Predictive power for life and residual life of the zinc rich primer coatings with electrical measurement

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Progress in Organic Coatings 58 (2007) 253–258

Predictive power for life and residual life of the zinc richprimer coatings with electrical measurement

R.N. Jagtap ∗, Rakesh Nambiar, S. Zaffar Hassan, V.C. MalsheSurface Coating Division, University of Mumbai, Institute of Chemical Technology (UICT), Mumbai 400019, India

Received 8 March 2006; accepted 9 August 2006

bstract

Zinc rich primers with lamellar and spherical shaped zinc pigments and their combinations were formulated. These zinc rich primers were usedo measure the electrical conductance and subjected to the corrosion analysis for 3000 h. The lamellar shaped pigments at lower PVC were foundo be better than spherical shaped pigments, whereas the typical combination of spherical and lamellar zinc was the best in providing corrosion

rotection. A set of equations were developed to predict the life and residual life for pigment of both shapes using PVC, conductance and corrosionesistance for 3000 h in salt spray as parameters. Finally the influence of zinc particle shape on the flow and orientation behavior was evaluatedsing rheological measurements.

2006 Elsevier B.V. All rights reserved.

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eywords: Corrosion; Spherical and lamellar zinc pigments; Salt spray; Orient

. Introduction

Protection of metals from corrosion is a serious issue. Themount of metal lost due to corrosion is of the tune of 2.5–4% ofhe gross world’s production [1–3]. As a consequence corrosionrotection is important and its implications are observed in threereas. First area of significance is economics, inclusive of thebjective of reducing the material losses, second the improvedife and safety of operations while third is the conservation ofhe metal resources [4,5].

Zinc primers have been used throughout the world for overalf a century. They have reached a level of performance inonjunction with modern coating systems where these can beonsidered as practical solution to virtually any environmentemanding long term corrosion protection [6–9]. From a com-arative evaluation of inorganic and organic zinc coating, itas established that only metallic zinc is capable of providingalvanic protection [10].

Organically coated galvanized steels are increasingly popular

n the construction and automotive industry due to their low cost,exibility and speed for construction [11,12]. Zinc rich primersan be used as a single coat shop primer for temporary protec-

∗ 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 © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2006.08.015

ion or as a primer in a multilayer coating system for long-termrotection. The protective mechanism of single coat zinc richrimers has been studied for zinc rich epoxy polyamide and zincich ethyl silicate paints [13]. Anti-corrosive behavior was eval-ated on series of acrylic and epoxy waterborne coating systemssing different test methodologies and study concluded that theest anti-corrosive behavior was obtained by epoxy-polyamideaint systems including zinc rich primers [14]. Giudice et al.15] have studied the effect of particle shape of zinc in epoxyrimer, at varied PVC followed by a titanium dioxide finishedoat. A combination of spherical and lamellar shape zinc pig-ents may also be the answer for improved protection [16]. The

mportance of electrical resistance and capacitance in predictingorrosion protective performance in coatings has been shown byalentinelli et al. [17]. Porosity and electrochemical behavior are

ntimately related to the conductive pigment present in the coat-ng [18]. If primer films are tested in a salt spray test, corrosionailure largely reflects the presence of channels or pores in theoating [19,20].

In this study we have employed zinc pigments manufac-ured by different techniques, producing particles of sphericalnd lamellar shape. Using these pigments, zinc primers of dif-

erent PVC were prepared, applied on MS surface and theoatings were then used for measurement of electrical conduc-ance. These panels were exposed to the salt spray test accordingo ASTM B-117 and evaluated for corrosion resistance by salt

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254 R.N. Jagtap et al. / Progress in Organ

Table 1Physical characteristics of the pigment

S. no. Physical characteristics Spherical Lamellar

1 Form Solid powder Solid powder2 Color Dark gray Light gray3 Density (g/cm3) 7.1 7.14 O.A. (g/100 gm) 7.4 9.85 Purity 99.02 99.976

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Particle size distribution.Mean size (�m)

7.89 18.38

pray according to ASTM D-1654 test and blistering tendencyt high humidity levels. Finally, flow and leveling ability waseasured by measuring rheological properties of the primers to

nderstand the orientation of the pigment particles.

. Experimental work

.1. Raw materials

Zinc was obtained from Transpek Silox Industries Limitedndia. Spherical zinc dust was manufactured by spraying ofolten zinc metal, which on cooling causes them to condense

s spheres of varying particle sizes while lamellar zinc dust wasanufactured by pulverizing zinc metal by ball milling. The

hysical characteristics of these pigments are shown in Table 1.The epoxy resin, OC 5460 with epoxy equivalent 185–200

nd polyamide, Replamide-315 having amine value 220 mg ofOH/g were obtained from Resins and Plastics (India) Limited.olvents like xylene and n-butanol were procured from S.D.Finehemicals (India) which were used without any purification.

.2. Preparation

The pigments and epoxy binder were mixed and thoroughlyispersed in high-speed agitation equipment supplied by Tudorachine, India. The solvents xylene and n-butanol mixture were

dded to control viscosity, the hardener was added to it in theatio suggested by the manufacturer (1:1.3) prior to application.he PVCs of primers were varied from 45 to 70%. The recipesf the primers formulation are given in Table 2.

The panels were made of mild steel and obtained from a localupplier. The size of the panels was 100 mm × 150 mm. These

anels were first cleaned with solvents and dried and then rubbedith the help of an emery paper. The primer was applied on theanels to uniform film thickness of 75 �m by brush. The edgesere protected by dipping in molten wax and rear side of the

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able 2ecipes for the primers

omponent (gm) PVC (%)

45 50

n Pigment 84.62 87.04poxy resin 6.75 5.68olyamide hardener 8.63 7.26otal 100 100

ic Coatings 58 (2007) 253–258

anels was well protected from corrosion. These panels wereured as per recommendation of the resin supplier before theests.

.3. Measurement of conductance of zinc primers

Besides being a good barrier to oxygen and water zinc richrimer should also be able to protect the surface galvanically.or better corrosion resistance the conductivity should be high;pproaching to that of metal conductivity. Spherical shape ofarticles and presence of inorganic non-conducting particles actgainst the purpose [21]. The conductance of the primer panelsere measured with the help of a multimeter by bringing theointers of the probes in contact with the coating on the panelurface between two points separated by 5 cm. Several readingsere recorded at various positions on the coating and the aver-

ge of these readings was then noted for lamellar, spherical andixed zinc primers at various PVC concentrations.

.4. Film density measurement

Film densities of the primer films on the metallic surface wereeasured according to the hydrostatic weighing method [22].

.5. Salt spray test

The coated mild steel panels were then tested for their cor-osion resistance properties according to ASTM B-117 method.wo crossed scribes were made down to the metal surface forbserving the protective action of the primers. The panels werelaced in the cabinet and were inspected periodically for signsf blistering, staining, loss of adhesion and progression of cor-osion from the scratch. The panels were exposed for 3000 h ofontinuous salt spray and inspected for corrosion on the scribeds well as unscribed area according to ASTM D-1654.

.6. Rheological properties

The rheological properties of samples were studied withaake RT 10 Rheometer, Germany.

. Results and discussion

.1. Conductance versus PVC

The conductance measurements at various PVC concentra-ions for lamellar and spherical shape are as shown in Table 3.

55 60 65 70

89.16 91.00 92.3 94.004.76 3.96 3.24 2.636.09 5.06 4.14 3.36100 100 100 100

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R.N. Jagtap et al. / Progress in Organic Coatings 58 (2007) 253–258 255

Table 3Conductance measurements of spherical and lamellar Zn rich Epoxy Primer

PVC (%) Conductance (× 10−6 �−1)

Spherical Lamellar

45 0 050 0 055 1.02 2.4860 1.78 6.8567

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Table 5Film density at 60% PVC

Lamellar:spherical Film density (g/cm3)

0:100 5.413525:75 5.5928571

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bPgCPVC. The results are as shown in Fig. 1. The scanned pictureof the coated panel with 60% PVC is as shown in Fig. 2.

Lamellar zinc pigment: The results are as shown in Fig. 3.The corrosion resistance of lamellar pigments was substantially

Fig. 1. Rating of spherical zinc primers for scribed and unscribed panels w.r.t.time.

5 5.38 35.720 11.9 83.33

Spherical zinc pigments: As the PVC increased the conduc-ivity or connectivity of the pigment increased. There was rapidump in the conductivity at PVC 65 and 70%.

Lamellar zinc pigments: Similar trend was observed, the con-uctivity of lamellar pigment was 3.8 times higher than sphericaligment. The CPVC of spherical and lamellar pigments were7 and 64 respectively. It implied that the packing density ofamellar pigment was definitely more than spherical shaped zincarticles and it was confirmed from the observed value of filmensities.

Mixed spherical and lamellar pigments: For the primerncluding combination of lamellar and spherical shape parti-les the average CPVC of spherical and lamellar pigments wasound to be 60%. As a result the primers were prepared with theombination of these pigments in the proportions varying from5 to 75% at 60% PVC. The corresponding conductivity resultsre as shown in Table 4.

The conductance increased with increase in lamellar pig-ents content. The conductance of 75:25 lamellar:spherical

ombination was 4.5 times more than spherical and 1.2 timesore than lamellar pigment alone. This may be due to the better

acking of spherical pigments and the shape factor which helpsn proper flow and leveling of the lamellar pigments and contactith one another.

.2. Film density

Measured film densities of coated primers on metal surfacerepared at 60% PVC for various combinations of spherical andamellar shaped zinc pigments are as shown in Table 5. The flat,laty lamellar zinc can align parallel to one another formingense compact structures resulting into higher packing densityompared to the spherical pigment, which at the most will have

2 contact points and the adjoining areas will be left out withmpty space resulting into voids, making the film less com-act. In the mixed pigments containing different combinations of

able 4onductance measurement of primer containing combination of Lamellar andpherical Zn at 60% PVC

omposition lamellar:spherical Conductance (X 10−6 �−1)

5:75 2.620:50 3.475:25 7.94

0:50 5.73435:25 5.983900:0 6.0268

amellar and spherical pigments, the void volume was decreaseds the weight percentages of lamellar pigment increased. All theamellar pigment packing densities were more than for sphericaligments resulting in higher film density.

.3. Evaluation of panels for scribed area

For the sensitive area, i.e. scribed area the extent of corrosionas represented in an arbitrarily numerical scale from 0 to 10.

n this 10 represented no progression and 0 represented failure.he results are as shown in Figs. 1, 3 and 5.

Spherical zinc pigment: With time the corrosion increased,ut more corrosion was seen for primers at PVC 55%, as thisVC was less than CPVC. The corrosion resistances were fairlyood up to 1000 h exposure for primers with PVC greater than

Fig. 2. Salt spray tested panel coated with spherical Zn at PVC 60%.

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256 R.N. Jagtap et al. / Progress in Organic Coatings 58 (2007) 253–258

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ig. 3. Rating of lamellar zinc primers for scribed and unscribed panels w.r.time.

ore as compared to spherical ones. One can assume, lamellarigments align parallel to the surface and connectivity and thusonductivity was very well achieved thus barrier properties wereuperior. The corrosion resistance was very high, i.e. no failurep to 1500 h. The scanned picture of the coated panel with 60%VC is as shown in Fig. 4.

Mixed spherical and lamellar pigments: The results are ashown in Fig. 5. The corrosion resistance of lamellar zinc pig-ent rich primers was very good up to 1250 h and was better than

pherical zinc pigment primers. This was because of proper flownd leveling of lamellar pigments in primers with 75:25 lamel-ar:spherical zinc pigments. Corrosion resistance of primers

ecreased on more addition of spherical pigments. The scannedictures of the coated panels are as shown in Fig. 6.

Fig. 4. Salt spray tested panel coated with lamellar Zn at PVC 60%.

ig. 5. Rating of mixed zinc primers for scribed and unscribed panels w.r.t. time.

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ig. 6. Salt spray tested panel coated with mixed Zn primer at PVC 60%:a) lamellar:spherical, 25:75; (b) lamellar:spherical, 50:50 and (c) lamel-ar:spherical, 75:25.

Effect of conductance measured at various PVC concentra-ions (spherical or lamellar zinc primer) on corrosion resistancef scribed panels are as shown in Fig. 7.

.4. Evaluation of panels for unscribed area

Even for the unscribed area the extent of corrosion is repre-ented in an arbitrarily numerical scale from 0 to 10. In this 10epresent no corrosion and 0 represent spots of 16 mm diameter,.e. failure. The results are as shown in Figs. 1, 3 and 5. Forhe evaluation of the unscribed area the paint was completelytripped off by application of a strong solvent and the corrosionn the unexposed area was assessed.

Spherical zinc pigment: The results are as shown in Fig. 1.

mall pitting could be seen right after 500 h of exposure. Theesistance was very low for primer with PVC 55% and itecreased with rise in PVC and was lowest at 70% PVC. Nolisters were found up to 1000 h at 70% PVC.

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R.N. Jagtap et al. / Progress in Organ

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ig. 7. Effect of conductance measured at various PVC concentrations of spher-cal and lamellar Zn primers on corrosion resistance.

Lamellar zinc pigment: The results are as shown in Fig. 3.here were no signs of corrosion at 65 and 70% PVC up to 1500 hnd the conductance increased with PVC increase. The blisteringas more for the lamellar pigments due to random orientationsf the pigments which resist the escape of the hydrogen (theorrosion product).

Mixed lamellar and spherical zinc pigment: The results ares shown in Fig. 5. As the lamellar pigments increased, the cor-osion resistance increased. Also the blistering was very lowhen compared to the previous lamellar and spherical shaped

inc primers.

.5. Rheological properties of zinc rich primers

Rheological studies were performed in order to understandhe effect of shapes of the particles on the flow and orientationf the pigments in the primer.

Fig. 8 depicts the viscosities of primers consisting of pig-ents with 60% PVC. Three primers were prepared consisting of

pherical, lamellar and 50:50 combination of spherical:lamellar

igments. The lamellar pigments encapsulate all the particlesecause of high surface area, thus require more binders andherefore the lubrication between them was low resulting intoigh viscosity at low shear rate, while spherical pigments which

ig. 8. Viscosity curves at PVC 60% of spherical (Zinc 1), spherical:lamellaratio 50:50 (Zinc 2) and lamellar (Zinc 3).

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ic Coatings 58 (2007) 253–258 257

ossess low surface area and relatively less contact points com-ared to lamellar pigments resulted in low viscosity at higherhear rate. Whereas a 50:50 combination of spherical and lamel-ar pigments was showing viscosity in between the two pigmentsut more inclined towards the lamellar pigments. The sphericaligments were acting as rollers for the lamellar pigments whileredominantly helping in reducing the viscosity and at the sameime improving the orientation of lamellar pigments.

. Design equations (for scribed panels)

A design equation was developed for predicting the life ofpherical and lamellar shape zinc rich epoxy primers, using thisquation one can predict the life of coating after 1000 h testn term’s of corrosion rating and can extrapolate safely upto000 h of test. The design equation for the corrosion rating wasormulated assuming linear behavior for all the systems. Theean sum of square R2 ∼ 1, precisely R2 ≈ 0.98.

= 0.59381 + 0.02318 × X1 − 0.0036 × X2

+ 0.20175 × X3 − 6.72641E − 4 × X4

here, Y is the corrosion rating (0–10 value); X1 the concentra-ion of zinc (for spherical, X1 = 0 and lamellar, X1 = 100); X2 thealt spray time duration (X2 > 1000 h); X3 the PVC %; X4 is theonductance, ×10−6 �−1

Similarly an equation was designed for the prediction ofesidual life (approximately after 2000 h test) of mixed zincrimers at 60% PVC by assuming a nonlinear relationship forll the systems and then solved for R2 ∼ 1, precisely R2 ≈ 0.99.ne can predict the residual life of coating if conductance values

re known.

= 8.02918 − 5.37

1 + (X/3.17874)5.689

here Y is the corrosion rating (0–10 value); X is the conduc-ance, ×10−6 �−1

. Conclusions

Lamellar pigments have more surface area than spherical zincigments because of which it has better connectivity and conduc-ivity. This connectivity resulted into better corrosion protectionecause of barrier as well as sacrificial cathodic protection tohe base metal. The spherical shaped filler in the lamellar shapeigment at 25:75 ratio provided the best results which werelso supported by the rheological measurements. And the designquations based on PVC and conductivity analysis could be aimple tool in predicting the salt spray resistance of the pigmentnder consideration.

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