Progress in Organic Coatings Effect of zinc oxide in combating

Progress in Organic Coatings 63 (2008) 389–394

Contents lists available at ScienceDirect

Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat

Effect of zinc oxide in combating corrosion in zinc-rich primer R.N. Jagtap ∗ , P.P. Patil, S.Z. Hassan Surface 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 2007 Received in revised form 17 June 2008 Accepted 24 June 2008 Keywords: Corrosion Lamellar zinc Spherical ZnO Salt spray Ecorr

a b s t r a c t During the production of zinc pigments, some zinc may get oxidized and remains as impurity. In the present study, the effect of spherical ZnO on corrosion protection properties was evaluated along with lamellar zinc. ZnO was added in various weight fractions in epoxy primer coating at 60% PVC. The coated panels were evaluated among others for their conductance, packing density and morphology. The corrosion 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 obtained with 15 wt.% of ZnO. © 2008 Published by Elsevier B.V.

1. Introduction Steel is the most suitable construction material for a wide range of structures, as it has favorable mechanical properties and high speed fabrication [1]. However, structures made of steel, sooner or later gets damaged by corrosion and fouling, resulting in direct and indirect losses which could be as high as 5% of the Gross National Production [2]. Combating corrosion of metals or alloys is of paramount importance as during the corrosion reaction the pure metal goes back to its ore state. Coating is one of the most convenient methods for corrosion protection of metals by isolating them from the contact with environment (O2 and moisture). Hence, protection of metals could be done by sacrificial cathodic and/or barrier mechanisms. Primers with metallic zinc dust provides cathodic as well as barrier protection [3]. Therefore, zinc is widely used in coatings throughout the world for over half a century and has reached a level of high performance [4–6]. Zinc-rich coatings, often called zinc-rich primers, are a unique class of convertible coatings having inorganic or organic binders highly loaded with metallic zinc-dust pigment. In our previous study, we found that lamellar zinc is better than spherical zinc and their combination performed far better in anti-corrosive properties [7]. Coating systems that include these high-performance primer components can provide prolong anticorrosion protection, depending on the severity of the environment [8].

∗ Corresponding author. Tel.: +91 22 2414 5616; fax: +91 22 2414 5614. E-mail addresses: [email protected], [email protected] (R.N. Jagtap). 0300-9440/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.porgcoat.2008.06.012

The inherent advantages of the organic coatings over the inorganic coatings are flexibility, cost and speed of construction [9]. Organic zinc-rich primers are commonly formulated from epoxy, epoxy ester, urethane, vinyl and chlorinated-rubber as binders. The most widely used organic zinc-rich coatings are based on epoxy chemistry. Epoxy resins are characterized by ease of cure and processing, excellent moisture, solvent and chemical resistance and good adhesive strength [10]. The manufacturing of zinc involves atomization of the molten zinc to obtain spherical particles [11,12]. During the zinc production, molten zinc may get oxidize to ZnO when it comes in contact with oxygen and eventually pure zinc dust contains traces of ZnO. The electrical properties of ZnO are interesting and complex [13]. ZnO acts as an inhibitor to chloride-induced corrosion, seals pores in primer and improves the barrier properties [14]. Therefore, it would be an interesting idea to include ZnO in the zinc-rich primer to study its effect on corrosion protection properties. 2. Experimental work 2.1. Raw materials Lamellar zinc dust was prepared by ball milling the spherical zinc dust obtained from M/s Forage Chemical Industries, India. ZnO was procured from S.D. Fine Chemicals (India). The physical characteristics of these pigments are shown in Table 1. The epoxy resin, PER 524 with epoxy equivalent 181–198 g/equiv. and polyamine, pH 541 having amine value 290–325 g/equiv. were obtained from Pliogrip, India. The solvents used for improving processability like xylene, methyl ethyl ketone (MEK), n-butanol (AR grade) were

390

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

Table 1 Physical characteristics of the pigments Serial No.

Physical characteristics

Zinc

ZnO

1 2 3 4 5

Form Color Shape Density (g/cm3 ) Particle size distribution Mean size (␮m)

Solid powder Dark gray Lamellar 7.1 42.458

Powder White Spherical 5.6 7.764

purchased from S.D. Fine Chemicals (India) and were used without any purification. 2.2. Primer preparation The effects of ZnO were measured at lower concentrations ranging from 0 to 20 wt.% with the increment of 5 wt.% and at higher concentrations from 30 to 100 with increments of 20 wt.%. The pigment mixture at 60% PVC was dry blended and then dispersed into epoxy resin (prior dissolved in xylene–butanol solvents mixture) using high speed disperser to ensure proper dispersion. The polyamine hardener, dissolved in solvent mixture, was mixed uniformly in proper ratio. Finally primers were applied by brush on 150 mm × 100 mm × 1 mm MS panels. These panels were duly abraded with a 100 No. emery paper and subsequently cleaned with solvents and dried. Dry film thickness was kept at 70 ± 5 ␮m. The recipes of the primers formulation are given in Table 2.

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

2.5. Open circuit potential measurement (corrosion potential) The ability of the zinc-rich coatings to protect the steel by sacrificial cathodic protection was investigated. One side primed panels were completely immersed in vertical position in a stagnant aqueous electrolyte (3.5 wt.% NaCl). The bare steel surface was monitored regularly; the appearance of pitting signified the end of cathodic protection. The registration of the first rust spot on the steel surface is acknowledged as loss of cathodic protection. Schematic representation of corrosion potential measurement setup is shown in Fig. 1.

2.3. Porosity and film density 2.6. Study of morphology In all the formulations PVC was fixed at 60% using equation: volume of pigment PVC = volume of binder + volume of pigment To find out the critical PVC of the pigments, consisting of Zn and ZnO combinations at various weight fractions, their densities and oil absorption (O.A.) values were determined according to ASTM D-153 and ASTM D-281, respectively. CPVC =

93.5 93.5 + (oil absorption × pigment density)

The porosity of the coatings was calculated using the following equation: Porosity = 1 −

CPVC PVC

Porosity and film density are inter-related to each other, therefore, film density of the primer films were also measured according to the hydrostatic weighing method [15]. 2.4. Electrical properties of the coatings Zinc-rich primer besides being a good barrier to oxygen and water should also protect the metal galvanically. For better corrosion resistance the conductivity should be high; approaching to that of metal conductivity. The conductance of the clean primed panels was measured with the help of a multimeter by the probes in contact with the coating on the panels. The distance between the probes was 5 cm. Several readings were recorded at various positions on the coating and the average of these readings was then noted. The standard deviation was found to be ±5%.

Surface morphology of coatings was observed under polarizing optical microscope (OLYMPUS BX41). SEM images (JEOL, 6380LA, Japan) of cross-section of cast primer sample were also recorded. 2.7. Salt spray test The coated mild steel panels were tested for their corrosion resistance properties according to ASTM B-117 method. The crossscribes were made down to the metal surface in order to observe the protective action of the primers. The panels were exposed for continuous 3000 h in the salt spray and were inspected for corrosion on the scribed area according to ASTM D-1654 at an interval of 250 h along with signs of blistering, staining and loss of adhesion. 3. Results and discussion 3.1. Porosity and film density The particle size of ZnO was 7.76 ␮m which was small compare to 42.45 ␮m for Zn. Density of ZnO and Zn was 5.6 and 7.1 g/cm3 , respectively. This resulted in moderate difference in pigments oil absorption values: 15 and 10.3 g/100 g for ZnO and Zn, respectively as shown in Fig. 2. While for all pigment combinations O.A. values were less than that of Zn pigment. In pigment mixtures O.A. decreased up to 30 wt.% ZnO additions but beyond this value increased. This trend of O.A. value may be explained by to the covering of zinc surface and filling of voids within the Zn pigments by ZnO. This was very well supported by SEM photographs (Fig. 3). CPVC and porosity at 60% PVC are also plotted in Fig. 2. Porosity of the coatings has to be minimum to retard the percolation of the water and oxygen molecules into the coating and prevent corrosion of metal. According to the relation between porosity and


391

Table 2 Recipe of Zn–ZnO primer Components (g)

Zinc Zinc oxide Resin Hardener Total

Zn:ZnO (wt.%) 100:0

95:05

90:10

85:15

80:20

70:30

50:50

30:70

0:100

90.79 00 5.42 3.79 100

86.14 4.53 5.49 3.84 100

81.50 9.06 5.55 3.89 100

76.88 13.57 5.62 3.93 100

72.27 18.07 5.68 3.98 100

63.08 27.04 5.81 4.07 100

44.84 44.84 6.07 4.25 100

26.77 62.47 6.33 4.43 100

00 88.60 6.70 4.70 100

increased. Negative values of porosity could be considered equal to zero because PVC less than CPVC results primers having less or no porosity. CPVC of primer at 30 wt.% ZnO addition was 69.4% which was much higher than the formulated PVC (=60%). A PVC lower than CPVC results in loss in connectivity between the pigments and ultimately little or no cathodic protection. The CPVC with 10 wt.% ZnO was equal to 60%. This means that more sacrificial protection could be assumed at this composition but with the addition of ZnO as barrier protective or semi-conducting material, conductivity would be lower than 100% Zn primers and thus influences the cathodic protection. 100% zinc-rich primer CPVC was less than the formulated PVC. If a coating is formulated such that PVC > CPVC the dry film will contain voids and pores. Thus 100% zinc-rich primer’s porosity was higher than primers containing a low percentage of ZnO. Increase in porosity will also increase the corrosion rate and thus more consumption of zinc in the coating which will cut short the actual service life of the coating. Density of the primer films were also measured according to the hydrostatic weighing method (Fig. 4(a)). As the wt.% of ZnO increased, the film density decreased because of reduction in high density Zn pigment and addition of low density ZnO pigment. But

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

CPVC, porosity of the various coatings formulated at fixed PVC will decrease with increase in coating CPVC. From Fig. 2, porosity of the formulated primers (at 60% PVC) decreased as CPVC of coating 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.

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

392


to 80 wt.%. According to Schmidt et al., ZnO at atmospheric pressure exhibits high resistivity although the same ZnO under reduced pressure performs as an electrical conductor [17]. 3.3. Surface morphology Surface morphology of coatings observed under polarizing optical microscope is shown in Fig. 6. The prominent presence of ZnO on the coating surface was visible at 15 wt.% and beyond. The difference in density and shape of these pigments were significant as discussed previously. They behave differently during flow and leveling what resulted in more ZnO particles at the coating surface. 3.4. Evaluation of panels (salt spray test)

Fig. 5. Conductance vs. ZnO concentration.

the difference in film density between 10 and 20 wt.% was insignificant. For the purpose of understanding the packing of coating film, the difference between experimental density and theoretical density calculated on the basis of formulations was plotted (Fig. 4(b)). More deviation of experimental density from theoretical density reflects better packing. This deviation increased at 15 wt.% revealing better packing and less void volumes. 3.2. Conductance Zn is viewed as a “p-metal” and thought to have the character of a p-semiconductor, whereas ZnO is a n-type semiconductor [16]. Thus, the combination of Zn–ZnO may form a p–n junction, which permits the flow of electrons and can very well control the electrochemical reaction of corrosion. The conductance of the coatings, with various weight fractions of ZnO is depicted in Fig. 5. When the concentration of Zn decreased from 100 to 80 wt.%, the conductance decreased drastically. It became insulating when Zn wt.% was less than or equal

The extent of corrosion was represented in an arbitrarily numerical scale from 0 to 10. In the scale 10 represents no corrosion and 0 represents failure. The results are shown in Fig. 7. With time corrosion increased although corrosion resistance was fairly good up to 1000 h exposure for all primers. Poor corrosion resistance was observed at 5 wt.% of ZnO. Upto 2000 h, corrosion resistance of 100% zinc was better compare to the other formulations, whereas after 3000 h of exposure surprisingly 15 wt.% followed by 10 wt.% ZnO primer exhibited better protection than the others. After 3000 h of exposure corrosion protection properties of 20 wt.% ZnO primer was equivalent to that of 100% zinc-rich primer. The scanned pictures of the primer panels after 3000 h of exposure are shown in Fig. 8. Zinc-rich primer protects the steel substrate by cathodic protection only when the particles are in contact with the steel substrate and in contact with each other. Zinc reacts with oxygen to form zinc oxide/hydroxide, and with CO2 and water to give basic carbonates. Once corrosion products form, they seal pores in the primer improving the barrier properties [18]. In case of 100% zinc primer (i.e. no ZnO) cathodic protection plays initially a vital role in corrosion protection and then the barrier effect provides protection. Whereas in case of other primers cathodic as well as barrier protection act simultaneously right from the beginning. Incredibly the best corrosion protection was obtained for the primers with 15 wt.%

Fig. 6. Surface topology by optical microscope at 100×.


393

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

3.5. Open circuit potential measurement (corrosion potential)

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

followed by 10 wt.% of ZnO. At 15 wt.% better packing was observed by film density measurement which further enhanced the cathodic as well as barrier protection due to improved connectivity within pigments and less void volumes. Hence, very well synchronized cathodic and barrier protection was observed right from the beginning which may be responsible for the decrease in corrosion rate of Zn providing effective corrosion resistance of the base substrate for prolong period.

The cathodic protection duration of the substrate can be easily determined by measuring the corrosion potential. The potential of zinc in sea water is approximately −1.050 VSCE , while steel has a potential of approximately −0.650 VSCE [3]. According to thermodynamics, cathodic protection of steel is attained when potential is lower than −0.850 VSCE [19]. Measured potentials are mixed potentials between the steel substrate and the “active” zinc pigments. If only few zinc pigments are active, the anode area will be small, and the potential will be close to that of steel. On the other hand, if the area of active zinc particles is large, the potential will be close to that of zinc. The open circuit potentials, i.e. Ecorr for the coated panels are plotted vs. the immersion time in days (Fig. 9). The corrosion potentials of coatings are initially close to that of zinc metal what indicates that all coatings provide a cathodic protection to the substrate in the early days of immersion. The plot shows that for higher concentration of Zn in coatings, the corrosion potential was highly negative, thus large anodic areas are accessible to the electrolyte leading to small current density being drawn from active Zn. Corrosion products are formed on the surface, the protective action decreases with time hence the corrosion potential increases. At zero concentration of Zn (i.e. 100% ZnO), Ecorr increases rapidly to positive values indicating the inefficiency in combating corrosion. Very interesting results were obtained from the Ecorr plots ZnO-rich combinations like Zn:ZnO; 30:70, 50:50 and 70:30; did not perform

Fig. 8. Scanned pictures of salt spray-tested panels coated with different combination of Zn and ZnO.

394


well although the combinations like 80:20, 85:15 and 90:10 showed interesting behavior. After 65 days of immersion only 15 wt.% followed by 10 wt.% ZnO primers had corrosion potential less than −0.85 V, i.e. cathodic protection duration was longer in the case of these coatings. As discussed in Section 3.2, when Zn and ZnO are mixed together they quite likely formed a p–n junction at their interphase. At the contact established between a metal and a semiconductor (known as a Schottky junction), electrons will flow from the material with the highest Fermi level to that with the lowest Fermi level, creating a positive charge in the semiconductor [20]. This will allow the electrons transport in a control manner on the top of the coating surface providing better cathodic protection for the Zn–ZnO combinations rather than for Zn only. The packing density was better at 15 wt.% ZnO which implies that the compactness was enhancing the junction formation. This data are in agreement with results obtained from the salt spray test and the rheological evaluation. 4. Conclusion Interestingly the best corrosion protection was obtained at 15 wt.% ZnO addition followed by 10 wt.% ZnO in zinc-rich primer bearing 60% PVC. Dual effect of cathodic (metallic zinc) and barrier protection (ZnO) in combination of p–n junction formation give better corrosion resistance properties than either cathodic or barrier protection act alone. Hence, Zn–ZnO combination would be the better choice for single coat shop primer.

References [1] W.F. Smith, J. Hashemi, Foundations of Materials Science and Engineering, 4th ed., McGraw-Hill, 2006. [2] S. Anand Kumar, T. Balakrishnan, M. Alagar, Z. Denchev, Progress in Organic Coatings 55 (2006) 207–217. [3] O.O. Knudsen, U. Steinsmo, M. Bjodal, Progress in Organic Coatings 54 (2005) 224–229. [4] C.G. Munger, L.D. Vincent, Corrosion Prevention by Protective Coatings, 2nd ed., NACE, 1999. [5] R.J. Brodd, V.E. Leger, A.J. Bard (Eds.), Encyclopedia of Electrochemistry of the Elements, vol. 6, Marcel Decker, New York, 1976, p. 35. [6] T.K. Ross, J. Lingard, Transactions of the Institute of Metal Finish 40 (1983) 186. [7] R.N. Jagtap, R. Nambiar, S Zaffar Hassan, V.C. Malshe, Progress in Organic Coatings 58 (2007) 253–258. [8] K.B. Tator, Journal of Architectural Coatings (October/November) (2006) 64–69. [9] A.M. Van Londen, B.P. Alblas, PCE (November) (1997) 26–34. [10] S. Ahmad, A.P. Gupta, E. Sharmin, M. Alam, S.K. Pandey, Progress in Organic Coatings 54 (2005) 248–255. [11] P. Nylen, E. Sunderland, Modern Surface Coating, John Wiley and Sons Ltd., New York, 1965. [12] C.H. Mathewso, Zinc, Reinhold Publishing Corporation, New York, 1959. [13] H.E. Brown, Zinc Oxide Properties and Applications, International Lead Zinc Research Organization, NY, 1986. [14] V. Zivica, Bulletin Material Science 25 (5) (2002) 371–373. [15] R. Castells, J. Meda, J. Caprati, M. Damia, Journal of Coatings Technology 55 (707) (1983) 53–59. [16] W.C. Dunlap, Introduction to Semiconductors, Wiley, 1957, p. 56. [17] O. Schmidt et al., Proceedings of E-MRS Spring Meeting, Symposium G, Strasbourg, 2005. [18] S. Felui, R. Barajas, J. Bastidas, M. Morcillo, Journal of Coatings Technology 61 (1989) 71–76. [19] C.M. Abreu, et al., Electrochimica Acta 41 (15) (1996) 2405–2415. [20] N. Sato, Electrochemistry at Metal and Semiconductor Electrodes, 1st ed., Elsevier, Amsterdam, 1998.