J. Coat. Technol. Res. DOI 10.1007/s11998-014-9588-5

A comparative study on corrosion-resistant performance of halogenated copolymers S. Mohammed Safiullah, K. Abdul Wasi, K. Anver Basha

 American Coatings Association 2014 Abstract Methacrylate-based copolymers are frequently used as anticorrosive organic coatings. Polymeric coating on metallic surfaces provides protection by a barrier action. Thermal properties have a significant influence on corrosion-resistance. This paper deals with the effect of thermal properties of 2,4,6tribromophenyl methacrylate-co-glycidyl methacrylate and N-(p-bromophenyl)-2-methacrylamide-co-glycidyl methacrylate copolymers in corrosion-resistant behavior on low nickel stainless steel (LNSS). Hence attempts have been made to synthesize a set of copolymers by free radical polymerization and compare their corrosion-resistance properties. The copolymers were structurally characterized by Fourier transform-infrared, and 1H-nuclear magnetic resonance spectroscopic techniques. The molecular weight of the copolymers was determined by gel permeation chromatography. Thermal studies were carried out using thermogravimetric analysis and differential scanning calorimetry. Corrosion performances of LNSS coated with two different copolymers was investigated in 1 M H2SO4 using potentiodynamic polarization and electrochemical impedance spectroscopic methods. The corrosion study reveals that poly(TBPMA-coGMA) showed better corrosion-resistance than poly (PBPMA-co-GMA). Keywords Anticorrosion, Polymeric coating, Methacrylate copolymer, Thermal studies S. Mohammed Safiullah Department of Chemistry, C. Abdul Hakeem College of Engineering & Technology, Melvisharam, Vellore District 632509 Tamil Nadu, India K. Abdul Wasi, K. Anver Basha (&) P. G. & Research Department of Chemistry, C. Abdul Hakeem College, Melvisharam, Vellore District 632509 Tamil Nadu, India e-mail: [email protected]

Introduction Corrosion is a serious global problem that could be controlled by different methods like cathodic protection,1 passivation,2 and barrier protection.3 Corrosion of iron and its alloys in acidic media is a continuing issue in many industrial sectors. The protective coatings are preferred due to their low toxicity towards the environment. Coatings provide a barrier against the corrosive species on the metallic surfaces. The use of corrosion inhibitors is one of the most practical methods of protecting metal from corrosion; however, their toxicity has led to extensive study of organic coatings as alternatives. The protection of metal against corrosion by using organic coatings has been the subject of considerable research in recent years.4–12 The effectiveness of polymeric coating is another important topic of interest.13–16 When the coating is disturbed in the operating condition either by mechanical forces or by changes in the environment (temperature, pH, etc.), it becomes porous due to degradation of polymer which leads to the penetration of corrosive media through the permeable structure, and the polymeric coating deteriorates. Under these conditions, the coating efficiency has to be improved. Thus, there is a demand for an effective coating with extended protection. One possibility is developing a thermally stable polymer, which will not degrade under stress. The effectiveness of the coating depends on its chemical composition, permeability towards corrosive species, and the environment. The permeability of polymers changes significantly at the glass transition temperature. The polymer possesses the permeable structure generally at its Tg. Polymers having higher Tg than service temperature give comparatively better barrier properties, because above Tg the free volume increases due to cooperative movements of the segments.3 Therefore, the significance of the present

J. Coat. Technol. Res.

investigation is to formulate an environmentally favorable halogenated methacrylate polymeric coating to protect metals from corrosion with good adherence and thermal properties under severe conditions. In the literature several studies have been done on the synthesis of new methacrylate monomers17,18 and their radical copolymerization with functional monomers. These studies clearly show that the hydrophobic nature, as well as presence of the substituent in the comonomer, had a large effect on both glass transition temperatures and anticorrosive properties.19 Nowadays, copolymers with reactive or functional monomers are steadily gaining importance. Generally poly(phenyl methacrylate) copolymers possess high tensile strength, high thermal stability, and glass transition temperature higher than their acrylates polymer due to the presence of the methyl group on the main chain. Glycidyl methacrylate (GMA) is hydrophobic with an adhesive nature moiety. Saric et al.20 discussed the thermal stabilities of brominated acrylates with styrene and observed that tribromophenyl acrylatebased copolymers are more thermally stable than other acrylates. Very few works have been reported about the thermal stability and corrosion protection properties of the halogenated methacrylate copolymer.21 The main theme of this article is to compare the anticorrosive properties of thermally stable halogenated copolymers. Thus a set of copolymers namely 2,4,6tribromophenyl methacrylate-co-glycidyl methacrylate poly(TBPMA-co-GMA) and N-(p-bromophenyl)-2methacrylamide-co-glycidyl methacrylate poly(PBPMAco-GMA) were synthesized. The copolymer structures were characterized by Fourier transform-infrared (FTIR), and 1H-nuclear magnetic resonance (1H NMR) spectroscopic technique. The thermal properties of the copolymers were studied using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The molecular weight was determined by gel permeation chromatography (GPC). A corrosion performance study of the copolymer-coated low nickel stainless steel (LNSS) was done by potentiodynamic polarization technique and electrochemical impedance spectroscopy (EIS) method.

Experimental methods and materials Materials Methacryloyl chloride (MAC) was prepared by using the procedure of Stampel et al.22 GMA was purchased from Sigma-Aldrich and freed from inhibitor by

distilling at 60–78C under reduced pressure. Benzoyl peroxide (BPO) was recrystallized from chloroform– methanol (1:1). 2,4,6-tribromophenol and p-bromoaniline were purchased from Sigma-Aldrich and used as such. The AR grade benzene, ethyl methyl ketone (EMK), hexane, chloroform, and methanol were distilled before use. The properties of raw materials used for experimentation are provided in Table 1. Synthesis of poly(TBPMA-co-GMA) Synthesis of 2,4,6-tribromophenyl methacrylate For the synthesis of 2,4,6-tribromophenyl methacrylate (TBPMA) (Scheme 1), stoichiometric amounts of 2,4,6tribromophenol and triethylamine were dissolved in EMK. The mixture was placed in a round-bottom flask with stirring occurring in an ice bath. MAC dissolved in EMK was added to the flask at 0–5C. The reaction was allowed to proceed for 6 h at constant stirring. The reaction mixture was washed and dried. The resultant product was a very dark brown colored liquid.

Copolymerization of 2,4,6-tribromophenyl methacrylate-co-glycidyl methacrylate Required quantities of the monomers TBPMA and GMA, along with BPO, were dissolved in benzene. The mixture was flushed with dry N2 gas. The reaction vessel was immersed in a thermostatic water bath maintained at 70 ± 1C. The copolymerization reaction was allowed to proceed for an appropriate duration (Scheme 2). The solution was poured into excess methanol to precipitate the copolymer. The copolymers were purified by repeated precipitation by methanol from solution in chloroform. It was then dried in pressure at 40C for 24 h. Synthesis of poly(PBPMA-co-GMA) N-(p-Bromophenyl)-2-methacrylamide (PBPMA) monomer was synthesized using the procedure reported.23 Required quantities of the monomer PBPMA and GMA, along with BPO, were dissolved in benzene. The reaction mixture was flushed with oxygen-free N2 gas. The reaction vessel was immersed in a thermostatic water bath maintained at 70 ± 1C. The copolymerization reaction was allowed to proceed for an

Table 1: Properties of raw materials Particulars Monomer 1 Monomer 2 Monomer 2¢ Initiator

Name

Monomer Feed

Property

Glycidyl methacrylate Tribromo phenyl methacrylate (TBPMA) p-Bromo phenyl methacrylamide (PBPMA) Benzoyl peroxide (BPO)

0.80 0.20 0.20 0.75%

Good adhesiveness Thermally stable Thermally stable Free radical initiator

J. Coat. Technol. Res. OH Br

Br

Br

O

Cl O EMK/Et3N 0–5°C

Br 2,4,6-Tribromophenol

O Br

Br

2,4,6-Tribromophenyl methacrylate

Methacryloyl chloride

Scheme 1: Synthesis of 2,4,6-tribromophenyl methacrylate

n

O O

O

O

O Br

Br

O

EMK/BPO

O

Br

O

Br

70 ± 1°C O 2,4,6-Tribromophenyl methacrylate (TBPMA)

O

Br

Br Glycidyl methacrylate (GMA)

Poly(TBPMA-co-GMA)

Scheme 2: Copolymerization of 2,4,6-tribromophenyl methacrylate-co-glycidyl methacrylate

n

O NH

O

O NH O

O O

Benzene/BPO

70 ± 1°C Br

p-Bromophenyl methacrylamide

(PBPMA)

O

Br

Glycidyl methacrylate (GMA)

O

Poly(PBPMA-co-GMA)

Scheme 3: Copolymerization of p-bromophenyl methacrylamide-co-glycidyl methacrylate

appropriate duration (Scheme 3). The solution was poured into excess hexane to precipitate the copolymer. The copolymers were purified by repeated precipitation by hexane from solution in chloroform. They were then dried in a vacuum oven at 40C for 24 h. Spectral characterization of poly(TBPMA-co-GMA) and poly(PBPMA-co-GMA) The synthesized copolymers were characterized by FTIR spectrometer using KBr-pressed pellet (Model:

Perkin-Elmer paragon 1000), 1H NMR spectroscopic techniques (Model: Jeol GSX—400 MHz). Deuterated chloroform was used as a solvent in the NMR studies. Thermal stability and Tg of the copolymers were determined by thermogravimetric analysis (TGA, Model: Mettler TA 3000 thermal analyzer) at a heating rate of 20C min1 under nitrogen atmosphere and DSC, respectively. The molecular weight (Mw) of the copolymers was obtained from gel permeation chromatography (GPC; Model: waters 501). The surface morphology of the copolymer-coated austenitic LNSS was investigated using a scanning electron microscope

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(JEOL Model JSM-6390LV). The thickness of the coating on LNSS specimens was measured by using an Elcomaster thickness meter. Preparation of electrodes In the present work, LNSS metal was used as the substrate for the corrosion studies. The chemical composition of LNSS was determined by atomic absorption spectroscopic technique and its composition is illustrated in Table 2. All the test specimens of LNSS were cut into an overall apparent size of 1 9 1 9 0.3 cm and embedded using epoxy resin with an electrical connection and exposed area of 1 cm2. These specimens were polished with different grits of emery papers. Polymers were dissolved in CHCl3 and coated on the electrode surface by dropping and spreading the solution, which finally dried at room temperature.

Solubility test The solubility of the newly prepared polymers was tested in various polar and nonpolar solvents. About 5– 10 mg of the polymer was added to about 2 ml of different solvents in a test tube and kept overnight with the test tube tightly closed. The solubility of the polymer was noted after 24 h. The poly(TBPMA-coGMA) was easily soluble in dimethyl sulphoxide, chloroform, carbon tetrachloride, dichloromethane, tetrahydrofuran, acetonitrile. Similarly poly(PBPMAco-GMA) was easily soluble in solvents, namely toluene, benzene, chloroform, acetone, and acetonitrile. Both the polymers were insoluble in methanol and ethanol. The solubility test clearly shows that there is wide possibility for using different solvents for the copolymers to be used in coating applications.

Result and discussion Electrochemical studies All the electrochemical measurements were performed using the Electrochemical Workstation (Model No: CHI 760, CH Instruments, USA) and a constant temperature of 28 ± 2C was maintained with 1 M H2SO4 as an electrolyte. A platinum electrode and a saturated calomel electrode (SCE) were used as auxiliary and reference electrodes, respectively, while the working electrode was composed of LNSS of 1 cm2 exposed area. The tip of the reference electrode was positioned very close to the surface of the working electrode by the use of a fine Luggin capillary in order to minimize the ohmic potential drop. The remaining uncompensated resistance was also reduced by the electrochemical workstation. Potentiodynamic polarization studies were carried out at a scan rate of 0.1 mV s1 and at a potential range of 1000 to 1000 mV for uncoated and copolymer-coated LNSS. The electrochemical impedance studies were carried out using the same setup as that of potentiodynamic polarization studies, and the applied AC perturbation signal was approximately 10 mV within the frequency range 100 kHz–1 Hz. All the electrochemical impedance measurements were carried out at open circuit potential (OCP). The percent of inhibition efficiency (I.E. %) was calculated as follows24:

I:E:% ¼

ðRct Þ1 R1 ctðcoatÞ ðRct Þ1

 100:

Characterization of poly(TBPMA-co-GMA) and poly(PBPMA-co-GMA) FTIR spectroscopy The FTIR spectra of the poly(TBPMA-co-GMA) and poly(PBPMA-co-GMA) are shown in Figs. 1 and 2, respectively. From the figures it is confirmed that the copolymerization of the two monomers was successfully achieved. The FTIR spectra of the TBPMA and poly (TBPMA-co-GMA) are given in Figs. 1a and 1b. The symmetrical ring breathing vibration of the epoxy group appeared at 1282 cm1 and the asymmetrical stretching of the epoxy ring was observed at 994 cm1.25,26 The C–H out-of-plane bending vibration of aromatic ring appeared at 859 and 743 cm1. The appearance of peak at 907 cm1 indicates the incorporation of the GMA unit in the copolymer. The ester carbonyl of both the TBPMA and GMA overlapped at 1725 cm1, which shifted to higher frequency due to the loss of conjugation after the polymerization of the monomer.27 The FTIR spectra of the PBPMA and poly (PBPMA-co-GMA) are provided in Figs. 2a and 2b. The sharp band at 3318 cm1 is attributed to N–H stretching of the amide group. The peak at 3088 cm1 corresponds to the C–H stretching of the aromatic system. The symmetrical and asymmetrical stretching due to the methyl and methylene groups are observed at 2993, 2972, and 2952.48 cm1. The shoulder at 1725 cm1 and a peak at 1700 cm1 are attributed to

Table 2: Composition of low nickel stainless steel Element Composition (%)

C

Si

Mn

P

S

Cr

Ni

N

Cu

Fe

0.063

0.35

7.05

0.054

0.01

16.03

4.16

0.1

1.24

Balance

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(a)

(b)

548.4 684.10 3511.72 3069.59

708.48

2943.38 2995.29

907.55

1553.95 1372.25

3115.41

994.05

1391.55 1725.64

3500

3000

2500

2000

1436.74

1225.85

1500

743.96

859.54 1041.72

1000

500

Wave number cm–1 Fig. 1: FTIR spectra of (a) TBPMA and (b) poly(TBPMA-co-GMA)

(a)

(b)

3088 3318 2952

2993

2972

950 1725 1700 1460

3500

3000

2500

2000

Wave number (cm–1) Fig. 2: FTIR spectra of (a) PBPMA and (b) poly(PBPMA-co-GMA)

1500

1385 1000

500

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the ester carbonyl stretching of GMA and amide carbonyl stretching of PBPMA. The ring breathing vibrations of the aromatic nuclei are observed around at 1600 cm1. The asymmetrical and symmetrical bending vibrations of methyl groups are seen at 1460 and 1385 cm1. The symmetrical stretching of the epoxy group is observed at 1260 and 950 cm1. The C–O stretching is observed at 1165 and 1200 cm1. The C–H and C=C out-of-plane bending vibrations of the aromatic nuclei are observed at 790 and 565 cm1, respectively. The appearance of two sharp bands at 1725 and 950 cm1 in the case of GMA (ester carbonyl group, symmetrical stretching of the epoxy group) confirms that the epoxy group remains intact during the polymerization. Disappearance of C=C stretching frequency peak at 1636 cm1 in the Figs. 1b and 2b confirms the formation of copolymer. 1

spond to the NH protons of the PBPMA unit. The aromatic protons show signals between 7.82 and 6.93 ppm. The spectrum shows two signals at 4.42 and 3.81 ppm, which are due to –COOCH2– group at GMA units. The peak at 3.22 ppm is due to the methyne proton of the epoxy group. The methylene protons of the epoxy group show signals at 2.72 and 2.60 ppm. The backbone methylene groups show signals from 1.54–2.51 ppm. Thermal analysis TGA and DSC thermograms of poly(TBPMA-coGMA) and poly(PBPMA-co-GMA) are shown in Figs. 4 and 5, respectively. Thermal analysis data obtained from TGA and DSC are given in Table 3. The thermograms clearly indicate that both the copolymers undergo single stage decomposition. The initial decomposition temperatures (IDT) for poly(TBPMAco-GMA) and poly(PBPMA-co-GMA) were around 260 and 248C, respectively, which is consistent with the literature.20,23,28 The obtained IDT values from the figures confirm that the enhancement in thermal stability of poly(TBPMA-co-GMA) over poly(PBPMA-coGMA) was due to the presence of three bromine atom in the former. The higher stability of poly (TBPMA-co-GMA) is due to the three stronger C–Br bonds in the aromatic compound than in poly(PBPMAco-GMA).20 Hence the decomposition temperature range depends upon the composition of the constitutional monomeric units in the copolymer. Polymer glass transition temperature represents the molecular mobility of the polymer chain, which is an important phenomenon that influences the material properties

H NMR Spectroscopy

The 1H NMR spectrum of the poly(TBPMA-co-GMA) is shown in Fig. 3a. The spectrum shows resonance signals at 1.96 and 0.93 ppm due to the presence of the a-methyl group. The spectrum shows two signals at 7.7 and 7.776 ppm due to the presence of the aromatic ring proton. The methylene protons of the epoxy group were observed at 2.8 and 2.63 ppm. The signals at 3.22 ppm are due to the presence of methyne proton of the epoxy group. The 1H NMR spectrum of the copolymer poly (PBPMA-co-GMA) is provided in Fig. 3b. The chemical shift assignments for the copolymers were based on the chemical shifts observed for the respective homopolymers. The resonance signals at 9.51 ppm corre-

(a)

(b)

10

9

8

7

6

5

4

3

δ (ppm) Fig. 3:

1

H NMR spectra of (a) poly(TBPMA-co-GMA) and (b) poly(PBPMA-co-GMA)

2

1

0

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mobility of the copolymer due to the high dipolar character of the structural unit.27 100

Molecular weight determination 80

The number average molecular weight (Mn) and weight average molecular weight (Mw) of copolymers were determined by GPC and the data are given in Table 4. The polydispersity indices of the copolymer depend on the mole ratio of the comonomers in the polymer chain. The polydispersity index (Mw/Mn) of the poly(TBPMA-co-GMA) and poly(PBPMA-coGMA) were 2.16 and 3.15, respectively. The Mw/Mn of these copolymers suggests a strong tendency for chain termination by disproportionation rather than radical combination. Molecular weight has an influence on glass transition temperature. It was revealed that the polymer’s molecular weight up to 20,000 has a significant influence on Tg.30 But as Table 4 shows, the molecular weight of the copolymers is above 20,000, which indicates that improved Tg is not due to the high molecular weight of copolymers.

Weight (%)

60 240

250 260

40

(a)

20

(b) 0

0

100

200

300

400

Temperature (°C) Fig. 4: TGA curve of copolymers (a) poly(TBPMA-co-GMA) and (b) poly(PBPMA-co-GMA)

Heat flow (mW)

Exo

Properties of copolymer coating

(a) (b)

Endo 0

50

100

150

200

250

The SEM image of poly(TBPMA-co-GMA) and poly(PBPMA-co-GMA) film developed on austenitic LNSS is presented in Figs. 6a and 6b, respectively. It is evident from the micrograph that a layer of copolymer covers the entire surface of the LNSS, which prevents its deterioration in the corrosive solution. As a result, pits are seldom seen on the surface, and no detachment of polymer coverage is noted. The thickness of the copolymer coating on LNSS was measured by using an Elcomaster thickness meter. An average of four readings was noted and the thickness of the polymer film was around 1–1.5 lm.

300

Potentiodynamic polarization studies

Temperature (°C)

Fig. 5: DSC thermogram of copolymers (a) poly(TBPMAco-GMA) and (b) poly(PBPMA-co-GMA)

Table 3: TGA and DSC data of developed copolymers Copolymer and its composition

IDT (C)

Tg (C)

Poly(TBPMA-co-GMA) (20:80) Poly(PBPMA-co-GMA) (20:80)

260 248

125 109

and potential application of a given polymer.29 The increase in the Tg of poly(TBPMA-co-GMA) due to the presence of a bulky tribromophenyl group in the copolymer indicates substantial decrease of chain

The cathodic and anodic polarization curves of LNSS in 1 M H2SO4 without and with coating of copolymers are shown in Figs. 7a and 7b, respectively. It is evident from the figure that the polarization curves for LNSS coated with copolymers showed reduced current density in both cathodic and anodic regions from that of uncoated LNSS. The corrosion parameters such as corrosion potential Ecorr and corrosion current density Icorr obtained from Tafel plots are given in Table 5. From the table, it can be seen that the Ecorr values increased for LNSS coated with poly(TBPMA-coGMA) over poly(PBPMA-co-GMA). This observation clearly shows that LNSS coated with poly(TBPMA-coGMA) possesses improved corrosion-resistance over poly(PBPMA-co-GMA). Even though the composition of both copolymers with respect to GMA is the

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Table 4: Molecular weight measurement of synthesized polymers Composition Poly(TBPMA-co-GMA) (20:80) Poly(PBPMA-co-GMA) (20:80)

Mw

Mn

PDI

37,806 21,897

9,131 4,058

2.16 3.17

Fig. 6: SEM pattern of (a) poly(TBPMA-co-GMA)-coated austenitic LNSS and (b) poly(PBPMA-co-GMA)-coated austenitic LNSS

(a) 10–1

(b) Uncoated 10–4

Current density (A)

Current density (A)

10–2

10–3

10–4

PBPMA-co-GMA TBPMA-co-GMA

10–5

10–5 1.0

0.8

0.6

0.4

0.2

0

–0.2 –0.4 –0.6 –0.8 –1.0

Potential (V vs SCE)

0.8

0.6

0.4

0.2

0

–0.2 –0.4 –0.6 –0.8 –1.0

Potential (V vs SCE)

Fig. 7: (a) Potentiodynamic polarization curve of uncoated LNSS in 1 M H2SO4. (b) Potentiodynamic polarization curve of copolymers-coated LNSS in 1 M H2SO4

same, LNSS coated with poly(TBPMA-co-GMA) showed a slight shift towards positive potential region in anodic and cathodic curves compared to the poly(PBPMA-co-GMA)-coated. This behavior was attributed to the presence of a bulky bromine atom directly attached to the phenyl ring of TBPMA, which restricts the permeability of the corrosive media towards the metal.

The corrosion rate calculated for LNSS specimens coated with copolymers are also shown in Table 5. It is observed that the LNSS coated with the poly(TBPMAco-GMA) showed improved corrosion-resistance compared to poly(PBPMA-co-GMA). In this study, the corrosion rate for the uncoated LNSS specimen was found to be 11.5 mpy and it was minimized by coating with poly(TBPMA-co-GMA) to 1.93 mpy.

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Table 5: Potentiodynamic polarization parameters obtained for copolymers-coated and uncoated low nickel stainless steel in 1 M H2SO4 Sample

Ecorr (mV)

Uncoated Poly(TBPMA-co-GMA) (20:80) Poly(PBPMA-co-GMA) (20:80)

500 472 552

icorr (lA cm2)

ba (V)

bc (V)

CR (mpy)

25.12 3.44 5.71

0.08 0.31 0.34

0.11 0.12 0.13

11.5 1.93 2.52

CPE

(b)

Cn

(a)

Cn Rs

Cn Rs

R1

R1 R2

R2

Fig. 8: The equivalent circuit of (a) copolymers-coated LNSS and (b) uncoated LNSS

Electrochemical impedance spectroscopy Practically, EIS provides information about the corrosion-resistance of the organic coatings to aqueous and ionic transport.31 Usually impedance is described with an equivalent electrical circuit constructed, preferably by circuit elements with proper electrochemical background.32 The protective effect of the studied methacrylatebased copolymer coating is evaluated using the electrochemical impedance spectroscopy (EIS). An equivalent circuit, such as that shown in Fig. 8, was used to consider all the process involved in the electrical response of the system. The first element in the circuit is a solution resistance (Rs), which corresponds to the ohmic resistance of the system. In Fig. 8a the Cdl represents the double layer capacitance, and R1 represents a resistance of the coating which is a passive film in the case of coated austenitic LNSS. The second subsystem corresponds to the resistance (R2) and the capacitance to the charge transfer of the oxidation of the alloy. The different elements were evaluated by a fitting procedure. In the case of uncoated LNSS in 1 M H2SO4 environment, the equivalent circuit, such as that shown in Fig. 8b, was used to consider the electrical response of the system. The first resistance (R1) corresponds to the oxidation of the metal after this oxidation iron and chromium oxides film is formed. This film has a higher resistance due to passive characteristics. The capacity constant phase element (CPE) of this film was simulated as a CPE too. This was often attributed to roughness and nonhomogeneity of the solid surface in an aggressive environment.33 Hence, a CPE is used

instead of a capacitive element to get a more accurate fit of experimental data sets using generally more complicated equivalent circuits. The second subsystem corresponds to the resistance (R2) and the capacitance of the passive oxide layer. Figure 9 shows the impedance spectra of the uncoated and copolymer-coated LNSS specimens recorded at OCP in the frequency range of 100 kHz– 1 Hz; the amplitude was 10 mV. For the coated specimen the semicircle region was attributed to the sum of charge transfer resistance Rct against LNSS dissolution and the oxide film resistance.34 It was found that the two different copolymer-coated samples exhibited good protection against corrosion. In both cases, the copolymer coating was homogeneous, adherent, and a porous-free coating which was clearly observed from the high impedance of the sample. Various impedance parameters such as Rct, double layer capacitance Cdl, and percentage inhibition efficiency (% I.E.) are given in Table 6. The Rct value calculated for uncoated LNSS was 61.2 X and Cdl was 0.442 lF cm2. The increased Rct value for both the copolymer-coated LNSS suggests the higher corrosionresistance of the copolymer. However, the poly (TBPMA-co-GMA) showed lesser Cdl and higher Rct value compared to poly(PBPMA-co-GMA), which might be due to the restrictions in segmental mobility of the bulky pendant group in the backbone of poly (TBPMA-co-GMA) which leads to better barrier properties. The tendency toward a decrease in Cdl which can result from a decrease in local dielectric constant and/or increase in the thickness of the double layer suggested that the copolymer molecules function effectively by strong adhesion at the metal/surface interface.35 The

J. Coat. Technol. Res.

(a)

(b)

600

Uncoated poly(PBPMA-co-GMA)

2.7

poly(TBPMA-co-GMA)

Uncoated PBPMA-co-GMA TBPMA-co-GMA

log (Z/Ohm)

500

–Z ''/Ohm

400

300

1.8

0.9

200

100 0 0 0

100

200

300

400

500

600

Z '/Ohm

0

1.0

2.0

3.0

4.0

5.0

log (Freq/Hz)

Fig. 9: (a) Nyquist plot for uncoated and coated low nickel stainless steel with two different copolymers. (b) Bode plot for uncoated and coated low nickel stainless steel with two different copolymers

Table 6: Electrochemical impedance parameters obtained for copolymers-coated and uncoated low nickel stainless steel in 1 M H2SO4 Sample

Rct (X)

Cdl (lF cm2)

% I. E.

Uncoated Poly(TBPMA-co-GMA) (20:80) Poly(PBPMA-co-GMA) (20:80)

61.2 574.2 501.1

0.442 0.126 0.140

– 89.35 87.76

poly(PBPMA-co-GMA) poly(TBPMA-co-GMA) n n

O NH

O

O

O O

O O

O

Br

(a)

O

Metal Substrate

Metal Substrate

poly(PBPMA-co-GMA) (87.76%) were improved over the nonhalogenated copolymers of GMA reported by AP Srikanth et al.25,26

(b)

Scheme 4: Conceptual illustration of the noncovalent bonding interaction between (a) poly(TBPMA-co-GMA) and LNSS, (b) poly(PBPMA-co-GMA) and LNSS

decrease in Cdl value was caused by adhesion of the polymer chains on the metal surface, reducing the extent of the dissolution reaction.36 Therefore, these results suggested that corrosion protection of the poly(TBPMA-co-GMA)-coated specimen exhibits a higher impedance value compared to the poly (PBPMA-co-GMA)-coated specimen. The % inhibition efficiency of the poly(TBPMA-co-GMA) (89.35%) and

Mechanism of corrosion The possible mechanism involving the effectiveness of the copolymer on LNSS depends on the polymeric structure. It is known that adsorption of polymer chain is influenced by the electronic structure of the molecules and also by the steric factors, aromaticity, electron donating at the donor atom, and the presence of functional groups such as =NH; –N=N–; –CHO; R–R; etc. in the inhibition molecule.37–39 In both the copolymers poly(TBPMA-co-GMA) and poly (PBPMA-co-GMA), the epoxy group of GMA unit, ester carbonyl and ester oxygen atom, and amide group of PBPMA contribute to the corrosion inhibition by virtue of the electron-donating nature of these groups to the metal surface. In the present investigation among the two copolymers which were employed for coating, poly(TBPMAco-GMA) on LNSS has been found to give better performance when compared to poly(PBPMA-co-GMA). The adherence of the copolymer film to the metal surface depends on the nature of the functional groups, electron-donating heteroatom, composition, and sequence

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of the monomer units. The possible reason for the good behavior of the corrosion inhibition efficiency of poly(TBPMA-co-GMA) may be tacticity. TBPMA contains three bulky Br atoms directly attached to the phenyl ring that are oriented in the opposite side of GMA due to the electronic cloud arrangement. Therefore, GMA adsorbs effectively on a metal surface, which would explain the improved corrosion inhibition efficiency of poly(TBPMA-co-GMA) (Scheme 4). An alternate explanation could be the high Tg of poly (TBPMA-co-GMA), which is thermally stable and restricts the segmental mobility of the adsorbed copolymer. Therefore the poly(TBPMA-co-GMA) restricts the interaction of corrosive media to a metal surface better than poly(PBPMA-co-GMA).

Conclusion An attempt has been made to synthesize novel halogenated methacrylate-based anticorrosive copolymers, namely 2,4,6-tribromophenyl methacrylate-co-glycidyl methacrylate and N-(p-bromophenyl)-2-methacrylamide-co-glycidyl methacrylate, by the free radical solution copolymerization method. The spectral studies from FTIR and 1H NMR have confirmed the participation of two monomers. Thermal stability and molecular weight of the copolymers were determined by TGA, DSC, and GPC technique. The poly(TBPMA-coGMA) has a good thermal stability and possesses higher Tg than poly(PBPMA-co-GMA). The molecular weight determination confirmed that the improved Tg does not necessarily correspond to the higher molecular weight of the copolymer. The potentiodynamic polarization technique studies and EIS showed that poly(TBPMAco-GMA) has better corrosion-resistance properties than poly(PBPMA-co-GMA). Therefore the studies reveal that the thermal property of the polymers had an influence on corrosion-resistance performance. Acknowledgments The authors are grateful for the financial support by University Grants Commission (UGC) (F. No. 39-808/2010 (SR)), New Delhi, India and also thank to the Management of C. Abdul Hakeem College of Engineering & Technology, Melvisharam, Vellore, Tamil Nadu, India for the encouragement to carry out this work.

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