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Inhibition of mild steel corrosion by hexadecylpyridinium bromide in 0.5 M H2SO4
Materials Chemistry and Physics 98 (2006) 83–89
Inhibition of mild steel corrosion by hexadecylpyridinium bromide in 0.5 M H2SO4 Mahmoud M. Saleh ∗ Chemistry Department, Faculty of Science, Cairo University, Cairo, Egypt Received 20 February 2005; received in revised form 17 June 2005; accepted 29 August 2005
Abstract The inhibiting action of hexadecylpyridinium bromide (HDPB) on mild steel in 0.5 M H2 SO4 solution in the temperature range of 30–60 ◦ C was studied using potentiodynamic technique and weight loss measurements. The collected polarization curves were used to study the effects of the inhibitor concentration and temperature on the corrosion behavior of mild steel. The inhibitor was found to be a mixed-type inhibitor. Maximum range of protection efficiency of HDPB was obtained at its critical micelle concentration (cmc). Weight loss measurements confirmed the above results. The inhibitor showed stronger influence on the anodic branch by shifting the free corrosion potential to more positive values. The surface coverage values were found to fit with Bockris–Swinkels isotherm with molecular ratio equal to 3. The apparent activation energy of corrosion, Ea is lower in presence than in absence of the inhibitor. Also, higher negative values of the free energy of adsorption, Goads and higher positive o value of the enthalpy of adsorption, Hads were obtained. The above results helped us to predict the mode and extent of adsorption of HDPB on the iron surface. © 2005 Elsevier B.V. All rights reserved. Keywords: Corrosion inhibitor; Steel; Hexadecyl; Pyridinium
1. Introduction Organic inhibitors have long been used in acid pickling, industrial acid cleaning and acid de-scaling to control acid corrosion of metals [1]. Surfactants represent an important category of organic inhibitors for corrosion protection of metals in different corroding media. They have been used as corrosion inhibitors for steel in different acidic solutions [2–6]. Surfactants exert its inhibition action through its adsorption on the metal/solution interface. The strength of adsorption and hence the extent of inhibition is dependent on the nature of the surfactant, nature of the surface and the corrosion medium. Cationic surfactants as a section of the surfactants have been used as corrosion inhibitors for steel both in HCl [7,8] and H2 SO4 [9,10] solutions. Maximum ranges of protection efficiency were obtained in most of the cases near and at the critical micelle concentration (cmc) of the surfactant. The molecular structure of the cationic surfactant affects the mode and extent of adsorption on the metal surface. Also, the behavior in HCl is different than in H2 SO4 ∗
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solution. Mainly, in HCl solution, the presence of Cl− facilitates the adsorption of the cationic surfactant through pre-adsorption of the halide ion on the iron surface. In H2 SO4 solution, on the other hand, cationic surfactant has lower extent of adsorption due to low adsorbability of the SO4 2− ions [11,12]. However, in the presence of a halide ion either as a counter ion or in solution, it helps to increase the extent of adsorption due to the well-known synergistic effects [9,13,14]. The presence of bonding electrons via aromatic ring in the molecular structure of the cationic surfactant has a great influence on the mode and extent of adsorption of the cationic surfactant on iron surface. Chemical adsorption was suggested in these cases [15,16]. Study of the corrosion behavior of mild steel in presence of such kind of inhibitor is important both from academic and industrial point of view. It is the aim of the present work to study the effects of hexadecylpyridinium bromide on the corrosion inhibition of mild steel in 0.5 M H2 SO4 solution. Polarization curves are collected at different inhibitor concentration and different temperatures. Weight loss measurements are achieved to confirm the electrochemical results. Also, the mode and extent of adsorption will be explained on the light of applying an adsorption isotherm.
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2. Experimental Iron samples had a composition of 0.15% C, 0.27% Mn, 0.06% Si, 0.01% S, 0.015% P and the remainder iron. The iron sample was polished gradually with wet SiC paper down to 00 grade. It was then washed with bi-distilled water and finally degreased by rinsing with acetone and dried. Weight loss measurements were achieved on circular iron discs of 1.8 cm diameter and thickness 0.5 cm. Two samples were immersed in 100 ml of the corroding solutions for 1 h. The rate of corrosion in absence and presence of the surfactant was determined for each sample and the mean value was taken. Surfactant, hexadecylpyridinium bromide, HDPB was obtained from Aldrich and used as received. The molecular structure of the used surfactant is C5 H5 NBr (CH2 )15 CH3 . A stock solution of the surfactant was prepared in 0.5 M H2 SO4 and the desired concentrations were obtained by appropriate dilution. Bi-distilled water was used in preparation of the solutions. De-aeration of the solution for 30 min was performed using N2 gas before measurements. The temperature was adjusted to ±0.2 ◦ C using a water thermostat. Electrochemical measurements were performed using an EG&G Princeton Applied Research Model 273A potentiostat/galvanostat controlled by m352 electrochemical analysis software. Polarization experiments were carried out in a conventional three-electrode cell. The iron electrode was fitted into a glass tube of proper internal diameter by using epoxy resins. The exposed surface area of the electrode was 0.50 cm2 . The iron electrode was polished gradually with emery paper down to 00 grade and treated as mentioned before. The counter electrode was made of a platinum sheet. The reference electrode was of the type Ag/AgCl/KCl (sat.) with a Luggin probe positioned near the electrode surface. The iron electrode was immersed for 30 min at the free corrosion potential, Ecor in the solution before the polarization curves were recorded. The polarization curves were recorded using potentiodynamic technique with a constant scan rate of 1 mV s−1 . The cathodic polarization measurements were recorded first in a potential range from Ecor to higher negative potentials. Anodic measurements were then recorded. Current densities were calculated on the basis of the apparent surface area of the electrode. The measurements were repeated to test the reproducibility of the results.
3. Results and discussion 3.1. Free corrosion potential Fig. 1 shows the effects of the inhibitor concentration and temperature on the free corrosion potential, Ecor . The Ecor values were recorded after reaching a practical constant value of
the open circuit potential (OCP) with time. In most of the cases it took about 30 min to reach to Ecor . The used concentration range of HDPB was 10−6 to 10−3 M at 30–60 ◦ C. The measured Ecor values of iron in blank solution were −0.4963, −0.4897, −0.4876 and −0.4805 V at 30, 40, 50 and 60 ◦ C, respectively. Generally, the values of Ecor shift to more anodic (positive) values as the inhibitor concentration and/or the temperature increases. The shifts are more pronounced at higher inhibitor concentration and/or high temperatures. Also, no more shift was observed after [HDPB] = 5 × 10−4 M. The maximum shift in Ecor as shown in Fig. 1 is less than 80.0 mV. According to Riggs [17] and Ferreira et al. [18], it is possible to classify certain inhibitor as cathodic or anodic type if the displacement in Ecor (inhibitor) is at least 85 mV with respect to Ecor (blank). It is evident from the present results that HDPB could be classified as a mixed type inhibitor. This is in agreement with the results reported for similar compounds at similar condition [11]. 3.2. Weight loss results The protection efficiency (PEw ) was calculated by using the rates of corrosion data obtained from the weight loss measurements. It could be written as: R0 − Rinh PEw = 100 × (1) R0 where Rinh and R0 are the corrosion rates in presence and absence of the inhibitor. The results are shown in Table 1. The value of the rate of corrosion of mild steel in 0.5 M H2 SO4 (blank) is comparable with the reported values in literatures at similar conditions [19]. As the concentration of HDPB increases PEw increases until it reaches constant values at the cmc of the surfactant [20,21]. Also, the protection efficiency increases with the increase in temperature. As the temperature increases the rate of corrosion increases but to much higher values in blank solution than in presence of the inhibitor. 3.3. Polarization curves
Fig. 1. Free corrosion potential, Ecor in different concentrations of HDPB in 0.5 M H2 SO4 at different temperatures.
Figs. 2 and 3 show the polarization curves of mild steel in 0.5 M H2 SO4 in absence and presence of different concentrations of HDPB at 30 and 40 ◦ C, respectively. The presence of the inhibitor shifts both the anodic and cathodic branches towards lower currents, i.e. towards lower rates of the metal corrosion. The inhibitor is of the mixed-type. However, it affects the anodic branch of the polarization curves more than the cathodic branch, i.e., it has stronger influence on inhibiting the anodic dissolution reaction. The effects of the inhibitor depend on its concentration and temperature. Polarization curves at 5 × 10−4 and 1 × 10−3 M of HDPB are similar (specially for the cathodic branches) and no more significant shifts in Tafel lines were observed. This concentration is corresponding to the cmc of the surfactant [20,21]. Similar polarization curves were collected at temperatures of 50 and 60 ◦ C. The data were analyzed to obtain different electrochemical parameters as listed in Table 2. These parameters are; corrosion current density, icor , exchange current density of HER, io ,
M.M. Saleh / Materials Chemistry and Physics 98 (2006) 83–89
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Table 1 Weight loss measurements: corrosion rates and protection efficiency of HDPB for mild steel corrosion in 0.5 M H2 SO4 t (◦ C)
[HDPB] (M)
30
Blank (0.5 M H2 SO4 ) 1 × 10−6 5 × 10−6 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
40
50
60
Rate of corrosion (mg cm−2 h−1 )
PEw (%)
6.15 5.25 4.73 4.24 2.77 1.48 0.49 0.49
– 14 23 31 55 76 92 92
Blank 1 × 10−6 5 × 10−6 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
14.95 12.67 11.80 10.24 6.58 3.59 0.89 0.89
– 15 21 31 56 76 94 94
Blank 1 × 10−6 5 × 10−6 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
28.77 23.59 21.0 18.42 9.49 5.04 1.29 1.15
– 18 28 36 67 82 95 96
Blank 1 × 10−6 5 × 10−6 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
58.52 47.69 42.37 33.6 16.9 7.60 2.05 1.76
– 17 27 42 72 87 96 97
Fig. 3. Polarization curves for mild steel in different concentrations of HDPB in 0.5 M H2 SO4 at 40 ◦ C. (1) Blank, 0.5 M H2 SO4 ; (2) 1 × 10−6 M; (3) 5 × 10−6 M; (4) 1 × 10−5 M; (5) 5 × 10−5 M; (6) 1 × 10−4 M; (7) 5 × 10−4 M; (8) 1 × 10−3 M HDPB in 0.5 M H2 SO4 .
cathodic and anodic Tafel slopes, βc and βa , respectively, and the polarization resistance, Rp . The corrosion current density was determined by extrapolating the linear part of the Tafel plots to the free corrosion potential. On the other hand, the polarization resistance was calculated by taking the slope of the non-linear part of the Tafel plot, i.e. using the linear polarization technique. This was done by the help of the m352 corrosion software which is provided with the EG&G Potentiostat. The obtained values of icor and Rp in blank (0.5 M H2 SO4 ) at 30 ◦ C are comparable with the literature values at the same conditions. For instance, results in [22,19] are comparable with the obtained values of icor and Rp , respectively. The io,H was calculated by extrapolating the cathodic Tafel line to the equilibrium potential of the hydrogen evolution reaction (HER), EH . The EH value is −0.05 V (NHE) at 30 ◦ C [23,24]. The corrosion current decreases with HDPB concentration and increases with temperature. The Tafel slopes, βc and βa remain unchanged in presence of different inhibitor concentrations. The presence of HDPB does not change the mechanism of both anodic and cathodic reactions. This may lead to the conclusions that the action of the inhibitor is simple blocking of the metal surface by adsorption. The polarization resistance, Rp is an important kinetic parameter. Higher values of Rp indicate lower rate of metal corrosion reaction. The results shown in Table 2 indicate that Rp increases with the concentration of HPDB and decreases with the temperature. 3.4. Protection efficiency The protection efficiency, PEicor of the inhibitor can be calculated from icor values according to the following equation:
Fig. 2. Polarization curves for mild steel in different concentrations of HDPB in 0.5 M H2 SO4 at 30 ◦ C. (1) Blank, 0.5 M H2 SO4 ; (2) 1 × 10−6 M; (3) 5 × 10−6 M; (4) 1 × 10−5 M; (5) 5 × 10−5 M; (6) 1 × 10−4 M; (7) 5 × 10−4 M; (8) 1 × 10−3 M HDPB in 0.5 M H2 SO4 .
PEicor =
icor1 − icor2 icor1
× 100
(2)
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M.M. Saleh / Materials Chemistry and Physics 98 (2006) 83–89
Table 2 Electrochemical parameters and inhibition efficiency for the HER on mild steel in 0.5 M H2 SO4 at different concentrations of HDPB and temperatures T (◦ C)
[HDPB] (mol L−1 )
30
0 1 × 10−6 5 × 10−6 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
40
0 1 × 10−6 5 × 10−6 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
50
60
i0 × 106 (A cm−2 )
βa (mV dec−1 )
βc (mV dec−1 )
Rp (Ohm cm2 )
IEio (%)
3.99 3.31 3.07 2.75 1.79 0.95 0.15 0.15
132 121 127 137 145 126 120 128
145 135 137 142 155 136 130 138
6.5 5.6 8.2 11.4 40.6 158 206 343
– 3 16 25 51 79 86 86
13 12.4 10.6 9.3 5.3 2.5 1.4 1.4
8.60 7.05 6.45 5.67 3.35 1.72 0.25 0.25
135 125 121 132 150 134 122 120
139 133 131 132 154 131 127 132
4.1 4.7 6.1 15.4 61 131 200 201
– 5 18 28 59 81 89 89
0 1 × 10−6 5 × 10−6 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
20 18.5 15.6 13.6 6.8 3.4 2.0 2.0
15.8 12.8 11.4 10.0 4.76 2.54 0.31 0.31
133 118 127 132 144 124 121 126
141 131 129 136 151 137 121 134
3 3.5 5.6 12 35 51 70 72
0 1 × 10−6 5 × 10−6 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
46.7 42.4 35.3 29.8 14.4 7.0 4.2 4.2
33.6 26.9 23.5 18.8 9.08 4.37 0.50 0.50
133 125 129 138 145 126 122 123
141 123 131 132 147 127 123 118
2 2.5 3.0 4.6 12 24 171 133
6.8 6.6 5.7 5.1 3.3 1.4 0.95 0.95
icor (mA cm−2 )
where icor2 and icor1 are the corrosion current densities in presence and absence of the inhibitor, respectively. Fig. 4 depicts the effects of the inhibitor concentration and temperature on the corrosion protection efficiency, PEicor . The figure reveals
– 9 24 36 69 85 91 91
that PEicor increase with the increase in [HDPB] and/or temperature. The protection efficiency increases with [HDPB] until it reaches a constant maximum ranges (plateau) at the critical micelle concentration of HDPB. The corrosion protection is a result of adsorption of the organic inhibitor on the metal surface. Adsorption of organic adsorbate at a metal/solution interface is regarded as a substitutional adsorption process between the organic molecules in the aqueous solution, Org(sol) and the water molecules adsorbed on the electrode surface, H2 O(ads) : Org(sol) + xH2 O(ads) = Org(ads) + xH2 O
Fig. 4. Effects of HDPB concentrations on PEicor for iron corrosion in 0.5 M H2 SO4 at different temperatures.
– 7.5 22 32 66 83 90 90
(3)
where Org(sol) and Org(ads) are the organic molecules in the aqueous solution and adsorbed on the metallic surface, respectively, H2 O(ads) is the water molecules on the metallic surface and x the size ratio representing the number of water molecules replaced by one molecule of the organic adsorbate. At lower inhibitor concentration, the HDPB molecules replace the adsorbed water molecules from the iron surface. HDPB hinders acid attack on the iron surface giving rise to a corrosion protection. The surfactant molecules adsorb by its head (hydrophilic) directing its tail (hydrophobic) to the solution face leading to decrease in the corrosion rate. As [HDPB] increases, it replaces more H2 O molecules leading to higher degree of sur-
M.M. Saleh / Materials Chemistry and Physics 98 (2006) 83–89
face coverage and hence to higher protection efficiency. In the range between 1 × 10−6 M and up to the start of the plateau, the degree of surface coverage increases with the concentration and consequently higher corrosion inhibition was obtained. Adsorption is enhanced due to the inter-hydrophobic chain interaction resulting in the formation of a thin film of surfactant molecules at the iron surface. This film is hydrophobic in nature due to anchoring of the hydrophobic chains into the solution. A plateau is obtained at the cmc of the surfactant due to the formation of a bimolecular layer of surfactant through the interaction of the hydrocarbon chains by tail–tail orientation at the metal/solution interface [25,26]. The bilayer is amenable to desorb away from the surface due to repulsion of the polar head group. At constant PE range, the surfactant prefer to form micelles rather than adsorbing on the iron surface and hence no further increase of PEicor was noted. The figure also reveals that the inhibitor is efficient at higher as well as at lower temperatures. The inhibitor efficiency for HER, IEio was calculated using Eq. (2) but with io2 and io1 instead of icor2 and icor1 , respectively. The values are listed in Table 2 at different concentration and temperatures. Although IEio has the same trends as PEicor , it has lower values than PEicor at the same [HDPB] and temperature. This indicates that HDPB has stronger influence on the anodic reaction than on the cathodic reaction which confirm the above results. 3.5. Activation energy The apparent activation energy, Ea of the corrosion reaction was determined using Arrhenius plots. Arrhenius equation could be written as: −Ea icor = A exp (4) RT where icor is the corrosion current density, Ea the apparent activation energy of the corrosion reaction and A the Arrhenius pre-exponential factor. The apparent activation energy of the corrosion reaction in presence and absence of the inhibitor could be determined by plotting log icor with 1/T which gives a straight line with a slope permitting the determination of Ea . Fig. 5 shows those plots in absence and presence of 5 × 10−4 M of HDPB. A maximum protection efficiency was obtained at this concentration as shown in Fig. 4. The value of Ea was found to be 63 kJ mol−1 in absence of the inhibitor, i.e. in 0.5 M H2 SO4 . This value is comparable with literature values at similar conditions [4,27]. The apparent activation energy in presence of HDPB was calculated from Fig. 5 as 38.0 kJ mol−1 . The lower value of Ea in presence of the inhibitor suggest chemical adsorption of the HDPB molecules on the iron surface [13,28,29]. 3.6. Adsorption isotherm The polarization curves were analyzed to study the inhibition efficiency and mode of adsorption of HDPB for corrosion of mild steel in 0.5 M H2 SO4 . The degree of surface coverage, θ of the metal surface was calculated from the following relation at
87
Fig. 5. Arrhenius plots for the corrosion current of mild steel in blank (0.5 M H2 SO4 ) and inhibitor (5 × 10−4 M HDPB in 0.5 M H2 SO4 ).
constant potential [30,31]: θ=
i1 − i2 i1
(5)
where i1 and i2 are the current densities for the blank and the inhibited solutions, respectively, at specific anodic potential of −0.35 V. This equation is valid under the condition of equal slopes of Tafel lines, a condition which is evident in the present Tafel lines. The anodic branches were taken here because the anodic Tafel lines are more affected by the inhibitor than the cathodic ones. Of the known adsorption isotherms, the data of θ values were found to fit well with Bockris–Swinkels isotherm. The isotherm is given by [32]: Goads Corg θ [θ + x (1 − θ)](x−1) exp − = (1 − θ)x xx 55.4 RT (6) where Corg is the concentration of the inhibitor in the bulk of the solution and Goads the free energy of adsorption. According to this isotherm a plot of the logarithm of the LHS of Eq. (6), F(θ) against logarithm of Corg should give a straight line with a slope of unity. Fig. 6 shows such these plots at different temperatures. For all the plots, straight lines were obtained with slopes of 1.0 ± 0.1 only when using a value of x = 3. This value means that one molecule of the HDPB adsorbed on the iron surface replaces three water molecules. The predicted value of x is supported by considering the reported cross-sectional areas of HDPB and H2 O ˚ 2 , respectively [33,34]. which are 35 and 12 A o The values of Gads at different temperature were estimated from the intercept of Fig. 6 and plotted against T. This plot is shown in Fig. 7. The slope and intercept of Fig. 7 were o and the entropy used to calculate the heat of adsorption, Hads o change of adsorption, Sads according to the basic equation, o − T S o . The predicted values of H o and Goads = Hads ads ads o Sads are 34.4 kJ mol−1 and 248 J mol−1 K−1 , respectively. The
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M.M. Saleh / Materials Chemistry and Physics 98 (2006) 83–89
Fe atom. Concerning the adsorption and/or corrosion inhibition processes, the orientation of the adsorbed organic molecules at the metal surfaces is of special interest. It has been reported that the adsorption of the heterocyclic compounds occur with the aromatic rings sometimes parallel but mostly normal to the metal surface. Those orientations could be dependant on pH values and/or electrode potentials [36]. In our case the pyridinium ring is parallel to the metal surface as evident from the area which corresponds to a planar orientation of the molecule. However, more efforts should be done to confirm the above arguments. Since there is a strong adsorption of the HDPB on the metal surface suggests the coadsorption of the positive moiety, N+ of the pyridinium ring on the negative cathodic sites induced by the metal surface [26]. 4. Summary and conclusions Fig. 6. Bockris–Swinkels isotherm at different temperatures.
large negative values of the free energy of adsorption and the positive value of the heat of adsorption are characteristic of a strong interaction between the HDPB molecule and the metal surface [28,29]. The adsorption process usually is accompanied by ordering of the adsorbed organic molecules. The positive o is attributed to the increase of disorder due to value of Sads desorption of three-water molecules from the iron surface. This disordering overweighs the ordering resulting from the adsorption of only one surfactant molecule [35]. In the present study, chemisorption is evident from, the increase in protection efficiency with temperature, the apparent activation energy of the corrosion is lower in presence of HDPB than in its absence, the large negative values of Goads and the o [28,29]. According to the above large positive value of Hads argument, chemisorption of the HDPB molecules on the iron surface, may take place through the donor–acceptor links between the -electrons of the pyridine ring and the empty d-orbital of the
Fig. 7. Free energy of corrosion of steel at different temperatures.
Hexadecylpyridinium bromide acts as a mixed type inhibitor for mild steel in 0.5 M H2 SO4 . The protection efficiency increases with the inhibitor concentration and temperature. Maximum efficiency was obtained at the cmc of the surfactant. The apparent activation energy was found to be lower in presence of the inhibitor than in absence of the inhibitor. The surface coverage values fit with Bockris–Swinkels isotherm with molecular ratio of 3 (water to inhibitor). Large negative values of Goads were obtained. The above results suggested chemical adsorption of HDPB on the iron surface. References [1] G. Schmitt, Br. Corr. J. 19 (4) (1984) 165. [2] M. Hosseini, S.F.L. Mertens, M.R. Arhadi, Corros. Sci. 45 (2003) 1473. [3] A. Frignani, M. Tassinari, C. Monticelli, G. Trabanelli, Werkst. Korros. 42 (1991) 208. [4] T. Szauer, A. Brandt, Electrochim. Acta 26 (1981) 1219. [5] G.N. Mu, T.P. Zhao, M. Liu, T. Gu, Corrosion 52 (1996) 853. [6] M. Elachouri, M.S. Hajji, M. Salem, S. Kertit, J. Aride, R. Coudert, E. Essassi, Corrosion 52 (1996) 103. [7] L.-G. Qiu, A.-J. Xie, Y.-H. Shen, Corros. Sci. 47 (2005) 273. [8] F. Zucchi, G. Trabanelli, G. Brunoro, Corros. Sci. 33 (1992) 1135. [9] D.P. Schweinsberg, V. Ashworth, Corros. Sci. 28 (1988) 539. [10] M.M. Osman, Anti-corros. Meth. Mat. 45 (1998) 176. [11] A. Frignani, F. Zucchi, C. Monticelli, Br. Corros. J. 18 (1983) 19. [12] F. Bentiss, M. Traisnel, M. Lagrenee, J. Appl. Electrochem. 31 (2001) 41. [13] A. Popova, E. Sokolova, S. Raicheva, M. Christov, Corros. Sci. 45 (2003) 33. [14] L. Larabi, Y. Harek, M. Traisnel, A. Mansri, J. Appl. Electrochem. 34 (2004) 833. [15] A.A. Atia, M.M. Saleh, J. Appl. Electrochem. 33 (2003) 177. [16] D. Chebabe, Z. Ait Chikh, N. Hajjaji, A. Srhiri, F. Zucchi, Corros. Sci. 45 (2003) 309. [17] O.L. Riggs Jr., Corrosion Inhibitors, second ed., C.C. Nathan, Houston, TX, 1973. [18] E.S. Ferreira, C. Giancomelli, F.C. Giacomelli, A. Spinelli, Mater. Chem. Phys. 83 (2004) 129. [19] S.Y. Sayed, M.S. El-deab, B.E. El-Anadouli, B.G. Ateya, J. Phys. Chem. B 107 (2003) 5575. [20] M.M. Saleh, A.A. Atia, Adsorp. Sci. Technol. 17 (1999) 53. [21] D. Myer, Surfactants Science and Technology, VCH, New York, 1988.
M.M. Saleh / Materials Chemistry and Physics 98 (2006) 83–89 [22] S.T. Selvi, V. Raman, N. Rajendran, J. Appl. Electrochem. 33 (2003) 1175. [23] J.O’M. Bockris, Bo. Yang, J. Electrochem. Soc. 138 (1991) 2237. [24] B.G. Ateya, B.E. El-Anadouli, F.M. El-Nizamy, Corros. Sci. 24 (1984) 497. [25] N. Hajjaji, I. Rico, A. Srhiri, A. Lattes, M. Soufiaoui, A.B. Bachir, Corrosion 49 (1993) 326. [26] M. Elachouri, M.S. Hajji, S. Kertit, E.M. Essassi, M. Salem, R. Coudert, Corros. Sci. 37 (1995) 381. [27] B.E. El-Anadouli, B.G. Ateya, F.M. El-Nizamy, Corros. Sci. 26 (1986) 419. [28] S. Sankarapapavinasam, F. Pushpanadan, M.F. Ahmed, Corros. Sci. 32 (1991) 193.
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[29] F. Bentiss, M. Traisnel, M. Lagrenee, J. Appl. Electrochem. 31 (2001) 41. [30] J. Lipkowski, Galus, J. Electroanal. Chem. 61 (1975) 11. [31] B.G. Ateya, B.E. El-Anadouli, F.M. El-Nizamy, Corros. Sci. 24 (1984) 509. [32] J.O’M. Bockris, D.A.J. Swinkels, J. Electrochem. Soc. 111 (1964) 736. [33] D.N. Rubingh, P.M. Holland, Cationic Surfactants, vol. 37, Marcel Dekker, New York, 1991, pp. 115–116. [34] M.J. Jaycock, R.H. Ottewill, Adsorption of Ionic Surface Active Agentsby Charged Solids, Trans. IMM 72 (1962–1963) 497–506. [35] E. Khamis, Corrosion 46 (1990) 476. [36] Lj.M. Vracar, D.M. Drazic, Corros. Sci. 44 (2002) 1669.
Inhibition of mild steel corrosion by hexadecylpyridinium bromide in 0.5 M H2SO4 Mahmoud M. Saleh ∗ Chemistry Department, Faculty of Science, Cairo University, Cairo, Egypt Received 20 February 2005; received in revised form 17 June 2005; accepted 29 August 2005
Abstract The inhibiting action of hexadecylpyridinium bromide (HDPB) on mild steel in 0.5 M H2 SO4 solution in the temperature range of 30–60 ◦ C was studied using potentiodynamic technique and weight loss measurements. The collected polarization curves were used to study the effects of the inhibitor concentration and temperature on the corrosion behavior of mild steel. The inhibitor was found to be a mixed-type inhibitor. Maximum range of protection efficiency of HDPB was obtained at its critical micelle concentration (cmc). Weight loss measurements confirmed the above results. The inhibitor showed stronger influence on the anodic branch by shifting the free corrosion potential to more positive values. The surface coverage values were found to fit with Bockris–Swinkels isotherm with molecular ratio equal to 3. The apparent activation energy of corrosion, Ea is lower in presence than in absence of the inhibitor. Also, higher negative values of the free energy of adsorption, Goads and higher positive o value of the enthalpy of adsorption, Hads were obtained. The above results helped us to predict the mode and extent of adsorption of HDPB on the iron surface. © 2005 Elsevier B.V. All rights reserved. Keywords: Corrosion inhibitor; Steel; Hexadecyl; Pyridinium
1. Introduction Organic inhibitors have long been used in acid pickling, industrial acid cleaning and acid de-scaling to control acid corrosion of metals [1]. Surfactants represent an important category of organic inhibitors for corrosion protection of metals in different corroding media. They have been used as corrosion inhibitors for steel in different acidic solutions [2–6]. Surfactants exert its inhibition action through its adsorption on the metal/solution interface. The strength of adsorption and hence the extent of inhibition is dependent on the nature of the surfactant, nature of the surface and the corrosion medium. Cationic surfactants as a section of the surfactants have been used as corrosion inhibitors for steel both in HCl [7,8] and H2 SO4 [9,10] solutions. Maximum ranges of protection efficiency were obtained in most of the cases near and at the critical micelle concentration (cmc) of the surfactant. The molecular structure of the cationic surfactant affects the mode and extent of adsorption on the metal surface. Also, the behavior in HCl is different than in H2 SO4 ∗
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solution. Mainly, in HCl solution, the presence of Cl− facilitates the adsorption of the cationic surfactant through pre-adsorption of the halide ion on the iron surface. In H2 SO4 solution, on the other hand, cationic surfactant has lower extent of adsorption due to low adsorbability of the SO4 2− ions [11,12]. However, in the presence of a halide ion either as a counter ion or in solution, it helps to increase the extent of adsorption due to the well-known synergistic effects [9,13,14]. The presence of bonding electrons via aromatic ring in the molecular structure of the cationic surfactant has a great influence on the mode and extent of adsorption of the cationic surfactant on iron surface. Chemical adsorption was suggested in these cases [15,16]. Study of the corrosion behavior of mild steel in presence of such kind of inhibitor is important both from academic and industrial point of view. It is the aim of the present work to study the effects of hexadecylpyridinium bromide on the corrosion inhibition of mild steel in 0.5 M H2 SO4 solution. Polarization curves are collected at different inhibitor concentration and different temperatures. Weight loss measurements are achieved to confirm the electrochemical results. Also, the mode and extent of adsorption will be explained on the light of applying an adsorption isotherm.
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2. Experimental Iron samples had a composition of 0.15% C, 0.27% Mn, 0.06% Si, 0.01% S, 0.015% P and the remainder iron. The iron sample was polished gradually with wet SiC paper down to 00 grade. It was then washed with bi-distilled water and finally degreased by rinsing with acetone and dried. Weight loss measurements were achieved on circular iron discs of 1.8 cm diameter and thickness 0.5 cm. Two samples were immersed in 100 ml of the corroding solutions for 1 h. The rate of corrosion in absence and presence of the surfactant was determined for each sample and the mean value was taken. Surfactant, hexadecylpyridinium bromide, HDPB was obtained from Aldrich and used as received. The molecular structure of the used surfactant is C5 H5 NBr (CH2 )15 CH3 . A stock solution of the surfactant was prepared in 0.5 M H2 SO4 and the desired concentrations were obtained by appropriate dilution. Bi-distilled water was used in preparation of the solutions. De-aeration of the solution for 30 min was performed using N2 gas before measurements. The temperature was adjusted to ±0.2 ◦ C using a water thermostat. Electrochemical measurements were performed using an EG&G Princeton Applied Research Model 273A potentiostat/galvanostat controlled by m352 electrochemical analysis software. Polarization experiments were carried out in a conventional three-electrode cell. The iron electrode was fitted into a glass tube of proper internal diameter by using epoxy resins. The exposed surface area of the electrode was 0.50 cm2 . The iron electrode was polished gradually with emery paper down to 00 grade and treated as mentioned before. The counter electrode was made of a platinum sheet. The reference electrode was of the type Ag/AgCl/KCl (sat.) with a Luggin probe positioned near the electrode surface. The iron electrode was immersed for 30 min at the free corrosion potential, Ecor in the solution before the polarization curves were recorded. The polarization curves were recorded using potentiodynamic technique with a constant scan rate of 1 mV s−1 . The cathodic polarization measurements were recorded first in a potential range from Ecor to higher negative potentials. Anodic measurements were then recorded. Current densities were calculated on the basis of the apparent surface area of the electrode. The measurements were repeated to test the reproducibility of the results.
3. Results and discussion 3.1. Free corrosion potential Fig. 1 shows the effects of the inhibitor concentration and temperature on the free corrosion potential, Ecor . The Ecor values were recorded after reaching a practical constant value of
the open circuit potential (OCP) with time. In most of the cases it took about 30 min to reach to Ecor . The used concentration range of HDPB was 10−6 to 10−3 M at 30–60 ◦ C. The measured Ecor values of iron in blank solution were −0.4963, −0.4897, −0.4876 and −0.4805 V at 30, 40, 50 and 60 ◦ C, respectively. Generally, the values of Ecor shift to more anodic (positive) values as the inhibitor concentration and/or the temperature increases. The shifts are more pronounced at higher inhibitor concentration and/or high temperatures. Also, no more shift was observed after [HDPB] = 5 × 10−4 M. The maximum shift in Ecor as shown in Fig. 1 is less than 80.0 mV. According to Riggs [17] and Ferreira et al. [18], it is possible to classify certain inhibitor as cathodic or anodic type if the displacement in Ecor (inhibitor) is at least 85 mV with respect to Ecor (blank). It is evident from the present results that HDPB could be classified as a mixed type inhibitor. This is in agreement with the results reported for similar compounds at similar condition [11]. 3.2. Weight loss results The protection efficiency (PEw ) was calculated by using the rates of corrosion data obtained from the weight loss measurements. It could be written as: R0 − Rinh PEw = 100 × (1) R0 where Rinh and R0 are the corrosion rates in presence and absence of the inhibitor. The results are shown in Table 1. The value of the rate of corrosion of mild steel in 0.5 M H2 SO4 (blank) is comparable with the reported values in literatures at similar conditions [19]. As the concentration of HDPB increases PEw increases until it reaches constant values at the cmc of the surfactant [20,21]. Also, the protection efficiency increases with the increase in temperature. As the temperature increases the rate of corrosion increases but to much higher values in blank solution than in presence of the inhibitor. 3.3. Polarization curves
Fig. 1. Free corrosion potential, Ecor in different concentrations of HDPB in 0.5 M H2 SO4 at different temperatures.
Figs. 2 and 3 show the polarization curves of mild steel in 0.5 M H2 SO4 in absence and presence of different concentrations of HDPB at 30 and 40 ◦ C, respectively. The presence of the inhibitor shifts both the anodic and cathodic branches towards lower currents, i.e. towards lower rates of the metal corrosion. The inhibitor is of the mixed-type. However, it affects the anodic branch of the polarization curves more than the cathodic branch, i.e., it has stronger influence on inhibiting the anodic dissolution reaction. The effects of the inhibitor depend on its concentration and temperature. Polarization curves at 5 × 10−4 and 1 × 10−3 M of HDPB are similar (specially for the cathodic branches) and no more significant shifts in Tafel lines were observed. This concentration is corresponding to the cmc of the surfactant [20,21]. Similar polarization curves were collected at temperatures of 50 and 60 ◦ C. The data were analyzed to obtain different electrochemical parameters as listed in Table 2. These parameters are; corrosion current density, icor , exchange current density of HER, io ,
M.M. Saleh / Materials Chemistry and Physics 98 (2006) 83–89
85
Table 1 Weight loss measurements: corrosion rates and protection efficiency of HDPB for mild steel corrosion in 0.5 M H2 SO4 t (◦ C)
[HDPB] (M)
30
Blank (0.5 M H2 SO4 ) 1 × 10−6 5 × 10−6 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
40
50
60
Rate of corrosion (mg cm−2 h−1 )
PEw (%)
6.15 5.25 4.73 4.24 2.77 1.48 0.49 0.49
– 14 23 31 55 76 92 92
Blank 1 × 10−6 5 × 10−6 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
14.95 12.67 11.80 10.24 6.58 3.59 0.89 0.89
– 15 21 31 56 76 94 94
Blank 1 × 10−6 5 × 10−6 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
28.77 23.59 21.0 18.42 9.49 5.04 1.29 1.15
– 18 28 36 67 82 95 96
Blank 1 × 10−6 5 × 10−6 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
58.52 47.69 42.37 33.6 16.9 7.60 2.05 1.76
– 17 27 42 72 87 96 97
Fig. 3. Polarization curves for mild steel in different concentrations of HDPB in 0.5 M H2 SO4 at 40 ◦ C. (1) Blank, 0.5 M H2 SO4 ; (2) 1 × 10−6 M; (3) 5 × 10−6 M; (4) 1 × 10−5 M; (5) 5 × 10−5 M; (6) 1 × 10−4 M; (7) 5 × 10−4 M; (8) 1 × 10−3 M HDPB in 0.5 M H2 SO4 .
cathodic and anodic Tafel slopes, βc and βa , respectively, and the polarization resistance, Rp . The corrosion current density was determined by extrapolating the linear part of the Tafel plots to the free corrosion potential. On the other hand, the polarization resistance was calculated by taking the slope of the non-linear part of the Tafel plot, i.e. using the linear polarization technique. This was done by the help of the m352 corrosion software which is provided with the EG&G Potentiostat. The obtained values of icor and Rp in blank (0.5 M H2 SO4 ) at 30 ◦ C are comparable with the literature values at the same conditions. For instance, results in [22,19] are comparable with the obtained values of icor and Rp , respectively. The io,H was calculated by extrapolating the cathodic Tafel line to the equilibrium potential of the hydrogen evolution reaction (HER), EH . The EH value is −0.05 V (NHE) at 30 ◦ C [23,24]. The corrosion current decreases with HDPB concentration and increases with temperature. The Tafel slopes, βc and βa remain unchanged in presence of different inhibitor concentrations. The presence of HDPB does not change the mechanism of both anodic and cathodic reactions. This may lead to the conclusions that the action of the inhibitor is simple blocking of the metal surface by adsorption. The polarization resistance, Rp is an important kinetic parameter. Higher values of Rp indicate lower rate of metal corrosion reaction. The results shown in Table 2 indicate that Rp increases with the concentration of HPDB and decreases with the temperature. 3.4. Protection efficiency The protection efficiency, PEicor of the inhibitor can be calculated from icor values according to the following equation:
Fig. 2. Polarization curves for mild steel in different concentrations of HDPB in 0.5 M H2 SO4 at 30 ◦ C. (1) Blank, 0.5 M H2 SO4 ; (2) 1 × 10−6 M; (3) 5 × 10−6 M; (4) 1 × 10−5 M; (5) 5 × 10−5 M; (6) 1 × 10−4 M; (7) 5 × 10−4 M; (8) 1 × 10−3 M HDPB in 0.5 M H2 SO4 .
PEicor =
icor1 − icor2 icor1
× 100
(2)
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Table 2 Electrochemical parameters and inhibition efficiency for the HER on mild steel in 0.5 M H2 SO4 at different concentrations of HDPB and temperatures T (◦ C)
[HDPB] (mol L−1 )
30
0 1 × 10−6 5 × 10−6 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
40
0 1 × 10−6 5 × 10−6 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
50
60
i0 × 106 (A cm−2 )
βa (mV dec−1 )
βc (mV dec−1 )
Rp (Ohm cm2 )
IEio (%)
3.99 3.31 3.07 2.75 1.79 0.95 0.15 0.15
132 121 127 137 145 126 120 128
145 135 137 142 155 136 130 138
6.5 5.6 8.2 11.4 40.6 158 206 343
– 3 16 25 51 79 86 86
13 12.4 10.6 9.3 5.3 2.5 1.4 1.4
8.60 7.05 6.45 5.67 3.35 1.72 0.25 0.25
135 125 121 132 150 134 122 120
139 133 131 132 154 131 127 132
4.1 4.7 6.1 15.4 61 131 200 201
– 5 18 28 59 81 89 89
0 1 × 10−6 5 × 10−6 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
20 18.5 15.6 13.6 6.8 3.4 2.0 2.0
15.8 12.8 11.4 10.0 4.76 2.54 0.31 0.31
133 118 127 132 144 124 121 126
141 131 129 136 151 137 121 134
3 3.5 5.6 12 35 51 70 72
0 1 × 10−6 5 × 10−6 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
46.7 42.4 35.3 29.8 14.4 7.0 4.2 4.2
33.6 26.9 23.5 18.8 9.08 4.37 0.50 0.50
133 125 129 138 145 126 122 123
141 123 131 132 147 127 123 118
2 2.5 3.0 4.6 12 24 171 133
6.8 6.6 5.7 5.1 3.3 1.4 0.95 0.95
icor (mA cm−2 )
where icor2 and icor1 are the corrosion current densities in presence and absence of the inhibitor, respectively. Fig. 4 depicts the effects of the inhibitor concentration and temperature on the corrosion protection efficiency, PEicor . The figure reveals
– 9 24 36 69 85 91 91
that PEicor increase with the increase in [HDPB] and/or temperature. The protection efficiency increases with [HDPB] until it reaches a constant maximum ranges (plateau) at the critical micelle concentration of HDPB. The corrosion protection is a result of adsorption of the organic inhibitor on the metal surface. Adsorption of organic adsorbate at a metal/solution interface is regarded as a substitutional adsorption process between the organic molecules in the aqueous solution, Org(sol) and the water molecules adsorbed on the electrode surface, H2 O(ads) : Org(sol) + xH2 O(ads) = Org(ads) + xH2 O
Fig. 4. Effects of HDPB concentrations on PEicor for iron corrosion in 0.5 M H2 SO4 at different temperatures.
– 7.5 22 32 66 83 90 90
(3)
where Org(sol) and Org(ads) are the organic molecules in the aqueous solution and adsorbed on the metallic surface, respectively, H2 O(ads) is the water molecules on the metallic surface and x the size ratio representing the number of water molecules replaced by one molecule of the organic adsorbate. At lower inhibitor concentration, the HDPB molecules replace the adsorbed water molecules from the iron surface. HDPB hinders acid attack on the iron surface giving rise to a corrosion protection. The surfactant molecules adsorb by its head (hydrophilic) directing its tail (hydrophobic) to the solution face leading to decrease in the corrosion rate. As [HDPB] increases, it replaces more H2 O molecules leading to higher degree of sur-
M.M. Saleh / Materials Chemistry and Physics 98 (2006) 83–89
face coverage and hence to higher protection efficiency. In the range between 1 × 10−6 M and up to the start of the plateau, the degree of surface coverage increases with the concentration and consequently higher corrosion inhibition was obtained. Adsorption is enhanced due to the inter-hydrophobic chain interaction resulting in the formation of a thin film of surfactant molecules at the iron surface. This film is hydrophobic in nature due to anchoring of the hydrophobic chains into the solution. A plateau is obtained at the cmc of the surfactant due to the formation of a bimolecular layer of surfactant through the interaction of the hydrocarbon chains by tail–tail orientation at the metal/solution interface [25,26]. The bilayer is amenable to desorb away from the surface due to repulsion of the polar head group. At constant PE range, the surfactant prefer to form micelles rather than adsorbing on the iron surface and hence no further increase of PEicor was noted. The figure also reveals that the inhibitor is efficient at higher as well as at lower temperatures. The inhibitor efficiency for HER, IEio was calculated using Eq. (2) but with io2 and io1 instead of icor2 and icor1 , respectively. The values are listed in Table 2 at different concentration and temperatures. Although IEio has the same trends as PEicor , it has lower values than PEicor at the same [HDPB] and temperature. This indicates that HDPB has stronger influence on the anodic reaction than on the cathodic reaction which confirm the above results. 3.5. Activation energy The apparent activation energy, Ea of the corrosion reaction was determined using Arrhenius plots. Arrhenius equation could be written as: −Ea icor = A exp (4) RT where icor is the corrosion current density, Ea the apparent activation energy of the corrosion reaction and A the Arrhenius pre-exponential factor. The apparent activation energy of the corrosion reaction in presence and absence of the inhibitor could be determined by plotting log icor with 1/T which gives a straight line with a slope permitting the determination of Ea . Fig. 5 shows those plots in absence and presence of 5 × 10−4 M of HDPB. A maximum protection efficiency was obtained at this concentration as shown in Fig. 4. The value of Ea was found to be 63 kJ mol−1 in absence of the inhibitor, i.e. in 0.5 M H2 SO4 . This value is comparable with literature values at similar conditions [4,27]. The apparent activation energy in presence of HDPB was calculated from Fig. 5 as 38.0 kJ mol−1 . The lower value of Ea in presence of the inhibitor suggest chemical adsorption of the HDPB molecules on the iron surface [13,28,29]. 3.6. Adsorption isotherm The polarization curves were analyzed to study the inhibition efficiency and mode of adsorption of HDPB for corrosion of mild steel in 0.5 M H2 SO4 . The degree of surface coverage, θ of the metal surface was calculated from the following relation at
87
Fig. 5. Arrhenius plots for the corrosion current of mild steel in blank (0.5 M H2 SO4 ) and inhibitor (5 × 10−4 M HDPB in 0.5 M H2 SO4 ).
constant potential [30,31]: θ=
i1 − i2 i1
(5)
where i1 and i2 are the current densities for the blank and the inhibited solutions, respectively, at specific anodic potential of −0.35 V. This equation is valid under the condition of equal slopes of Tafel lines, a condition which is evident in the present Tafel lines. The anodic branches were taken here because the anodic Tafel lines are more affected by the inhibitor than the cathodic ones. Of the known adsorption isotherms, the data of θ values were found to fit well with Bockris–Swinkels isotherm. The isotherm is given by [32]: Goads Corg θ [θ + x (1 − θ)](x−1) exp − = (1 − θ)x xx 55.4 RT (6) where Corg is the concentration of the inhibitor in the bulk of the solution and Goads the free energy of adsorption. According to this isotherm a plot of the logarithm of the LHS of Eq. (6), F(θ) against logarithm of Corg should give a straight line with a slope of unity. Fig. 6 shows such these plots at different temperatures. For all the plots, straight lines were obtained with slopes of 1.0 ± 0.1 only when using a value of x = 3. This value means that one molecule of the HDPB adsorbed on the iron surface replaces three water molecules. The predicted value of x is supported by considering the reported cross-sectional areas of HDPB and H2 O ˚ 2 , respectively [33,34]. which are 35 and 12 A o The values of Gads at different temperature were estimated from the intercept of Fig. 6 and plotted against T. This plot is shown in Fig. 7. The slope and intercept of Fig. 7 were o and the entropy used to calculate the heat of adsorption, Hads o change of adsorption, Sads according to the basic equation, o − T S o . The predicted values of H o and Goads = Hads ads ads o Sads are 34.4 kJ mol−1 and 248 J mol−1 K−1 , respectively. The
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M.M. Saleh / Materials Chemistry and Physics 98 (2006) 83–89
Fe atom. Concerning the adsorption and/or corrosion inhibition processes, the orientation of the adsorbed organic molecules at the metal surfaces is of special interest. It has been reported that the adsorption of the heterocyclic compounds occur with the aromatic rings sometimes parallel but mostly normal to the metal surface. Those orientations could be dependant on pH values and/or electrode potentials [36]. In our case the pyridinium ring is parallel to the metal surface as evident from the area which corresponds to a planar orientation of the molecule. However, more efforts should be done to confirm the above arguments. Since there is a strong adsorption of the HDPB on the metal surface suggests the coadsorption of the positive moiety, N+ of the pyridinium ring on the negative cathodic sites induced by the metal surface [26]. 4. Summary and conclusions Fig. 6. Bockris–Swinkels isotherm at different temperatures.
large negative values of the free energy of adsorption and the positive value of the heat of adsorption are characteristic of a strong interaction between the HDPB molecule and the metal surface [28,29]. The adsorption process usually is accompanied by ordering of the adsorbed organic molecules. The positive o is attributed to the increase of disorder due to value of Sads desorption of three-water molecules from the iron surface. This disordering overweighs the ordering resulting from the adsorption of only one surfactant molecule [35]. In the present study, chemisorption is evident from, the increase in protection efficiency with temperature, the apparent activation energy of the corrosion is lower in presence of HDPB than in its absence, the large negative values of Goads and the o [28,29]. According to the above large positive value of Hads argument, chemisorption of the HDPB molecules on the iron surface, may take place through the donor–acceptor links between the -electrons of the pyridine ring and the empty d-orbital of the
Fig. 7. Free energy of corrosion of steel at different temperatures.
Hexadecylpyridinium bromide acts as a mixed type inhibitor for mild steel in 0.5 M H2 SO4 . The protection efficiency increases with the inhibitor concentration and temperature. Maximum efficiency was obtained at the cmc of the surfactant. The apparent activation energy was found to be lower in presence of the inhibitor than in absence of the inhibitor. The surface coverage values fit with Bockris–Swinkels isotherm with molecular ratio of 3 (water to inhibitor). Large negative values of Goads were obtained. The above results suggested chemical adsorption of HDPB on the iron surface. References [1] G. Schmitt, Br. Corr. J. 19 (4) (1984) 165. [2] M. Hosseini, S.F.L. Mertens, M.R. Arhadi, Corros. Sci. 45 (2003) 1473. [3] A. Frignani, M. Tassinari, C. Monticelli, G. Trabanelli, Werkst. Korros. 42 (1991) 208. [4] T. Szauer, A. Brandt, Electrochim. Acta 26 (1981) 1219. [5] G.N. Mu, T.P. Zhao, M. Liu, T. Gu, Corrosion 52 (1996) 853. [6] M. Elachouri, M.S. Hajji, M. Salem, S. Kertit, J. Aride, R. Coudert, E. Essassi, Corrosion 52 (1996) 103. [7] L.-G. Qiu, A.-J. Xie, Y.-H. Shen, Corros. Sci. 47 (2005) 273. [8] F. Zucchi, G. Trabanelli, G. Brunoro, Corros. Sci. 33 (1992) 1135. [9] D.P. Schweinsberg, V. Ashworth, Corros. Sci. 28 (1988) 539. [10] M.M. Osman, Anti-corros. Meth. Mat. 45 (1998) 176. [11] A. Frignani, F. Zucchi, C. Monticelli, Br. Corros. J. 18 (1983) 19. [12] F. Bentiss, M. Traisnel, M. Lagrenee, J. Appl. Electrochem. 31 (2001) 41. [13] A. Popova, E. Sokolova, S. Raicheva, M. Christov, Corros. Sci. 45 (2003) 33. [14] L. Larabi, Y. Harek, M. Traisnel, A. Mansri, J. Appl. Electrochem. 34 (2004) 833. [15] A.A. Atia, M.M. Saleh, J. Appl. Electrochem. 33 (2003) 177. [16] D. Chebabe, Z. Ait Chikh, N. Hajjaji, A. Srhiri, F. Zucchi, Corros. Sci. 45 (2003) 309. [17] O.L. Riggs Jr., Corrosion Inhibitors, second ed., C.C. Nathan, Houston, TX, 1973. [18] E.S. Ferreira, C. Giancomelli, F.C. Giacomelli, A. Spinelli, Mater. Chem. Phys. 83 (2004) 129. [19] S.Y. Sayed, M.S. El-deab, B.E. El-Anadouli, B.G. Ateya, J. Phys. Chem. B 107 (2003) 5575. [20] M.M. Saleh, A.A. Atia, Adsorp. Sci. Technol. 17 (1999) 53. [21] D. Myer, Surfactants Science and Technology, VCH, New York, 1988.
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