Enhancement of Surface Coating Characteristics of Epoxy Resin by

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Enhancement of surface coating characteristics of epoxy resin by dextran: An electrochemical approach B Ponnappa Charitha, Arunchandran Chenan, and Padmalatha Rao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03274 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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Industrial & Engineering Chemistry Research

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Enhancement of surface coating characteristics of epoxy resin by dextran: An electrochemical approach B Ponnappa Charitha1, Arunchandran Chenan2 and Padmalatha Rao1* 1. Department of Chemistry, Manipal Institute of Technology, Manipal, University Karnataka INDIA 2. Laboratory of Synthetic and Materials Chemistry, Manipal Centre for Natural Sciences, Manipal University, Karnataka, INDIA. Corresponding author’s mail id: [email protected]

ABSTRACT The work describes the use of polymer dextran to enhance the corrosion protection of epoxy coating applied on 6061 Al-15%(v) SiC(p) composite material (Al-CM). Initially, conditions were established to obtain optimum inhibition efficiency of dextran. Electrochemical techniques such as potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) were adopted for the corrosion rate measurements. Later, same techniques were employed to study the corrosion rate of the epoxy resin coated composite material (EC-CM) before and after the addition of environmentally benign dextran polymer. Detailed investigation of surface morphology of the coated Al-CM was carried out using scanning electron microscopy (SEM), energy dispersion X-ray (EDX) analysis and atomic force microscopy (AFM) techniques. The work which is done with dual purpose, establishes not only the anticorrosive property of dextran, but also proves its ability to improve the coating characteristics of epoxy resin on Al-CM. Keywords: Dextran, Epoxy resin coating, 6061 Al-15%(v) SiC(p) composite, Corrosion inhibition. 1

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INTRODUCTION Aluminum alloys are light weight material and they have wide range of applications in various industries such as aircraft, automobile, aerospace and electronics due to their high electrical and thermal conductivities, low density, high reflectivity, high ductility and relatively low cost. Moreover, Al alloys have reasonably good corrosion resistance due to the presence of protective oxide film over the surface of aluminum. Nevertheless, aluminum alloys are soft and they have less length to weight ratio and hence their applications in many modern engineering systems are limited1–6. Therefore, it is required to improve the properties of aluminum alloy by reinforcing it with SiC particulate matter and make a new generation metal matrix composite to meet the challenging demands of modern engineering applications. The reinforcement increases the strength to weight ratio and make them hard. In comparison with all other aluminum composite materials, aluminum alloys reinforced with SiC particulates have immense application in various sectors like defense, aerospace, aviation etc.7-10. They also found extensive applications in radiators, water cooling systems and heat exchangers11. However, reinforcement of SiC depletes the protective oxide film of the metal and leads to corrosion. Moreover, as a mandatory practice to meet industrial application standards it is required to clean the surface of metals and alloys to remove residual oxides or impurities present on the metal/alloy surface. This cleaning is generally carried out using hydrochloric acid and sulphuric acid or a mixture of these two mineral acids and this process is known as acid pickling. During, this processes acids removes the extraneous matters deposited on the material surfaces and also leads to dissolution of materials, thereby enhances the rate of corrosion. The presence of chlorides and extraneous anions also facilitates the damage to the metal12, 13.


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Application of protective coatings is one of the ways to combat corrosion. Generally epoxy based coatings have been applied to protect various metals and alloys from corrosion in aggressive environments14,

15

. Epoxy coatings have been used as a structural or engineering

adhesive for the construction of aircrafts, automobiles, radiators and boats etc. The excellent chemical resistance, good adhesion to the underlying metal surfaces, and electrical insulating properties make epoxy resins an ideal coating material for various applications16 – 18. Epoxy resin reduces the corrosion rate of the underlying metals and alloys by acting as an effective physical barrier between the metals/alloys and corrosive environment. However, owing to wear and abrasion, epoxy based coatings also fail to offer long term corrosion protection. During the curing process of epoxy coatings, the shrinkage of epoxy resin takes place and absorbs water, air from the environment and this would in turn make microspores in the epoxy coatings. The pores in the coating facilitates the migration of corrosive species and water molecules at metal/epoxy interface and initiate the corrosion process and depletes the coating19–21. This can be overcome by increasing the toughness of the epoxy resin. Toughness can be enhanced through chemical modifications of epoxy backbone, which leads to increase the flexibility and also can be enhanced by increasing the molecular weight of the epoxy 22. The corrosion resistance of epoxy coating can be increased by the incorporation of substances which are miscible with epoxy resin. Biopolymers, which are non-toxic, and environmentally benign substances, can be used to enhance the toughness of the epoxy coating. Biopolymers are the giant molecules having high molecular weight and high degree of flexibility. Thus biopolymers can enhance the toughness of the epoxy coating and reduces the corrosion rate of underlying Al-CM. The average molecular weight of dextran is 9,000-11,000



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which is 10 times higher than the average molecular weight of epoxy.

Thus it may provide

toughness to the epoxy coating and protects the Al-CM corrosion Stadler et al. worked on corrosion inhibition using sulphate-reducing bacteria and showed that dextran could prevent corrosion on metals23. Van Leeuwen et al., found ⍺ (1 → 3), ⍺ (1 • 6) -linked D-glucan produced by lactobacillus reuteri that inhibited corrosion while dispersed in an electrolyte solution rather than as a coating24. Work by Victoria L. Finkenstadt et al highlighted the corrosion protection of carbon steel from the coating of leuconostoc mesenteroids and showed corrosion inhibition appears to be strain-specific with dextran-producing bacteria such as leuconostoc mesenteroids25. In the present study, we describe the use of dextran as a green corrosion inhibitor for 6061 Al-15%(v) SiC(p) composite and find the inhibition efficiency of dextran for 6061 Al-15%(v) SiC(p) composite for optimized concentration. Later the dextran molecules were incorporated into epoxy coatings to improve its corrosion resistance to protect 6061 Al-15%(v) SiC(p) composite. Dextran

EXPERIMENTAL Materials The material used for the study is 6061 Al-15%(v) SiC(P) having the composition given in Table 1. Table 1. Composition of base metal 6061 aluminum-alloy

Elements

Cu

Mg

Cr

Si

Al

Composition(%wt) 0.02 0.61 0.01 1.00 Balance


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The epoxy resin and hardener used for the work were from Huntsman Int. India Private Ltd., Araldite (Standard epoxy adhesive). The liquid epoxy is whitish in color and consists of Bis-phenol Diglycidyl ether. The liquid hardener is yellowish and consists of 1 to 3% Triethylene glycol dimercaptan N,N(3-dimethylaminopropyl)-1,3-propylenediamine, 2,4,6tris(dimethylaminomethyl)phenol, 4-dimethylaminoethyl-1-methyl-4-piperazine, n-butyl acetate.

Preparation of test coupon The cylindrical test coupon of 4.5 cm length was molded by using cold setting resin and made to expose 1 cm2. The exposed surface was polished with 200, 400, 800 and 1000 graded emery papers and further polishing was on disc polisher using levigated alumina paste as abrasive in order to obtain mirror surface.

Preparation of coating The epoxy coating on the 6061 Al-15%(v) SiC(p) composite was obtained as per reported elsewhere26. The epoxy resin and hardener of 1:1 weight ratio was taken separately and diluted with acetone. Subsequently, dextran was added to the acetone solution of epoxy resin and stirred at a speed of 700 rpm for 25 min. The curing agent was then added slowly and stirred for 25 min. The Al-composite material was dipped into the mixture at one time for 60 s at right angle to the container27 and dried for 7days at room temperature. The material was then used for electrochemical studies.



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Electrochemical measurements Electrochemical measurements were conducted with three electrode Pyrex glass cell. It consists of platinum as auxiliary, saturated calomel as reference electrode and 6061 Al-15%(v) SiC(P) as working electrode connected to potentiostat(CH600 D-series, U.S. model with CH instrument beta software). Immediately after the electrochemical impedance spectroscopy studies without further surface treatment potentiodynamic polarization studies were carried out. The PDP studies were conducted for coated and uncoated working electrodes immersed in 1 M hydrochloric acid solution at 303 K by polarizing it from ─250 mV cathodically and +250 mV anodically with respect to open circuit potential (OCP) at a scan rate of 1 mV sec─1. The steady state OCP was recorded at end of 1800 s. The EIS studies were performed at OCP with amplitude of 10 mV, with frequency ranging from 10,000 to 0.01Hz.

Surface characterization of coating The surface characterization of the uncoated Al-CM and coated Al-CM were carried out using SEM, EDX and AFM. The main idea of these investigations is to know the extent of corrosion attack on the metal surface after immersing the specimens in 1 M HCl. Surface Morphology studies of Al-CM were carried out by using analytical scanning electron microscope (JEOL JSM─6380L). Surface morphology of coated samples were obtained before and after the 2 h of immersion in 1 M HCl elemental analysis were carried out using Energy Dispersive X-Ray analysis technique. Surface roughness of the coated samples was analyzed by using AFM analysis technique (1B342 innova model). The analysis was carried out for coated samples before and after 2 h immersion in 1M HCl.


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RESULTS AND DISCUSSIONS Potentiodynamic polarization (PDP) studies Figure 1(a) depicts the potentiodynamic polarization plots obtained for corrosion of 6061 Al-15%(v) SiC(P) composite material (Al-CM) in 1M HCl after the addition of varying concentrations of inhibitor. Figure 1(b) depicts potentiodynamic polarization plots obtained for epoxy coated composite material (EC-CM) and epoxy coating containing dextran on composite material (ECD-CM) in 1 M HCl. Results of potentiodynamic polarization measurements are given in Table 2 and 3.

Figure 1. Potentiodynamic polarization plots obtained for 6061 Al-15%(v) SiC(P) composite material in 1M HCl at 303 K (a) at varying concentrations of dextran and (b) Al-CM; EC-CM; ECD-CM From the potentiodynamic polarization plots various parameters such as corrosion current density (icorr), corrosion potential (Ecorr) and Tafel slope (-βc) values were obtained. The Figure 1



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shows that anodic current plateau is not well defined so icorr values were obtained by the extrapolation of cathodic slopes and the change in the shape of the anodic slope was observed because of the passivation28 occurring in the potential region ranging from ─0.5 V to ─0.6 V. Thus % I.E can be calculated by using equation (1),

I. E. (%) =

୧ౙ౥౨౨ ష ౟ౙ౥౨౨(౟౤౞) ୧ౙ౥౨౨

× 100

(1)

Table 2 shows negligible change in the values of cathodic slopes this indicates coating slightly alters the hydrogen evolution reaction rate without altering the mechanism of corrosion process29, 30

. As per the reported literature31, if the shift in the Ecorr is more than ±85 mV then the inhibition

effect is on either cathodic or anodic reaction process, but here the shift in the Ecorr is less than 85 mV this indicates the controlling of both cathodic and anodic reactions. The ECD-CM inhibition efficiency substantially increased when compared with EC-CM.

Table 2. Results of Potentiodynamic polarization for the corrosion of 6061 Al-15%(v) SiC(P) composite after the addition of varying concentration of dextran at 303 and 313 K Temp (K) 303

[Dextran] ( gL ─1) Blank 0.025 0.050 0.1 0.2 0.4 0.6

Ecorr (mV/ SCE) ─710 ─669 ─668 ─698 ─700 ─687 ─662

icorr ( m Acm─2) 9.97 2.00 1.40 1.22 1.03 0.86 1.81

-βc (mVdec-1) 448 549 569 576 636 645 335


CR (mmy−1) 37.6 9.83 5.36 4.61 3.63 3.37 9.62

I.E (%) ── 79.9 85.9 87.7 89.6 91.3 81.8

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313

Blank 0.025 0.050 0.1 0.2 0.4 0.6

─714 ─671 ─670 ─668 ─681 ─667 ─736

16.3 4.62 3.99 3.02 2.80 2.52 3.63

498 537 580 549 612 594 633

61.5 14.7 14.5 12.2 10.7 9.54 12.7

── 71.6 75.4 81.4 82.8 84.5 77.7

From Table 2 it was observed that, at 303 K the inhibition efficiency of dextran increased with increase in concentration and reached maximum inhibition efficiency of 91.3% at a concentration of 0.4 g L−1. Further increase in the concentration of dextran (0.6 g L−1) resulted in decrease in the inhibition efficiency. Hence it can be concluded that optimum concentration required to achieve maximum efficiency is 0.4 g L−1. Increase in the temperature from 303 K to 313 K resulted in the decrease in inhibition efficiency both for EC-CM and ECD-CM. The decrease in the inhibition efficiency with increase in the temperature may be due to desorption of the adsorbed inhibitor molecule. At higher temperature. Results are tabulated in Table 3.

Table 3. Results of Potentiodynamic polarization for the corrosion of epoxy coated 6061 Al15%(v) SiC(P) composite after the addition of 0.4 gL−1 of dextran at 303 K and 313 K Temp (K) 303 313

Coating EC-CM ECD-CM

Ecorr (mV/ SCE) ─712 ─719

icorr ( mA cm─2) 6.50 0.72

-βc (mVdec-1) 487 518

CR (mmy−1) 24.5 2.74

I.E (%) 34.8 92.7

EC-CM ECD-CM

─694 ─701

12.3 9.83

423 520

46.6 2.74

24.5 39.6

From Table 3 it is clear that, when the same amount of the inhibitor was to EC, its corrosion current density decreased from 6.5 mA cm─2 to 0.72 mA cm─2. Corrosion rate also decreased drastically. It showed inhibition efficiency of 92.7%. From these observations it could



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be concluded that the addition of small amount of dextran enhances the anticorrosive characteristics of epoxy resin based coating.

Electrochemical impedance spectroscopy (EIS) studies Nyquist plots for corrosion of Al-CM in 1M HCl at various concentrations of dextran inhibitor at 303 K is shown in Figure 2(a). Nyquist plots for EC-CM and ECD-CM at 303 K is given in Figure 2 (b). It can be seen from Figure 2 (a) and 2 (b), the impedance plots are semicircle; this indicates corrosion process is controlled by transfer of charges. Two processes can be observed from the Nyquist plot and they appeared at high frequency region and low frequency region respectively. The first process appeared at high frequency region is due to a double layer loop and it could be characterized by a parallel combination of double layer capacitance and charge transfer resistance. An inductive loop appeared at the low frequency region. The EIS results obtained in our study are very well agreed with reported EIS results in earlier studies by other research groups32-37.


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Figure 2. Nyquist plots for corrosion of 6061 Al-15%(v) SiC(P) composite 1M HCl at 303 K: (a) at various concentrations of dextran (b) Al-CM; EC-CM; ECD-CM The capacitive loop appeared at high frequency region could be attributed for the oxidation at M+/oxide/solution interface. During corrosion process, Al1+ ions are formed at the metal/oxide interface later gets oxidized to Al3+. Through oxide/electrolyte interface Al1+ migrates from M+/oxide interface and gets oxidized to Al3+. At oxide/solution interface OH─ and O2─ ions are also formed38-41. This region could also be attributed for the formation of passivating oxide layer on the surface of the Al-composite material42. It is assumed that the surface of aluminum have covered by the passivating oxide film, arise due to the ex-situ pretreatment of aluminum alloy. Thus it is difficult to study on oxide free Al-surface. The O2 leads repassivation of Al-surface very fast43. It could also be attributed for the formation of [metal/oxide/hydroxide/inhibitor]ads 36. Another loop at LF region is induction loop, which could be attributed for the bulk or surface species relaxation44

- 46

and it also could be due to the adsorbed inhibitor molecule

relaxation over the aluminum surface or due to the incorporation of Cl─ ions34, adsorption of Hads+

44

, oxide ions47 or inhibitor species 48 onto the Al-surface. It also could be attributed to the

dissolution of aluminum48 or due to the re-dissolution of surface oxide film46. The inductive loop might also be attributed to the modulation of surface area and for the active pitted state50. Nevertheless, the diameter of the semicircle capacitive loop and inductive loop increased considerably with increasing the concentration of dextran and it also showed increase in the diameter from EC-CM to ECD-CM, this might be due to the slowdown of charge transfer process leading to decrease in the corrosion rate in the presence of dextran. No change in the



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shape of the nyquist plots throughout the studies; this indicates corrosion inhibition action is not altering the mechanism of corrosion process after the addition of dextran and coating studies. Figure 3a is the five element equivalent circuit which was used to simulate the impedance plots for 6061Al-15%(V) SiC(P) composite material. Figure 3b is the circuit fitment for the obtained impedance values.

Figure 3. a) Simulated equivalent circuit and b) Circuit fitment for impedance values. The simulated circuit is of five elements having solution resistance (RS), charge transfer resistance (Rct), inductive resistance (RL) and inductive element (L). It also have CPE (Constant Phase Element, Q), which is parallel to resistors and inductive element shown in the Figure 3a. The presence of inductive loop leads to the calculation of RP value using equation (2),

ܴ௉ =

ோಽ ଡ଼ ோ೎೟ ோಽ ା ோ೎೟

(2)

Semicircles of the Nyquist plots have depressed capacitive loops. This depression is due to the surface inhomogeneity 41, 42. Under this situation electrical double layer at the metal solution interface is not behaves as ideal capacitor. In other words, depressed semicircle shows


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deviation from ideal behavior. Accordingly, in the mathematical analysis of impedance plots, constant phase element (Q) which is a frequency-dependent, used to denote the deviation from ideal behavior. The constant phase element (Q) is related to the ideal capacitor C by the following equation ‫ܥ‬ௗ௟ =ܳௗ௟ × (2ߨ݂௠௔௫ ) (a-1)

(3)

where, ݂௠௔௫ is the frequency at which the imaginary part of the impedance (Z″) is maximum and ‘a’ is a constant phase element exponent. The value of ‘a’ provides a measure of unevenness or roughness of the electrode surface. The value of ‘a’ is given by (−1 ≤ a ≤ 1). When ‘a’ =1 constant phase element behaves as an ideal capacitor Bode diagram gives a clear explanations to how the electrochemical system behaves based upon the frequency. Figure 4 is the bode plot obtained for Al-CM without and with various concentration of dextran at 303 K. The plot showed that the increase in the values of phase with increase in concentration of added Dextran up to their optimal concentration. From the Figure 5a, it is clear that impedance value is larger in case of ECD-CM when compared with blank and EC-CM. This indicated the enhancement of anticorrosive performance of ECD. Figure 5b, revels the plots of Bode magnitude. The plots showed only single slope for all the studied systems, the Rp values were obtained from the difference between HF limit and LF limit in Bode plots52. The difference in Rp value increased in the order of ECD-CM > EC-CM> Blank. It also increased as the concentration of the inhibitor increased.



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Figure 4. (a) Bode phase and (b) Bode magnitude plots for the corrosion of 6061 Al-15%(v) SiC(P) composite after the addition of varying concentration of dextran at 303 K

Figure 5. (a) Bode phase and (b) Bode magnitude plots for the corrosion of epoxy coated 6061 Al-15%(v) SiC(P) composite after the addition of 0.4 gL−1 of dextran at 303 K


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As polarization resistance (RP) and corrosion current density (icorr) are inversely related. The percentage I.E. can be obtained by using the equation (4),

I. E(%) =

ୖౌ(ౙ౥౗౪౛ౚ)─ ౎ౌ ୖౌ(ౙ౥౗౪౛ౚ)

x 100

(4)

RP and RP(coated) are the polarization resistance for uncoated and coated samples. RP values obtained from PDP measurement were in good agreement with RP values obtained from EIS studies indicates that rate of corrosion does not depends upon the technique used but it depends upon the behavior of metal due to coating49. Results are given in the Tables 4 and 5. Polarization resistance (RP) increased and double layer capacitance (Cdl) values decreased significantly after the addition of the inhibitor. This is because of increased thickness of electrical double layer at M+/solution interface and decreased dielectric constant. At the interface of charged metal surface and the solution, there will be formation of electrical double layer. Usually, as a result of adsorption of the inhibitors on the metal surface, capacitance of electrical double layer (Cdl) decreases. This is because adsorption of inhibitor by the displacement of water molecules and other ions originally adsorbed on the metal surface. Capacitance of electrical double layer (Cdl) is related to thickness of the double layer (d) and the local dielectric constant (ε) by following equation (5) C dl = ε / 4π d

(5)



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Table 4. Results of EIS measurements for the corrosion of 6061 Al-15%(v) SiC(P) composite after the addition of varying concentration of dextran at 303 and 313 K Temp (K) 303

[Dextran] ( g L ─1) Blank 0.025 0.050 0.1 0.2 0.4 0.6

Cdl (µF cm─2 ) 493.3 320.1 312.7 301.0 296.8 289.7 317.8

Rp (ߗ cm2) 5.1 19.5 28.9 32.5 41.7 52.3 25.6

I.E (%) ── 74.3 82.3 84.3 87.7 90.2 80.0

313

Blank 0.025 0.050 0.1 0.2 0.4 0.6

427 329 320 317 316 313 326

3.60 11.1 13.5 15.8 17.6 19.6 14.6

── 67.5 73.3 77.2 79.5 81.6 75.3

Table 5. Results of EIS measurements for the corrosion of epoxy coated 6061 Al-15%(v) SiC(P) composite after the addition of 0.4 g L−1 of dextran at 303 and 313 K

Temp (K) 303 313

Coating EC-CM ECD-CM EC-CM ECD-CM

Cdl (µF cm─2 ) 77.37 41.37 112.9 76.82

Rp (ߗ cm2) 8.40 57.0 4.65 6.10

Surface characterization of coatings SEM and elemental analysis


I.E (%) 39.2 91.2 22.5 40.9

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SEM image of freshly polished aluminum composite material and same immersed in HCl medium for 2 h is shown in the Figure 6(a) and 6(b) respectively. The surface of the metal seems to be rough. The Figure 6(a) is also relatively rough; this could be due to the presence of reinforced SiC particulates. After immersion in HCl solution without any inhibitor, the surface appeared rougher in Figure 6(b). Following explanation can be offered to the observed increase in the roughness of the surface of Al-CM immersed in corrosive. Galvanic corrosion may result because of the presence of SiC as reinforcing agent. SiC act as cathode (ESi = ─ 0.14 V) in m aluminum matrix (anode). The SiC facilitates hydrogen evolution in acid medium and it may result in galvanic corrosion53, 54 During the preparation of Al-CM, SiC reacts with molten Al and forms intermetallic compound Al4C3 55. Al4C3 reacts with acid by forming AlCl3. Further interaction of AlCl3 with Cl- gives AlCl4─

44

. When AlCl4─ detaches from the matrix, it results in the formation of pits.

The aluminum matrix defects like voids may also occur at the gaps exist in the interfacial region i.e., matrix / SiC

56

. The formation of corrosion product at the junction of matrix / SiC

interface increases the concentration of H+, Cl─ and Al3+ with decreasing the O2 in the crevice. The increase in the pH and chloride ions leads to crevice corrosion resulting in the voids on metal surface57 – 58. The reinforcement of SiC particulates may rupture the passive film present on the aluminum matrix. The defective passive film makes the direct exposure of underlying materials into corrosive and enhances corrosion. . Figure 6(c) and 6(d) shows the SEM images of epoxy coated composite (EC-CM) with and without dextran respectively. The surface of the epoxy coated composite containing dextran



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(ECD-CM) is relatively smooth when compared with EC-CM. This observation can be explained by considering the cross sectional view of coated samples depicted in Figure 6(e) and 4(f). It can be observed that, the thickness of EC-CM is 41.95 ߤ m while, the thickness of ECD-CM is 79.32

ߤ m. This increase in the thickness can be attributed to the reduction in the viscosity of the cured epoxy resin solution when dextran is present. Figure 6(g) and 6(h) depicts the SEM images of EC-CM and ECD-CM, when in contact with 1M HCl. The surfaces of both the coated samples are rough as observed in the Figure 6(g) and 6(h). The roughness of the EC-CM is due to the depletion of the epoxy resin film and removed flakes of the epoxy coating can be observed from the image. The depleted protective coating exposes the material to undergo corrosion. On the other hand, the ECD-CM is relatively smoother and no coating flakes are observed after immersion in 1 M HCl.


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Figure 6. SEM image of (a) Freshly polished Al-CM (b) Al-CM immersed in 1M HCl. (c) ECCM (d) ECD-CM, (e) and (f) : Cross sectional view of (c) and (d), (g) EC-CM immersed in 1 M HCl; (h) ECD-CM immersed in 1M HCl

Figure 7 depicts EDX spectrum of freshly polished, corroded, EC-CM, ECD-CM before and after immersing in HCl medium and the results are tabulated in Table 6. From the Table 6, it can be observed that the amount of Al is less for coated specimens compared to that of uncoated specimens. Moreover, the ECD-CM shows least (0.01%). This observation implies that the Al-


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CM surface is uniformly covered by a thick epoxy + dextran coatings and its thickness is higher compared to that of plain epoxy coating. This increase in the thickness makes the detector incapable to detect underlying Al and Si composition. The amount of carbon is very high for ECD-CM, and increase in the carbon contents indicates the presence of dextran. When the coated samples were immersed in HCl medium, the % composition of Al, and carbon decreased. This can be due to the detachment of Al4C3. The peak for chlorine indicates the interaction of HCl with specimen while immersion.



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Figure 7. EDX spectrum of (a) Freshly polished Al-CM (b) Al-CM immersed in 1 M HCl. (c) EC-CM (d) ECD-CM, (e) EC-CM immersed in 1 M HCl; (f) ECD-CM immersed in 1 M HCl

Table 6. EDX results obtained for anticorrosive performance of dextran coating on Al-composite Samples. Freshly polished Al-CM

(%) Composition Al Si O 79.19 13.88 6.93

Cl ---

Al-CM + HCl

53.24

6.32

21.30

0.17 19.13

EC-CM

34.59

6.76

8.69

---

49.97

ECD-CM

0.01

---

20.14

---

79.71

EC-CM +1M HCl

26.05

4.12

5.54

2.94 35.05

ECD-CM + 1M HCl

37.04

5.58

12.09

0.07 43.22

C 24.48

AFM analysis Figure 8(a), (b) and (c), (d) are the 3-dimensional (3D) AFM images of EC-CM and ECDCM before and after immersion in 1M HCl respectively. In comparison with EC-CM, ECD-CM have smooth surface, this confirms the uniform coating with decrease in surface roughness. The average surface roughness (Ra), root mean square (RMS) roughness (Rq) and peak-valley maximum (P-V) values are calculated and given in the Table 7. The decrease in the Ra, Rq and


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(P-V) values for EC-CM indicates coating reduces the roughness of materials for certain limit. In order to reduce the roughness and to increase the toughness for higher extent dextran was incorporated to the epoxy resin. For the ECD-CM the Ra, Rq and (P-V) values decreased, indicating decrease in the overall roughness of the specimen and increase in the protective capacity of the coating. The tabulated surface roughness values increased for the EC-CM and ECD-CM immersed in HCl medium. This increase is because of roughness occurred due to the dissolution of material and also due to the depletion of the coating. For ECD-CM, the Ra, Rq and (P-V) values were less in comparison with EC-CM; this indicates improved toughness owing to the greater anticorrosive ability than ECD-CM.



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Figure 8. AFM images: (a) EC-CM (b) ECD-CM (c) EC-CM immersed in 1M HCl (d) ECDCM immersed in 1M HCl

Table 7. AFM results obtained for anticorrosive performance of dextran coating on Alcomposite

Samples

Ra

Rq

(P-V)

Samples

(nm) (nm) (nm)

Ra

Rq

(P-V)

(nm) (nm)

(nm)

Freshly polished Al-CM

52.6

68.3

1513

Al-CM + 1M HCl

78.9

146

2522

EC-CM

67.3

130

838

EC-CM +1M HCl

100

154

972

ECD-CM

8.29

11.5

122

ECD-CM + 1M HCl

23.8

31.0

271


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Mechanism Based on the results obtained from electrochemical studies and surface analysis of the specimens when immersed in 1 M HCl, we propose the following mechanism for the corrosion protection offered by ECD. Anodic dissolution reaction In HCl medium, the protective oxide layer of Al will get ruptured and this leads to the entry of corrosive species and increase the rate of corrosion. Moreover, the presence of SiC also increases the corrosion due to the formation of a galvanic couple with Al-matrix. In general at the anodic area metal dissolution takes place as given below,

Al + Cl─ ⇆ AlCl─(ads) AlCl─(ads) + Cl─ → AlCl2+ + 3e─

(6)

The increased chloride ions increases the dissolution of metal and forms a different forms of chloro and oxohydroxo complexes as given below, Al [ Ox (OH)y (H2O)z ] + Cl─ →Al [Ox (OH)y-1 Cl (H2O)z ]+ OH─

(7)

(AlOOH)4. H2O + Cl─ → (AlOOH)3. AlOCl. H2O + OH─

(8)

AlOOH +Cl─ → AlOCl + OH─

(9)

Al(OH)3 + Cl─ → Al (OH)2 Cl + OH─

(10)

Cathodic reaction The evolution of hydrogen gas takes place at cathodic area as follows, H+ + e− → H(ads) H(ads) +H(ads)→ H2

(11)



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However, when Al-composite material was coated with epoxy resin and epoxy resin containing dextran, the corrosion rate get decreased. The ECD-CM showed maximum inhibition efficiency compared to EC-CM. There can be multiple reasons for the enhanced corrosion protection. First, epoxy resin can easily cross link with oxidized dextran and get adsorb onto the metal surface as shown in Figure 9. Dextran linked with epoxy is used as water resistant component to improve the wet shear strength. Second, dextran improves the coating quality of the resin, reduces the porosity of the coating, increases the flexibility and decreases the viscosity of epoxy resin. Third, resin containing dextran improves the adherence property of the cured resin coating to the Al-composite by altering the physiochemical properties of the resin-dextran coating/Al-composite interface. Dextran is not only biodegradable, but also reported to be stable for more than 2-3 years under amiable conditions. As a consequence of this unique property it may provide toughness to the epoxy coating for pro longed period of time and protect Al-CM from corrosion.

Figure 9. (a) Structure of dextran linked with epoxy (b) Adsorption of dextran linked epoxy resin onto the surface of composite material.


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CONCLUSIONS •

Dextran showed 91% inhibition efficiency for the corrosion control studies of Alcomposite in 1 M HCl at 303 K.

•

Epoxy coating modified with dextran showed significant enhancement in the corrosion resistance.

•

Dextran enhanced the coating quality of epoxy resin, decreased the porosity of the coating, increased the flexibility and decreased the viscosity of epoxy resin

ACKNOWLEDGEMENTS Ms. Charitha B P is grateful to Manipal University for Research Fellowship and Department of Chemistry, M.I.T. Manipal for laboratory facilities.

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Enhancement of Surface Coating Characteristics of Epoxy Resin by

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