Grain Refiner Effect of Black Wattle Tannin in Iron and ZincPhosphate CoatingsRafael S. Peres,*,† Eduardo Cassel,‡ Carlos A. Ferreira,† and Denise S. Azambuja§

†Escola de Engenharia/PPGE3M − Laboratorio de Materiais Polimericos, UFRGS, Av. Bento Goncalves 9500, 91501-970, PortoAlegre, Brasil‡Faculdade de Engenharia − Departamento de Engenharia Química, PUCRS, Av. Ipiranga 6681, 90619-900, Porto Alegre, Brasil§Instituto de Química − Laboratorio de Eletroquímica, UFRGS, Av. Bento Goncalves 9500, 91501-970, Porto Alegre, Brasil

ABSTRACT: This article reports a study of the addition of black wattle tannin to a phosphating bath as an environmentallyfriendly grain refiner additive. Scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, and electrochemicalimpedance spectroscopy analysis were carried out to verify the tannin effect. Adhesion was measured according to the ASTMD3359 standard. The presence of tannin in the zinc phosphating bath changed the direction of phosphate crystal growth andfavored phosphophylite phase formation (XRD analyses). SEM images showed a reduction in the size of the zinc phosphatecrystals when tannin was present in the bath. Because of this decrease in size, the adherence of the final coating improved. Theoptimal concentration of black wattle tannin in the zinc phosphating bath used in this work was 2 g L−1, as higher values reducedthe corrosion resistance of the coating. Grain size reduction was also observed in iron phosphating.

1. INTRODUCTION

One of the most well-known surface pretreatments, still largelyused in the automotive1 and industrial fields, is the phosphatingprocess.2−5 This type of treatment is used to modify the surfaceof ferrous and nonferrous materials.1−6 An insoluble metalphosphate film is formed and deposited on the metal surface,modifying its properties.4,7 This treatment may also increasethe corrosion resistance of the phosphatized sample andimprove the adhesion of organic coatings.4,6 The phosphatingprocess has been widely studied, as shown in a review byNarayanan.8 However, several modifications and improvementshave been proposed since the first phosphating bath wasformulated.8 The coating formed in the phosphating process ishighly porous, which allows electrolytes to attack thesubstrate.1,9 Thus, phosphate coatings alone do not ensurecorrosion resistance of the underlying material and need to beused concomitantly with other treatments, such as organiccoatings or pore sealers.2,6,10 The use of grain refiners improvesthe adhesion of subsequent paints by increasing the surfacecontact between the coating and the phosphated crystals.8

Some heavy metals such as manganese11 are used as grainrefiners, but nonharmful environmental refiners such as Ca12

and SiO213 have also been developed. Sheng et al.13 used nano-

SiO2 to replace the nitrite additive in the zinc phosphating bathand reported better corrosion resistance of the resulting coatingand smaller crystals when compared with the traditionalmethod.13

On the other hand, tannins have been used for many years asnatural corrosion inhibitors for carbon steel because of theirefficiency in acid mediums and their formation of a metalliccomplex.14−16 Tannins comprise two different classes ofpolyphenolic compounds: hydrolyzable and condensed tannins.The condensed tannins, found in the bark of the black wattletree (Acacia mearnsii), consist of tricyclic and hydroxylated

units containing 15 carbons.17,18 The hydrolyzable tannins are amixture of simple phenols and sugar esters.18

The use of tannins in phosphating baths has been describedin commercial patents.19−22 Emeric et al.19 developed aphosphating treatment for preoxidized steel which contains atannin and a surfactant in its formulation. This phosphatingbath is useful for application in areas where it is not possible touse abrasive blasting because of the capacity of tannin to reactwith rust. Dodd et al.20 proposed the use of tannins inconjunction with metallic compounds to accelerate theformation of phosphate coatings. Pedrazzini21 employedhydrolyzable tannins to form insoluble chelates on steelsurfaces. Recently, Hsu et al.22 developed a phosphatingtreatment for magnesium alloys containing tannin, phosphoricacid, and dihydrogen phosphate magnesium. To the best of ourknowledge, the effects of tannins on crystal size duringphosphating have not been reported to date.The goal of this work was to investigate the use of condensed

black wattle tannin, obtained from the bark of the black wattletree (A. mearnsii), as an environmentally friendly grain refinerin zinc phosphate coatings for carbon steel and its effect on thefinal coating. To verify the influence of tannin in anotherphosphating process, the modification of grain size in ironphosphating was also studied in the absence of an accelerator oradditional additives.

2. EXPERIMENTAL PROCEDURE2.1. Materials. All solutions and samples were prepared

using analytical grade reagents. To prepare the zincphosphating bath, zinc oxide (Vetec, Brazil), nitric acid

Received: November 11, 2013Revised: January 6, 2014Accepted: January 23, 2014Published: January 23, 2014

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(Merck, Germany), nickel sulfate (Vetec, Brazil), andphosphoric acid (Merck, Germany) were used. The ironphosphating bath was prepared with sodium phosphatemonobasic (Vetec, Brazil) and phosphoric acid (Merck,Germany). Hydrochloric acid (Vetec, Brazil) was used toprepare an acid pickling bath, and sodium sulfate (Vetec,Brazil) was employed in the preparation of the electrolytesolutions for electrochemical impedance spectroscopy (EIS)measurements.A two-component epoxy primer, Intergard 269 (Akzo Nobel,

U.S.), was used as a finishing coating in adherence tests. Thecondensed tannin obtained from the black wattle used in thisstudy was supplied by TANAC (Montenegro, Brazil). The sizesof carbon steel samples were 11 × 50 × 1 mm3 for theadherence tests and 15 × 25 × 1 mm3 for the scanning electronmicroscopy (SEM), EIS, and X-ray diffraction (XRD) analyses.The composition of the steel samples is listed in Table 1.

2.2. Sample Preparation and Experimental Setup. Thecarbon steel surface was prepared by degreasing with anacetone/chloroform mixture, polishing with sandpaper up tograde #1200, and degreasing and drying with hot air. Before thezinc phosphating process, the steel samples were immersed inan acid pickling bath (HCl 30% (v/v) solution at room

temperature) for 2 min. The zinc phosphate bath used in thiswork was formulated with 17.5 g L−1 of H3PO4, 20.0 g L−1 ofHNO3, 11.5 g L−1 of ZnO, and 0.25 g L−1 of NiSO4. Theconcentrations are referred to deionized water. The zincphosphating treatment was carried out by immersing the steelsample in the bath described above for 10 min at 80 °C in thepresence of 2, 4, and 8 g L−1 of black wattle tannin (tannin zincphosphating coating TZn2, TZn4 and TZn8, respectively), and inthe absence of tannin (conventional zinc phosphating coating,CZn). After the phosphatization process, the samples wererinsed with deionized water and dried with hot air.For the iron phosphating treatment, the degreased and

abraded steel sample was immersed in a solution of 10 g L−1

NaH2PO4 at pH 3.5 (acidified with H3PO4) in the absence(conventional iron phosphating coating CFe) and presence(tannin iron phosphating coating TFe) of 0.5 g L−1 of blackwattle tannin. The concentration of NaH2PO4 is also referredto deionized water. The total time of immersion for the ironphosphating procedure was 24 h. After the immersion, thesamples were rinsed with deionized water and dried with hotair.The composition of the zinc phosphate samples was

investigated by X-ray diffraction analysis using a Philipsdiffractometer (model X′Pert MPD) and Cu Kα radiation. X-ray patterns were created (with a step size of 0.02°) to identifythe material at a 2θ angle (4°−70°). Compounds wereidentified by comparing the experimental diffractograms withJoint Committee on Powder Diffraction Standards (JCPDS)using the X′Pert HighScore software.

Table 1. Chemical Composition of Steel Samples

chemical composition

element C Mn P S Cu Cr

wt % 0.103 0.46 0.013 0.096 0.01 0.18

Figure 1. XRD patterns of zinc phosphate coatings (a) CZn, (b) TZn2, (c) TZn4 and (d) TZn8 obtained from baths containing different concentrationsof black wattle tannin (0 , 2, 4, and 8 g L−1, respectively).

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Working electrodes with a size of 25 × 15 × 1 mm3 wereprepared from the zinc phosphatized steel samples. An area of1.0 cm2 was delimited by epoxy resin. A saturated calomelelectrode (SCE, E = 0.241 V/NHE) was used as the referenceelectrode, and a Pt rod was used as the auxiliary electrode. TheEIS measurements were carried out under naturally aeratedconditions at room temperature using an AUTOLAB PGSTAT30 potentiostat (Echo Chemie, Netherlands) coupled to afrequency response analyzer (FRA 2). The software used forthe analysis of impedance spectra and data fitting was NOVA1.7 (Echo Chemie, Netherlands). The EIS measurements wereperformed in potentiostatic mode at the open circuit potential.The amplitude of the EIS perturbation signal was 10 mV andthe frequency studied ranged from 105 to 10−2 Hz. The fittingof the EIS data was considered ideal only when errors smallerthan 5% for all components of the equivalent electrical circuitwere achieved.The SEM instrument was a PHILIPS XL30 coupled to an

Edax energy dispersive spectrometer (EDS). All samples werecovered with a thin gold layer prior to analysis to avoid contactproblems. The SEM system used an accelerating voltage of 20kV, a working distance of 11.2 mm, and a spot size of 5.3.The epoxy primer was applied by brush onto samples that

were used in the adhesion test. The dry paint film thickness wasmeasured using a Byko-test 7500 electromagnetic film thicknessgauge (BYK Gardner, Germany). The mean value of sixdifferent areas of the steel samples was used to calculate thefinal dry film thickness. Adhesion measurements were carriedout following the ASTM D3359 standard test23 instructionsand those of Lakshmi et al.24 To measure coating thickness, 22cuts spaced 1 mm apart were made in the organic coating inperpendicular directions (11 vertical and 11 horizontal) to formsmall squares. The residual coating was then cleaned andScotch 880 (3M, U.S.) tape was attached over the cuts. Thetape was then pulled off with parallel movement relative to thesurface. Finally, the sample was compared and classifiedaccording to the standard.

3. RESULTS AND DISCUSSION3.1. X-ray Diffraction Analysis. Different concentrations

of black wattle tannin (0, 2, 4, and 8 g L−1) were added to thezinc phosphating baths. To verify possible changes in thecoating structure of CZn, TZn2, TZn4, and TZn8, X-ray diffraction(XRD) analysis was performed. The XRD patterns are shownin Figure 1. The main phases present in the four coatingsappeared as expected for this type of phosphating: iron,phosphophylite, and hopeite, in accordance with theliterature.1,6,8,13 However, variation in several peak behaviorswas observed when the tannin was added to the phosphatingbath. The hopeite peaks at a 2θ angle of 9.8° and 19.5° ((020)and (040) planes, respectively) and phosphophylite peaks at10° and 20° ((100) and (−111) planes, respectively) reachedhigher values as the tannin concentration increased. The insetdiffractogram in Figure 1 shows more clearly the intensity ofthe detached planes. The value of the iron peaks ((110) and(200) planes) was similar in ∼1200 counts. Several peaks (e.g.,between a 2θ angle of 16.7° and 18.6° and 50.0° and 60.0°)assumed lower values and even disappeared when more tanninwas added to the bath. These variations in the XRD peaksconfirmed a modification in the direction of crystal growthwhen more black wattle tannin was added to the phosphatingbath. An alteration in the coating morphology was alsoexpected. A change in the growth direction of the crystals

was also reported by Sheng et al. when nano-SiO2 was added tothe zinc phosphating bath.13

Modifications in peak intensities can be reflected in the totalphase amount. Thus, semiquantitative analyses using thereference intensity ratio (RIR)25 in the X′Pert HighScorePlus software were conducted. According to Figure 2, the

addition of black wattle tannin to the zinc phosphating bathinfluenced the phase quantification. Figure 2 shows an increasein the percentage phosphophylite and consequently a decreasein the amount of hopeite when more tannin was added to thebath. According to Reynolds,26 when more iron is present inthe bath, more phosphophylite is formed because ofreplacement of a zinc atom by an iron atom in thephosphophylite (Zn2Fe(PO4)2·4H2O) phase. The substrateused in this work is a carbon steel, which is a source of iron inthe bath. Thus, the presence of tannin in the bath can improvethe iron release. On the other hand, if the black wattle tannincould act as tailor-made additive, an increase in iron amount isnot mandatory. The addition of organic tailor-made additivescan control the crystal size and shape in several crystallizationprocedures.27,28 A modification in the active sites of a growingcrystal substrate (steel) can be made in the presence of anorganic additive, and consequently a change in the orientationand size of the crystals can be observed.29 The tailor-madeadditives can inhibit or promote the crystal growth being usedin many applications.29 As an example, Graham et al.27 usedthis kind of additive to predict the morphology of bulk-heterojunction photovoltaic cells. Some authors reported thattannins can be adsorbed on a metal surface depending on thepH value of the medium.30−33 Thus, the tannin can also modifythe steel active sites, influencing the crystal orientation and size.

3.2. Electrochemical Impedance Spectroscopy. Theinfluence of black wattle tannin on the corrosion resistance ofthe final zinc phosphating coating was verified by EISmeasurements. The EIS spectra for the phosphatized samplesimmersed in aerated 0.1 mol L−1 Na2SO4 solution for 4 h areshown in Figures 3 and 4.The RS(CPE1R1)(CPE2R2) equivalent circuit (EC) previ-

ously proposed in the literature10,34 was used to fit theelectrochemical experimental data (inset of Figure 4b). Ohmicresistance between the SCE and sample electrode symbolizedby RS, R1, and R2 are the charge transfer resistance in physicaldenotation.35 CPE1 and CPE2 are the constant phase element

Figure 2. Variation in phase composition (as a percentage) for zincphosphating samples according to the black wattle tannin concen-tration in a zinc phosphating bath.

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(CPE)36 and can be attributed to the electrode surface(heterogeneities and roughness) or adsorbed species.35−37

Two time constants (R1CPE1 and R2CPE2) appear in Bodeplots (Figure 4a) with maximum phase angles of around −45°.These values of maximum phase angles are attributed todiffusion process, especially in a porous layer that is the case ofzinc phosphate coating.10,34 The first time constant isassociated with the internal conversion layer and the secondwith the external porous layer.35

For this work, the overall impedance (Zoverall) is obtained bythe sum of the internal and external layer impedances, asproposed in the literature.10,34 The data obtained from theequivalent electrical circuit are summarized in Table 2.The effect of tannin concentration on the corrosion

resistance (Rcorr) of the zinc phosphating coatings is illustratedin Figure 5, which shows the variation in the Zoverall valuesobtained from the simulated parameters (sum of R1 and R2).The high values of impedance found for CZn and TZn2 (0 and 2g L−1 of tannin, respectively) indicated better corrosionresistance. This was supported by a small increase in thecorrosion resistance (∼12%) of the TZn2 coating. On the otherhand, when the tannin concentration exceeded 2 g L−1, thisparameter started to decrease. Gorecki38 reported that anincrease in corrosion resistance is observed when an idealconcentration of iron is present in the bath. However, when the

iron content exceeds the optimum, corrosion resistancedecreases.38 However, the corrosion resistance of the TZn4

and TZn8 coatings was reduced; thus, the optimal concentrationof black wattle tannin in the zinc phosphating bath used in thiswork was 2 g L−1.

3.3. Scanning Electron Microscopy. Because 2 g L−1 wasdefined as the best concentration of tannin for the studiedphosphating process, the surfaces of the TZn2 and CZn sampleswere analyzed by SEM in a backscattering electron (BSE)mode. Figure 6 shows the micrographs taken at differentmagnifications of zinc phosphate coating obtained in thepresence (TZn2) and absence (CZn) of 2 g L−1 of tannin in thephosphating bath. The addition of black wattle tannin changedthe shape of the zinc phosphate crystals, and the TZn2 coatinghad crystal sizes smaller than those of the CZn coating.According to Reynolds,26 phosphophylite crystals are smallerthan hopeite, which can explain the morphology shown inFigure 6.

3.4. Adhesion Analyses. The results of the ASTM D3359standard23 are listed in Table 3. No primer was removed fromthe TZn2 sample, which corresponded to the 5B classification.The CZn sample was classified as 4B, which means primerremoval in some areas. As expected, the unphosphated samplehad the worst adherence rate (1B), as more than 50% of theprimer detached. Reynolds26 reported that phosphophylitegrows in organized directions, unlike hopeite which grows inrandom orientations. This behavior of hopeite generated moreareas without crystal covering, impairing the adherence oforganic coating over the zinc phosphate layer.26 An increase inthe amount of phosphophylite coupled with crystal grain sizereduction enhances the adherence of paint.26 Thus, the additionof black wattle tannin reduced the size of the zinc phosphatecrystals and increased the phosphophylite content, improvingthe adherence of epoxy primer.

3.5. Verification of Grain Size Reduction in IronPhosphate Coating. The effect of black wattle tannin ongrain size reduction was also studied in iron phosphate coating.The purpose of this experiment was to investigate the behaviorof black wattle tannin in another kind of phosphating bath. Forthis phosphatization process, no accelerator or other additivewas used. The best concentration of black wattle tannin for theiron phosphate coating proposed in this work was 0.5 g L−1.

Figure 3. Nyquist plots for CZn (closed square), TZn2 (circle), TZn4(closed star), and TZn8 (open square) samples immersed for 4 h in 0.1mol L−1 Na2SO4.

Figure 4. Bode plots for CZn (closed square), TZn2 (open circle), TZn4 (closed star) and TZn8 (open square) samples immersed for 4 h in 0.1 mol L−1

Na2SO4. The inset in panel b represents the equivalent electrical circuit used to simulate the EIS experimental data of zinc phosphated steel samples.

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Higher concentrations hindered the formation of the coatingon the steel surface, and lower concentrations had no effect.Figure 7 shows micrographs taken in BSE mode at different

magnifications of iron phosphate coating obtained in thepresence (TFe) and absence (CFe) of 0.5 g L−1 of tannin in theiron phosphating bath. A considerable modification in the

morphology of the coating was observed: the TFe crystals wereclearly smaller than the crystals in the CFe sample. The decreasein the light areas in the TFe sample indicated that it had crystalcoverage of the steel surface better than that of CFe.Energy dispersive spectroscopy analysis was carried out to

verify the chemical composition in different areas of the TFe andCFe samples (Figure 8). The EDS spectra in Figure 8a,bconfirmed that dark areas corresponded to iron phosphatecrystals. Thus, the size of areas without protection (exposedsteel) decreased in the presence of black wattle tannin, showingbetter coverage of the steel. As mentioned before, the tannincan be adsorbed in the steel surface in aqueous medium.30−33

The adsorption can modify the active sites of substrate,

Table 2. Fitting Parameters Used to Simulate the EIS Plots for the Phosphatized Steel Samples Immersed in Aerated 0.1 molL−1 Na2SO4 Solution for 4 h

fitting parameters

sample CPE1 (F cm−2 sn−1) n1 R1 (Ω cm2) CPE2 (F cm−2 sn−1) n2 R2 (kΩ cm2)

CZn 1.57 × 10−5 0.62 235.10 1.78 × 10−5 0.65 8.24TZn2 5.77 × 10−6 0.74 420.36 1.38 × 10−5 0.61 9.08TZn4 2.84 × 10−5 0.63 152.16 4.72 × 10−5 0.56 1.56TZn8 2.78 × 10−5 0.54 266.70 1.18 × 10−5 0.69 0.840

Figure 5. Variation in the overall impedance (Zoverall) for phosphatizedcarbon steel samples immersed for 4 h in aerated 0.1 mol L−1 Na2SO4according to the black wattle tannin concentration in a zincphosphating bath.

Figure 6. Micrographs taken in BSE mode at a magnification of 100× for (a) CZn and (b) TZn2 samples. Magnification of 400× for (c) CZn and (d)TZn2 samples.

Table 3. Comparison of the Adhesion Ratings of the PrimerCoating in the Phosphatized Samples

sampleprimer thickness(μm) ± SDa

adhesion rating(ASTM D3359)

without phosphating 37.45 ± 0.41 1Bconventional zinc phosphatecoating (CZn)

38.11 ± 0.35 4B

tannin zinc phosphate coating(TZn2)

38.31 ± 0.37 5B

aStandard deviation.

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influencing the crystal size and distribution over the steelsurface in this kind of phosphating. The grain size reductionbecomes more evident in high magnification as shown in themicrographs of Figure 8.

4. CONCLUSIONS

The use of black wattle tannin as grain refiner of zinc phosphateand iron phosphate coating was evaluated in this study. Theaddition of black wattle tannin changed the orientation of thezinc phosphate crystals over the steel surface as shown by XRDanalyses. The presence of tannin reduced the size of the zincphosphate crystals and favored the formation of phosphophy-lite. Because of this size reduction, the adhesion properties of

the phosphate coating were improved (amelioration of theorganic coating adherence in the phosphated layer). The bestconcentration of black wattle tannin in the zinc phosphatingbath proposed in this work was 2 g L−1, as higher tanninconcentrations reduced the corrosion resistance of thephosphate coating. In iron phosphate coating, black wattletannin also reduced the size of the iron phosphate crystals andenhanced their distribution over the steel surface. Thus, blackwattle tannin can be used as an environmentally friendly andlow-cost grain refiner in zinc and iron phosphating baths at itsideal concentration. This is a simple, low-cost procedure thatcan improve the durability of coating and substrate systems byimproving adhesion properties.

Figure 7.Micrographs taken in BSE mode at a magnification of 100× for (a) CFe and (b) TFe samples. Magnification of 400× for (c) CFe and (d) TFesamples.

Figure 8. Micrographs taken in BSE mode at a magnification of 1600× and EDS spectrum for (a) CFe and (b) TFe sample.

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■ AUTHOR INFORMATION

Corresponding Author*E-mail: rafael.s.peres@gmail.com. Tel.:+55 51 3308 9412.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The Brazilian government agency CNPq, which provided thefinancial support for this study, the Microscopy Center of thePontifical Catholic University (PUC) of Rio Grande do Sul,and LACER from the Federal University of Rio Grande do Sulare gratefully acknowledged.

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