a College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, Chinab Integrated Composites Laboratory (ICL), Chemical and Biomolecular Engineering Department, University of Tennessee, Knoxville, TN 37996, USAc College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, Chinad Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Mass AV., Cambridge, MA, 02139, USA
G R A P H I C A L A B S T R A C T
The plate, treated with RC-GAE, has a dense black protective chelate film with a better anticorrosion performance than the untreated.
A novel water-based rust converter 2-hydroxypropyl 3, 4, 5-trihydroxybenzoate (named as RC-GAE) was pre-pared by using an esterification reaction between 1, 2-propylene glycol (PG) and gallic acid (GA). The effects ofPG and GA mass ratio in the RC-GAEs on the anticorrosion were investigated by the Tafel polarization with therusty plate samples coated with RC-GAEs in a 3.5 wt% NaCl solution. The rust was converted to a compact blackprotective rust-conversion film with this rust converter. The metallographic microstructure, morphology, andcrystalline phase of the rust-conversion films were conducted systematically by optical microscopy, X-ray dif-fraction, and scanning electron microscopy with energy-dispersive spectrometry. The corroded plate with un-even and rough brown surface became smoother and darker after treated with converter. X-ray photoelectronspectroscopy indicated that the RC-GAE reacted with Fe2+/Fe3+ and formed a compact Fe2+/Fe3+ chelate filmthat played the role of anticorrosion. Additionally, new water-based anticorrosion coating was prepared by
http://dx.doi.org/10.1016/j.colsurfa.2017.10.041Received 5 June 2017; Received in revised form 16 October 2017; Accepted 18 October 2017
applying a mixture of RC-GAE and other additives to form polymer composites on the rusty steel plates. Bothadhesion test and neutral salt spray test indicated that the anticorrosion coating presented a good corrosionresistance and a remarkably strong film adhesion of the new water-based anticorrosion polymer compositescoating.
1. Introduction
Corrosion protection and prevention are remarkably important,since considerable parts of the gross national products are consumedannually for corrosion and corrosion prevention [1–3]. Among thefrequently reported protection methods such as organic or inorganiccoating [4,5], electrochemical protection [6] and corrosion inhibition[7–10] used to prevent metal corrosion, the most effective and eco-nomic corrosion prevention technique is the application of coating,particularly organic coatings. Ideally, organic coatings would providelong-lasting corrosion protection [11–13]. However, most organiccoatings used previously in anticorrosion systems are solvent-basedcoatings, which contain large amount of organic solvent. With the in-crease of environmental awareness, zero-volatile organic compounds(VOC) or low-VOC water-based anticorrosion coatings are demanded tosave natural resources and protect the environments. High-molecularweight organic corrosion inhibitors (such as a castor oil derivative) [14]and barrier anticorrosive pigments (such as calcium metasilicate) [15]and convertible anticorrosive pigments (such as zinc chromate) [16]are active protective elements in paint formulations [17]. The primarypaints that protect the steel structures from corrosion normally containcorrosion-inhibiting pigments, which are mostly lead oxide (red lead)or chromate [18,19]. These compounds are hazardous to the environ-ments because of their toxicity, and must be substituted by environ-ment-friendly corrosion inhibitors.
Although tannic acid and their derivatives such as gallic acid (GA, 3,4, 5-trihydroxy benzoic acid) have been studied as non-toxic and non-polluting multipurpose rust converters [20–24], its weak rust-conver-sion capacity, poor water resistance and low solubility limit their ap-plications [25,26]. Modifications need be taken for its potential uses forrust conversion. The reported improvement approaches [26–28] in-volved mixing with other organic or inorganic acids such as phosphoricacid and oxalic acid, which can improve the conversion capacity of rustconverter. Most studies regarding water-based anticorrosion coating arebased on a mixture of tannic acid and phosphoric acid because of theirconvenient applications and simple operation [29,30]. However, excessacids can introduce a large number of active groups and consequentlycause instability and water resistance of the coating. GA is a naturalplant polyphenol that is widely distributed in fruits and plants [31,32].GA and its derivatives possessing antioxidative, antineoplastic proper-ties and synergistic inhibition effects [33–35] are widely used inbiology, pharmacy and chemistry. The structure of GA causes strongchelating capability with metal ions through hydroxyl groups [36,37].Although a GA ester with excellent water solubility and the existence ofhydroxyl groups could have great chelating capability with rust, thesynthesis of GA ester as a rust converter for residual rust coatingtreatments has not been reported yet.
Herein, a novel water-based rust converter 2-hydroxypropyl 3, 4, 5-
trihydroxybenzoate (named as RC-GAE in this paper) based on GA wassynthesized. The rust conversion behaviors were examined on the rustysamples by preparing an anticorrosion painting with the as-preparedRC-GAE. The success of RC-GAE synthesis was confirmed by Fouriertransform infrared spectroscopy (FT-IR). The rust conversion capabilityof the as-prepared RC-GAE and the corrosion resistance behaviors of theanticorrosion polymer composite coatings were studied by scanningelectron microscopy (SEM), X-ray diffraction (XRD), polarization curve,and neutral salt spray test. The anticorrosion mechanism of the as-prepared RC-GAE was explained by considering the formed iron che-lation film on the surface of the plate. The adhesion test and the saltspray corrosion resistance test were used to evaluate the barrier prop-erties and protective capacity of the RC-GAE coating, and to reveal thepenetration of aggressive species during the corrosion test over a longtime.
2. Materials and methods
2.1. Materials
GA (powder, Analytical Reagent) was purchased from TianjinDaguche Chemical Co. Ltd., Tianjin, China. 1, 2–Propylene glycol (PG,Analytical Reagent), p-toluene sulfonic acid (p-TSA, AnalyticalReagent), sodium chloride (NaCl, Analytical Reagent), ethanol(Analytical Reagent), and 2-Butoxy ethanol (Analytical Reagent) werepurchased from Tianjin Bodi Chemical Co. Ltd., Tianjin, China. Vinylacrylic water-based resin (Haloflex 202) was obtained from DSM Co.Ltd., Shanghai, China. Texanol with the effective chemical compositionof 2,2,4-Trimethyl-1,3-pentanediolmono (2-methylpropanoate) waspurchased from EASTMAN Chemical Company, Shanghai, China.Aluminum tripolyphosphate (AlH2P3O10, 45 μm), sericite (45 μm),talcum powder (45 μm), barium sulfate (45 μm), and nanosilica fillers(30 nm) were purchased from Guangzhou Yifeng chemical Co. Ltd.,Guangdong, China. All the other chemicals were of industrial grade andused as received without any further purification. Distilled water wasused throughout the experiment.
2.2. Preparation of the water-based rust converter
The synthesis of the as-prepared RC-GAE is illustrated in Scheme 1.GA reacted with PG through an esterification method with p-TSA ascatalyst.
In a typical synthesis, PG (30.6 g) and GA (10.2 g) were added into afour-necked flask with a condenser and thermometer. The mixture wasstirred at 70 °C in a temperature-controlled water bath. Approximately,1.02 g p-TSA was added into the flask, and GA was all dissolved. Thesuspension was stirred at 70 °C for 1 h and heated to 105 °C. After 3 hcontinuous agitation and reaction, the four-necked bottle was naturally
Scheme 1. Synthesis of RC-GAE from the reactionbetween GA and PG.
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cooled down to room temperature. The obtained product was a paleyellow liquid. PG was used as a film-coalescing aid in the anticorrosionpainting; thus, it did not need be separated from the as-prepared rustconverter.
The mass ratio of PG to GA was important for the performance ofthe novel water-based rust converter. GA could not react completelywhen PG was not more than GA; otherwise, the anticorrosion perfor-mance of the rust converter was not high when PG was more than GA.Therefore, RC-GAEs were prepared with different mass ratios of PG andGA to evaluate the influence of mass ratio on the anticorrosion per-formance of RC-GAE. These samples were marked as RC-GAEs (m:n) todesignate the samples prepared with the mass ratio (m:n) of PG and GA.
2.3. Preparation of samples
The cold rolled plates (The compositions of the cold rolled steelplates are listed as follows (%): C: ≤0.10, Si: ≤0.05, Mn: ≤0.50, P:≤0.035, S:≤0.025, Fe: Residual dose.) of 6 cm × 12 cm× 0.05 cmwere degreased with alkaline degreasing agent. The degreasing agentwas made in our lab. To be more specific, its main compositions arelisted as follows (for example to make one g degreasing agent):Na4P2O7∙10H2O: 0.03 g, Na2CO3: 0.12 g, NaOH: 0.05 g, NaCl: 0.01 g,sodium gluconate: 0.04 g, triethanolamine: 0.08 g, sodium carbox-ymethylcellulose: 0.002 g, polyoxyethylene nonylphenol ether (NP-10):0.02 g, and distilled water: residual dose. The plates were then rinsedwith distilled water, and then exposed to air for three months to rust.The pre-rusted plates were only roughly cleaned to remove the floatingrust and dust on their surface using a metallic brush and rinsed withethanol before the test. The pre-treatment of the rusted plates couldmake a good effort to improve the adhesion performance of the coatingfilm. Then the plate was coated with the rust converter synthesized inSection 2.2 using a banister hairbrush when the ethanol was evapo-rated. The coated samples were allowed to be dried for 48 h at roomtemperature.
A new water-based anticorrosion coating based on RC-GAE wasprepared. The composition of the anticorrosion coating is shown inTable 1. Briefly, distilled water, dispersant and appropriate amount ofdefoamer and Texanol with the amount described in Table 1 wereadded into a beaker with a mixer. Then, aluminum tripolyphosphate(AlH2P3O10), sericite, talcum powder, barium sulfate, and nanosilicafillers were added into the beaker sequentially to make the rust con-verter paint. The suspension was stirred for 15 min. Afterward, the RC-GAE prepared in Section 2.2, Haloflex 202, 2-Butoxy ethanol and ap-propriate amount of defoamer, Texanol and thickener were added intothe suspension with stirring for 30 min.
The new water-based anticorrosion coating containing RC-GAE andother additives was painted on the prepared corroded plates to formpolymer composites on the rusty steel plates. Before the coating filmperformance test, the samples were dried for 120 h at room tempera-ture.
2.4. Characterization
FT-IR spectra of the samples were recorded on a NICOLTE 380 FT-IRspectrometer ((Thermo, USA)) using the KBr method. The crystal-lographic information of the samples was analyzed by XRD (RigakuUltima IV) equipped with graphite monochromatized Cu Kα radiation(λ = 0.15418 nm), with a scanning rate of 4°/min in 2θ range of10°–80°. X-ray photoelectron spectroscopy (XPS) analyses were de-termined with an X-ray spectrometer K-Alpha (Thermo Scientific, USA),using Mg Kα X-ray radiation (1486.6 eV).
The morphology and microstructure of the samples were observedby a field-emission scanning electron microscope (SEM, SU-70, Hitachi,Japan) equipped with energy dispersive spectrometer (EDS) at an ac-celerating voltage of 30 kV.
Metallographic test of sample profiles was done on a microscope
(DVM2500, Leica, Germany). The samples were prepared by en-capsulating the sample prepared in Section 2.2 in black phenolic resin,sanding on #400, #500, #600, #800, #1000 and #1200 sandpaper,polishing with alumina suspensions of particle sizes between 1.0 μmand 0.3 μm and etching with nital solution.
2.5. Tafel polarization measurements
The Tafel polarization measurements were carried out on aLK2005A electrochemical workstation (Lanlike, Tianjin, China) in a3.5 wt% NaCl solution, with a saturated calomel electrode (SCE) asreference and platinum electrode as counter electrode. The sampleplates prepared in section 2.3 were cut to fixed size(1.2 cm × 1.2 cm × 0.05 cm), and then coated with wax and madesure the surface area of the plate to be 1 cm2. Then it can be used as theworking electrode for the Tafel studies. The open circuit potential(OCP) of plate samples coated with the as-prepared rust converter wasmeasured before the Tafel polarization measurements to confirm therange of the sweeping potential. The Tafel polarization measurementswere performed by sweeping the potential between −0.25 and 0.25 Vfrom OCP at a scan rate of 0.0005 V/s. The specimen was stabilized inthe electrolyte for 30 min before the experiment. The same amount ofRC-GAEs (m:n) was used to treat the unrusted plate samples. All themeasurements were repeated at least three times to ensure good re-producibility of the results.
2.6. Polymer composite coating film performance test
2.6.1. Film adhesion experimentThe adhesion test was conducted to evaluate the adhesion of the
anticorrosion coating in the plate samples according to the ASTMD3359 standard. Briefly, a lattice pattern with six cuts in each directionwas made in the film to the substrate, the pressure-sensitive tape wasapplied over the lattice and then removed, and the adhesion wasevaluated by comparison with the descriptions and illustrations Thereare six levels in total, 0B, 1B, 2B, 3B, 4B and 5B, in which 0B is theworst while 5B is the best. The level of the adhesion in accordance withthe following scale is illustrated as following. For level of “5B”, theedges of the cuts are completely smooth, none of the squares of thelattice is detached. For level of “4B”, small flakes of the coating aredetached at intersections, less than 5% of the area is affected. For levelof “3B”, small flakes of the coating are detached along edges and atintersections of cuts; the area affected is 5–15% of the lattice. For levelof “2B”, the coating has flaked along the edges and on parts of thesquares, the area affected is 15–35% of the lattice. For level of “1B”, thecoating has flaked along the edges of cuts in large ribbons and wholesquares have detached, the area affected is 35–65% of the lattice. For
Table 1The composition of the prepared water-based anticorrosioncoatings.
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level of “0B”, flaking and detachment are worse than Grade 1.
2.6.2. Salt spray corrosion resistance testNeutral salt spray test was used to evaluate the corrosion resistance
of the anticorrosion coating according to the ASTM B117 standard. Thetested samples were placed in a salt spray corrosion test chamber(1100 × 780 × 1150 mm) at an angle of 45°. The instrument used forthe neutral salt spray test was a FQY015 pneumatic salt spray corrosiontest chamber (Shanghai Experimental Instrument Co. Ltd., China)containing sodium chloride (5 ± 0.5) wt% with a pH value range of6.5–7.2 and temperature of 47 ± 0.5 °C. The degree of protectionrating (Rp), which is defined by the area of defects (A(%)) in polymercomposite coating film, is used to represent the ability of the coating toprotect the base metal from corrosion according to ISO 10289–1999.The coating film was checked with an optical microscope to evaluatethe area of the substrate that could exhibit localized attacks. There areten levels in total, 10, 9, 8, 7, 6, 5, 4, 3, 2 and 1, in which 1 is the worstwhile 10 is the best (No defects: 10, 0 < A ≤ 0.1: 9, 0.1 < A ≤ 0.25:8, 0.25 < A ≤ 0.5: 7, 0.5 < A ≤ 1.0: 6, 1.0 < A ≤ 2.5: 5,2.5 < A≤ 5.0: 4, 5.0 < A ≤ 10: 3, 10 < A ≤ 25: 2, 25 < A≤ 50:1, 50 < A: 0). Moreover, the time of the salt spray corrosion resistancetest is important to evaluate the performances of the anticorrosioncomposite coating film, so the protection rating of the coating after saltspray corrosion resistance test of X h were marked as Rp/X h, for ex-ample, Rp/240 h means the protection rating of the coating after saltspray corrosion resistance test of 240 h.
3. Results and discussion
Fig. 1 shows the Tafel curves of plate samples coated with the as-prepared RC-GAEs (m:n). And the potentiodynamic polarization resultsobtained by the Tafel extrapolation method [38–40] are summarized inTable 2. The corrosion potential (Ecorr) of the samples coated with therust converter (Fig. 1c–i) remained positive with respect to the rustedplate (−1.157 V, Fig. 1a) and the unrusted plate (−1.060 V, Fig. 1b).Herein, the rusted plate is more susceptible to corrosion than the un-rusted plate and the samples coated with the rust converter, it will beeasier to be corroded. Moreover, all of the plate samples coated withRC-GAEs (m:n) provide a certain degree of galvanic or passivate pro-tection to the steel substrate. Therefore, the plate sample, when coatedwith RC-GAEs (m:n), is significantly less susceptible to corrosion andshows a better corrosion performance than the case when it is nottreated with RC-GAEs (m:n) [41,42].
All of the plate samples coated with RC-GAEs (m:n) showed lowercorrosion current density than that of the rusted plate(1.157× 10−3 A cm−2) and the unrusted plate (1.827× 10−4 A cm−2)This is attributed to the reaction between rust and RC-GAE that enhancesboth the kinetics of passivation and the stability of the protective passivefilm formed. Moreover, the sample coated with RC-GAE (3:1) (Fig. 1e)showed the lowest corrosion current density (7.194 × 10−7 A cm−2).This is in turn due to the higher relative content of RC-GAE in RC-GAE(3:1) with respect to others that speed up the formation of a stable andprotective passive film. This indicates that the rusty plate sample, whencoated with RC-GAE (3:1), has a significantly higher corrosion resistancethan the case when it is coated with other RC-GAEs (m:n). In addition, itscorrosion potential positively shifted to a higher value (−0.892 V) thanthat of other coated samples (−0.937 V for RC-GAE (1:1), −0.958 V forRC-GAE (7:1), etc). As the more negative corrosion potential and the largercorrosion current usually correspond to faster corrosion rates while themore positive corrosion potential and the smaller corrosion current density(icorr) mean a slower corrosion process, thus the plate sample, when coatedwith RC-GAE (3:1), is significantly less susceptible to corrosion than thecase when it is coated with other RC-GAEs (m:n). Furthermore, the pro-tection efficiency was increased from RC-GAE (1:1) to RC-GAE (3:1)(Fig. 1c–e) and decreased from RC-GAE (4:1) to RC-GAE (7:1) (Fig. 1f–i),which might be due to the fact that GA cannot react completely when the
relative content of PG was not more than that of GA (RC-GAE (1:1) andRC-GAE (2:1)), therefore, the rust had not been transformed by RC-GAEscompletely. Otherwise, when the amount of PG was higher than that ofGA, the relative content of the GA esters was small in the same dosage, theGA esters were not sufficient to convert all rust into iron chelate (for ex-ample, RC-GAE (4:1), RC-GAE (5:1), RC-GAE (6:1), and RC-GAE (7:1)),and the rust had not been transformed by RC-GAEs completely. As the as-prepared RC-GAE (3:1) showed the more positive corrosion potential andthe smaller corrosion current (−0.892 V in the Ecorr and7.194× 10−7 A cm−2 in the icorr) and possessed an excellent antic-orrosion property, thus the optimal mass ratio of PG and GA for thecoating was 3:1.
Fig. 2 shows the typical FT-IR spectra of PG (Fig. 2a), GA (Fig. 2b)and RC-GAE (Fig. 2c). The strong broad peak in all FT-IR spectra ataround 3300 cm−1 is due to the eOeH stretching vibration. The ab-sorptions around 1230 and 2975 cm−1 are the eCeH bending andstretching vibrations, respectively (Fig. 2a and c). The observed variouspeaks between 771 and 744 cm−1 correspond to the substituted ben-zene rings in RC-GAE [43]. The observed peaks at 1714 and 1230 cm−1
are owing to the formation of −COO− in RC-GAE, and the peak at1340 cm−1 corresponds to the eOeH in-plane bending vibration of thebenzene ring in RC-GAE. The stretching vibration peaks of eOeH at3334, 1136 and 1039 cm−1 are enhanced significantly, this is due tothe appearance of polyhydric phenol in RC-GAE. The stretching vibra-tion peak at 1652 cm−1 is disappeared, this is caused by the dis-appearance of aliphatic polyols structure in PG. For Fig. 2b and Fig. 2c,the characteristic peaks at 1714 and 1230 cm−1 arise from the eCOOestretching vibration [28,44], the peaks at 1451, 1539 and 1613 cm−1
are due to the eC]Ce bending and stretching vibrations of the ben-zene ring [44]. Moreover, when compared with Fig. 2b, the positionand intensity of absorption peaks in Fig. 2c have changed obviously.The eCeH stretching vibration peaks at 2973 and 2933 cm−1 are sig-nificantly enhanced, the stretching vibration peaks of eOeH peaks ataround 3300, 1136 and 1039 cm−1 are owing to the reaction betweenGA and PG, which results in the appearance of eCeH and eOeH offatty alcohol in RC-GAE. Therefore, the FT-IR analyses indicate thesuccessful synthesis of RC-GAE.
Fig. 3 shows the typical FT-IR spectra of rust (Fig. 3a), rust treatedwith rust converter (Fig. 3b), and rust converter (Fig. 3c). The char-acteristic peaks at 1022 and 598 cm−1 in Fig. 3a are the absorptions ofγ-FeOOH and Fe3O4 [23]. The peaks at 3440 and 1642 cm−1 are theeOeH stretching vibration of the rust. The absorption peaks at 1642,1022 and 598 cm−1 are disappeared, the absorption peaks at1382 cm−1 are enhanced, the absorption peaks at 3440 and 1642 cm−1
are significantly reduced, which are correspond to the decrease ofeOeH in the rust (γ-FeOOH, α-FeOOH). The observed absorption peaks
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at 1578, 1210 and 602 cm−1 are due to the new bond formation ofOeFeeO and the CeOeFe in the Fe2+/Fe3+ chelate in the reactionbetween rust and rust converter. Moreover, when compared withFig. 3c, the position and intensity of many absorption peaks in Fig. 3bhave changed obviously. The stretching vibration absorption peaks ofAreOeH at 3362, 1136 and 1034 cm−1 are significantly reduced owingto the decrease of eOeH in RC-GAE. There are new absorption peaks at1578, 1081 and 602 cm−1 observed, this is due to the new bond for-mation of OeFeeO and the CeOeFe in the Fe2+/Fe3+ chelate in thereaction between rust and rust converter. All these indicated that the
rust had reacted with rust converter.The rusted steel plate surface (Fig. 4a) would be transformed into
black (Fig. 4b) after applying the RC-GAE coating for about 15 min.This is due to the reaction between the rust and rust converter and theformation of the iron chelate (the color of the iron chelate of gallic acidand it derivative [22]). From Fig. 4b, we can see that a dense shinyblack iron chelate protective film was obtained when the rust converterreacted with rust.
Fig. 5 shows the metallographic microstructure of sample profilesbefore and after the treatment with the rust converter. The averagethickness of rust was about 3.5 μm for all the samples. This is due to therust on the surface of the sample plates is a short-term product, so it isuniform and there are no long-term products from seriously localizedattack and pitting corrosion on the surface of the sample plates. Asshown in metallographic microstructure of untreated rusted plate(Fig. 5a), there is an obvious dividing line between the rusted layer andthe steel substrate, which is not seen in the metallographic micro-structure of treated rusted plate (Fig. 5b). Furthermore, the rust con-verter seeped onto the substrate and reacted with the substrate(Fig. 5b), in this case, the converted rust layer will have better bondingforce with the steel substrate, so the rust converted layer will have agood adhesion performance. Moreover, the rust layer on the steelsubstrate is thin, thus the dense covered protective film is also thincompared with the steel substrate. As the dense protective film hasexcellent barrier and corrosion resistant properties, it will avoid thematerials from further corrosion and decrease in mass and thus makesno damage to the metal pieces, so the mechanical or physical propertiesof the metal pieces are well improved than the loose rust on the rustedmetal pieces.
Fig. 6 shows SEM and EDS images of the rusty plate samples beforeand after the treatment with the rust converter. Fig. 6a shows thesurface with fine grain morphology of rusts. The structures were alsohighly porous and rough, and they presented cavities of different sizes.Both lamellar regions and cactus-shaped particle regions can be ob-served in Fig. 6a. The morphologies are indicative of the presence of γ-FeOOH (lamellar regions) and traces amount of α-FeOOH (cactus-shaped particle regions) [45], which can also be confirmed by the XRDtest results. The crystalline morphology on the sample surface dis-appeared, and an amorphous compact and cracked layer was formedafter the treatment with the rust converter (Fig. 6c and inset). Thesechanges indicated that the rusted phases were transformed by the rustconverter. Fig. 6b and d show the EDS spectra of the plate samplesbefore and after the treatment with the rust converter. The EDS spectraindicate that the rusty plate samples before and after the treatment withthe rust converter are all composed of Fe, C and O elements, and thecomposition percentages of Fe and C of the plate sample after thetreatment both increase, while the composition percentage of O de-creases compared with the rusty plate sample after the treatment. Theresults indicate that the rust is converted by the rust converter [44].
Fig. 7 shows a typical XRD pattern of the plate samples before andafter the treatment with the rust converter. Only Fe (JCPDS 06-0696)and a small amount of γ-FeOOH (JCPDS 18-0639) can be identified onthe plate samples (Fig. 7a). No Fe2O3 peaks were observed, which mightbe caused by the thin thickness of rust layer, as well as low content,poor crystallinity, and amorphous characteristics of Fe2O3. Therefore,the rust powders obtained from the plate sample were also tested tocompare with other samples. The XRD pattern of the corrosion productstreated with RC-GAE revealed that the intensities of Fe2O3 (JCPDS 25-1402) and γ-FeOOH (JCPDS 18-0639) peaks diminished significantly ordisappeared (Fig. 7c), as easily observed in Fig. 7b. Only crystallized Fepeaks were observed, indicating that the rust reacted with the rustconverter, the crystal shape of the rusted surface changed obviously,the content of active rust on the rusty surface was considerably reduced,and the corrosion products treated with RC-GAE were mainly amor-phous iron chelate [46].
Fig. 8a shows the general XPS spectra of the rusty plate samples
Fig. 2. FT-IR spectra of (a) PG, (b) GA and (c) RC-GAE.
Fig. 3. FT-IR spectra of (a) Rust, (b) Rust treated with rust converter, (c) Rust converter.
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before and after the treatment with the as-prepared RC-GAE, and theresults are shown in Table 3. The electron binding energy of O de-creased from 531.58 to 530.88 eV after the treatment with the as-pre-pared RC-GAE (Fig. 8c), suggesting the formation of the FeeOeC bond.In the XPS high-resolution spectrum for the Fe2p transition (Fig. 8b),the energies of 712.14 and 722.80 eV, corresponding to Fe2p3/2 andFe2p1/2 levels, transformed to 711.25 and 722.23 eV after the
treatment with the rust converter, respectively. Gallic acid and theirderivatives are believed to possess reductive and antioxidative prop-erties, and most of gallic acid esters are often used as antioxidants [33].Moreover, the pH value strongly influences the property of gallic acidand their derivatives, when the pH value is low, the reductive proper-ties of gallic acid and their derivatives became stronger [22,24]. Herein,part of Fe3+ transformed into Fe2+ by RC-GAE in acidic conditions,
Fig. 4. Optical images of the rusty steel sample be-fore (a) and after (b) the treatment with the RC-GAErust converter. The brown rust in the sample plate isconverted into a dense black protective iron chelatefilm after the treatment of RC-GAE.
Fig. 5. Metallographic microstructures of sampleprofile (a) before and (b) after the treatment with theRC-GAE rust converter (BPR: Black phenolic resin,CRP: Cold rolled plates).
Fig. 6. SEM and EDS images of the plate samples before (a, b) and after (c, d) the treatment with the RC-GAE rust converter.
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which was in agreement with other reports [22,24], and Fe2+ reactedeasier than the Fe3+ during the reaction between the rust and the as-prepared RC-GAE. Additionally, the Fe3+ content decreased, which waswell consistent with the XRD pattern. In this case, we concluded thatthe conversion mechanism of rust was that the ortho-phenolic hydroxylgroups of the rust converter molecules reacted with Fe2+/Fe3+ andformed a Fe2+/Fe3+ chelate (Scheme 2). The Fe2+ chelate is morestable than Fe3+ chelate within a short period of time in an acidiccondition, which is in agreement with other reports [47].
Haloflex 202, a kind of acidic vinyl acrylic water-based resin, hasexcellent barrier and adhesion properties and often used as the filmforming material in the anticorrosion primer coating. Furthermore, thewater-based rust converter is also acidic, a kind of acidic resin has to beselected as the film forming material to ensure the stability of polymercomposite coating. When used as a rust converting agent, Haloflex 202does not show a better performance and cannot convert the rust into theinert material. The rust also exists in the substrate plate under thecoating film. In this case, the rust can also damage the substrate and theanticorrosion coating will be bubbling, then the anticorrosion coatingwill lose its effect. By contrast, RC-GAE can react with rust and convertthe rust into an inert dense protective film completely (Fig. 5) Herein,the polymer composite coating which contains RC-GAE and Haloflex202 can avoid the bubbling phenomenon and has higher corrosion re-sistance than the polymer composite coating which only contains Ha-loflex 202.
Lots of the traditional rust converters are well-known to have excessacids and are non-soluble, and may damage the substrate plate andcause pitting corrosion. The as-prepared RC-GAE presents an obviouscontrast against the traditional rust converters since it exhibits betterwater solubility and compatibility with the water-based anticorrosioncoating than the traditional rust converters, such as GA, due to the factthat RC-GAE can dissolve in water indefinitely while one g GA candissolve in 87 g water, so it can be well dispersed in the polymercomposite coating. Therefore, no organic solvents (isopropyl alcohol[26]) or strong acids (phosphoric acid [29], sulfuric acid [48]) areneeded to increase the solubility and penetrability of the RC-GAE in thepolymer composite anticorrosion coating. On the basis that RC-GAE canbe well dispersed in the anticorrosion coating, and there is no excessacid present in the polymer composite anticorrosion coating, the an-ticorrosion coating based on RC-GAE polymer composites show nodamage on the surface of steel structure and can well avoid the pittingcorrosion and localized corrosion.
In this case, the RC-GAE composite coating film may present a goodcorrosion resistance. Table 4 shows the coating film performances ofthe anticorrosion composite coating. We concluded that the coatingfilm exhibited a good adhesion performance as the adhesion level of allanticorrosion coating films reached 5B (Table 4). From Table 4, thecorrosion resistance of the samples in different coating thicknesses isimproved during the same time, this is due to the active composition(RC-GAE) in the polymer composite coating film increased from lowercoating thickness to higher coating thickness. Moreover, the corrosionresistance for the same coating thickness decreased, due to the corro-sion of NaCl on the surface of the polymer composite coating film. Thesalt spray corrosion resistance test is a key indicator of the corrosion
Fig. 7. XRD patterns of the samples before and after treatment with the rust converter.
Fig. 8. XPS spectra of the plate samples before and after the treatment with the rust converter.
Table 3Element composition and electron binding energy of the rust before and after the treat-ment with the rust converter.
Element Condition Content (Massfraction%)
Binding energy (eV)
Fe2p Before treatment 50.06 712.14 (Fe2p3/2) 722.80(Fe2p1/2)
After treatment 78.11 711.25 (Fe2p3/2) 722.23(Fe2p1/2)
O1s Before treatment 44.37 531.58After treatment 4.68 530.88
C1s Before treatment 5.57 285.44After treatment 17.21 285.71
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resistance, localized corrosion and pitting of the coating, and it is alsothe most classic and most widely used accelerated corrosion testmethod to predict the useful life of the coating by evaluating the cor-rosion resistance, localized corrosion and pitting of the coating in theworld. As the neutral salt spray corrosion resistance showed a highprotection rating (Rp/240 h = 10) and the protection rating did notdecrease obviously as time goes on, indicating that the coating film notonly can well avoid the pitting corrosion and localized corrosion butalso exhibits a superior salt spray resistance.
4. Conclusion
Novel RC-GAE was successfully prepared though an esterificationmethod. The mass ratio of PG and GA was studied to obtain the optimalanticorrosion performance, which was confirmed by the Tafel curves.The as-prepared RC-GAE shows superior rust removal, antirust, andcorrosion resistance capacities. The use of RC-GAE is expected to in-crease the barrier properties and protective capacity of the water-basedresidual rust coatings, and decrease the penetration of aggressive spe-cies promoting corrosion when the coated metal is exposed to a cor-rosive environment. Importantly, the gallic ester, used as the rustconverter, presents an excellent water solubility and compatibility withthe water-based anticorrosion coating than gallic acid. Therefore, it canavoid pitting corrosion and be used directly in the anticorrosion coating
without any cosolvent. Moreover, gallic ester shows no damage on thesurface of steel structure when used for steel structure surface withlarge difference in corrosive status. This method can be used for pre-paring multifunctional polymer composites coating for various appli-cations [49–56].
Acknowledgement
The authors wish to acknowledge Jiangxi Boshiming TechnologyIndustry Co., Ltd. (Shangrao, China) for financial support of this study.
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