An electroactive co-polymer as corrosion inhibitor for steel in sulphuric acid medium

Applied Surface Science 254 (2008) 5569–5573

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An electroactive co-polymer as corrosion inhibitor for steel in sulphuric acid medium Ganesha Achary a, Y. Arthoba Naik a,*, S. Vijay Kumar b, T.V. Venkatesha a, B.S. Sherigara b a b

Department of P. G. Studies & Research in Chemistry, School of Chemical Sciences, Kuvempu University, Jnana Sahyadri, Shankaraghatta 577451, India Department of P. G. Studies & Research in Industrial Chemistry, School of Chemical Sciences, Kuvempu University, Jnana Sahyadri, Shankaraghatta 577451, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 27 October 2007 Received in revised form 10 February 2008 Accepted 29 February 2008 Available online 6 March 2008

The corrosion behavior of mild steel in sulphuric acid solution containing various concentrations of a copolymer formed between maleic anhydride and N-vinyl-2-pyrrolidone (VPMA) was investigated using weight-loss, polarization and electrochemical impedance techniques. The polymer acts as an effective corrosion inhibitor for steel in sulphuric acid medium. The inhibition process is attributed to the formation of an adsorbed film of co-polymer on the metal surface which protects the metal against corrosion. Scanning electron microscopy (SEM) studies of the metal surfaces confirmed the existence of an adsorbed film. The adsorption followed the Langmuir isotherm. The protection efficiency increased with increase in inhibitor concentration and decreased with increase in temperature and acid concentration. The thermodynamic functions of the adsorption and dissolution processes were evaluated. ß 2008 Elsevier B.V. All rights reserved.

Keywords: Adsorption Corrosion inhibition Co-polymer and electroactive

1. Introduction Mild steel is one of the important iron-containing alloys used in many industrial aqueous systems in which water circulates. The use of inhibitors is one of the most practical methods for protection against corrosion in closed systems, especially in acidic media [1– 4]. Organic compounds are widely used as corrosion inhibitors in acidic environments in various industries [5–7]. Their role as barrier on metal surface and to prevent the access of corrosive environment to metal substrate is well known to reduce the corrosion rate. The need of the hour is non-toxic compounds that can protect metals against corrosion without causing environmental problems. Polymers find applications as effective corrosion inhibitors for steel [8,9]. These polymers being surface-active agents have a high chelating ability with metals [10]. A few polymers exhibit strong interaction with metal; this is attributed to high charge density on their polymer ion leading to chelation with metal ion [11]. Polyaniline has been used as an effective corrosion inhibitor for steel in HCl [12]. Poly(styrenesulphonic acid)-doped polyaniline has been synthesized and the influence of this polymeric compound on the inhibition of corrosion of mild steel in HCl has been investigated [13]. A study by Badran et al. reveals the effect of different polymers on the efficiency of water-borne methylamine adduct as corrosion * Corresponding author. E-mail address: [email protected] (Y.A. Naik). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.02.103

inhibitor for surface coatings [14]. Shestopalov et al. studied the corrosion inhibiting properties of a polymer containing lubricatingcooling liquids [15]. Poly(aniline-co-metanilic) acid was prepared as per a reported procedure and its soluble form was used as inhibitor for mild steel [16]. Several reports since then have shown that the polymer is a possible protective material, especially for steel [17,18]. Thin polymer coatings on steel materials were obtained by chemical methods and the coated specimens showed maximum corrosion protection [19]. The polymers are used as inhibitors and as surface modifiers. They exhibited superior corrosion inhibition properties in contrast to simple organic molecules [20]. The polymers possess long chain carbon linkage and thus block large area of corroding metal. The adsorbed film on metal provides barrier between metal and the medium. In this paper we report the inhibitive action of a synthetic copolymer, co-poly(maleic anhydride–N-vinyl-2-pyrrolidone) (VPMA) on corrosion of steel in sulphuric acid. The inhibition has been evaluated by Tafel extrapolation, electrochemical impedance spectroscopy and weight-loss methods. The thermodynamic parameters for the adsorption of inhibitor on the metal surface and for metal dissolution are discussed. 2. Experimental The co-polymer (VPMA) was synthesized by adopting the following procedure. MA (9.8 g, 0.1 M) and N-vinyl-2-pyrrolidone (11.12 g, 0.1 M) were dissolved in THF (100 cm3). The solution was flushed with argon and AIBN (0.20 g) was added. The polymerization

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was carried out in a water bath at 60 8C for 3.5 h. The reaction mixture was diluted with THF (40 cm3) and the polymer was then precipitated into excess ether. The polymer was redissolved and then reprecipitated into ether and dried under vacuum, 17.9 g (86%) and the molecular weight is found to be 17,530 [21]. The purity was checked by TLC and the formation of the product was confirmed by FTIR spectroscopy. The structure of the co-polymer is shown in Fig. 1. For the weight-loss measurements, rectangular specimens of mild steel (% composition: C = 0.14, Mn = 0.25, Si = 0.03, P = 0.10, S = 0.02, Ni = 0.01, Cu = 0.01 and Cr = 0.01) specimens of size 1 cm  5 cm  0.1 cm were used. A 2 mm diameter small hole was drilled near the upper edge of the specimen to accommod a suspension hook. The specimens were mechanically polished using emery paper of various grit sizes and rinsed with distilled water. They were then degreased with trichloro ethylene followed by washing with double distilled water, rinsed in alcohol and dried. The weights of the specimens were recorded before placing them in the test solution. After a known period of time the samples were removed and washed in a current of water followed by rinsing with alcohol and dried. The corrosive solution was prepared from Analar grade sulphuric acid and distilled water. All the tests were performed in aerated solution. Weight-loss was determined at different immersion times at 298 K by weighing the cleaned steel samples before and after immersing in 100 cm3 of 2 M sulphuric acid in the absence and presence of various concentrations of VPMA. The experiments were repeated at different temperatures, ranging from 298–323 K. The weight-loss measurements were performed in triplicate and the average weight-loss was recorded. The percent inhibition efficiency was calculated using the following relationship [22]. Wo  W %IE ¼  100 Wo where Wo and W are the weight-losses in the absence and presence of VPMA. For the polarization measurements a conventional threeelectrode cell containing steel specimen (of exposed area of 1 cm2), a platinum wire and a saturated calomel were used as working, auxiliary and reference electrodes, respectively. The anodic and cathodic polarization parameters were measured under galvanostatic conditions. The specimens were first polarized in cathodic direction up to a maximum shift from open circuit potential (OCP). After cathodic polarization the specimen was again kept at OCP for about 10 min and anodic polarization was carried out in a similar manner. The corrosion current densities were determined by extrapolating the cathodic and anodic Tafel lines. The percentage inhibition efficiency %IE was calculated using the relationship, Io  Icorr %IE ¼ corr o  100 Icorr

o where Icorr and Icorr are the corrosion current densities in the absence and presence of inhibitor, respectively. In order to determine the polarization resistance, RP, by the linear polarization method, the potential of the working electrode (E) was ramped 10 mV in the vicinity of the corrosion potential at a scan rate of 0.1 mV s1. The polarization resistance (RP) was obtained from the slope of the polarization curves at low potentials. Polarization resistance values were calculated by the following relation,

RP ¼ Aðslope of plot of E vs: IÞ where A is the electrode surface area. Impedance spectra were recorded (AUTOLAB, Eco-Chemie) at Ecorr in the frequency range 10 mHz to 100 mHz with an AC amplitude of 5 mV. Various impedance parameters such as polarization resistance (RP), solution resistance (RS) and double layer capacitance (Cdl) were determined using Nyquist plots for steel in acidic solutions containing different concentrations of VPMA. The RP values were used to calculate the inhibition efficiencies (%IERP ), using the relation, %IERP ¼

RP  RIP  100 RIP

where RP and RIP are the polarization resistances in the presence and absence of inhibitors, respectively. The surface morphology of the steel samples in the presence and absence of VPMA after anodic polarization was investigated using scanning electron microscopy (model: JEOL, JSM 6400). 3. Results and discussion Table 1 shows the variation of weight-loss and inhibition efficiencies (%IE) with the concentrations of VPMA. The weightloss decreased with increasing VPMA concentration. The maximum inhibition efficiency, of 91% was observed at 1.6% of VPMA and no change in %IE occurred above this concentration. The protection efficiency (%IE) decreased with increase in acid concentration. This may be attributed to the increased hydrogen evolution and consequent stripping of the adsorbed compound from the metal surface [23]. Table 2 presents the influence of acid and VPMA concentration on %IE. For a given strength of VPMA, %IE decreased slightly with acid strength. But for the same acid strength the %IE were directly depended on VPMA concentration. Table 1 Corrosion inhibition efficiencies by weight-loss method in 2 M H2SO4 VPMA (%)

Weight-loss (g)

IE (%)

0.0 0.2 0.4 0.8 1.2 1.6

0.240 0.083 0.069 0.043 0.031 0.018

– 65.00 71.00 82.00 87.00 91.00

Table 2 Variation of %IE with concentrations of VPMA and H2SO4 by weight-loss method for immersion period of 2 h VPMA (%)

IE (%) H2SO4 concentration

Fig. 1. Structure of VPMA.

0.2 0.4 0.8 1.2 1.6

1M

2M

3M

67 73 84 88 93

65 71 82 87 92

61 63 68 76 84

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Table 4 Corrosion parameters obtained from mass loss measurements for mild steel in 2 M H2SO4 containing various concentrations of inhibitors at 303, 313 and 323 K VPMA (%)

0.0 0.4 0.8 1.2 1.6

Fig. 2. Polarization behavior of steel in 2 M H2SO4 with the addition of various concentrations of VPMA.

Table 3 Corrosion parameters obtained from galvanostatic polarization measurements for mild steel in 2 M H2SO4 containing various concentrations of inhibitors at 298 K VPMA (%)

Ecorr (mV)

Icorr (mA cm2)

ba (mV dec1)

bc (mV dec1)

IE (%)

0.0 0.4 0.8 1.2 1.6

532 560 565 575 584

141 70 44 18 14

98 95 68 65 70

101 103 112 142 147

– 51 68 84 90

A typical galvanostatic polarization behavior for steel in the absence and presence of VPMA in 2 M H2SO4 is shown in Fig. 2. Table 3 gives the values of corrosion current (Icorr), corrosion potential (Ecorr), anodic and cathodic Tafel slopes (ba and bc) and percentage inhibition efficiency (%IE). From the Tafel slopes, it clear that the corrosion inhibition efficiency is higher in the presence of VPMA. The Icorr values decreased significantly with increase in VPMA concentration. The corrosion potential (Ecorr) shifted in the more noble direction in the presence of additive. The Ecorr values were only slightly shifted in the presence of VPMA. The cathodic curves shifted towards lower current density values in the presence of inhibitor. There is no considerable shift in the anodic polarization curves, which implies that the corrosion protection is mainly due to interaction of VPMA by simple blocking of the active sites of the metal surface. The adsorption of copolymer leads to the formation of a surface film and thus provides a barrier between metal and acid [24]. This indicates that the copolymer is a good surface modifier for the corrosion protection of steel in sulphuric acid. To verify the nature of adsorption and the effect of temperature on the corrosion behavior of mild steel in 2 M H2SO4 in the presence and absence of VPMA, weight-loss and polarization studies were undertaken. The range of temperature selected was 298–323 K. From Table 4, it can be seen that the corrosion rate increases with increase in temperature for a given concentration of inhibitor and the %IE also increases with increase in concentration. This implies that the corrosion inhibition is due to adsorption of VPMA. The degree of surface coverage (u) was determined using the expression [25]: IE u¼ 100 where u represents the degree of surface coverage by the inhibitor molecule.

Corrosion (mg cm2 h1)

rate

Inhibition efficiency (%)

Surface coverage (u)

303

313

323

303

313

323

303

313

323

1.74 0.81 0.54 0.35 0.30

2.871 1.23 0.9 0.65 0.61

3.35 2.18 1.75 1.40 1.31

– 57.0 68.0 84.0 88.0

– 55.0 63.0 77.0 85.0

– 48.0 58.0 71.0 81.0

– 0.57 0.68 0.84 0.88

– 0.55 0.63 0.77 0.85

– 0.48 0.58 0.70 0.81

The values of u/(1  u) vs. concentration of VPMA were plotted, gives straight lines. This follows Langmuir adsorption isotherm equation, given by u ¼ KC 1u where K is equilibrium adsorption constant, C is the concentration of VPMA and u is the surface coverage. The equilibrium adsorption constant, K, for the adsorption of the compound at different temperature were calculated from the slopes of straight lines (Fig. 3). This suggests that the inhibitor prevents the contact of the metal with electrolyte by forming a barrier [26]. The thermodynamic adsorption parameters are given in Table 5. The decrease in the values of K with temperature indicates that the interaction of molecule and the metal surface gets weakened and consequently the adsorbed molecules become easily removable at the high temperatures. The values of heat of adsorption in the presence of different inhibitor concentrations were computed from the slopes of lines obtained by plotting log (RI) vs. 1/T. The heat of adsorption increases with increase in concentration suggesting a more effective surface coverage at higher concentrations [27]. This

Fig. 3. Langmuir adsorption isotherm for mild steel in 2 M H2SO4 in the presence of different concentrations of VPMA at different temperatures 298–323 K.

Table 5 Thermodynamic parameters for the adsorption of inhibitors in 2 M H2SO4 on mild steel Temp. (K)

K  103 (mol1 dm3)

DG8 (kJ mol1)

298 303 313 323

6.40 4.15 2.60 1.80

31.67 31.10 30.90 30.66

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was further supported by the free energy values determined using the formulae: DGads ¼ RT lnð55:5 KÞ and DG ¼ DH  TDS The value of DG8 becomes slightly positive with increase in temperature. From this it is evident that the rate of adsorption and therefore %IE decrease with temperature. The thermodynamic functions for the dissolution of steel in 2 M H2SO4 both in the absence and presence of various concentrations of VPMA were obtained from the Arrhenius equation. The apparent activation energy was obtained from a plot of log of rate of corrosion vs. 1/T with slope Ea =2:303R (Fig. 4). logðrateÞ ¼

Table 7 Polarization resistance and inhibitor efficiencies obtained from linear polarization method for steel in 2 M H2SO4 at 298 K VPMA (%)

Ecorr (mV)

RP (V cm2)

IE (%)

0.0 0.4 0.8 1.2 1.6

530 560 565 575 580

15 30 46 93 180

– 62 80 86 91

Ea 2:303RT þ A

where Ea is the apparent activation energy and A is the preexponential factor. The calculated values of Ea, DH8, DG8 and DS8 are recorded in Table 6. The value of Ea increased with increase in VPMA concentration. The extent of increase was found to be proportional to the inhibitor concentration. This indicates that the energy barrier for the corrosion reaction increases with increase in VPMA concentration. The value of DS8 in the absence of VPMA is large and negative which implies that the activation complex in the rate determining step represents association rather than dissociation. This suggests a decrease in disorder taking place on going from reactants to the activated complex [22,28]. Polarization resistance values were determined from the slope of the linear polarization curves in the potential range 10 mV with respect to the corrosion potential at a sweep rate of 0.1 mV s1. Linear relationships were obtained in this potential range. In general Rp values increase with increase in VPMA concentration as seen in Table 7. The highest inhibition efficiency was 91% and at a concentration of 1.6%. The parallel increase in the inhibition efficiencies with increasing VPMA concentration can be explained on the basis of inhibitor adsorption [29].

Fig. 4. Arrhenius plot of corrosion rate of steel in 2 M H2SO4 in the presence of different concentrations of VPMA at different temperatures 298–323 K.

Table 6 Activation parameters for the dissolution of steel in the presence and absence of VPMA

Fig. 5. Nyquist plot for steel in 2 M H2SO4 in the absence and presence of different concentrations of VPMA.

The impedance spectra for steel in 2 M H2SO4 in the absence and presence of various concentrations of VPMA are similar in shape, as seen from Fig. 5. The semicircle radii depend on the additive concentration. The results show that Rp values increase with increasing VPMA concentration. This indicates the formation of a surface film with increasing concentration of VPMA [24]. The polarization resistance Rp and double layer capacitance Cdl, were determined by analysis of complex plane impedance plots and their values are given in Table 8. The increase in Rp and decrease in Cdl with increase in VPMA concentration indicates higher surface coverage by the inhibitor. The values of protection efficiencies indicate that there is an interaction of the compound with the active sites of metal [30]. As adsorption is of Langmuir character, the organic molecule is attached as a monolayer and through a physical mechanism. Polarization resistance is correlated to the corrosion current density in relatively simple corrosion systems characterized by a charge transfer controlled process [31]. Fig. 6A depicts the photomicrographs of the steel surface in 2 M H2SO4 medium. A number of pits observed in the figure are due to the attack of corrosive medium on the steel surface in the absence of VPMA. In the presence of VPMA the steel specimen is covered with a layer of inhibitor, which protects the surface from attack of aggressive medium (Fig. 6B). Table 8 Polarization resistance, capacitance and inhibitor efficiencies for steel in 2 M H2SO4 at 298 K, obtained using electrochemical impedance method

Conc. (w/v%)

Ea (kJ mol1)

DH8 (kJ mol1)

DS8 (J mol1)

VPMA (%)

RP (V cm2)

Capacitance (mF cm2)

IEEIS (%)

0.0 0.4 0.8 1.2 1.6

23.72 37.03 30.40 61.30 63.50

29.3 31.3 43.1 63.3 68.8

106.5 102.53 97.20 94.72 87.50

0.0 0.4 0.8 1.2 1.6

15 30 45 93 180

889 341 295 210 164

– 67 79 84 91

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Adsorption of inhibitor on the surface of steel is indicated by decrease in the double layer capacitance. The inhibition is due to the adsorption of the inhibitor on the steel surface and a resultant blocking of active sites. Acknowledgements The authors are grateful to the authorities of Kuvempu University and National Institute of Technology Karnataka, Surathkal, for providing facilities. References

Fig. 6. SEM photomicrographs of the steel surface taken after anodic polarization in 2 M H2SO4 in the absence and presence of VPMA.

4. Conclusion VPMA acts as an effective corrosion inhibitor for steel in sulphuric acid medium. The inhibition efficiencies increase by an increase in concentration of VPMA and decrease in temperature. The uniform increasing inhibition efficiency as the function of concentration deals with the adsorption phenomenon and this obeys the Langmuir adsorption isotherm. The change in values of enthalpies and the increase in activation energy with addition of inhibitor indicate that the adsorption is of a physisorption type.

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