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Corrosion behavior of organic epoxy-xinc coating with fly ash as an extender pigment
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 21629–21635
www.materialstoday.com/proceedings
The 3rd International Conference on Green Chemical Engineering Technology (3rd GCET_2017): Materials Science
Corrosion behavior of organic epoxy-xinc coating with fly ash as an extender pigment Engku Norfatima Engku Dahalana, Azizul Helmi Sofiana*, Arman Abdullaha, Norhazilan Md Noorb a
Faculty of Chemical Engineering & Natural Resources, Universiti Malaysia Pahang, 26300 Gambang, Pahang, Malaysia b Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia
Abstract The utilization of industrial waste like fly ash may reduce the environmental risk. The properties of fly ash are useful and may contribute in organic coating field. This paper study the new strategy for coating with enhanced cathodic and barrier protection simultaneously. The aim of this study is to show that the low pigment volume concentration (PVC) of the coating may have a great protection by introducing a small amount of fly ash as an extender pigment into the coating material. Specifically, we compare the protection offered by epoxy coating with 10% of PVC. The zinc pigment in the coating was replaced with fly ash at 10, 20, 30 and 40 vol/vol %. The replacing of fly ash in the formulation reduced the pigments percentage in the coating. Polarization test was conducted up to 30 days. The corrosion behaviors of the coated specimens were observed, and the corrosion rates were calculated based on the corrosion kinetics obtained from the test. The results demonstrated that addition of fly ash into the coating material, galvanizing the coated specimen, thus proved that fly ash has cathodic properties. The addition of fly ash changes the mechanism of protection from barrier protection mainly came from epoxy resin to cathodic protection from fly ash. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 3rd International Conference on Green Chemical Engineering and Technology (3rd GCET): Materials Science, 07-08 November 2017. Keywords: Fly ash; organic coating; cathodic protection; barrier protection; extender pigment
* Corresponding author. Tel.: +6017-8228387; fax: +609-5493382 E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 3rd International Conference on Green Chemical Engineering and Technology (3rd GCET): Materials Science, 07-08 November 2017.
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Nomenclature PVC
pigment volume concentration
1. Introduction Steels are the most popular and widely used in the world. However, deterioration resulting from chemical and electrochemical reaction with the environment must be considered during industrial or marine applications. The application of the organic coating is one of the most extensively used and established corrosion protection method [1-3]. This coating promotes both active protection of pigments/fillers and barrier protection to the metal substrates through the organic binder. The active cathodic protection is achieved by the sacrificial action of inorganic pigments/fillers while organic binder together with fillers contributes to the barrier protection of the metal substrates. The use of zinc pigments as a corrosion inhibitor in organic coating system is one of an effective way to develop corrosion protection especially when corrosion products evolves through the film after reacting with the corrosive environment [4,1]. The corrosion products can seal the porosity of the coating materials and emerge as adhering barrier layer that is highly resistant to normal atmospheric effects. The cathodic protection is restored every time after the mechanical damage occurred to the film, and this is represented as an active protection [5,6]. The anticorrosion properties are also owned by the corrosion products of zinc which are zinc oxide and zinc hydroxide [7]. Coating materials containing a high pigment concentration is unavoidable to provide electric conductivity between pigment particles and the metallic substrate. However, low binder content is incommensurate to obtain required physico-mechanical properties [8] because a very porous film is obtained [9]. There are some efforts in research field to substitute zinc pigments in coating material with other pigments, mainly for financial and environmental issues [10]. Reducing zinc pigments by adding extender pigment in coating material has been studied extensively during recent years based on its anticorrosion properties. Table 1 below shows the variety of extender pigment that has been studied and its finding or ability to improve coating properties. Table 1. Variation of extender pigments in organic coating. Extender pigments
Finding
References
Nanoclay
-Improves corrosion resistance by enhancing it barrier properties
[9]
Iron oxide
-Reducing the rate of reactivity
[11,12]
-Shortening the duration of cathodic protection -Increasing the barrier protection stage Iron oxide and aluminium
-Enhancement of corrosion protection properties without reducing its sacrificial properties
[7]
Nano silica
-Enhancing the conductivity of the coating in the corrosive environment
[13]
Carbon nanotubes
-Increasing resistance to corrosion test
[14]
Graphite
-Increasing resistance to mechanical stress
[14]
Alumina-silica
-Improving corrosion resistance and mechanical performances
[15]
-Enhancing the cross-linking of the cured film
[16,17]
Silica
-Enhancing anticorrosive performance because an effective dispersion in resin
Nowadays, everything that may contribute to green technologies is globally explored to reduce the negative effect on the environment. Fly ash is an industrial waste from the combustion of pulverized coal rich in silica, alumina, and
Dahalan et al./ Materials Today: Proceedings 5 (2018) 21629–21635
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iron (III) oxide content [18,19]. Enormous studies of fly ash have been done in geopolymer industry because of its mechanical strength and chemical resistance [20-23]. These abilities are the elements that needed in coating materials so that the coated steel is sturdier and long lasting. In this study, we focused on the waste as an extender pigment that contains alumina and silica which is fly ash. The aim of this study is to explore the potential of fly ash as an extender pigment in epoxy zinc coating. The effect of fly ash content in coating material has been studied in term of corrosion behavior from day 1 up to 30. 2. Methods 2.1. Metallic substrate pretreatment The coating substrates were prepared from commercially carbon steel sheets with the thickness of 0.5 mm. The surface area was 4 cm2. The specimens were polished with #180, #220, #280, #600, #800, #1000 and #1500 abrasive silicon carbide (SiC) paper to produce a clean and clear surface. After polishing, the specimens were washed with ethanol and ultrasonicated for 30 minutes. Ethanol was employed to degrease the surface of the metal substrates. The clean specimens were dried at room temperature. 10 cm copper wire in length was cut and then soldered on the back of the metal specimen. Copper wire was being used based on its ability to conduct electricity during a corrosion test. In order to strengthen the adhesion of copper wire on the metallic specimen, the soldered part was filled up with epoxy resin and dried at room temperature for 6 to 7 hours. 2.2. Organic coating formulation The organic zinc coating was formulated with 10 % PVC. The composition of fly ash in the coating was varied to 10, 20, 30 and 40 % vol/vol of fly ash and zinc oxide. The further contents designed in the coating was shown in Table 2, while the percentage of pigment was presented in Table 3. The painting process took place after the specimen was properly cleaned and dried. The process was conducted at room temperature to achieve optimum and similar coating. After that, the coated substrates will be dried at ambient temperature for 4-5 hours. 2.3. Polarization test Corrosion protection performance of the coated substrate at the various composition of fly ash was evaluated by electrochemical Tafel extrapolation of polarization data and anodic polarization curve measurements. The measurements of all electrochemical were made in a cell containing 3 electrodes. The coated substrate sample was used as the working electrode. A platinum electrode was employed as a counter or auxiliary electrode, and Ag/AgCl/KCl (saturated) is used as a reference electrode. All the three electrodes were connected to the test solution or electrolyte (NaCl solution). Polarization curve measurements of a sample are obtained in 3.5 wt./wt. % NaCl solution from -1.0V to 1.0V of potential. Electrode potentials were controlled by an Autolab (PGSTAT101) potentiostat/galvanostat. Table 2. Ingredients in the organic zinc coating. Sample
Pigment Volume Concentration, PVC1 (%)
Fly Ash Concentration (vol/vol) (%)
Epoxy Resin (g)
Zinc Oxide (g)
Fly ash (g)
Total (g)
FA10
10
10
10
5.43
0.12
15.55
FA20
10
20
10
4.97
0.25
15.22
FA30
10
30
10
4.56
0.34
14.90
FA40
10
40
10
4.24
0.42
14.66
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Dahalan et al.,/ Materials Today: Proceedings 5 (2018) 21629–21635 Table 3. Percentage of epoxy resin, zinc oxide and fly ash content in the formulated organic zinc coating. Sample
Pigment Volume Concentration, PVC (%)
Epoxy Resin Content (w/w %)
Zinc Oxide Content (w/w %)
Fly Ash Content (w/w %)
Total Pigments Content (w/w %)2
FA10
10
64.31
34.92
0.77
35.69
FA20
10
65.70
32.65
1.64
34.29
FA30
10
67.11
30.60
2.28
32.88
FA40
10
68.21
28.92
2.86
31.78
3. Results and discussion The results of Tafel extrapolation from electrochemical measurements for different composition of fly ash in coatings are given in Table 4. Corrosion potential of the samples can be used for coating’s comparison and corrosion behavior interpretation. Comparison of the results shows that FA40 has the lowest corrosion potential (-0.81 V) and high current density (2.35E-06 A/cm2). Ergo, FA40 will corrode before others and the corrosion is much faster than others. Based on the half-cell potential for the anodic oxidation of the samples, the order of fastest corrosion occurred is FA40 and then following by FA30, FA20, and FA10. One can see that the current density at the corrosion potential for FA40 is larger than the current density for others at the respective potential. Therefore, FA40 corrodes faster than FA20, FA10, and FA30 in orderly. An addition of fly ash to the zinc coating results in the reduction of the total pigments content (Table 3). Since total pigment and zinc content reduced, the corrosion potential should be more positive because lack of zinc which contributes to the less active in term of galvanic properties. However, the corrosion potential tends to be more negative as the increment of fly ash rise up. On the other hand, the trend for current density fluctuates as the fly ash content increase. This is due to the mechanism protection involves. Table 4. Dependence of corrosion potentials, Ecorr and current density, Icorr on coating configurations
1 2
Sample
Ecorr,V
Icorr, A/cm2
FA10
-0.48
1.10E-07
FA20
-0.65
2.65E-07
FA30
-0.67
1.05E-07
FA40
-0.81
2.35E-06
PVC = ((Vzinc oxide + Vfly ash) / (Vzinc oxide + Vfly ash + Vbinder)) X 100. Total Pigments Content = (Zinc Oxide Content) + (Fly Ash Content) 1.0
FA10 FA20 FA30 FA40
Potential (V)
0.5
0.0
-0.5
-1.0 1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
2
Current Density (A/cm )
Fig. 1. Representative potentiodynamic polarization curves of the four-different coating composition.
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The potentiodynamic polarization curve relative to the sample FA10 exhibits a corrosion potential of -0.48 V. The corrosion current density is about 1.0E-08 A/cm2. The curve displays an unstable passive behavior with breakdown potential at -0.3 and -0.22 V. The potentiodynamic polarization curve relative to the sample FA20 and FA30 exhibit a corrosion potential of -0.65 and -0.67 V respectively, while the current density is about 1.0E-08 and 1.0E-07 A/cm2. Both samples possess almost the same value for corrosion potential but differ in current density. However, the polarity trends are not identical with each other. FA20 shows passivity after the corrosion potential and breakdown at potential -0.35 V. Meanwhile FA30 shows the same trend as FA40, a spontaneously passive behavior with breakdown potential of -0.23 V. The behavior of passivity is resulting from the presence of a thin protective oxide layer on the metal surface. The breakdown potential after the passive behavior means that the coating is easier to attack by the local attack (pitting). FA10 FA20 FA30 FA40
-0.40 -0.45
BARRIER PROTECTION ZONE
-0.50
Ecorr (V)
-0.55
CATHODIC PROTECTION ZONE -0.60 -0.65 -0.70 -0.75 -0.80 0
5
10
15
20
25
30
Time (days)
Fig. 2. Dependence of corrosion potentials, Ecorr on coating configuration.
The OCP values for the samples are shown in Figure 2 as a function of immersion time. From the figure, it can be seen that the OCP values for FA20, FA30 and FA40 are below the OCP value of iron (-0.58 V), showing that the ability of these coatings at the initial stage of immersion is from the effective cathodic protection. Meanwhile, FA10 possess a barrier protection from the initial stage of immersion until the end of immersion. Comparing the OCP values of these samples, it is shown that inclusion of fly ash into the coating resulted in the shift of OCP values to more negative values, illustrating the increase of cathodic protection intensity at the first days of immersion. As the time progresses, the OCP values of FA10, FA20, and FA40 increase until day 7. After that, the reduction of OCPs value occurred up to day 19 and start to increase again. The increase of these OCPs is caused by the fast oxidation process of zinc particles and active pigments in fly ash leading to the creation of oxide corrosion products to seal coating porosity. The OCP values of beginning immersion are about the same as the OCP values of the end of immersion. The fluctuated trend of these coatings is same except for FA30. The OCP values of FA30 decrease until day 11 and increase until approach the barrier protection zone. Thus, decreasing the electrical contact of zinc particles and active pigments in the fly ash and increasing the performance of coating barrier. However, too much addition of fly ash to the coating formulation (FA30) reduces the rate of electrolyte diffusion and therefore the activation of active pigments takes place at longer immersion time. By adding the fly ash into the coating, the advantages are not only on the barrier performance of coating but also the electrical contact between the active pigments and the metallic substrate, resulting in the increase cathodic protection at long period of immersion. The variation of particle size and shape of fly ash gave effective filling of internal voids in the structure of the coatings, thus lowering the porosity of the coating. After the end of immersion, the OCP of FA30 became more positive than the OCP of iron, indicating that no cathodic protection but barrier protection capability of this coating. However, the FA20 and FA40 show cathodic protection performances until the end of immersion. Compared to the FA20 sample,
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the FA30 sample shows more negative OCP value at long immersion time, indicating effective cathodic protection performance of this sample. Table 5. Corrosion parameters of zinc coating after immersion in NaCl solution. Sample
Current density, Icorr (A/cm2)
Anodic slope, Ba
Cathodic slope, Bc
Proportionally constant, B
Polarization resistance, Rp
Corrosion rates (mm/yr)
FA10
1.10E-07
1.12E+05
1.97E+05
3.11E+04
2.83E+07
1.3E-03
FA20
2.65E-07
4.03E+04
2.10E+04
6.00E+03
2.26E+06
3.1E-03
FA30
1.05E-07
3.39E+04
6.20E+04
9.52E+03
9.07E+06
1.2E-03
FA40
2.35E-06
3.56E+03
4.58E+03
8.70E+02
3.70E+04
2.7E-02
From polarization curves in Figure 1, significant information on the kinetic of anodic and cathodic reaction can be obtained (Panagopoulos et al., 2013). Corrosion parameters including current density (icorr), the anodic and cathodic Tafel plot (ba and bc), polarization resistance (Rp) and corrosion rates are listed in Table 5. The Stern-Geary equation was used to calculate the Rp and corrosion rate was calculated in term of penetration rate. As listed in Table 5, at a high value of current density (FA40), the specimen tends to corrode faster. This is proved by the calculated corrosion rates. FA30 shows the lowest corrosion rate, followed by FA10, FA20 and the worst FA40. The value of icorr and corrosion rate for FA10 and FA30 are slightly similar to each other even though the mechanism of protection involves is different. The difference of corrosion rate between FA10 from barrier protection and FA20 from cathodic protection is 0.0022mm/yr, more than 2 times of FA10 corrosion rate value. While the difference of corrosion rate between FA20 and FA30, and FA30 and FA40 which both undergo cathodic protection are 0.0019 and 0.026. The understanding in corrosion parameter is very important. From this value, the small addition of fly ash may change the type of coating protection from barrier to cathodic protection. The increasing percentage of fly ash content in the coating material reduced the amount of zinc oxide, thus lowering the percentage of pigment content in that formulation. Therefore, too much fly ash in the coating material (FA40) may increase the number of pores on the surface of the coating and increase the rate of corrosion. The alumina and silica containing in fly ash improve the corrosion resistance and thermal stability of coating material (Cao et al., 2017). 4. Conclusion The polarization test in an aggressive medium showed that zinc pigments in the coating material can be partially substituted by fly ash without significant destructive effect in the protecting properties. The partial replacement of the inorganic anticorrosive zinc pigment produces a switch in the coating protection mechanism from barrier protection to cathodic protection. Once the coating surface sealed with the corrosion products, the barrier protection takes place. The polarization measurement showed that the addition of fly ash shifted the voltage to more negative. This work suggests that FA30 as the best coating material in term of corrosion rate. The result presented in this study provides a significant act that enthusiastically embraces the new regulations to minimize the use of zinc containing compound as anticorrosive pigments. Acknowledgements This work has been financially supported by the RACE grant RDU 1703185 and UMP Postgraduate Research Grant Scheme PGRS160375. References [1] Visser, P., Liu, Y., Zhou, X., Hashimoto, T., Thompson, G. E., Lyon, S. B., … Terryn, H. A. (2015). The corrosion protection of AA2024-T3 aluminium alloy by leaching of lithium-containing salts from organic coatings. Faraday Discuss., 180(0): 511–526. [2] Asemani, H. R., Ahmadi, P., Sarabi, A. A., and Eivaz Mohammadloo, H. (2016). Effect of zirconium conversion coating: Adhesion and anticorrosion properties of epoxy organic coating containing zinc aluminum polyphosphate (ZAPP) pigment on carbon mild steel. Progress in Organic Coatings, 94: 18–27.
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[3] Szociński, M., and Darowicki, K. (2016). Performance of zinc-rich coatings evaluated using AFM-based electrical properties imaging. Progress in Organic Coatings, 96: 58–64. [4] Liu, Y., Visser, P., Zhou, X., Lyon, S. B., Hashimoto, T., Curioni, M., … Terryn, H. (2016). Protective Film Formation on AA2024-T3 Aluminum Alloy by Leaching of Lithium Carbonate from an Organic Coating. Journal of The Electrochemical Society, 163(3): C45–C53. [5] Kalendová, A. (2003). Effects of particle sizes and shapes of zinc metal on the properties of anticorrosive coatings. Progress in Organic Coatings, 46: 324–332. [6] Schaefer, K., and Miszczyk, A. (2013). Improvement of electrochemical action of zinc-rich paints by addition of nanoparticulate zinc. Corrosion Science, 66: 380–391. [7] Arman, S. Y., Ramezanzadeh, B., Farghadani, S., Mehdipour, M., and Rajabi, A. (2013). Application of the electrochemical noise to investigate the corrosion resistance of an epoxy zinc-rich coating loaded with lamellar aluminum and micaceous iron oxide particles. Corrosion Science, 77: 118–127. [8] Shreepathi, S., Bajaj, P., and Mallik, B. P. (2010). Electrochemical impedance spectroscopy investigations of epoxy zinc rich coatings: Role of Zn content on corrosion protection mechanism. Electrochimica Acta, 55(18): 5129–5134. [9] Arianpouya, N., Shishesaz, M., Arianpouya, M., and Nematollahi, M. (2013). Evaluation of synergistic effect of nanozinc/nanoclay additives on the corrosion performance of zinc-rich polyurethane nanocomposite coatings using electrochemical properties and salt spray testing. Surface and Coatings Technology, 216: 199–206. [10] Armelin, E., Martí, M., Liesa, F., Iribarren, J. I., and Alemán, C. (2010). Partial replacement of metallic zinc dust in heavy duty protective coatings by conducting polymer. Progress in Organic Coatings, 69(1): 26–30. [11] Kakaei, M. N., Danaee, I., and Zaarei, D. (2013). Investigation of corrosion protection afforded by inorganic anticorrosive coatings comprising micaceous iron oxide and zinc dust. Corrosion Engineering, Science and Technology, 48(3): 194–198. [12] Naser Kakaei, M., Danaee, I., and Zaarei, D. (2013). Evaluation of cathodic protection behavior of waterborne inorganic zinc‐rich silicates containing various contents of MIO pigments. Anti-Corrosion Methods and Materials, 60(1): 37–44. [13] Wang, Z., Han, E., Liu, F., Qian, Z., and Zhu, L. (2014). Waterborne Epoxy Nanocoatings Modified by Nanoemulsions and Nanoparticles. Journal of Materials Science & Technology, 30(10): 1036–1042. [14] Kalendová, A., Veselý, D., Kohl, M., and Stejskal, J. (2015). Anticorrosion efficiency of zinc-filled epoxy coatings containing conducting polymers and pigments. Progress in Organic Coatings, 78: 1–20. [15] Cao, R., Zhu, L., Li, W., Hu, W., and Liu, H. (2017). The effect of alumina-silica sols on electrodeposited zinc coatings for sintered NdFeB. Journal of Alloys and Compounds, 726: 95–106. [16] Al-Sabagh, A. M., Abdou, M. I., Migahed, M. A., Fadl, A. M., and El-Shahat, M. F. (2017). Influence of surface modified nanoilmenite/amorphous silica composite particles on the thermal stability of cold galvanizing coating. Egyptian Journal of Petroleum. [17] Ma, Y., Di, H., Yu, Z., Liang, L., Lv, L., Pan, Y., … Yin, D. (2016). Fabrication of silica-decorated graphene oxide nanohybrids and the properties of composite epoxy coatings research. Applied Surface Science, 360: 936–945. [18] Risdanareni, P., Puspitasari, P., and Jaya, E. J. (2017). Chemical and Physical Characterization of Fly Ash as Geopolymer Material. In Chemical and Physical Characterization of Fly Ash as Geopolymer Material. In MATEC Web of Conference.Web of Conference (p. 8). [19] Thomas, M. D. A. (2007). Optimizing the Use of Fly Ash in Concrete. Portland Cement Association, 24. [20] Azevedo, A. G. de S., and Strecker, K. (2017). Brazilian fly ash based inorganic polymers production using different alkali activator solutions. Ceramics International, 43(12): 9012–9018. [21] Gwon, S., Jeong, Y., Oh, J. E., and Shin, M. (2017). Sustainable sulfur composites with enhanced strength and lightweightness using waste rubber and fly ash. Construction and Building Materials, 135: 650–664. [22] Mehta, A., and Siddique, R. (2017). Sulfuric acid resistance of fly ash based geopolymer concrete. Construction and Building Materials, 146: 136–143. [23] Yousefi Oderji, S., Chen, B., and Jaffar, S. T. A. (2017). Effects of relative humidity on the properties of fly ash-based geopolymers. Construction and Building Materials, 153: 268–273.
ScienceDirect Materials Today: Proceedings 5 (2018) 21629–21635
www.materialstoday.com/proceedings
The 3rd International Conference on Green Chemical Engineering Technology (3rd GCET_2017): Materials Science
Corrosion behavior of organic epoxy-xinc coating with fly ash as an extender pigment Engku Norfatima Engku Dahalana, Azizul Helmi Sofiana*, Arman Abdullaha, Norhazilan Md Noorb a
Faculty of Chemical Engineering & Natural Resources, Universiti Malaysia Pahang, 26300 Gambang, Pahang, Malaysia b Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia
Abstract The utilization of industrial waste like fly ash may reduce the environmental risk. The properties of fly ash are useful and may contribute in organic coating field. This paper study the new strategy for coating with enhanced cathodic and barrier protection simultaneously. The aim of this study is to show that the low pigment volume concentration (PVC) of the coating may have a great protection by introducing a small amount of fly ash as an extender pigment into the coating material. Specifically, we compare the protection offered by epoxy coating with 10% of PVC. The zinc pigment in the coating was replaced with fly ash at 10, 20, 30 and 40 vol/vol %. The replacing of fly ash in the formulation reduced the pigments percentage in the coating. Polarization test was conducted up to 30 days. The corrosion behaviors of the coated specimens were observed, and the corrosion rates were calculated based on the corrosion kinetics obtained from the test. The results demonstrated that addition of fly ash into the coating material, galvanizing the coated specimen, thus proved that fly ash has cathodic properties. The addition of fly ash changes the mechanism of protection from barrier protection mainly came from epoxy resin to cathodic protection from fly ash. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 3rd International Conference on Green Chemical Engineering and Technology (3rd GCET): Materials Science, 07-08 November 2017. Keywords: Fly ash; organic coating; cathodic protection; barrier protection; extender pigment
* Corresponding author. Tel.: +6017-8228387; fax: +609-5493382 E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 3rd International Conference on Green Chemical Engineering and Technology (3rd GCET): Materials Science, 07-08 November 2017.
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Dahalan et al.,/ Materials Today: Proceedings 5 (2018) 21629–21635
Nomenclature PVC
pigment volume concentration
1. Introduction Steels are the most popular and widely used in the world. However, deterioration resulting from chemical and electrochemical reaction with the environment must be considered during industrial or marine applications. The application of the organic coating is one of the most extensively used and established corrosion protection method [1-3]. This coating promotes both active protection of pigments/fillers and barrier protection to the metal substrates through the organic binder. The active cathodic protection is achieved by the sacrificial action of inorganic pigments/fillers while organic binder together with fillers contributes to the barrier protection of the metal substrates. The use of zinc pigments as a corrosion inhibitor in organic coating system is one of an effective way to develop corrosion protection especially when corrosion products evolves through the film after reacting with the corrosive environment [4,1]. The corrosion products can seal the porosity of the coating materials and emerge as adhering barrier layer that is highly resistant to normal atmospheric effects. The cathodic protection is restored every time after the mechanical damage occurred to the film, and this is represented as an active protection [5,6]. The anticorrosion properties are also owned by the corrosion products of zinc which are zinc oxide and zinc hydroxide [7]. Coating materials containing a high pigment concentration is unavoidable to provide electric conductivity between pigment particles and the metallic substrate. However, low binder content is incommensurate to obtain required physico-mechanical properties [8] because a very porous film is obtained [9]. There are some efforts in research field to substitute zinc pigments in coating material with other pigments, mainly for financial and environmental issues [10]. Reducing zinc pigments by adding extender pigment in coating material has been studied extensively during recent years based on its anticorrosion properties. Table 1 below shows the variety of extender pigment that has been studied and its finding or ability to improve coating properties. Table 1. Variation of extender pigments in organic coating. Extender pigments
Finding
References
Nanoclay
-Improves corrosion resistance by enhancing it barrier properties
[9]
Iron oxide
-Reducing the rate of reactivity
[11,12]
-Shortening the duration of cathodic protection -Increasing the barrier protection stage Iron oxide and aluminium
-Enhancement of corrosion protection properties without reducing its sacrificial properties
[7]
Nano silica
-Enhancing the conductivity of the coating in the corrosive environment
[13]
Carbon nanotubes
-Increasing resistance to corrosion test
[14]
Graphite
-Increasing resistance to mechanical stress
[14]
Alumina-silica
-Improving corrosion resistance and mechanical performances
[15]
-Enhancing the cross-linking of the cured film
[16,17]
Silica
-Enhancing anticorrosive performance because an effective dispersion in resin
Nowadays, everything that may contribute to green technologies is globally explored to reduce the negative effect on the environment. Fly ash is an industrial waste from the combustion of pulverized coal rich in silica, alumina, and
Dahalan et al./ Materials Today: Proceedings 5 (2018) 21629–21635
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iron (III) oxide content [18,19]. Enormous studies of fly ash have been done in geopolymer industry because of its mechanical strength and chemical resistance [20-23]. These abilities are the elements that needed in coating materials so that the coated steel is sturdier and long lasting. In this study, we focused on the waste as an extender pigment that contains alumina and silica which is fly ash. The aim of this study is to explore the potential of fly ash as an extender pigment in epoxy zinc coating. The effect of fly ash content in coating material has been studied in term of corrosion behavior from day 1 up to 30. 2. Methods 2.1. Metallic substrate pretreatment The coating substrates were prepared from commercially carbon steel sheets with the thickness of 0.5 mm. The surface area was 4 cm2. The specimens were polished with #180, #220, #280, #600, #800, #1000 and #1500 abrasive silicon carbide (SiC) paper to produce a clean and clear surface. After polishing, the specimens were washed with ethanol and ultrasonicated for 30 minutes. Ethanol was employed to degrease the surface of the metal substrates. The clean specimens were dried at room temperature. 10 cm copper wire in length was cut and then soldered on the back of the metal specimen. Copper wire was being used based on its ability to conduct electricity during a corrosion test. In order to strengthen the adhesion of copper wire on the metallic specimen, the soldered part was filled up with epoxy resin and dried at room temperature for 6 to 7 hours. 2.2. Organic coating formulation The organic zinc coating was formulated with 10 % PVC. The composition of fly ash in the coating was varied to 10, 20, 30 and 40 % vol/vol of fly ash and zinc oxide. The further contents designed in the coating was shown in Table 2, while the percentage of pigment was presented in Table 3. The painting process took place after the specimen was properly cleaned and dried. The process was conducted at room temperature to achieve optimum and similar coating. After that, the coated substrates will be dried at ambient temperature for 4-5 hours. 2.3. Polarization test Corrosion protection performance of the coated substrate at the various composition of fly ash was evaluated by electrochemical Tafel extrapolation of polarization data and anodic polarization curve measurements. The measurements of all electrochemical were made in a cell containing 3 electrodes. The coated substrate sample was used as the working electrode. A platinum electrode was employed as a counter or auxiliary electrode, and Ag/AgCl/KCl (saturated) is used as a reference electrode. All the three electrodes were connected to the test solution or electrolyte (NaCl solution). Polarization curve measurements of a sample are obtained in 3.5 wt./wt. % NaCl solution from -1.0V to 1.0V of potential. Electrode potentials were controlled by an Autolab (PGSTAT101) potentiostat/galvanostat. Table 2. Ingredients in the organic zinc coating. Sample
Pigment Volume Concentration, PVC1 (%)
Fly Ash Concentration (vol/vol) (%)
Epoxy Resin (g)
Zinc Oxide (g)
Fly ash (g)
Total (g)
FA10
10
10
10
5.43
0.12
15.55
FA20
10
20
10
4.97
0.25
15.22
FA30
10
30
10
4.56
0.34
14.90
FA40
10
40
10
4.24
0.42
14.66
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Dahalan et al.,/ Materials Today: Proceedings 5 (2018) 21629–21635 Table 3. Percentage of epoxy resin, zinc oxide and fly ash content in the formulated organic zinc coating. Sample
Pigment Volume Concentration, PVC (%)
Epoxy Resin Content (w/w %)
Zinc Oxide Content (w/w %)
Fly Ash Content (w/w %)
Total Pigments Content (w/w %)2
FA10
10
64.31
34.92
0.77
35.69
FA20
10
65.70
32.65
1.64
34.29
FA30
10
67.11
30.60
2.28
32.88
FA40
10
68.21
28.92
2.86
31.78
3. Results and discussion The results of Tafel extrapolation from electrochemical measurements for different composition of fly ash in coatings are given in Table 4. Corrosion potential of the samples can be used for coating’s comparison and corrosion behavior interpretation. Comparison of the results shows that FA40 has the lowest corrosion potential (-0.81 V) and high current density (2.35E-06 A/cm2). Ergo, FA40 will corrode before others and the corrosion is much faster than others. Based on the half-cell potential for the anodic oxidation of the samples, the order of fastest corrosion occurred is FA40 and then following by FA30, FA20, and FA10. One can see that the current density at the corrosion potential for FA40 is larger than the current density for others at the respective potential. Therefore, FA40 corrodes faster than FA20, FA10, and FA30 in orderly. An addition of fly ash to the zinc coating results in the reduction of the total pigments content (Table 3). Since total pigment and zinc content reduced, the corrosion potential should be more positive because lack of zinc which contributes to the less active in term of galvanic properties. However, the corrosion potential tends to be more negative as the increment of fly ash rise up. On the other hand, the trend for current density fluctuates as the fly ash content increase. This is due to the mechanism protection involves. Table 4. Dependence of corrosion potentials, Ecorr and current density, Icorr on coating configurations
1 2
Sample
Ecorr,V
Icorr, A/cm2
FA10
-0.48
1.10E-07
FA20
-0.65
2.65E-07
FA30
-0.67
1.05E-07
FA40
-0.81
2.35E-06
PVC = ((Vzinc oxide + Vfly ash) / (Vzinc oxide + Vfly ash + Vbinder)) X 100. Total Pigments Content = (Zinc Oxide Content) + (Fly Ash Content) 1.0
FA10 FA20 FA30 FA40
Potential (V)
0.5
0.0
-0.5
-1.0 1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
2
Current Density (A/cm )
Fig. 1. Representative potentiodynamic polarization curves of the four-different coating composition.
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The potentiodynamic polarization curve relative to the sample FA10 exhibits a corrosion potential of -0.48 V. The corrosion current density is about 1.0E-08 A/cm2. The curve displays an unstable passive behavior with breakdown potential at -0.3 and -0.22 V. The potentiodynamic polarization curve relative to the sample FA20 and FA30 exhibit a corrosion potential of -0.65 and -0.67 V respectively, while the current density is about 1.0E-08 and 1.0E-07 A/cm2. Both samples possess almost the same value for corrosion potential but differ in current density. However, the polarity trends are not identical with each other. FA20 shows passivity after the corrosion potential and breakdown at potential -0.35 V. Meanwhile FA30 shows the same trend as FA40, a spontaneously passive behavior with breakdown potential of -0.23 V. The behavior of passivity is resulting from the presence of a thin protective oxide layer on the metal surface. The breakdown potential after the passive behavior means that the coating is easier to attack by the local attack (pitting). FA10 FA20 FA30 FA40
-0.40 -0.45
BARRIER PROTECTION ZONE
-0.50
Ecorr (V)
-0.55
CATHODIC PROTECTION ZONE -0.60 -0.65 -0.70 -0.75 -0.80 0
5
10
15
20
25
30
Time (days)
Fig. 2. Dependence of corrosion potentials, Ecorr on coating configuration.
The OCP values for the samples are shown in Figure 2 as a function of immersion time. From the figure, it can be seen that the OCP values for FA20, FA30 and FA40 are below the OCP value of iron (-0.58 V), showing that the ability of these coatings at the initial stage of immersion is from the effective cathodic protection. Meanwhile, FA10 possess a barrier protection from the initial stage of immersion until the end of immersion. Comparing the OCP values of these samples, it is shown that inclusion of fly ash into the coating resulted in the shift of OCP values to more negative values, illustrating the increase of cathodic protection intensity at the first days of immersion. As the time progresses, the OCP values of FA10, FA20, and FA40 increase until day 7. After that, the reduction of OCPs value occurred up to day 19 and start to increase again. The increase of these OCPs is caused by the fast oxidation process of zinc particles and active pigments in fly ash leading to the creation of oxide corrosion products to seal coating porosity. The OCP values of beginning immersion are about the same as the OCP values of the end of immersion. The fluctuated trend of these coatings is same except for FA30. The OCP values of FA30 decrease until day 11 and increase until approach the barrier protection zone. Thus, decreasing the electrical contact of zinc particles and active pigments in the fly ash and increasing the performance of coating barrier. However, too much addition of fly ash to the coating formulation (FA30) reduces the rate of electrolyte diffusion and therefore the activation of active pigments takes place at longer immersion time. By adding the fly ash into the coating, the advantages are not only on the barrier performance of coating but also the electrical contact between the active pigments and the metallic substrate, resulting in the increase cathodic protection at long period of immersion. The variation of particle size and shape of fly ash gave effective filling of internal voids in the structure of the coatings, thus lowering the porosity of the coating. After the end of immersion, the OCP of FA30 became more positive than the OCP of iron, indicating that no cathodic protection but barrier protection capability of this coating. However, the FA20 and FA40 show cathodic protection performances until the end of immersion. Compared to the FA20 sample,
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the FA30 sample shows more negative OCP value at long immersion time, indicating effective cathodic protection performance of this sample. Table 5. Corrosion parameters of zinc coating after immersion in NaCl solution. Sample
Current density, Icorr (A/cm2)
Anodic slope, Ba
Cathodic slope, Bc
Proportionally constant, B
Polarization resistance, Rp
Corrosion rates (mm/yr)
FA10
1.10E-07
1.12E+05
1.97E+05
3.11E+04
2.83E+07
1.3E-03
FA20
2.65E-07
4.03E+04
2.10E+04
6.00E+03
2.26E+06
3.1E-03
FA30
1.05E-07
3.39E+04
6.20E+04
9.52E+03
9.07E+06
1.2E-03
FA40
2.35E-06
3.56E+03
4.58E+03
8.70E+02
3.70E+04
2.7E-02
From polarization curves in Figure 1, significant information on the kinetic of anodic and cathodic reaction can be obtained (Panagopoulos et al., 2013). Corrosion parameters including current density (icorr), the anodic and cathodic Tafel plot (ba and bc), polarization resistance (Rp) and corrosion rates are listed in Table 5. The Stern-Geary equation was used to calculate the Rp and corrosion rate was calculated in term of penetration rate. As listed in Table 5, at a high value of current density (FA40), the specimen tends to corrode faster. This is proved by the calculated corrosion rates. FA30 shows the lowest corrosion rate, followed by FA10, FA20 and the worst FA40. The value of icorr and corrosion rate for FA10 and FA30 are slightly similar to each other even though the mechanism of protection involves is different. The difference of corrosion rate between FA10 from barrier protection and FA20 from cathodic protection is 0.0022mm/yr, more than 2 times of FA10 corrosion rate value. While the difference of corrosion rate between FA20 and FA30, and FA30 and FA40 which both undergo cathodic protection are 0.0019 and 0.026. The understanding in corrosion parameter is very important. From this value, the small addition of fly ash may change the type of coating protection from barrier to cathodic protection. The increasing percentage of fly ash content in the coating material reduced the amount of zinc oxide, thus lowering the percentage of pigment content in that formulation. Therefore, too much fly ash in the coating material (FA40) may increase the number of pores on the surface of the coating and increase the rate of corrosion. The alumina and silica containing in fly ash improve the corrosion resistance and thermal stability of coating material (Cao et al., 2017). 4. Conclusion The polarization test in an aggressive medium showed that zinc pigments in the coating material can be partially substituted by fly ash without significant destructive effect in the protecting properties. The partial replacement of the inorganic anticorrosive zinc pigment produces a switch in the coating protection mechanism from barrier protection to cathodic protection. Once the coating surface sealed with the corrosion products, the barrier protection takes place. The polarization measurement showed that the addition of fly ash shifted the voltage to more negative. This work suggests that FA30 as the best coating material in term of corrosion rate. The result presented in this study provides a significant act that enthusiastically embraces the new regulations to minimize the use of zinc containing compound as anticorrosive pigments. Acknowledgements This work has been financially supported by the RACE grant RDU 1703185 and UMP Postgraduate Research Grant Scheme PGRS160375. References [1] Visser, P., Liu, Y., Zhou, X., Hashimoto, T., Thompson, G. E., Lyon, S. B., … Terryn, H. A. (2015). The corrosion protection of AA2024-T3 aluminium alloy by leaching of lithium-containing salts from organic coatings. Faraday Discuss., 180(0): 511–526. [2] Asemani, H. R., Ahmadi, P., Sarabi, A. A., and Eivaz Mohammadloo, H. (2016). Effect of zirconium conversion coating: Adhesion and anticorrosion properties of epoxy organic coating containing zinc aluminum polyphosphate (ZAPP) pigment on carbon mild steel. Progress in Organic Coatings, 94: 18–27.
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