Effect of zinc oxide in combating corrosion in zinc-rich primer

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Progress in Organic Coatings 63 (2008) 389–394

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Progress in Organic Coatings

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Effect of zinc oxide in combating corrosion in zinc-rich primer

R.N. Jagtap ∗, P.P. Patil, S.Z. HassanSurface Coating Division, Institute of Chemical Technology (UICT), University of Mumbai, Mumbai 400019, India

a r t i c l e i n f o

Article history:Received 5 November 2007Received in revised form 17 June 2008Accepted 24 June 2008

Keywords:Corrosion

a b s t r a c t

During the production of zinc pigments, some zinc may get oxidized and remains as impurity. In thepresent study, the effect of spherical ZnO on corrosion protection properties was evaluated along withlamellar zinc. ZnO was added in various weight fractions in epoxy primer coating at 60% PVC. The coatedpanels were evaluated among others for their conductance, packing density and morphology. The corro-sion resistance was measured using salt spray test for 3000 h of exposure. Open circuit potential (i.e.corrosion potential) was measured in 3.5 wt.% NaCl salt solution. The best corrosion protection was

Lamellar zincSpherical ZnOSE

obtained with 15 wt.% of ZnO.© 2008 Published by Elsevier B.V.

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. Introduction

Steel is the most suitable construction material for a wide rangef structures, as it has favorable mechanical properties and highpeed fabrication [1]. However, structures made of steel, soonerr later gets damaged by corrosion and fouling, resulting in directnd indirect losses which could be as high as 5% of the Grossational Production [2]. Combating corrosion of metals or alloys isf paramount importance as during the corrosion reaction the pureetal goes back to its ore state. Coating is one of the most conve-

ient methods for corrosion protection of metals by isolating themrom the contact with environment (O2 and moisture). Hence, pro-ection of metals could be done by sacrificial cathodic and/or barrier

echanisms. Primers with metallic zinc dust provides cathodic asell as barrier protection [3]. Therefore, zinc is widely used in coat-

ngs throughout the world for over half a century and has reachedlevel of high performance [4–6]. Zinc-rich coatings, often called

inc-rich primers, are a unique class of convertible coatings havingnorganic or organic binders highly loaded with metallic zinc-dustigment. In our previous study, we found that lamellar zinc is bet-er than spherical zinc and their combination performed far bettern anti-corrosive properties [7]. Coating systems that include these

igh-performance primer components can provide prolong anti-orrosion protection, depending on the severity of the environment8].

∗ Corresponding author. Tel.: +91 22 2414 5616; fax: +91 22 2414 5614.E-mail addresses: [email protected], [email protected] (R.N. Jagtap).

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300-9440/$ – see front matter © 2008 Published by Elsevier B.V.oi:10.1016/j.porgcoat.2008.06.012

The inherent advantages of the organic coatings over the inor-anic coatings are flexibility, cost and speed of construction [9].rganic zinc-rich primers are commonly formulated from epoxy,poxy ester, urethane, vinyl and chlorinated-rubber as binders. Theost widely used organic zinc-rich coatings are based on epoxy

hemistry. Epoxy resins are characterized by ease of cure and pro-essing, excellent moisture, solvent and chemical resistance andood adhesive strength [10].

The manufacturing of zinc involves atomization of the molteninc to obtain spherical particles [11,12]. During the zinc production,olten zinc may get oxidize to ZnO when it comes in contact with

xygen and eventually pure zinc dust contains traces of ZnO. Thelectrical properties of ZnO are interesting and complex [13]. ZnOcts as an inhibitor to chloride-induced corrosion, seals pores inrimer and improves the barrier properties [14]. Therefore, it woulde an interesting idea to include ZnO in the zinc-rich primer to study

ts effect on corrosion protection properties.

. Experimental work

.1. Raw materials

Lamellar zinc dust was prepared by ball milling the sphericalinc dust obtained from M/s Forage Chemical Industries, India. ZnOas procured from S.D. Fine Chemicals (India). The physical char-

cteristics of these pigments are shown in Table 1. The epoxy resin,ER 524 with epoxy equivalent 181–198 g/equiv. and polyamine,H 541 having amine value 290–325 g/equiv. were obtained fromliogrip, India. The solvents used for improving processability likeylene, methyl ethyl ketone (MEK), n-butanol (AR grade) were

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390 R.N. Jagtap et al. / Progress in Organic Coatings 63 (2008) 389–394

Table 1Physical characteristics of the pigments

Serial No. Physical characteristics Zinc ZnO

1 Form Solid powder Powder2 Color Dark gray White3 Shape Lamellar Spherical45

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tratvOieZnO. This was very well supported by SEM photographs (Fig. 3).

Density (g/cm3) 7.1 5.6Particle size distributionMean size (�m)

42.458 7.764

urchased from S.D. Fine Chemicals (India) and were used withoutny purification.

.2. Primer preparation

The effects of ZnO were measured at lower concentrations rang-ng from 0 to 20 wt.% with the increment of 5 wt.% and at higheroncentrations from 30 to 100 with increments of 20 wt.%. Theigment mixture at 60% PVC was dry blended and then dispersed

nto epoxy resin (prior dissolved in xylene–butanol solvents mix-ure) using high speed disperser to ensure proper dispersion. Theolyamine hardener, dissolved in solvent mixture, was mixed uni-ormly in proper ratio. Finally primers were applied by brushn 150 mm × 100 mm × 1 mm MS panels. These panels were dulybraded with a 100 No. emery paper and subsequently cleaned witholvents and dried. Dry film thickness was kept at 70 ± 5 �m. Theecipes of the primers formulation are given in Table 2.

.3. Porosity and film density

In all the formulations PVC was fixed at 60% using equation:

VC = volume of pigmentvolume of binder + volume of pigment

o find out the critical PVC of the pigments, consisting of Zn andnO combinations at various weight fractions, their densities andil absorption (O.A.) values were determined according to ASTM-153 and ASTM D-281, respectively.

PVC = 93.593.5 + (oil absorption × pigment density)

he porosity of the coatings was calculated using the followingquation:

orosity = 1 − CPVCPVC

orosity and film density are inter-related to each other, therefore,lm density of the primer films were also measured according tohe hydrostatic weighing method [15].

.4. Electrical properties of the coatings

Zinc-rich primer besides being a good barrier to oxygen andater should also protect the metal galvanically. For better cor-

osion resistance the conductivity should be high; approaching tohat of metal conductivity. The conductance of the clean primedanels was measured with the help of a multimeter by the probes

n contact with the coating on the panels. The distance between therobes was 5 cm. Several readings were recorded at various posi-ions on the coating and the average of these readings was thenoted. The standard deviation was found to be ±5%.

ioc

Fig. 1. Schematic presentation of corrosion potential measurement setup.

.5. Open circuit potential measurement (corrosion potential)

The ability of the zinc-rich coatings to protect the steel byacrificial cathodic protection was investigated. One side primedanels were completely immersed in vertical position in a stag-ant aqueous electrolyte (3.5 wt.% NaCl). The bare steel surfaceas monitored regularly; the appearance of pitting signified the

nd of cathodic protection. The registration of the first rust spotn the steel surface is acknowledged as loss of cathodic protec-ion. Schematic representation of corrosion potential measurementetup is shown in Fig. 1.

.6. Study of morphology

Surface morphology of coatings was observed under polarizingptical microscope (OLYMPUS BX41). SEM images (JEOL, 6380LA,apan) of cross-section of cast primer sample were also recorded.

.7. Salt spray test

The coated mild steel panels were tested for their corrosionesistance properties according to ASTM B-117 method. The cross-cribes were made down to the metal surface in order to observehe protective action of the primers. The panels were exposed forontinuous 3000 h in the salt spray and were inspected for corro-ion on the scribed area according to ASTM D-1654 at an interval of50 h along with signs of blistering, staining and loss of adhesion.

. Results and discussion

.1. Porosity and film density

The particle size of ZnO was 7.76 �m which was small compareo 42.45 �m for Zn. Density of ZnO and Zn was 5.6 and 7.1 g/cm3,espectively. This resulted in moderate difference in pigments oilbsorption values: 15 and 10.3 g/100 g for ZnO and Zn, respec-ively as shown in Fig. 2. While for all pigment combinations O.A.alues were less than that of Zn pigment. In pigment mixtures.A. decreased up to 30 wt.% ZnO additions but beyond this value

ncreased. This trend of O.A. value may be explained by to the cov-ring of zinc surface and filling of voids within the Zn pigments by

CPVC and porosity at 60% PVC are also plotted in Fig. 2. Poros-ty of the coatings has to be minimum to retard the percolationf the water and oxygen molecules into the coating and preventorrosion of metal. According to the relation between porosity and

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R.N. Jagtap et al. / Progress in Organic Coatings 63 (2008) 389–394 391

Table 2Recipe of Zn–ZnO primer

Components (g) Zn:ZnO (wt.%)

100:0 95:05 90:10 85:15 80:20 70:30 50:50 30:70 0:100

Zinc 90.79 86.14 81.50 76.88 72.27 63.08 44.84 26.77 00Zinc oxide 00 4.53 9.06 13.57Resin 5.42 5.49 5.55 5.62Hardener 3.79 3.84 3.89 3.93Total 100 100 100 100

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izpwCmwbrltPwiImactual service life of the coating.

Density of the primer films were also measured according tothe hydrostatic weighing method (Fig. 4(a)). As the wt.% of ZnOincreased, the film density decreased because of reduction in highdensity Zn pigment and addition of low density ZnO pigment. But

Fig. 2. Oil absorption values/CPVC/porosity vs. ZnO concentration.

PVC, porosity of the various coatings formulated at fixed PVC willecrease with increase in coating CPVC. From Fig. 2, porosity ofhe formulated primers (at 60% PVC) decreased as CPVC of coat-ng increased up to 30 wt.% ZnO addition and beyond that porosity

Fig. 3. SEM image of cross-section of cast film containing 15 wt.% ZnO. F

18.07 27.04 44.84 62.47 88.605.68 5.81 6.07 6.33 6.703.98 4.07 4.25 4.43 4.70

100 100 100 100 100

ncreased. Negative values of porosity could be considered equal toero because PVC less than CPVC results primers having less or noorosity. CPVC of primer at 30 wt.% ZnO addition was 69.4% whichas much higher than the formulated PVC (=60%). A PVC lower thanPVC results in loss in connectivity between the pigments and ulti-ately little or no cathodic protection. The CPVC with 10 wt.% ZnOas equal to 60%. This means that more sacrificial protection coulde assumed at this composition but with the addition of ZnO as bar-ier protective or semi-conducting material, conductivity would beower than 100% Zn primers and thus influences the cathodic pro-ection. 100% zinc-rich primer CPVC was less than the formulatedVC. If a coating is formulated such that PVC > CPVC the dry filmill contain voids and pores. Thus 100% zinc-rich primer’s poros-

ty was higher than primers containing a low percentage of ZnO.ncrease in porosity will also increase the corrosion rate and thus

ore consumption of zinc in the coating which will cut short the

ig. 4. Film density: (a) experimental film density (b) film density difference.

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Fig. 5. Conductance vs. ZnO concentration.

he difference in film density between 10 and 20 wt.% was insignif-cant. For the purpose of understanding the packing of coating film,he difference between experimental density and theoretical den-ity calculated on the basis of formulations was plotted (Fig. 4(b)).ore deviation of experimental density from theoretical density

eflects better packing. This deviation increased at 15 wt.% revealingetter packing and less void volumes.

.2. Conductance

Zn is viewed as a “p-metal” and thought to have the charac-er of a p-semiconductor, whereas ZnO is a n-type semiconductor16]. Thus, the combination of Zn–ZnO may form a p–n junction,hich permits the flow of electrons and can very well control the

lectrochemical reaction of corrosion.The conductance of the coatings, with various weight frac-

ions of ZnO is depicted in Fig. 5. When the concentration of Znecreased from 100 to 80 wt.%, the conductance decreased dras-ically. It became insulating when Zn wt.% was less than or equal

(rWtb

Fig. 6. Surface topology by opt

Coatings 63 (2008) 389–394

o 80 wt.%. According to Schmidt et al., ZnO at atmospheric pres-ure exhibits high resistivity although the same ZnO under reducedressure performs as an electrical conductor [17].

.3. Surface morphology

Surface morphology of coatings observed under polarizing opti-al microscope is shown in Fig. 6. The prominent presence of ZnOn the coating surface was visible at 15 wt.% and beyond. The dif-erence in density and shape of these pigments were significants discussed previously. They behave differently during flow andeveling what resulted in more ZnO particles at the coating surface.

.4. Evaluation of panels (salt spray test)

The extent of corrosion was represented in an arbitrarily numer-cal scale from 0 to 10. In the scale 10 represents no corrosion andrepresents failure. The results are shown in Fig. 7. With time cor-

osion increased although corrosion resistance was fairly good upo 1000 h exposure for all primers. Poor corrosion resistance wasbserved at 5 wt.% of ZnO. Upto 2000 h, corrosion resistance of 100%inc was better compare to the other formulations, whereas after000 h of exposure surprisingly 15 wt.% followed by 10 wt.% ZnOrimer exhibited better protection than the others. After 3000 hf exposure corrosion protection properties of 20 wt.% ZnO primeras equivalent to that of 100% zinc-rich primer. The scanned pic-

ures of the primer panels after 3000 h of exposure are shown inig. 8.

Zinc-rich primer protects the steel substrate by cathodic protec-ion only when the particles are in contact with the steel substratend in contact with each other. Zinc reacts with oxygen to forminc oxide/hydroxide, and with CO2 and water to give basic carbon-tes. Once corrosion products form, they seal pores in the primermproving the barrier properties [18]. In case of 100% zinc primer

i.e. no ZnO) cathodic protection plays initially a vital role in cor-osion protection and then the barrier effect provides protection.

hereas in case of other primers cathodic as well as barrier pro-ection act simultaneously right from the beginning. Incredibly theest corrosion protection was obtained for the primers with 15 wt.%

ical microscope at 100×.

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R.N. Jagtap et al. / Progress in Organic Coatings 63 (2008) 389–394 393

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ppiscnlrdecreases with time hence the corrosion potential increases. Atzero concentration of Zn (i.e. 100% ZnO), Ecorr increases rapidly to

Fig. 7. Corrosion rating of primers: (a) rating vs. time (b) rating vs. ZnO wt.%.

ollowed by 10 wt.% of ZnO. At 15 wt.% better packing was observedy film density measurement which further enhanced the cathodics well as barrier protection due to improved connectivity withinigments and less void volumes. Hence, very well synchronizedathodic and barrier protection was observed right from the begin-

ing which may be responsible for the decrease in corrosion ratef Zn providing effective corrosion resistance of the base substrateor prolong period.

pVc

Fig. 8. Scanned pictures of salt spray-tested panels co

Fig. 9. Open circuit potential vs. immersion days.

.5. Open circuit potential measurement (corrosion potential)

The cathodic protection duration of the substrate can be easilyetermined by measuring the corrosion potential. The potential ofinc in sea water is approximately −1.050 VSCE, while steel has aotential of approximately −0.650 VSCE [3]. According to thermo-ynamics, cathodic protection of steel is attained when potential is

ower than −0.850 VSCE [19]. Measured potentials are mixed poten-ials between the steel substrate and the “active” zinc pigments. Ifnly few zinc pigments are active, the anode area will be small, andhe potential will be close to that of steel. On the other hand, if therea of active zinc particles is large, the potential will be close tohat of zinc.

The open circuit potentials, i.e. Ecorr for the coated panels arelotted vs. the immersion time in days (Fig. 9). The corrosionotentials of coatings are initially close to that of zinc metal what

ndicates that all coatings provide a cathodic protection to the sub-trate in the early days of immersion. The plot shows that for higheroncentration of Zn in coatings, the corrosion potential was highlyegative, thus large anodic areas are accessible to the electrolyte

eading to small current density being drawn from active Zn. Cor-osion products are formed on the surface, the protective action

ositive values indicating the inefficiency in combating corrosion.ery interesting results were obtained from the Ecorr plots ZnO-richombinations like Zn:ZnO; 30:70, 50:50 and 70:30; did not perform

ated with different combination of Zn and ZnO.

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ell although the combinations like 80:20, 85:15 and 90:10 showednteresting behavior. After 65 days of immersion only 15 wt.% fol-owed by 10 wt.% ZnO primers had corrosion potential less than0.85 V, i.e. cathodic protection duration was longer in the case of

hese coatings.As discussed in Section 3.2, when Zn and ZnO are mixed together

hey quite likely formed a p–n junction at their interphase. At theontact established between a metal and a semiconductor (knowns a Schottky junction), electrons will flow from the material withhe highest Fermi level to that with the lowest Fermi level, creat-ng a positive charge in the semiconductor [20]. This will allow thelectrons transport in a control manner on the top of the coatingurface providing better cathodic protection for the Zn–ZnO com-inations rather than for Zn only. The packing density was bettert 15 wt.% ZnO which implies that the compactness was enhanc-ng the junction formation. This data are in agreement with resultsbtained from the salt spray test and the rheological evaluation.

. Conclusion

Interestingly the best corrosion protection was obtained at5 wt.% ZnO addition followed by 10 wt.% ZnO in zinc-rich primer

earing 60% PVC. Dual effect of cathodic (metallic zinc) and barrierrotection (ZnO) in combination of p–n junction formation giveetter corrosion resistance properties than either cathodic or bar-ier protection act alone. Hence, Zn–ZnO combination would be theetter choice for single coat shop primer.

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Coatings 63 (2008) 389–394

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