large period of time (30 days). Finally, an abrupt drop of the potential suddenly oc- curred, reaching a similar potential value to the one obtained at beginning of the test (−0.55 V) after 100 h. minimum value (105 Ωcm2 ), indicating that most probably the corrosion inhibition was exhausted and the metal/film interface on the artificial defect was not protected anymore. Polymers 2021, 13, x 10 of 14 Figure 6. Open circuit potential of both Bioacrylic binders with an artificial defect after exposure to Figure 3.5 6. Open circuitsolution. wt% NaCl potential of both Bioacrylic binders with an artificial defect after exposure to 3.5 wt% NaCl solution. 7 Apparently, the completely different behavior shown by both films, in terms of OCP, might be attributed to the role of CeO2 as a corrosion inhibitor. Therefore, a tailored anal- 6 ysis of the EIS diagrams was done for the different times of interest (i.e., before and after the variation of the potential values in Figure 6). Figure 7 shows the EIS diagrams for the 5 CeO2-Bioacrylic film with an artificial defect after 1, 11, 12, 13, 94 and 95 h of exposure, log Z (Ω·cm2) respectively. If the impedance modulus is compared at low frequency (|Z|0.01Hz) for each time, it can be observed that |Z|0.01Hz was 6 × 105 Ωcm2 after 1 h of exposure. It was de- 4 creasing slightly to around 105 Ωcm2 after 11 h, indicating that the corrosion process was taking place up toCeO 2-Bioacrylic+Defect_1 h then. However, this trend was drastically changed when a sudden in- 3 crease of |Z|0.01Hz CeO occurred at 12 h—impedance reaching 106 Ωcm2 and 6 × 106 Ωcm2 at 12 2-Bioacrylic+Defect_11 h and 13 h, respectively. A steady state valuehwas maintained above 5 × 106 Ωcm2 from 13 h CeO2-Bioacrylic+Defect_12 until 94 h2of exposure, thanks to the corrosion CeO2-Bioacrylic+Defect_13 h inhibition capabilities of CeO2 nanoparti- cles. This is in agreement with the delay CeO2-Bioacrylic+Defect_94 h of the corrosion onset shown on coatings formu- lated with CeO2 nanoparticles having an CeO2-Bioacrylic+Defect_95 h artificial defect: the corrosion activity is limited 1 to the vicinity of the defective area according to the results obtained by localized electro- -2 -1 0 1 2 3 4 5 chemical techniques. Finally, the impedance dropped down again to the minimum value (105 Ωcm2), indicating that most log Freq (Hz) the corrosion inhibition was exhausted probably and the metal/film Figure 7. Bodeinterface on the plots of CeO artificial defect 2 —Bioacrylic binderwas withnot protected an artificial anymore. defect after different immersion times in Figure 7. 3.5 Bodewt% NaCl plots solution. of CeO 2—Bioacrylic binder with an artificial defect after different immersion times in 3.5 wt% NaCl solution. On the other hand, Figure 8 shows the EIS diagrams for the Bioacrylic film with On the other an artificial defecthand, Figure after 1, 11,8 shows 57, 65 the andEIS95diagrams for the Bioacrylic h of exposure film with to salty water, an respectively. artificial Initially,defect aftervalue a similar 1, 11, 57, of 65 theand 95 h of exposure impedance modulus to salty waswater, respectively. obtained Ωcm2 ) at low (3 × 105Initially, afrequency similar value of the impedance modulus was obtained (3·× 1055 Ωcm22) at low frequency (|Z| 0.01Hz ) compared to the hybrid film (6 × 10 Ωcm ), which also decreased (|Z| 0.01Hz) compared 5to the hybrid to values below 10 Ωcm2 after film 11 h(6·× 105 Ωcm2), In of exposure. which also decreased contrast to the CeO to values be- 2 -Bioacrylic film, low 10 5 Ωcm 2 after 11 h of exposure. In contrast to the CeO 2-Bioacrylic film, the impedance the impedance value slightly varied until a minimum was reached at 57 h (7 × 104 Ωcm2 ) value slightly varied until a minimum was reached at 57 h (7·× 104 Ωcm2) due to the ab- due to the absence of any corrosion inhibition into the film. The slight |Z|0.01Hz increase sence of any corrosion inhibition into the film. The slight |Z|0.01Hz increase (3·× 105 Ωcm2) (3 × 105 Ωcm2 ) observed after 65 h may be related to the presence of corrosion products observed after 65 h may be related to the presence of corrosion products that are blocking that are blocking the pinhole/damage that was created with the artificial defect. It can the pinhole/damage that was created with the artificial defect. It can be observed that 104 /3test, and Ωcm ) be observed that |Z|0.01Hz remains in a10very narrow × entire × 10 5 2 |Z| 0.01Hz remains in a very narrow range (7·× 4/3 × 10 5 Ωcm2range ) during(7the during it confirmsthethat entire test, and itcan no protection confirms thatinnothe be achieved protection absence ofcan CeObe achieved in the absence 2 nanoparticles in the film formulation. Indeed, a |Z|0.01Hz value of 9·× 104 Ωcm2 was obtained after 95 h of expo- sure, indicating the absence of protection in the interface. Therefore, these results confirm that homogeneously distributed CeO2 nanoparticles are required into the film to provide an efficient corrosion protection of the metal/film interface. Polymers 2021, 13, 848 10 of 13 Polymers 2021, 13, x of CeO2 nanoparticles in the film formulation. Indeed, a |Z|0.01Hz value of 9 ×11 10of4 14 Ωcm2 was obtained after 95 h of exposure, indicating the absence of protection in the interface. Therefore, these results confirm that homogeneously distributed CeO2 nanoparticles are required into the film to provide an efficient corrosion protection of the metal/film interface. 7 Bioacrylic+Defect_1 h Bioacrylic+Defect_11 h 6 Bioacrylic+Defect_57 h Bioacrylic+Defect_65 h Bioacrylic+Defect_95 h 5 log Z (Ω·cm2) 4 3 2 1 -2 -1 0 1 2 3 4 5 log Freq (Hz) Figure 8. Bode plots of Bioacrylic binder with an artificial defect after different immersion time in 3.5 wt% Figure 8. NaCl solution. Bode plots of Bioacrylic binder with an artificial defect after different immersion time in 3.5 wt% NaCl solution. 4. Conclusions Novel waterborne hybrid CeO2 biobased acrylic binders were synthesized by miniemul- 4. Conclusions sion Novel polymerization. waterborne CeO hybrid2 nanoparticles CeO2 biobasedinteracted with were acrylic binders the phosphate synthesizedmoieties by minie- of the surfactant and migrated to the interphase leading to polymer particles mulsion polymerization. CeO2 nanoparticles interacted with the phosphate moieties of the with Pickering morphology. surfactant and Thus, migratedCeOto 2 nanoparticles the interphasedid not aggregate leading to polymerand were well particles with distributed Pickering in morphology. Thus, CeO2 nanoparticles did not aggregate and were well distributed inwith the surface of the polymer particles. The biobased acrylic copolymer, together the the surface of the polymer particles. The biobased acrylic copolymer, together with the phos- low phosphate surfmer used in the synthesis of the polymer particles, produced films with water surfmer phate uptake andusedhigh contact in the angle synthesis of to thewater. polymerEIS particles, results also showedfilms produced enhanced with lowbarrier properties water of and uptake bothhigh films, independently contact of theEIS angle to water. presence of CeO results also 2 nanoparticles. showed However, enhanced barrier the long-term properties durability of both of the intact hybrid films, independently of the film was higher presence of CeOthan the neat one. 2 nanoparticles. This can be However, attributed the long-termto the corrosion durability inhibition of the capabilities intact hybrid film wasof higher CeO2 nanoparticles, than the neat one. according This canto the results be obtained attributed when to the both types corrosion of films inhibition had an artificial capabilities defect. EIS diagrams of CeO2 nanoparticles, accordingshowed to the results obtained an increase when both types of the impedance of films modulus in had one an artificial order defect. EIS(|Z| of magnitude diagrams 0.01Hzshowed went from an 105 to 6of×the 6 ×increase 6 Ωcm2 after 10impedance modulus in one to 13 h) thanks order keyofrole magnitude of CeO2(|Z| 0.01Hz went from nanoparticles. 6·× the In fact, 10 5 to 6·× 106 Ωcm2 after 13 h) thanks to key role of CeO2 nanoparticles. In fact, the neat neat film did not show any protection of the interface in the presence of an artificial defect. film did not show any protection of the interface in the presence of an artificial defect. Supplementary Materials: The following are available online at https://www.mdpi.com/2073-436 Supplementary 0/13/6/848/s1,Materials: Figure S1:The XRDfollowing patternsare ofavailable online atand neat Byoacrylic www.mdpi.com/xxx/s1, Figure hybrid CeO2 -Bioacrylic S1: film. XRD patterns of neat Byoacrylic and hybrid CeO2-Bioacrylic film. Author Contributions: Conceptualization, J.R.L. and M.P.; methodology, E.G.; investigation, R.S., Author E.G. and Contributions: Conceptualization, J.M.V.; data curation, J.R.L. E.G., J.R.L., andand M.P. M.P.; methodology, J.M.V.; E.G.; investigation, writing—original R.S., draft preparation, E.G. and J.M.V.; data curation, E.G., J.R.L., M.P. and J.M.V.; writing—original draft preparation, E.G., M.P. and J.M.V.; writing—review and editing, E.G., J.R.L., M.P. and J.M.V.; supervision, E.G.-L., E.G., M.P. and J.M.V.; writing—review and editing, E.G., J.R.L., M.P. and J.M.V.; supervision, E.G.- H.-J.G.; funding acquisition, J.R.L. and M.P. All authors have read and agreed to the published L., H.-J.G.; funding acquisition, J.R.L. and M.P. All authors have read and agreed to the published version of the manuscript. version of the manuscript. Thisresearch Funding: This Funding: researchwas wasfunded fundedbyby thethe Spanish Spanish Government, Government, grant grant numbers numbers MINECO MINECO CTQ-CTQ- 2017-87841-R and CER-20191003, and by the Basque Government “Grupos Consolidados 2017-87841-R and CER-20191003, and by the Basque Government “Grupos Consolidados del del Sistema Universitario Sistema Vasco”,Vasco”, Universitario grant number IT999-16. grant number IT999-16. Acknowledgments:The Acknowledgments: The authors authors would would likelike also also to express to express thanksthanks for microscopy for microscopy analysisanalysis and hu- and human man support support provided provided by SGIker by SGIker of UPV/EHU of UPV/EHU and European and European funding funding (ERDF (ERDF and ESF). and ESF). Polymers 2021, 13, 848 11 of 13 Conflicts of Interest: The authors declare no conflict of interest. References 1. Bowman, E. 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