Edited by: Flavio Deflorian, University of Trento, Italy
Reviewed by: Rita Bacelar Figueira, University of Minho, Portugal; Fatima Montemor, Instituto Superior Técnico, Portugal
This article was submitted to Environmental Materials, a section of the journal Frontiers in Materials
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This study focuses on the design, development, and validation of two coating systems for corrosion protection of hot dip galvanized steel substrates. The coatings consist of epoxy-based resin reinforced with core-shell microcapsules, either cerium oxide or cuprous oxide core and a polymeric shell doped with cerium ions. The effect of the modification of the epoxy resin with a liquid rubber polymer has also been studied. Corrosion studies via electrochemical impedance spectroscopy (EIS) revealed that the coatings have enhanced barrier properties. Moreover, EIS studies on coatings with artificial scribes, demonstrated an autonomous response to damage and a self-healing effect. Heat-induced material re-flow has also been observed after exposure to temperature higher than the Tg of the system, which offered an additional self-healing mechanism, partially inhibiting the underlying corrosion processes when the liquid rubber is present in the system.
Corrosion phenomena on metallic structures result eventually in the degradation of the metal and the deterioration of its properties. Failure of a metallic operating structure compromises the safety, which is a critical consideration during an equipment design. The degradation of engineering structures such as bridges, automobiles, airplanes, and ships due to their exposure in corrosive environments may contribute to life-threatening situations. Moreover, direct and indirect economic losses are linked to corrosion phenomena such as maintenance or replacement costs, expenses related to industry temporary shutdown, efficiency, and product losses. According to the literature, about 25–30% of this total could be avoided if currently available corrosion technology was effectively applied (Uhlig,
Eventually, all coatings develop defects which are responsible for the direct access of corrosive agents to the metallic substrate. Chromate-based conversion coatings are the most popular all over the world as a pretreatment strategy for a variety of substrates because they offer re-passivation of the corroded area, so hexavalent chromium remains the most effective corrosion preventive compound to date (Gharbi et al.,
Thus, along with the barrier properties, a corrosion protection mechanism responsive to damage has become a promising strategy in order to deliver a coating system with long-term corrosion protection effect. A more sophisticated system dictates, ideally, the occurrence of multiple self-healing events in an autonomous way based on the intrinsic characteristics of the coating system (Yin et al.,
However, thermoset epoxy-based networks, which are one of the most widely used in coating applications, have many undesirable features, i.e., they have poor resistance to impact and crack growth, which limits their application to certain technological areas. The fracture energy of an epoxy resin is two and three orders of magnitude smaller than thermoplastic polymers and metals. This suggests the need to strengthen these systems in order to expand their applications (Comstock et al.,
Hence, the modification of epoxy resins to enhance their brittleness has received considerable research interest. Many research efforts have been reported since 1970, where the first resin modifications with secondary elastomeric phases was initiated by McGarry (
In addition, another limitation arises from the direct addition of inorganic capsules in self-healing epoxy coatings which is their limited compatibility of the inert inorganic surface with the organic coating. The coating's protective performance and adhesion properties are severely affected due to particle agglomeration and defect formation at the interface of these incompatible materials, which are also a function of the concentration and size of the containers and the coating's thickness, as reported by recent research and review articles (Kartsonakis et al.,
The motivation for this work is to develop a new multifunctional smart coating system, with self-healing characteristics which contribute to its protective anticorrosion properties. Thus, the main aim of this study is to evaluate the self-healing responsiveness and corresponding barrier properties of two developed advanced coating systems. Both samples consist of a hybrid organic-inorganic epoxy coating toughened by the addition of an organically modified silicate. Core-shell modified cerium oxide (CeO2) and cuprous oxide (Cu2O) microcapsules were incorporated into the epoxy-based matrices. In addition, in one of the studied systems, the incorporation of a liquid rubber modifier in free form was selected to investigate its influence on the electrochemical response of the obtained coating. The hybrid organic-inorganic coatings were applied onto hot dip galvanized (HDG) steel panels.
The core-shell microcapsules were characterized with respect to their morphology and chemical composition through transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), gel permeation chromatography (GPC), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The protective and self-healing ability of intact and scribed coatings as well as their responsiveness and their ability to restore their anticorrosion properties after thermal treatment were evaluated through electrochemical impedance spectroscopy (EIS) while immersed in selected corrosive electrolytes.
Acetonitrile (Acros Organics), toluene (Acros Organics), acetone (Acros Organics), absolute ethanol (Acros Organics), methanol (MeOH, Acros Organics), absolute ethanol (Sigma Aldrich), N,N-dimethylformamide (DMF, Sigma Aldrich), copper acetate (Sigma Aldrich), hydrazine hydrate (N2H4, 50–60%, Sigma Aldrich), cerium acetyl acetonate [Ce(acac)3, Sigma Aldrich], potassium peroxodisulfate (KPS, Sigma Aldrich), α-bromoisobutyryl bromide (BIBB, Sigma Aldrich), (3-aminopropyl)triethoxy silane (APTES, Sigma Aldrich), triethylamine (Sigma Aldrich), copper (II) bromide [Cu(II)Br, Sigma Aldrich], cerium nitrate [Ce(NO3)3, Sigma Aldrich], ammonium cerium(IV) nitrate [Ce(NH4)2(NO3)6), Sigma Aldrich], trifluoroacetic acid (CF3COOH, Merck, >99%), dichloromethane (Sigma Aldrich, 99.8%), 2,2′-Bipyridyl (bpy, Sigma Aldrich), diethylenetriamine (Sigma Aldrich), ascorbic acid (Sigma Aldrich), epoxy resin based on phenol 4,4'-(1-methylethylidene) bis- (Ciba-Geigy), sodium hydroxide (Sigma Aldrich), and sodium chloride (Sigma Aldrich) were used as received. The monomers methacrylic acid (MAA) and n-butyl acrylate (nBA) and tert-butyl acrylate (tBA) were double distilled under reduced pressure prior to use.
The synthesis of Cu2O cores was performed by a simple one-pot wet chemical route using copper acetate as the precursor and hydrazine as the reducing agent. The experiment was conducted at room temperature by adding hydrazine hydrate (0.03 mol) in a copper acetate solution (0.042 M). The reaction was left to proceed under vigorous stirring for 10 min, while the solution's color changed from blue to green and finally to bright orange. The products were left to precipitate, the supernatant solution was discarded and the sediments were repeatedly washed though multiple centrifugations. The Cu2O cores were left to dry in a vacuum designator.
For the production of CeO2 cores a two-step process was followed as reported elsewhere (Kartsonakis et al.,
Inorganic-organic core-shell microcapsules were synthesized through surface-initiated activators regenerated by electron transfer atom transfer radical polymerization (ARGET-ATRP) process. The inorganic core materials, CeO2 or Cu2O, were selected due to their inherent corrosion protective and antifouling characteristics, respectively. A diblock copolymer, poly(n-butyl acrylate-b-acrylic acid) [
The synthesis of the diblock copolymer onto the surface of either CeO2 or Cu2O was performed according to the following procedure (schematic representation in
Either CeO2 or Cu2O cores were dispersed in absolute ethanol using ultra sonication in a three-neck flask equipped with a condenser and a magnetic stirrer. After heating to 70°C, APTES was added under N2 atmosphere and the dispersion was left for 24 h under stirring. The APTES-modified inorganic cores were centrifuged, washed with ethanol and redispersed in DMF. Then BIBB was added drop-wise in the flask using triethylamine as the catalyst and left in an ice bath for 1 h under stirring in N2 atmosphere and then at room temperature for 24 h. The macroinitiators (initiator-functionalized inorganic particles) were isolated and rinsed through centrifugation. A septum-sealed Schenk flask was used during surface initiated ARGET-ATRP. For each experiment, CeO2 or Cu2O cores were dispersed in toluene followed by the complete dissolution of the Cu(II)Br/bpy complex and the addition of the deoxygenated monomer. The nBA was used as monomer and ascorbic acid served as reducing agent for initiating the polymerization process. Then, t-BA was added to the above mixture resulting in a diblock polymer shell comprising of PnBA and PtBA. The AA block resulted from an acidolysis reaction, as reported elsewhere (Colombani et al.,
The conditions used in the preparation of core/ shell microcapsules.
CeO2 or Cu2O cores | 1.0 (g) |
(3-aminopropyl)triethoxy silane | 1.0 (ml) |
Absolute ethanol | 20.0 (ml) |
N,N-dimethylformamide | 20.0 (ml) |
Triethylamine | 1.0 (ml) |
α-bromoisobutyryl bromide | 2.0 (ml) |
Toluene | 20.0 (ml) |
Copper (II) bromide | 0.1 (g) |
2,2′-Bipyridyl | 1.0 (g) |
N-butyl acrylate | 10.0 (ml) |
Ascorbic acid | 1.0 (g) |
The HDG steel substrates pre-treatment prior to coating application included degreasing and cleaning using acetone and NaOH solution at pH 11 for 5 min at 50°C, thoroughly washing with distilled water and their storage in a vacuum designator. Two coating systems were prepared; named
Schematic representation of the produced two-layer coating systems.
The conditions used in the preparation of the epoxy solution.
(3-aminopropyl)triethoxy silane | 2.0 |
2,2-Bis[4-(glycidyl oxy)phenyl]propane | 25.0 |
Diethylenetriamine | 2.0 |
PnBA | 5.0 |
Absolute ethanol | 34.0 |
Acetone | 30.0 |
2.0 |
Transmission electron microscope (TEM; JEM2000 FX, 200 KV, resolution 0.28 nm) and Fourier Transform Infrared Spectroscopy (FT-IR) were utilized for the chemical composition determination of the hybrid composites through Attenuated Total Reflectance (ATR) method with Agilent Cary 630 spectrometer.
The thermal degradation of the synthesized microcapsule materials was analyzed via thermogravimetric analysis (TGA) using a thermal analyzer apparatus (STA 449 F5 Jupiter). The samples were measured under constant nitrogen flow (50 ml/min) plus nitrogen flow as protective gas (20 ml/min) from 25 to 1,000°C at a heating rate of 10°C/min. Prior to the non-isothermal experiments the instrument was calibrated both for temperature and sensitivity. The exported data were manipulated through Proteus 6.1 Software. The polymer content was determined from the total of weight loss the microcapsules. The total organic content was then used to calculate the grafting density (σ
where the weight percentages of the shell and core materials were obtained by TGA for the temperature range 300–500°C.
The crystal structure was identified by powder X-ray diffraction (XRD) using X Bruker D8 Advance Twin Twin, employing Cu-Kα radiation (λ = 1.5418 Å). The thickness of the final coating system was measured with a coating thickness test instrument through the magnetic induction method (ACUTECH LTD, DUALSCOPE MP0R) and through SEM at the coatings' cross-section. In addition, Differential Scanning Calorimetry (DSC) testing was conducted on selected coating sample using Perkin Elmer Pyris 6 DSC apparatus calibrated with indium for temperature and heat capacity. Measurements were carried out in high purity nitrogen atmosphere on samples of ~8 mg in mass closed in standard aluminum Perkin Elmer pans. Measurement was performed on fresh sample that was heated from 20°C up to 250°C with a heating rate of 20°C/min, stayed there isothermally for 2 min, cooled down to −110°C at 30°C/min and, finally, heated from −110°C to 350°C at 5°C/min.
The microstructure, thickness and qualitative elemental analysis of the coatings were studied by Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-Ray Spectroscopy (EDS) using a PHILIPS Quanta Inspect (FEI Company) microscope with 149 W (tungsten) filament 25 KV equipped with EDAX GENESIS (AMETEX PROCESS & 150 ANALYTICAL INSTRUMENTS). Electrochemical Impedance Spectroscopy (EIS) measurements were conducted to assess the anti-corrosive properties of coatings and to verify its corrosion mechanism. A potentiostat/galvanostat connected to a Frequency Response Analyzer (VersaStat 3/FRA, PAR AMETEK) was used with an electrochemical cell of three electrodes (K0235 Flat Cell Kit, AMETEK), consisting of a working electrode (~1 cm2 exposed sample surface), a saturated silver/ silver chloride electrode [Ag/ AgCl, KCl (sat)] as a reference electrode and a platinum mesh electrode as the counting electrode for the measurements. The measurement frequency ranged from 100 kHz to 0.01 Hz and the sinusoidal perturbation applied was 10 mV. The spectra were analyzed through ZView® software (Scribner Associates) using the appropriate equivalent electrochemical circuit each time. The study of the anti-corrosion properties of the intact coating systems on galvanized steel samples included exposure to corrosive artificial ocean water prepared in accordance with ASTM D1141-98. The chemical composition of the solution is shown in
A second set of EIS measurements was performed on artificially scribed coated samples exposed to 5 mM NaCl solution in order to understand the mechanism of corrosion and its inhibition. For the in-depth study of the coatings and the investigation of their potential use as self-healing coating systems, artificial scribes of 1–2 mm length were created so that the metal substrate remains exposed to the electrolyte. This was confirmed for each sample by SEM analysis and the corresponding line elemental EDS analysis of the HDG metal substrate elements, Fe and Zn (
where Y0 is the admittance of the CPE and
Using the Cole-Cole approach together with CPE (Cole and Cole,
In addition, the investigation of a thermally-induced self-healing mechanism was conducted. Both the two-layer coating systems were subjected to the formation of a defect in a way that the underlying metal is exposed. Microscopic observation was then performed through SEM along with elemental mapping. The anticorrosion protection performance of each scribed coating was evaluated through EIS immediately after SEM analysis through the exposure to 5 mM NaCl solution. The samples were dried and were thermally treated at 90°C for 30 min and their corrosion behavior via EIS was again measured once. The low frequency impedance values were compared before and after heat treatment in order to evaluate the recovery of the barrier properties, the main function of the coatings studied.
According to
The successful synthesis of the polymer shell was confirmed through FT-IR analysis which revealed the characteristic adsorption peaks of poly(n-butyl acrylate;
FT-IR diagrams of the
The broad adsorption peak at about 1,658 cm−1 refers to the stretching vibration of the carbonyl group (C = O) of PAA. The methylene bending mode vibration occurs at 1,460 cm−1 (main polymer chain), 1,438 cm−1, 1,395 cm−1 (n-butyl groups) and the CH2 scissoring vibration at 1,454 cm−1 (polyacrylic acid). The two peaks at 1,373 and 1,320 cm−1 correspond to the symmetric methyl bending mode and the C-H bending mode of the tertiary C-H groups, respectively.
The C-O stretching mode shows two strong bands in the region 1,300–1,000 cm−1 in esters. In our case, the first adsorption band appears at 1,200 cm−1 and the second one at 1,147 cm−1. These bands are mainly attributed to asymmetrical and symmetrical C-O-C stretching modes of the n-butyl ester groups, respectively. Moreover, the doublet near 930 cm−1 is ascribed to the vibration corresponding to the n-butyl groups of the polymer and the skeletal stretching mode of the polymer main chain. The sharp band at 806 cm−1 also corresponds to the n-butyl groups. Finally, the double peaks at 767 and 726 cm−1 is due to the methylene rocking vibration mode (Kawasaki et al.,
The thermal behavior of the synthesized polymer shell was investigated through TGA in N2 atmosphere as illustrated in
TGA diagrams of the
With respect to the
EIS is used as a powerful tool to investigate the barrier properties of the coating systems developed in the current study and to assess the impact of the addition of a liquid rubber polymer either as a container surface modified or as an additive incorporated in free form in the epoxy matrix. As it has been well-documented previously in literature, the magnitude of impedance modulus at low frequency (|Z|0.01 Hz) can be used to assess the overall anti-corrosive behavior of an organic coating system, while the charge transfer resistance (
EIS Bode diagrams of intact
The EIS results of the artificially scribed
Equivalent circuits used for numerical simulation of the EIS data for the
Taking into account the aforementioned Equation (3), the
The evolution of the EIS fitting parameters as a function of time of artificially scribed
The evolution of the EIS fitting parameters as a function of time of artificially scribed
The fitting results for the scribed
The pore resistance (
Concluding, in both studied systems, the charge transfer resistance is decreased after the contact with the corrosive environment, yet the values are recovered to some point due to the beneficial effect of cerium-based additives.
In general, the literature agrees that rare earths behave as cathodic corrosion inhibitors for many metal alloys when added to a corrosive electrolyte (de Damborenea et al.,
Iron and zinc undergo severe localized dissolution at a number of anodic sites forming hydroxides of limited solubility according to the following equations (Pourbaix and de Zoubov,
Trivalent ions of rare earths in an aqueous environment undergo gradual hydrolysis and form complexed hydroxylated ions, as shown in Equation (7), and for Ce in Equation (8).
The exact chemical composition of these compounds depends on the type of the lanthanide cation added to the medium and the specific anion of the solution. This complex precipitates and results in the formation of a hydroxide layer as the pH of the solution locally increases at the cathodic areas, where the main reaction is the reduction of oxygen (Equations 9, 10).
Which of these two reaction pathways predominates, depends on the composition of the metal substrate and the presence, or absence, of any surface oxide film (Bockris and Khan,
The rare earth hydroxides formed are stable in alkaline solutions and dissolve under acidic conditions. The critical pH values at which the hydroxides precipitate is a function of the concentration of the trivalent ion (Ce3+), which cannot be assumed or calculated in our case (de Damborenea et al.,
Additionally, as shown in related works, in the case of zinc, the gradual dissolution occurring in the early stages of immersion is altered due to the adsorption of cerium ions. According to Aramaki and Arenas and de Damborenea cerium will react with the hydroxylated surface of the metal, thus impeding adsorption of the chlorides. Then, a complex is formed on the metallic surface such as [Zn]OCe(III)OH2 acting as a precursor to form the corrosion inhibiting layer. In addition, the precipitation of Ce(OH)3 in the cathodic regions is favored due to the lower solubility compared to zinc hydroxide (1.6 × 10−20 and 7.28 × 10−17; Aramaki,
A conflicting issue in literature is related with the oxidative state of the cerium ions found in the film. In detail, some claim that both oxygen and hydrogen peroxide (as produced by Equation 9) can act as oxidizing agents producing Ce4+ and CeO2, as shown by the following reactions (Equations 7–10)
Although in the present study no local pH measurement techniques and electrode potential measurements were utilized, according to literature (Böhm et al.,
In other related works, it is said that hydrogen peroxide presence contributes to the oxidation of Ce3+ to Ce4+, promoting the formation of oxides and hydroxides from both species, under prolonged time exposure conditions (Thierry et al.,
It is also reported in literature that the reaction between amine and epoxy groups is catalyzed by increased pH producing a new protective film (Kartsonakis et al.,
In the current study, a low Tg liquid rubber elastomer was used in order to add a second self-healing mechanism into the coatings. The polymer which was incorporated either as a surface modification of the microcapsules or as a polymer in free form is expected to modify the Tg of the system. The main aim is to induce self-healing of an artificial scribe upon heating the film above its Tg, where the polymeric chains have enhanced mobility. In this way, the deformation can be cured, possibly affecting positively and restoring the corrosion protection performance of the under-study film. Thus, the term “self-healing” is related with the recovery of the anti-corrosion protection of a coating system after being damaged due to a non-autonomous self-healing mechanism triggered by temperature. Theoretically, if the ratio between the defect size and the coating thickness is smaller than one, the aforementioned recovery may be complete and repeatable (D'Hollander et al.,
SEM surface images and line elemental analysis of the scribed
SEM surface images and line elemental analysis of the scribed
The separated crack surfaces are in spatial proximity after heating for samples
Fitting parameters according to equivalent circuits of
CPEcoat-T (nF cm−2 s−n) | – | 0.13 | – | 1.79 |
Error (%) | – | 15.7 | – | 50.0 |
CPEcoat-P | – | 0.97422 | – | 0.75727 |
Error (%) | – | 1.36 | – | 5.46 |
– | 37.3 | – | 36.4 | |
Error (%) | – | 0.85 | – | 5.99 |
– | 903.3 | – | 814.1 | |
CPEdl-T (μF cm−2 s−n) | 25.5 | 1.27 | 1.52 | 0.13 |
Error (%) | 2.56 | 4.91 | 6.69 | 8.92 |
CPEdl-P | 0.68035 | 0.63769 | 0.73103 | 0.76513 |
Error (%) | 0.57 | 1.76 | 1.18 | 2.06 |
2.04 | 102.9 | 4.77 | 234.1 | |
Error (%) | 0.71 | 1.68 | 0.85 | 2.07 |
6.36 | 0.40 | 0.24 | 0.05 | |
Chi-squared | 0.00237 | 0.00299 | 0.00833 | 0.01198 |
To conclude, the EIS measurements showed that the liquid rubber addition either in free form (
Two advanced coating systems have been presented in this work incorporating a type of core-shell cerium-based microcapsules which were compared with an epoxy-based coating including cerium microcapsules. The main aim was to investigate the influence of a liquid rubber polymer addition either as a surface-modifier or as an additive in free form on the anti-corrosion protection offered and their potentially heat-triggered material re-flow/self-healing ability.
The first autonomous self-healing mechanism refers to the well-known and studied anti-corrosion effect of cerium-based containers that contribute to the formation of stable chelating complexes on the surface of galvanized steel. On the other hand, their polymer-based surface modification enhanced the interaction of the microcapsules' surface with the epoxy matrix and the overall EIS response upon defect formation.
With the addition of a liquid rubber polymer in free form it is possible to incorporate a second, non-autonomous mechanism which was studied and presented in this work based on activation by increasing the temperature to values higher than the Tg of the coating system. In this case, except from the material re-flow ability of the system also shown in the coating incorporating the core-shell microcapsules, the protection performance of the coating is increased, offering a partial suppression of the underlying corrosion process.
Future plans of the current work include localized impedance spectroscopy measurements for the assessment of the local electrochemical corrosion reaction under the artificially scribed coating, along with complementary antifouling performance evaluation.
The datasets generated for this study are available on request to the corresponding author.
EK and IK conceived, designed and performed the analysis. EK collected the data and wrote the paper. IK performed the Scanning Electron Microscopy Analysis and also contributed during the experiments and the data analysis. CC had the overall responsibility for the final manuscript and contributed with critical comments and suggestions during the experimental process.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The Supplementary Material for this article can be found online at:
Schematic representation of the synthesis of the diblock copolymer onto the surface of either CeO2 or Cu2O.
SEM surface images and line elemental EDS analysis of the artificially scribed:
XRD pattern of the
XRD pattern of the of the
SEM surface images of the:
Topographic maps with the average coating thicknesses of the:
SEM cross-section images of the coating thicknesses of the:
EIS Bode diagrams of artificially scribed
EIS Bode diagrams of artificially scribed
EIS Bode diagrams of artificially scribed
EIS Nyquist diagrams of artificially scribed Epoxy-PBA-CSmc and Epoxy-CSmc coatings, before and after heat-induced healing after exposure to 5 mM NaCl solution.
The DSC measurements conducted on the scribed
Chemical Composition of Substitute Ocean Water (according to ASTM D1141-98).
The morphological and structural characteristics of the
The morphological and structural characteristics of
Fitting parameters according to equivalent circuit of