ZnO Nanoparticles Epoxy Resin Hybrid Nanocomposite with Anticorrosive and Antifouling Properties as Coatings for Naval Steel

Transcription

1 ZnO Nanoparticles Epoxy Resin Hybrid Nanocomposite with Anticorrosive and Antifouling Properties as Coatings for Naval Steel VIOREL PANAITE *1,2, SIMONA BOICIUC 1, VIORICA MUSAT 1 * 1 Dunãrea de Jos University of Galati,Faculty of Materials and Environment Engineering, Centre of Nanostructures and Functional Materials, 47 Domneascã Str.,800008, Galati, Romania, 2 Bureau Veritas Romania Controle International 165b Galati, Brailei Str., , Galati, Romania Hybrid nanocomposite epoxy-based coatings containing ZnO nanoparticles ( 50nm) were deposited on Grade A naval steel substrates by dip-coating. The morphology, microstructure, elemental composition and chemical interactions between the components of the nanocomposite coatings were analyzed by optical microscopy and scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM- EDX), grazing incidence X-ray diffraction (GIXRD) and Fourier transform infrared spectroscopy (FTIR). The coated steel samples were electrochemically monitorized over 30 days of immersion in 5 wt. % NaCl solution. The effect of ZnO nanoparticles (NPs) on the corrosion resistance was investigated by potentiodynamic polarization method. The incorporation of small concentration of ZnO nanoparticles (1 wt.%) shows a beneficial role, significantly improving the anticorrosion behaviour of the new coatings, in terms of polarization resistance and corrosion rate, parameters obtained from Tafel curves. The marine antifouling behaviour was evaluated during 120 days of immersion in Black sea water. Keywords: ZnO nanoparticle, epoxy coating, Tafel curves, corrosion parameters, antifouling behaviour The international convention on the control of harmful anticorrosion and antifouling systems on ships, which was adopted on 5 October 2001, prohibits the use of harmful paints used on ships. Among the eco-friendly naval protective coatings, the use of epoxy based coatings appears to be the most convenient method for preventing corrosion and fouling of steel surfaces which work in very demanding conditions [1]. Generally, epoxy coatings reduce the corrosion of a metallic substrate subject to an aggressive electrolyte, by two ways: first, they act as a physical barrier layer against the ingress of a deleterious species; second, they can serve as a reservoir for corrosion inhibitors to aid the steel surface in resisting to attack initiated by aggressive species [2]. Unfortunately, the usual epoxy coatings have susceptibility to damage by surface abrasion and wear [3, 4] and also they show poor resistance to the propagation of cracks [5]. The defects appeared on coating surface can act as pathways, accelerating the ingress of water, oxygen and chloride anions onto metallic substrate. Being hydrophilic in nature, the epoxy coatings have large volume shrinkage upon curing and can absorb water from surroundings [6, 7], leading to the increasing of number of pores and crevices, defects that create corrosion products. The barrier features of epoxy coatings can be enhanced by the incorporation of nanosized inorganic filler particles, dispersed within the epoxy matrix, to form an epoxy based nanocomposite. Thereby, the following benefits can be obtained: -improved integrity and durability of coatings by dispersion of fine particles in the cavities of epoxy matrix [8-10]. -preventing epoxy disaggregation, obtaining a more homogeneous coating, due to tendency of nanoparticles to occupy small hole defects, interconnecting more molecules, reducing total free volume and increasing crosslinking density [11-13]; * panaite_viorel@yahoo.com; Tel.: ; vmusat@ugal.ro -offering significant barrier properties for corrosion protection [14, 15] and reducing the tendency of coating for blistering or delaminating. This paper discusses the effect of ZnO nanoparticles (NPs) on the surface morphology, on the chemical interactions of ZnO NPs with epoxy matrix inside the hybrid coatings and on the anticorrosion and antifouling properties of the resulted nanocomposite coatings for protection of naval steel structures that work in very demanding marine conditions. Experimental part Materials and method The epoxy resin, the hardener and the solvent used for coating deposition, commercially known as DER 353, I 3100 and D 309 respectively, were obtained from Policolor S.A. (Bucharest). The DER 353 liquid epoxy resin is based on an aliphatic glycidyl ether modified bisphenol A/F, whereas I 3100 hardener is a polyaminoamidic compound. ZnO NPs with diameter of less than 50 nm, (3- Glycidyloxypropyl) trimethoxysilane (GPTMS) used as surfactant and tethrahydrofurane (THF) solvent were purchased from Sigma Aldrich. Sodium chloride (NaCl) used for corrosion test was purchased from Silal Trading S.R.L. (Bucharest). As substrates were used steel sheet samples (50x20x1.5mm) of grade S235JR+AR according to EN having in composition: 0.12% C, 0.48% Mn, 0.015% Si, 0.013% P, 0.006% S, 0.039% Al, 0.016% Cu, 0.029% Cr, 0.024% Ni, 0.001% V, 0.003% Mo, 0.001% Ti, 0.001% Nb. Sol preparation and coating deposition The epoxy resin used as matrix, (2,2-bis[4-(2,3-epoxy propoxy) phenyl] propane) commonly called diglycidyl ether of bisphenol A (DGEBA), derived from the reaction of bisphenol A and epichlorohydrin according to reaction (1). The hybrid sols used for the deposition of the nanocomposite coatings have been prepared in two steps. REV. CHIM. (Bucharest) 66 No

2 (1) In the first step, the epoxy precursor solution was prepared with the weight ratio of the epoxy resin to the hardener of 1.6:1. Before mixing, both resin and hardener were diluted with solvent at a 1:0.5 weight ratio [2]. After that, the two diluted solutions were mixed under stirring (1400 rpm) and sonication for 10 min. ZnO NPs (1wt%) were added into the resin solution and the obtained mixture was again stirred (1400 rpm) and sonicated for 10 min. Finally, the hybrid sols used for the deposition of nanocomposite coatings were prepared by adding the hardener solution, to initiate the curing process, followed by stirring (1400 rpm) and sonication for 10 min. Before the deposition of coatings, the steel substrates were cleaned with acetone, sand-blasting to the grade Sa 21/2 and finally immersed in acetone and dried. To remove the microparticles remained on the surface after sandblasting, the substrates were sonicated in acetone for 10 min. The coatings were deposited using a home-made dipcoater at withdrawn speed of 5 cm/min. After each layer deposition, the samples were kept at room temperature for 30 min, then placed in an oven (Model FN 055 Nüve) at 90 o C for 30 min and after that, again at room temperature for 30 min. All the coated samples were kept at room temperatures for 14 days, to allow full curing for the subsequent mechanical and anticorrosion tests. In order to compare the effect of the nanoparticles embedded in the coating, unmodified epoxy coating (Sample I) and epoxy modified with ZnO NPs coatings with different thickness (Samples II and III) have been prepared. Characterization of coatings The surface morphology and elemental composition of the nanocomposite coatings were analyzed by optical microscopy and scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM- EDX) using a Zeiss EVO MA 15 microscope. The thickness of film (Dry Film Thickness - DFT) was measured by a nondestructive testing method, using a 345 type Elcometer device. The microhardness measurements were carried out using the PMT 3 model microhardness instrument, and the hardness is measured according to the equation (EN ISO 6507/1 2002) [16]: HVF=18.19 * F/d 2 [MPa], (2) where F is the result of strength test and d is diagonal of square trace remained after test. The corrosion protection behaviour of the nanocomposite coatings was investigated by potentiodynamic polarization method. The samples were kept immersed in corrosive solution (5wt% aqueous NaCl solution) over the 30 days and the measurements of corrosion parameters were carried out periodically using Voltalab PGP 201 Radiometer Analytical equipment. The cell contained three electrodes: the epoxy coated steel sample served as working electrode, and for counter electrode and reference electrode were used a platinum plate and Ag/AgCl, respectively. This special cell assures the reproducibility of the experiments, especially keeping the distances between electrodes and a constant exposed sample surface (0.5 cm 2 ) in contact with the corrosive solution. The steel was polarized around its corrosion potential, in general within the potential range of -1000mV +200mV, at a scan rate of 2 mv/s. The results were processed with the VoltaMaster software-version 4, using the following experimental parameters: steel sample density (7.8 kg/dm 3 ) and iron valence (2). The antifouling behaviour of the nanocomposite coating was analyzed by visual aspect and by measuring the mass of marine deposits at different periods of time. The coatings were kept immersed in real work conditions (Black Sea, Agigea Port) for over 120 days. Results and discussions Structure and morphology The chemical structure of the hybrid coating achieved by chemical interaction between ZnO NPs (nano-fillers) was functionalized with GPTMS and the epoxy matrix was examined by FTIR spectra. Figure 1 presents the FTIR spectra, in 4000 to 400 cm 1 range, of ZnO nanoparticles used as nano-filler (fig. 1 a) and ZnO-epoxy nanocomposites coatings after drying at room-temperature followed by 30 min in oven at 90 C (fig. 1b). For the identification of the interaction between metal oxide nanoparticles and epoxy chains, in table 1 are compared the absorbtion bands from the FTIR spectra of ZnO-epoxy nanocomposite prepared in this work with some literature data [17]. The FTIR spectrum of unfunctionalised ZnO NPs (fig. 1 a) shows specific peak at about 500 cm -1 attributed to the stretching vibration of Zn-O bond, peaks at 1629 and 3448 cm -1 cm -1 that correspond to the deformation vibration and stretching vibrations, respectively, of OH groups of the water molecules attached at the surface of ZnO NPs (fig. 2). The FTIR spectrum of epoxy-zno nanocomposite shows peaks at 556 cm -1 characteristic for zinc oxide and at 1645 cm -1 and 3347 cm -1 that correspond to the bending vibrations of water molecules. The O-H bonds observed in the spectrum of nanocomposites can be attributed to the O-H groups that remain free during the functionalizing of ZnO NPs with GPTMS (fig. 2) and to water molecules resulted during functionalization, that remain inside the nanocomposite coating, due to the affinity of H 2 O molecules to get bonded to the metal oxide surface through hydrogen bonding [17]. H 2 O is a polar molecule and the oxygen is highly prone to hydrogen bonding with free hydroxyl groups present on the nanoparticle surface, as shown in figure 2. A comparison of spectra of the unfilled epoxy [17] and ZnO-epoxy nanocomposite prepared in this work, does not show relevant differences, very probably due to very small amount of filler (1%). The only difference that can be noticed is with respect to the amplitude of the water peak from 3340 cm -1 ; the FTIR spectrum of the nanocomposite shows a larger peak, due to the increasing amount of water resulted during the functionalization of a 10 times larger number of ZnO NPs (concentration increased from 1 to 10%) in the antifouling coating. Another difference can be seen from the bandwidth at 556 cm -1, corresponding to Zn-O bond vibration. Based on the previous discussion, it can be concluded that the mechanism of interactions between nanoparticles and epoxy matrix is through hydrogen bonding, involving the free hydroxyl (O-H) groups present on the nanoparticle REV. CHIM. (Bucharest) 66 No

3 Fig. 1. FTIR spectra of ZnO-NPs (a) and of epoxy-zno nanocomposite coatings without (Samples III) and with a topantifouling (AF) layer (Sample III+AF) (b) Table 1 FTIR PEAKS CORRESPONDING TO FUNCTIONAL GROUPS IN EPOXY SYSTEMS surface functionalized with GPTMS and the oxygen atom from the epoxy cycle (fig. 2). The hydrogen bonding does not result in the appearance of new peaks in the FTIR spectra, because it overlaps with the broadband from ~3500 cm -1 that already exists in the epoxy spectrum. An uncured DGEBA epoxy base resin is polar in nature and contains two epoxide groups at both ends, as shown in reaction (1); an epoxide group is a dipole on its own with pairs of electrons on the oxygen [17]. When the functionalized nanoparticles are added to the resin solution, the epoxide group forms hydrogen bonds with the free O-H groups of the GPTMS molecules attached to the surface of the nanoparticles during the functionalisation process (fig. 2). Topographic analysis of coatings deposited on sandblasted steel substrate was performed by overlapping the 3D images (fig. 3). Simple epoxy coating (fig. 3a) has a Fig. 2. Schematic representation of the chemical interactions at the interface region between ZnO NPs and epoxy matrix, via GPTMS ligand REV. CHIM. (Bucharest) 66 No

4 Fig. 3. 3D image on the top of epoxy coating (a), ZnO-epoxy nanocomposite coating (Sample III) (b) and ZnO-epoxy nanocomposite coating (Sample III) covered with antifouling layer (c) Fig. 4. SEM image of nano-zno modified epoxy coating, obtained at withdrawn speed of 10 cm/min (a); chemical analysis specific for region 1 (b) lower surface roughness than ZnO NPs-modified epoxy coating (fig. 3b), due to higher number of layers in the first case, each deposited layer contributing to the improving of surface quality. It should be noted that each layer deposited with undoped epoxy solution is thinner than the layer deposited with the solution doped with ZnO NPs. Therefore, to achieve the same thickness of coatings, about 50 µm, had to deposit more layers with the un-doped epoxy solution than for ZnO nanocomposite coating. The topographic aspect of nanocomposite coating after the deposition of antifouling layers shows a smoother surface (fig. 3c). Figure 4 shows the SEM image of the surface of ZnO NPs-modified epoxy coating (fig. 4a) together with the elemental composition obtained from EDX analysis on selected area (fig. 4b). In figure 4a, even some agglomerated nanoparticles can be observed and one can notice a smooth surface without cracks. The mapping of elemental composition by EDX in the selected area is displayed in figure 5. Although SEM image shows some agglomerated nanoparticles, the EDX mapping on the same area shows a continuous and relatively uniform distribution of zinc all over the investigated area, demonstrating a good dispersion of small ZnO NPs inside the composite coatings. The grazing incidence X-ray diffraction pattern of ZnO NPs-modified epoxy coating indicates the presence of nanocrystalline zinc oxide phase (fig. 6). The characteristic peaks of wurtzite type ZnO nanocrystals were observed in the GIXRD pattern at 2θ of 32.3 o (100), o (102), 37.4 o (200), o (101), o (111), (102), o (110) and o (103) [18]. The average measured thickness (DFT) values of unmodified epoxy coating (Sample I) and ZnO NPsmodified epoxy coatings (Samples II and III) are 48, 50 and 69 μm, respectively. The values obtained for the microhardness are 67.8 and 73.3 MPa for ZnO modified epoxy coating with DFT medium value of 50 μm (Sample II) and with DFT medium value of 69 μm (Sample III), respectively. Effect of ZnO NPs on the corrosion resistance of coatings The experimental corrosion parameters (corrosion potential, corrosion current, polarization resistance and instantaneous corrosion rate) of the steel samples coated with metal oxide-epoxy nanocomposite were estimated from Tafel curves. Figure 7 shows the variation of corrosion rate (fig. 7a) and polarization resistance, Rp (fig. 7b) of the Fig. 5. Elemental contents of oxygen and zinc mapped by EDX on the selected area of nano-zno modified epoxy coating (Sample III) 216 Fig. 6. GIXRD pattern of epoxy-zno nanocomposite (Sample III) REV. CHIM. (Bucharest) 66 No

5 Fig. 7. Variation of the corrosion rate (a) and polarization resistance (b) of coated steel samples vs the time of immersion in 5wt% aqueous NaCl solution coated steel samples, during over 30 days immersion in 5wt% NaCl solution. One can notice that in the case of unmodified epoxy coating there is a continuing trend of increasing the corrosion rate and a decreasing of the polarization resistance. This could be attributed to the fact that at this type of coating, the corrosion process once activated by destruction of physical barrier, it continuously develops. The data in figure 7 demonstrate the improvement of the anticorrosion properties of epoxy coatings by adding ZnO NPs (Sample II) even at very low concentrations (1%) and also by increasing the thickness of the nanocomposite coating (Sample III). In the case of nano-zno modified epoxy coating (Samples II), for both corrosion parameters it can be distinguished three different domains of variation: - from the beginning to around 800 h of immersion in aqueous NaCl solution, in which the corrosion rate grows up accelerated; -between h of immersion, in which there is a less rapid growth of corrosion rate; -between h of immersion, in which the corrosion rate decreases or it is maintained between close limits. The incorporation into the epoxy coating of 1wt. % ZnO NPs (Sample II) significantly reduced the corrosion rate of the coated steel over the 30-day immersion in 5wt.% NaCl solution, by 2 to 40 times, after 1272 h. Consequently, the polarization resistance of the epoxy-coated increases by 2 to 43 times, for the same period of immersion. The same behavior was observed for sample III, for which the incorporation of 1% of ZnO NPs significantly reduced the corrosion rate by 5 to 189 times after 1272 h and consequently leads to the adequate improving polarization resistance by 15 to 367 times for the same period of immersion. For the nanocomposite coating with higher thickness (Sample III), the first site of corrosion was distinguished only after 336 h of immersion in NaCl solution. Four domains in the corrosion rate diagram were distinguished, as follows: from the beginning until 800 h of immersion (the corrosion rate grows up accelerated), from 800 until around 1400 h of immersion (the corrosion rate fluctuates within relatively close values), from 1400 to 2100 h (the corrosion rate grows up accelerated) and the last domain in which the corrosion rate is maintained between close limits. Consequently, the trend of variation of polarization resistance is similar. The corrosion protection behaviour of nano-zno modified epoxy coatings (Samples II and III) is significantly improved for both corrosion parameters (corrosion rate and polarization resistance). The tendency of nano-zno modified epoxy coatings (Samples II and III) to keep a constant good level for anticorrosion resistance on longterm, starting from 2000 h of immersion, could be explained by the occurrence of corrosion products that can slow the corrosion process, but also by the beneficial presence of ZnO NPs which could act as reservoir for corrosion inhibitors to aid the steel surface in resisting to the attack initiated by aggressive species. Effect of ZnO NPs on the antifouling properties Figure 8 shows the comparative antifouling behaviour, in different stages of immersion in sea water, of three types of samples: unmodified epoxy-coating (Sample I) deposited on polished steel substrate without antifouling layer, ZnO-epoxy nanocomposite coating of 60 μm thickness without antifouling layer (Sample III) and with additional antifouling layer (Sample III+AF). The antifouling layer is also an epoxy-based nanocomposite richer in zinc oxide (10%) NPs. Fig. 8. Aspects of marine biofouling deposition on the top of epoxy-based coatings (on steel substrates), after different stages of immersion in sea water REV. CHIM. (Bucharest) 66 No

6 Fig. 9. Evolution in time of the mass of bio-fouling deposit on the top of nanocomposite coatings on naval steel samples, during immersion in Black Sea water. Concerning the epoxy coating (Sample I), one can observe that after 7 days of immersion the first points of corrosion appear, localized especially on the edges. After 45 days, there are some areas in which the top layer of coating was removed, but there were not visible signs of bio-fouling deposition due to very smooth surface of the samples that were deposited on a polish steel substrate. After 90 and 120 days of immersion, the bio-fouling material deposition was accelerated also due to the expansion of corrosion process. The samples III without and with additional antifouling layer rich in ZnO NPs (fig. 8) reveal, after 7 and 45 days of immersion, just few points of corrosion localized especially at the edges. Both samples showed some areas covered by marine bio-fouling after 45 days of immersion that is due to higher surface roughness of the coatings deposed on sand-blasted substrates that favours the adhesion of bio-fouling. On longer-term, after 91 and 120 days of immersion, the bio-fouling material deposition is lower than in the case of unmodified epoxy coating. The sample III covered with antifouling coating confirmed best antifouling behaviour in all stages. This qualitative observation is confirmed by the quantitative results presented in figure 9, that show the evolution in time of the mass of bio-fouling deposited on the top of the nanocomposite coating. From figure 9 one can notice that until 45 days of immersion the unmodified epoxy coating (Sample I) shows lower bio-mass depositions. That can be explained not only by lower fouling deposit due to glossy surface, but also by losing a small part of coating (fig. 9, Sample I- 45 days), due to erosion in sea water. Conclusions ZnO NPs were successfully dispersed in epoxy resin matrix, at a concentration of 1%, for the deposition of hybrid nanocomposite coatings on Grade A naval steel substrates by dip-coating method. The surface morphology shows smooth, continous and compact coatings, a good compliance and adherence to the substrate, without cracks or boundaries. The anticorrosion properties of the coatings were electrochemical monitoring over the 30 days of immersion in 5wt% NaCl aqueous solution and the bio-fouling material deposition was monitored over the 120 days of immersion in Black Sea water (real work conditions). Improvement of the anticorrosion properties of epoxy coatings by adding ZnO NPs even at very low concentrations (1%) and also by increasing the thickness of the nanocomposite coating was demonstrated. A nanocomposite coating of the same type, ZnO-epoxy, but with high (10%) ZnO NPs content was also prepared with anti-fouling role. Antifouling investigation has shown that with the addition of ZnO NPs, the bio-fouling material deposition decreases, this behaviour is improved with increasing concentration of ZnO NPs. For future research, it would be important to investigate ways to improve the dispersion of a mixture of different oxide nanoparticles to provide different features for epoxy coating, the effect of increasing ZnO NPs content and the potential application of the nanoparticles as reservoirs of corrosion inhibitors. References 1. YEBRA, D.M., KIIL, S., JOHANSEN, K.D., Prog. Org. Coat., 50, 2004, p XIANMING, S., TUAN, N.A., ZHIYONG, S., YAJUN, L., RECEP, A., Surf. Coat. Tehnol., 204, 2009, p WETZEL, B., HAUPERT, F., ZHANG, M.Q., Comp. Sci. Technol., 63, 2003, p ZHANG, M.Q., RONG, M.Z., YU, S.L., WETZEL, B., FRIEDRICH, K., Macromol. Mater. Eng., 287, 2002, p YAMINI, S., YOUNG, R.J., Polymer, 18, 1977, p PERREUX, D., SURI, C., Comp. Sci. Technol., 57, 1997, p LOOS, C., SPRINGER, G.S., J. Comp. Mater., 13, 1979, p LAM, K., LAU, K.T., Comp. Struc., 75, 2006, p SHI, G., ZHANG, M.Q., RONG, M.Z., WETZEL, B., FRIEDRICH, K., Wear, 254, 2003, p HARTWIG, M., SEBALD, D., PUTZ, L., ABERLE, Macromol. Symp., 221, 2005, p DIETSCHE, F., THOMANN, Y., MULHAUPT, R., J. Appl. Polym. Sci., 75, 2000, p HUONG, N., Improvement of bearing strength of laminated composites by nanoclay and Z-pin reinforcement, PhD. Dissertation, University of New South Wales, Australia, BECKER, O., VARLEY, R., SIMON, G., Polymer, 43 (16), 2002, p YANG, L.H., LIU, F.C., HAN, E.H., Prog. Org. Coat., 53, 2005, p LAMAKA, S.V., ZHELUDKEVICH, M.L., YASAKAU, K.A., SERRA, R., POZNYAK, S.K., FERREIRA, M.G.S., Prog. Org. Coat., 58, 2007, p *** EN ISO 6507/ standard 17. SINGHA, S., THOMAS, M.J., IEEE Trans. Dielectr. Electr. Insul., 16, nr. 2, p GONZALEZ-CAMPO, A., ORCHARD, K. L., SATO, N. SHAFFER, M.S.P., WILLIAMS, C.K., Chem. Commun., 27, 2009, p Manuscript received: 8, REV. CHIM. (Bucharest) 66 No