effects+of+nano+pigments+on+the+corrosion+resistance+of+alkyd+coating

8/12/2019 Effects+of+Nano+Pigments+on+the+Corrosion+Resistance+of+Alkyd+Coating

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552 J. Mater. Sci. Technol., Vol.23 No.4, 2007

Fig.1 TEM micrographs of dispersed: (a) nano-TiO2, (b) conventional TiO2, (c) nano-ZnO and (d) conventionalZnO in alkyd varnish

emical impedance spectrum (EIS) experiment were

fabricated into 50 mm50 mm sheets. Substrateswere polished with 800 emery paper, then degreasedwith acetone and washed by ethanol. The coatingfilms were coated by spreader bar and were dried atroom temperature. The final thickness of coated filmswas about 20 m for mechanical test and 30 m forcorrosion test.

In order to solve the dispersion problem of nanopigments in alkyd coatings, nano-oxide concentratedslurry was firstly prepared[17]. The solvent, nanopigments and dispersants were mixed, and then themixture was ground to avoid the agglomeration ofnanoparticles. The preparation of nano-compositecoatings was as follows. The nano pigment incorpo-rated varnish and colored coatings were mixed anddispersed for 15 min, then ground for 8 h to obtain theproper fineness. In this work, P /B (pigment/binder)is 0.02. When colored coating was applied, the forma-tion is composed of alkyd resin (64.6 g), conventionalTiO2 (13 g), French chalk (7 g), conventional ZnO(0.8 g), kaoline (8 g), flatting agent (1.2 g), whitespirit (7.9 g) and cobalt dryer (3 g). A series ofamounts of nano-TiO2 (0.5%, 1% and 2%) was incor-porated into colored alkyd coating to determine theoptimal content.

2.3 Electrochemical test

All EIS measurements were done at room tem-perature in 3.5% NaCl solution. Test system con-sisted of a three-electrode cell, where a saturatedcalomel electrode (SCE), a stainless steel electrodeand a coated coupon were used as reference, counter,and working electrodes, respectively. The workingarea of cell was 12.56 cm2. Experimental tests wereperformed under the open circuit potential. EIS mea-surement instruments included an EG&G 273 Poten-tiostat/Galvanostat and an EG&G 5210 lock-in am-plifier (0.5 Hz-120 kHz) integrated with a PC system.Impedance spectra of coupon in different immersiontimes were recorded in the frequency range of 101

105

Hz with initial time delay of 100 s and sinusoidalalternating potential signal of 20 mV. The EIS datawere analyzed by Z-view 2.2 software.

2.4 Corrosion test

Salt spray exposure was carried out according toASTM B117. The coated panels were exposed to a5% NaCl solution at 352C for 96 h. For this exper-iment, the A3 steel plates were coated with varnishand pigmented varnish (P/B=0.02). The panels werestudied under scratched (using X shape) condition.

2.5 Viscosity measurementA DV-III ULTRA programmable rheometer from

Brookfield Company, USA, was used to measure theviscosity of varnish and pigmented varnish as a func-tion of rate of shear.

3. Results and Discussion

3.1 Corrosion analysis of alkyd coatings with TiO2andZnO pigments

3.1.1 EIS of varnish The Bode plots of EISobtained at different exposure times of the specimencoated with varnish immersed in 3.5% NaCl solutionare shown in Fig.2. It can be seen that the impedancevalues of varnish remained relatively high until im-mersed for 648 h, which indicates that after the spec-imen was cured at room temperature, it was hard forelectrolyte to penetrate through the coating film.

Fig.2 Bode plots of alkyd varnish immersed in 3.5%

NaCl solution at different immersion times


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J. Mater. Sci. Technol., Vol.23 No.4, 2007 553

Rs

Qc

Rpo

Rs

Qc

Rpo

Qdl

Rct

Fig.3 Equivalent electrical circuit in the initial stage ofimmersion

Fig.4 Equivalent electrical circuit in the middle and theend stage of immersion

Fig.5 Evolution of pore resistance (a) and coating ca-pacitance (b) vs immersion time for alkyd varnish

The interpretation of impedance spectra was per-formed after numerical fitting using the equivalentcircuits presented in Figs.3 and 4. The equivalentcircuits are composed of the solution resistance Rs,the pore resistance Rpo due to penetration of elec-

trolyte, and the capacitance of the coating Qc. Anal-ysis of impedance spectra according to the equiva-lent circuit of Figs.3 and 4 rendered the parametersRpo and Qc to be changed as a function of time asshown in Fig.5. TheRpo value showed a sharp de-crease during the first 12 h and then a sharp in-crease. After 144 h of immersion, Rpo slowly de-creased with time, which can be explained by the dis-solution of corrosion product from substrate to theNaCl solution. The decrease in Rpo during the ini-tial stage of immersion can be attributed to the up-take of saturated electrolyte in coated film. Con-trasting to the above decrease in Rpo, Qc increased

during the initial immersion stage. The followingsharp increase in Qc can be ascribed to the block-ing of transmission channels by the corrosion product.

Fig.6 Bode plots for alkyd varnish containing conven-tional TiO2 (a) and nano-TiO2 (b) immersed in3.5% NaCl solution after different times

Correspondingly, electrolyte penetrated into largerarea of interface of substrate/coating, characterisedby the increase in Qc in the end stage. Qc was ap-

proximately 4109

F after 312 h of immersion, twicemore than that in the initial stage.3.1.2 EIS of varnish with pigments Figure 6(a) and(b) show the Bode plots of conventional and nanome-ter TiO2pigmented alkyd coatings ofP/B=0.02 afterdifferent immersion times in 3.5% NaCl solution. Theimpedance value of the low frequency varied with thecoatings. As shown in Fig.6(a), alkyd coating con-taining conventional TiO2 exhibited relatively highresistance during the initial stage of immersion, withimpedance value in low frequency reaching approx-imately 109 orders of magnitude. The impedancevalue decreased with time. However, the impedanceof the prewashed cold rolled steel coated with conven-

tional TiO2 showed still high impedance value (above106 cm2) after 648 h of immersion. Such high


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Fig.7 Bode plots for alkyd varnish containing conventional ZnO (a) and nanometer ZnO (b) immersed in 3.5%NaCl solution after different times

Fig.8 Bode plots for alkyd varnish and pigmented alkydvarnish immersed in 3.5% NaCl solution after2164 h

impedance value shows good barrier effect of conven-tional TiO2pigmented alkyd coatings. Two time con-stants were present in the Bode plot of 648 h, whichcan be ascribed to the formation of Fe oxide or hy-drate during immersion. In the case of nano-TiO2pigmented alkyd coatings, higher impedance valueswere reflected in low frequencies in Bode plot un-til the immersion time was up to 648 h, showingthat nano-TiO2 as pigment performed excellent bar-rier effect for alkyd coating. Additionally, it is ev-ident that the impedance value in low frequency ofnano-TiO2 pigmented coatings fluctuated with im-mersion time, which was probably due to the disso-lution of old corrosion product and regeneration of

the new and thicker corrosion product in electrolyte.The above results can be also observed in Bode plotof conventional TiO2 pigmented alkyd coating. Fig-ure 7(a) and (b) show the Bode plots of conventionaland nanometer ZnO pigmented alkyd coatings forP/B=0.02 after different immersion times. Conven-tional ZnO pigmented alkyd film maintained the rela-tively high impedance values,i.e., approximately 107108 cm2. The impedance value of nano-ZnO pig-mented coating decreased with immersion time, butthe value was much lower than that of conventionalZnO pigmented coatings. It is worthy to note thatthe difference of impedance value in low frequency is

evident for nano-ZnO compared with other pigments.No apparent impedance rise in the latter stage of im-mersion can be observed. The reason was that the

coated film was delaminated from the substrate andthe corrosion product could not effectively block thesaturated area with immersion time.

After 2164 h of immersion, the Bode plots foralkyd varnish and pigments added varnish are shown

in Fig.8. It is clear that the low frequency impedanceof nano-TiO2 pigmented coating is the highest amongall coatings, indicating that the nano-TiO2 particlescan effectively block the pores formed in coating film.The excellent anticorrosion protection is attributed tothe size and shape of nano-TiO2, which has the small-est size and largest aspect ratio. For the small parti-cles, free space between the particles and resin is farlesser than that of larger particles. Thus electrolyteis harder to penetrate through the pores in coatingfilm with addition of nano pigment[18]. In addition,due to longer diffusion path around the nano-TiO2 inthe film, the water and ions need more time to arriveat the substrate. In contrast, the impedance value inlow frequency of coating film pigmented with conven-tional TiO2 is below 110

6 cm2, which indicatesthe coated film is poor for protection at this time.As for the conventional ZnO pigmented coating, theimpedance value is similarly lower than 106 cm2

as conventional TiO2. At this time, the severe rust-ing can be observed on the substrate. Indeed, thefilm was already delaminated from the substrate andwas thinner than the original one. Similar statecan be observed for substrate coated with varnish.Whereas for the coating pigmented with nano-ZnO,the impedance value decreased to 103 cm2, suggest-ing that the nano-ZnO pigmented film was seriously

corroded. The coated film thoroughly shelled off fromthe substrate.Fitting values of the coating pore resistance and

capacitance as a function of the exposure time for thepigmented alkyd coatings are shown in Fig.9. Thepore resistance, which reflects the resistance of coat-ing film to electrolyte penetration, generally deceaseswith time (Fig.9(a)). Clear difference is evident whenthe four samples are compared. For nano-TiO2 pig-mented coating, Rpo value gradually decreased withthe exposure time, but the coating maintained thehighest Rpo values. Furthermore, unlike the otherthree coatings, no sharp decrease was found in the endstage of the immersion and the ultimate Rpo value

was still above 107 . For the conventional TiO2 andZnO, a similar trend of decrease in Rpo with time was


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Fig.9 Evolution of pore resistance (a) and capacitance (b) of alkyd coatings loaded with different pigments duringthe immersion in 3.5% NaCl solution

Fig.10 Aspects of alkyd varnish (a), conventional TiO2 pigmented varnish (b), nano-TiO2 pigmented varnish (c),nano-ZnO pigmented varnish (d), and conventional ZnO pigmented varnish (e) after exposure in salt fogfor 96 h

Fig.11 Typical Nyquist plots of EIS of alkyd coating containing different nano-TiO2 content: (a) 0, (b) 0.5%,(c) 1%, (d) 2% at different times immersed in 3.5% NaCl solution


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Fig.12 Evolution of p ore resistance (a) and capacitance (b) of alkyd coating loading with different content nano-TiO2 pigments during the immersion in 3.5% NaCl solution

observed, and a rapid decrease after 648 h was appar-ent. For nano-ZnO pigmented alkyd coating, theRpovalues were sharply increased at the start, but wereobviously lower than the other three after 648 h.

The change of the capacitance of the coating (Qc)

related to water uptake is shown in Fig.9(b). In theend stage, Qc of alkyd coating containing nano-ZnOexhibited the highest value, showing that the coatedfilm was delaminated from substrate. The followingwere coatings containing conventional TiO2 and con-ventional ZnO, with the nano-TiO2 pigmented coat-ing showing the smallest value ofQc. From the re-sults ofQc and Rpo, it can be concluded that thenano-TiO2 pigmented alkyd coating exhibits the bestprotective property.3.1.3 Corrosion test The corrosion resistance ofpigmented coatings was determined by the rusts andblistering along the X marks. The aspects of alkyd

varnish and pigmented varnish after exposure in saltfog for 96 h are shown in Fig.10. As can be seen inthe figure, no apparent rust and blistering along thescribes were observed on the nano-TiO2 pigmentedcoating surface. The similar situation along X markoccurred on alkyd varnish surface, however, large areaof small rusts could be observed in the other part ofsurface. For conventional TiO2 pigmented coating,small blisters with diameter of 0.010.3 mm could beobserved along X mark. In the cases of nano-ZnOand conventional ZnO pigmented coating, water sat-urated zone with width of 0.75 cm was found alongthe X marks.3.1.4 EIS of colored coating containing nano-

TiO2 In order to investigate the effect of nano-TiO2 content on the corrosion resistance of alkydcoatings, the EIS plots of alkyd colored coatingswith a series of nano-TiO2 contents (wt pct) from0 to 2% were compared. Typical Nyquist impedanceplots after different immersion times for alky coat-ings with different contents of nano-TiO2 are shownin Fig.11(a)(d), respectively for 0, 0.5%, 1%, and 2%.

For colored coating without nano-TiO2, EIS spec-tra show apparent two time constants after 48 h ofimmersion in Fig.11(a), which is related to the cor-rosion activity due to the Fe dissolution and O re-duction. For coating with 0.5% nano-TiO2, lower

impedance modulus values were reflected in Nyquistdiagram than the previous one. After 48 h of im-mersion, a 45 slope is obtained after the semi-circle,

as shown in Fig.11(b), which is the characteristic ofWarburg resistance, indicating that a possible diffu-sional progress occurred and corrosion product hadclogged the path of water transportation. For coat-ing with 1% nano-TiO2, from 0 to 1032 h, only one

semi-circle is observed with the values of radius big-ger than 107 cm2. The above results suggest thatthe coating was compact in structure with improvedanticorrosion performance. With the content of nano-TiO2 increased to 2%, it is evident that, althoughsemi-circle is as well presented in Nyquist plot, theradius of the semi-circle is much smaller and the ra-dius of the second one is below 5106 cm2, showingthe decrease in barrier effect of coating. It is pos-sible that the agglomeration of nano-TiO2 particlesoccurred with addition of 2% content. When nano-TiO2 content was more than 2%, the resistance valuedecreased fast in short time and more serious agglom-eration could occur.

Figure 12 shows the fitting values of coating poreresistance and capacitance as a function of immersiontime for the colored coatings. The pore resistancevalues similarly decreased with time. As shown inFig.12(a), the coating containing 1% nano-TiO2 ex-hibited the highest pore resistance among the fourcoatings in the immersion time. For 0.5% nano-TiO2, the coating showed the lowest resistance value.Whereas for 2% nano-TiO2, Rpo value fluctuated atfirst immersion stage and was a little more than thatof without nano-TiO2 in the end stage of immersion.As far as the pore resistances of nano-TiO2pigmentedare concerned, the above results show that the optimal

content of nano-TiO2 is 1%. If the content of nano-TiO2 was insufficient (less than 1%), nano particlescould not fully play the barrier effect and the corro-sion resistance was partly counteracted by the uptakeof water that reflected by the increase in Qc. Exces-sive dosage (more than 1%) would cause agglomera-tion. Thus, it can be concluded that a critical amountof nano pigment exists in nano-composite coatings.

As shown in Fig.12(b), the capacitance (Qc) val-ues of nano-TiO2pigmented colored coating graduallyincreased with time, which are higher values than thecoating without nano-TiO2, indicating that the addi-tion of hydrophilic nanoparticle could result in moreuptake of the electrolyte solution in coating film. In

the latter immersion stage, it can be seen that thecapacitance of the coating containing 2% nano-TiO2


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Fig.13 Viscosity of alkyd varnish

Fig.14 Viscosity of pigmented alkyd varnish

was higher than those of the other three ones. It can

be inferred that if agglomeration occurs, transmissionof water will transfer from zigzag path to big pore, andthus the capacitance of coating is enhanced. Further-more, with the addition of more nano-TiO2 particles,the area of absorbed points is increased, which resultsin the increase in absorbed electrolyte, and thus thecapacitance of coatings increases. From the capac-itance point of view, it can be also concluded thatthe amount of nano pigment is critical to corrosionresistance of nano-composite coatings.

3.2 Mechanical property of pigmented alkyd varnish

For P/B=0.02, the mechanical properties of pig-mented varnish were studied. The results show that

the coatings with addition of all the four pigmentswere high in flexibility and impact resistance due tothe characteristic of alkyd resin. The values of flexibil-ity and impact resistance were 1 mm and 120 kgcm,respectively. The hardness of the nano-TiO2 addedcoating is the highest, with value of HB (pencil hard-ness), whereas it is B for others. The improvement inhardness is probably due to the formation of a spacialframework by the uniform and stagger distribution ofnano-TiO2 particles in coating film, and the hardnessof nano-TiO2 in essence. Also, as shown in Fig.1(a),nanoparticles of TiO2 have higher aspect ratio, whichcould result in the higher strength[19].

3.3 Viscosity of pigmented alkyd varnish

Thixotropy is the reflection of the microstructure

of coatings[20]. In general, the change of thixotropycompanies with the change of pigment state. A bene-ficial attempt to observe the change of nano pigmentstate is to measure the viscosity related to thixotropyusing rheoviscometer. Figures 13 and 14 show the vis-cosity curve of varnish and varnish added with 2% (inwt pct) conventional TiO2 and ZnO, nano-TiO2 andnano-ZnO, respectively, as a function of rate of shear.As shown in Fig.13, the viscosity of varnish is very

low and decreases with the rate of shear, indicatingthat the alkyd resin is non-Newtonian fluid. Addi-tion of nano-ZnO caused the increase in viscosity andpresented coating as non-Newtonian fluid. Other pig-mented coatings show the characteristic of Newtonianfluid. Corresponding to results as shown in Fig.8, non-Newtonian coating,i.e. nano-ZnO pigmented coating,showed the poor corrosion resistance in the long im-mersion time. On the contrary, Newtonian coatings,i.e. conventional ZnO and TiO2 and nano-TiO2 pig-mented coatings presented the relatively higher re-sistance. Additionally, it seems that low-viscositycoatings represent higher corrosion resistance. It ispresumed that high viscosity will reduce the extent

of curing, which results in low crosslinking density,so electrolyte is prone to penetrate through coatingfilm and the corrosion resistance could be lowered.The further study is being carried out to investigatethe viscosity on anticorrosion performance of organiccoatings.

4. Conclusions

(1) For P/B (pigment/binder)=0.02, Nano-TiO2pigmented alkyd varnish exhibits the best cor-rosion resistance among the four nanometer andconventional ZnO and TiO2 pigmented coatings.

The order of anticorrosion performance of the fourcoatings was as follows: nano-TiO2>conventionalTiO2>conventional ZnO>nano-ZnO. Additionally, interms of the mechanical property, nano-TiO2 pig-mented varnish is the highest in hardness than others.Small size effect and high aspect ratio of nano-TiO2are the probable reasons to inhibit the corrosion andreinforce the barrier effect of coating film.

(2) The experiment on amount of nano-TiO2 in aformation evidences that 1% is the optimal amountfor alkyd colored coating. Excessive incorporationof nano-pigment could cause agglomeration. Also,it shows that the critical amount is a crucial factor

to improve the anticorrosion performance of nano-composite coatings.

(3) Except for nano-ZnO pigmented alkyd var-nish, conventional ZnO and TiO2 and nano-TiO2 pig-mented coatings behaved as Newtonian fluid. Viscos-ity of pigmented alkyd varnish probably has relationwith corrosion resistance when the coating is cured.Due to high viscosity, the extent of curing is reduced,which results in low crosslinking density, so that elec-trolyte is prone to penetrate through coating film andthe corrosion resistance could be lowered.

Acknowledgement

The authors are grateful for the financial supportfrom the National Natural Science Foundation of China(No. 50499334).


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