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Pyrazolone derivatives as corrosion inhibitors for C-steel in hydrochloric acid solution
Desalination 201 (2006) 1–13
Pyrazolone derivatives as corrosion inhibitors for C-steel in hydrochloric acid solution A.S. Foudaa*, A.A. Al-Sarawyb, E.E. El-Katorib a
Department of Chemistry, Faculty of Science, Mansoura University, Mansoura-35516, Egypt Tel. +20 (2) 50 2245730; Fax +20 (2) 50 2246781; email: [email protected] b Department of Mathematical and Physical Science, Faculty of Engineering, Mansoura University, Mansoura-35516, Egypt
Received 25 July 2005; accepted 11 March 2006
Abstract 4-phenylazo-3-methyl-2-pyrazolon-5-one and three of its derivatives have been investigated as corrosion inhibitors for C-steel in 2 M hydrochloric acid solution using weight-loss and galvanostatic polarization techniques. The efficiency of the inhibitors increases with the increase in the inhibitor concentration but decreases with a rise in temperature. The conjoint effect of the pyrozolone derivatives and KBr, KSCN and KI has also been studied. The apparent activation energy (Ea*) and other thermodynamic parameters for the corrosion process have also been calculated. The galvanostatic polarization data indicated that the inhibitors were of mixed-type, but the cathode is more polarized than the anode. The slopes of the cathodic and anodic Tafel lines (βc and βa) are approximately constant and independent of the inhibitor concentration. The adsorption of these compounds on C-steel surface has been found to obey the Frumkin’s adsorption isotherm. The mechanism of inhibition was discussed in the light of the chemical structure of the undertaken inhibitors. Keywords: Corrosion inhibitors; C-steel; Polarization; Weight loss; Synergistic effect; HCl; Pyrazolone derivatives
1. Introduction Corrosion and corrosion inhibition of iron and iron alloys, in general, and steel, in particular, have received a great attention in different media [1– 5] with and without various types of inhibitors. The corrosion inhibition of C-steel becomes of *Corresponding author.
such interest because it is widely used as a constructional material in many industries and this is due to its excellent mechanical properties and low cost. A number of studies have recently appeared in the literature [6–10] on the topic of the corrosion inhibition of C-steel in acidic media. But little work appears to have been done on the corrosion
0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.03.519
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A.S. Fouda et al. / Desalination 201 (2006) 1–13
inhibition of steel alloys in hydrochloric acid using pyrazolone derivatives. The present work was designed to study: i) corrosion inhibition of C-steel in hydrochloric acid solutions by some pyrazolone derivatives using weight-loss and galvanostatic polarization techniques; ii) the effect of substituted groups on the inhibition efficiency; iii) the effect of temperature on the corrosion rate in order to calculate some thermodynamic parameters related to the corrosion process. 2. Experimental techniques 2.1. Materials The experiments were performed with C-steel having the chemical compositions given in Table 1. The inhibitors used in this study were selected from pyrazolone derivatives and are listed below: N
H3C C
C
N N H
where: X = = = =
N
X
CH
OCH3 CH3 H NO2
O
(I) (II) (III) (IV)
2.1.1. Preparation of inhibitors used Pyrazolone derivatives (I)–(IV) were prepared by adding 0.01 mole of sodium nitrite acidified Table 1 Chemical composition of C-steel
Element
Weight (%)
C Mn P Si Fe
0.200 0.350 0.024 0.003 The rest
with 1:1 HCl dropwise with gradual stirring to the hydrochloric acid solution of 0.01 mole of both [p-methoxy aniline, p-methyl aniline , aniline, pnitro aniline] [11,12] and kept for about 20 min in ice bath .The formed diazonium chloride solution was added gradually with vigorous stirring to a 0.01 mole cold solution of 3-methyl-2-pyrazolin-5-one in 50 ml ethyl alcohol containing 5 g sodium acetate, respectively. After dilution the azo pyrazolone compounds formed were recrystallized from ethyl alcohol and then dried in vacuum desiccators over anhydrous calcium chloride. The important IR spectra bands of studied ligands are presented and discussed. The strong band at 3271 cm–1 was assigned to the ν (NH) of the pyrazolone ring. A broad band centered in the 3440–3450 cm–1 region assigned to the intramolecular hydrogen bonding (O–H.....N). The strong band in the 1655–1665 cm–1 region and amide at 1600 cm–1 are both assigned to the free keto-enol form of the side chain and pyrazolone ring. The ν (N=N) symmetric stretching vibration appears in 1550–1540 cm–1 region. The ν (C=C) bands was observed at 1500 cm–1. Finally, ν (N–N) band was observed at 1415 cm–1. 100 ml stock solutions (10–3 M) of compounds (I–IV) were prepared by dissolving an accurately weighed quantity of each material in an appropriate volume of absolute ethanol, then the required concentrations (1×10–6–11×10–6 M) were prepared by dilution with bidistilled water. Hydrochloric acid solution was prepared by diluting the appropriate volume of the concentrated chemically pure acid (BDH grade), with bidistilled water and its concentration was checked by standard solution of Na2CO3. 100 ml stock solutions (10–3 M) of KI, KBr and KSCN (BDH grade) were prepared by dissolving an accurately weighed quantity of each material in an appropriate volume of bidistilled water. Two different techniques have been employed for studying the corrosion inhibition of C-steel by pyrazolone derivatives, these are: i) chemical
A.S. Fouda et al. / Desalination 201 (2006) 1–13
technique (weight-loss method) and ii) electrochemical technique (galvanostatic polarization method). 2.2. Chemical technique (weight-loss method) The reaction basin used in this method was a graduated glass vessel with a 6 cm inner diameter and a total volume of 250 ml. 100 ml of the test solution were employed in each experiment. Three test pieces were cut into 2×2×0.2 cm. They were mechanically polished with emery paper (a coarse paper was used initially and then progressively finer grades were employed), degreased in acetone [13], rinsed with bidistilled water and finally dried between two filter papers and weighed. The three test pieces were suspended by suitable glass hooks at the edge of the basin, and under the surface of the test solution by about 1 cm. After specified periods of time, the three test pieces were taken out of the test solution, rinsed with bidistilled water, dried as before and weighed again. The average weight loss at a certain time for each set of three samples was taken. The weight loss was recorded to nearest 0.0001 g. 2.3. Electrochemical technique (galvanostatic polarization method) Three different types of electrodes were used during polarization measurements: The working electrode was C-steel electrode, which cut from C-steel sheets, thickness 0.2 cm. The electrode was of dimensions 1 cm × 1 cm and was weld from one side to a copper wire used for electric connection. The sample was embedded in a glass tube using epoxy resin [14]. The electrode was prepared before immersion in the test solution as in the case of weight loss. Saturated calomel electrode (SCE) and a platinum coil as reference and auxiliary electrodes, respectively, were used. A constant quantity of the test solution (100 ml) was taken in the polarization cell. A time interval of about 30 min was given for the system to attain steady state. Both cathodic and anodic polarization
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curves were recorded galvanostatically using Amel galvanostat (Model 549) and digital multimeters (Fluke-73) were used for accurate measurements of the potential and current density. All the experiments were carried out at 30±1°C by using an ultra circulating thermostat. 3. Results and discussion 3.1. Weight-loss measurements Weight-loss of C-steel was determined, at various time intervals, in the absence and presence of different concentrations of pyrazolone derivatives, compounds (I–IV). The obtained weight-loss time curves are represented in Fig. 1 for inhibitor (I), the most effective one. Similar curves were obtained for other inhibitors (not shown). The inhibition efficiency of corrosion was found to be dependent on the inhibitor concentration, the nature of the substituents and their positions in the phenyl ring, with respect to the phenylazo molecule derivatives. The curves obtained in the presence of inhibitors fall significantly below that of free acid. In all cases, the increase in the inhibitor concentration was accompanied by a decrease in weight-loss and an increase in the percentage inhibition. These results lead to the conclusion that the compounds under investigation are fairly efficient as inhibitors for C-steel dissolution in hydrochloric acid solution. Also, the degree of surface coverage (θ) by the inhibitor, calculated from Eq. (1), would increase by increasing the inhibitor concentration. θ = 1 − ( ∆Winh / ∆Wfree )
(1)
where ∆Winh and ∆Wfree are the weight losses per unit area in the presence and absence of the inhibitor, respectively. In order to get a comparative view, the variation of the percentage inhibition (In %) of the four inhibitors with their molar concentrations was calculated according to Eq. (2). The values obtained are summarized in Table 2.
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Fig. 1. Weight-loss time curves for C-steel dissolution in 2 M HCl in the absence and presence of different concentrations of compound (I) at 30ºC. Table 2 Values of % inhibition efficiencies of inhibitors for the corrosion of C-steel in 2 M HCl from weight-loss measurements at different concentrations at 30ºC
Concentration % Inhibition efficiency (M) I II III
IV
1×10–6 3×10–6 5×10–6 7×10–6 9×10–6 11×10–6
49.3 51.1 54.7 58.1 59.3 60.1
59.5 65.2 72.2 75.5 76.9 77.5
56.8 60.4 64.9 69.2 71.1 71.9
52.4 55.7 60.0 63.6 65.1 66.0
% In = θ × 100
(2)
Careful inspection of these results showed that, at the same inhibitor concentration, the order of inhibition efficiencies is as follows: I > II > III > IV The variation of surface coverage determined by weight-loss, θ, with the logarithm of the inhibitor concentration, log C, are represented in Fig. 2. These curves are of S-shape obeying the Frumkin adsorption isotherm [15] and are in good agreement with the Frumkin Eq. (3). As shown
A.S. Fouda et al. / Desalination 201 (2006) 1–13
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Fig. 2. θ–log c curves for C-steel dissolution in 2 M HCl in the presence of different concentrations of different inhibitors from weight-loss measurements at 30ºC.
from this figure, one can conclude that the degree of surface coverage increases as the concentration of the inhibitor increases and hence, the inhibition efficiency increases. The Frumkin equation is:
[θ /1 − θ] exp ( −2aθ ) = KC
(3)
where a is a molecular interaction parameter depending on the molecular interaction in the adsorption layer and on the degree of heterogeneity of the surface, C is the molar concentration of the inhibitor, K is the equilibrium constant of the adsorption reaction. 3.1.1. Effect of temperature The effect of temperature on both corrosion
and corrosion inhibition of C-steel in 2 M HCl solution in the absence and presence of different concentrations of inhibitors (I–IV) at different temperatures ranging from 30 to 50ºC was investigated. Arrhenius plot [Eq. (4)] of logarithm of the corrosion rate, log k, with the reciprocal of absolute temperature, 1/T, in the absence and presence of 5×10–6 M of inhibitors (I)–(IV) is shown graphically in Fig. 3. ln k = B − ( Ea* / RT )
(4)
where B is a constant depends on the metal type and electrolyte, and R is the universal gas constant. From the slopes of the plots, the respective activation energies (E*a) were calculated and are
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A.S. Fouda et al. / Desalination 201 (2006) 1–13 Table 3 Activation parameters for the dissolution of C-steel in 2 M HCl in the absence and presence of 5×10–6 M of different inhibitors Inhibitor
Ea* (KJ mol–1)
∆H* (KJ mol–1)
–∆S* (J mol–1 K–1)
Blank I II III IV
30.2 34.6 34.2 33.2 32.5
27.6 32.1 31.6 30.6 29.9
180.1 175.9 175.5 177.8 179.2
tion, on the one hand, and decreases with the rise of temperature, on the other hand. This means that these compounds are adsorbed physically on the C-steel surface. Enthalpy and entropy of activation (∆H*, ∆S*) were calculated from the transition state theory [17] (Table 3): k = ( RT / Nh ) exp ( ∆S * / R ) exp ( −∆H * / RT ) (5)
Fig. 3. log corrosion rate (k) vs.1/T for C-steel in the presence of 5×10–6 M inhibitor compounds.
tabulated in Table 3. The results show that the values of activation energy increase in the same order as the increase of the inhibition efficiency of the inhibitors. It is also indicated that the whole process is controlled by surface reaction, since the energy of the activation corrosion process is over 20 KJ mol–1 [16]. From the results of the effect of temperature, it was observed that the inhibition efficiency decreases with increasing the inhibitor concentra-
where h is the Plank constant, N is the Avogadros number. A plot of log (k/T) vs. (1/T), [Eq. (5)], gave straight lines as shown in Fig. 4 for C-steel dissolution in 2 M HCl in the absence and presence of 5×10–6 M of inhibitors (I). The order of the inhibition efficiency of the investigated compounds as gathered from the increase in E*a and ∆H* and the decrease in ∆S** values is the same as above. E*a and ∆H* of the inhibition process of C-steel in 2 M HCl in the presence of inhibitors are nearly the same (or slightly higher) as those in free 2 M HCl solution, indicating that no energy barrier is attained. These data reveal that the inhibition of the corrosion reactions is affected without changing the mechanism. The entropy of activation in the presence and absence of the inhibitor is large and negative. This implies that the activated complex in the rate-determining step represents association rather than dissociation, indicating that a decrease in disorder takes place, going from reactant to the activated complex.
A.S. Fouda et al. / Desalination 201 (2006) 1–13
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Fig. 4. log corrosion rate/T (k/T) vs.1/T for Csteel in the presence of 5×10–6 M inhibitor compounds.
3.1.2. Synergistic effect The corrosion behavior of C-steel in 2 M hydrochloric acid solution in the presence of 1×10–3 M potassium bromide, potassium thiocyanate and potassium iodide at different concentrations of inhibitors (I)–(VII) was studied. The obtained weight-loss time values are represented and summarized in Table 4. From these values, it is observed that % In of the inhibitors increases with increasing the concentration of inhibitors due to synergistic effects [18]. The synergistic effect of these
anions (Br–, SCN– and I–) has been observed [19]. This effect depends on the type of anion. From the results it was found that the order of decreasing the inhibition efficiency in the presence of these anions is KI > KSCN > KBr. The strong chemisorptions of (Br–, SCN– and I–) anions on the metal surface are responsible for the synergistic effect of bromide, thiocyanate and iodide anions in combination with cation of the inhibitor. The cation is then adsorbed by columbic attraction on the metal surface where bromide, thiocyanate and iodide
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Table 4 Inhibition efficiency of different inhibitors for C-steel in 2 M HCl containing 1×10–3 M KBr/KSCN/KI at 30ºC for an exposure period of 150 min Anions Concentration % Inhibition efficiency (M) I II III IV KBr
0.0 1×10–6 3×10–6 5×10–6 7×10–6 9×10–6 11×10–6
20.3 68.6 71.8 74.8 78.1 80.2 82.3
20.3 62.7 65.3 67.0 70.4 72.7 74.5
20.3 58.2 60.5 62.2 64.7 67.3 68.7
20.3 54.6 57.1 59.1 60.8 63.0 65.3
KSCN
0.0 1×10–6 3×10–6 5×10–6 7×10–6 9×10–6 11×10–6
28.4 76.7 78.5 81.2 83.0 85.3 86.7
28.4 68.4 70.1 72.4 73.8 75.8 77.8
28.4 63.0 64.5 66.7 68.2 70.1 71.8
28.4 59.7 61.1 62.8 64.8 67.0 68.7
KI
0.0 1×10–6 3×10–6 5×10–6 7×10–6 9×10–6 11×10–6
40.1 84.0 85.3 86.7 88.1 90.1 91.8
40.1 75.3 77.0 78.4 80.1 81.8 83.2
40.1 69.9 71.6 73.3 75.0 77.0 78.7
40.1 67.0 68.5 70.2 71.6 73.0 74.7
anions are already adsorbed by chemisorption. Stabilization of adsorbed iodide, bromide and thiocyanate anions with cations leads to a greater surface coverage and therefore greater inhibition. One can conclude that the addition of iodide, bromide and thiocyanate ions enhances the inhibition efficiency to a considerable extent, due to the increase of the surface coverage in the presence of iodide, bromide and thiocyanate ions. The order of investigated compounds remains unchanged, as before. From the previous results it is known that KI could be considered as one of the effective anions for synergistic action within the investigated inhibitors. 3.2. Galvanostatic polarization measurements Fig. 5 shows the galvanostatic polarization
curves for C-steel dissolution in 2 M HCl in the absence and presence of different concentrations of inhibitor (I) at 30ºC. Similar curves were obtained for other inhibitors (not shown). The numerical values of the variation of the corrosion current density (icorr.), the corrosion potential (Ecorr.), Tafel slopes (βa and βc), ), the degree of surface coverage (θ) and the inhibition efficiency (% In) with the concentrations of inhibitors (I)–(IV) are given in Table 5. The results indicate that: 1. The cathodic and anodic curves obtained exhibit Tafel-type behavior. Addition of pyrazolone derivatives increased both the cathodic and anodic overvoltages and the presence of pyrazolone derivatives in solution inhibits both the hydrogen evolution and the anodic dissolution processes. 2. The corrosion current density (icorr.) decreases with increasing the concentrations of the pyrazolone derivatives which indicates that these compounds act as inhibitors, and the degree of inhibition depends on the concentration and type of inhibitors present. 3. The slopes of anodic and cathodic Tafel lines (βa and βc) ), were slightly changed on increasing the concentration of the tested compounds. This indicates that there is no change of the mechanism of inhibition in the presence and absence of inhibitors. The pyrazolone derivatives are mixed-type inhibitors, but the cathode is more polarized than the anode when an external current is applied. 4. The orders of inhibition efficiency of all inhibitors at different concentrations as given by polarization measurements are listed in Table 5. The results are in good agreement with those obtained from weight-loss measurements. 3.3. Chemical structure and corrosion inhibition Variation in the structure of inhibitor molecules (I–IV) takes place through the phenylazo group. So, the inhibition efficiency will depend on this part of the molecule.
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Fig. 5. Polarization curves for the dissolution of C-steel in 2 M HCl in the presence and absence of different concentrations of compound I at 30ºC.
A skeletal representation of the proposed mode of adsorption of studied compounds is shown in Fig. 6 and it clearly indicates the active adsorption centers. This order of increased inhibition efficiency of the additives can be accounted for in terms of the polar effect [20] of the p-substituents on the phenylazo group. This behavior can be rationalized on the basis of the structure–corrosion inhibition relationship of organic compounds. Linear free energy relationship (LFER) has previously been used to correlate the inhibition efficiency of organic compounds with their Hammett substi-
tuent constant (σ) [21], the LFER or Hammett relation is given by [22,23]: log k or % In = −ρ ⋅ σ
(6)
where ρ is the reaction constant. Those substituents which attract electrons from the reaction center are assigned positive σ values and those which are electron donating have negative σ values. Thus σ is a relative measure of the electron density at the reaction center. The slope of the plot of % In or log k rate of corrosion vs. σ is ρ, and its sign indicates whether the process is
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Table 5 Electrochemical parameters for C-steel in 2 M HCl with and without different concentrations of inhibitors at 30°C Inhibitors
Concentration (M)
–Ecorr (mV)
icorr (mA cm–2)
ßa (mV dec–1)
–ßc θ (mV dec–1)
% In
I
0.00 1×10–6 3×10–6 5×10–6 7×10–6 9×10–6 11×10–6 1×10–6 3×10–6 5×10–6 7×10–6 9×10–6 11×10–6 1×10–6 3×10–6 5×10–6 7×10–6 9×10–6 11×10–6 1×10–6 3×10–6 5×10–6 7×10–6 9×10–6 11×10–6
468 458 457 457 456 455 455 460 460 459 458 458 459 462 461 461 460 459 460 463 462 461 461 462 461
179.2 77.9 68.0 57.5 51.6 47.1 44.9 83.1 74.8 68.5 62.8 57.5 55.0 91.1 83.1 77.1 72.8 68.2 65.6 96.7 91.5 86.8 83.0 78.9 76.6
62 75 77 78 80 83 85 73 75 77 79 81 83 70 73 76 78 80 80 68 72 75 77 79 80
107 122 125 128 130 131 133 120 122 125 127 128 130 116 119 122 124 126 127 114 116 119 121 122 124
— 56.5 62.1 67.9 71.2 73.7 75.0 53.6 58.3 61.8 64.9 67.9 69.3 49.2 53.6 56.9 59.4 62.0 63.4 46.1 49.0 51.6 53.7 56.0 57.3
II
III
IV
N
N
H3C N
N
— 0.57 0.62 0.68 0.71 0.74 0.75 0.54 0.58 0.62 0.65 0.68 0.69 0.49 0.54 0.57 0.59 0.62 0.63 0.46 0.49 0.52 0.54 0.56 0.57
X where: X = = = =
O
H
//////////
OCH3 (I) CH3 (II) H (III) NO2 (IV)
Fig. 6. Skeletal representation of the mode of adsorption of inhibitor compounds.
inhibited by an increase or a decrease of the electron density at the reaction center. The magnitude of ρ indicates the relative sensitivity of the inhibition process to electronic effects. The results
shown in Fig. 7 indicate that pyrazolone derivatives give a good correlation line (ρ = –11.7) which shows a weak dependence of adsorption character of the reaction center on the electron density of
A.S. Fouda et al. / Desalination 201 (2006) 1–13
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Fig. 7. Variation of the inhibition efficiency (In %) with the substitutent constant (σ) of the substitutent group in para position on the phenyl ring of inhibitor compounds.
the ring, and electron-releasing substituents result in an increase in inhibition. The weak dependence of the adsorption character of the reaction center of pyrazolone derivatives on the electron density of the ring may be due to the fact that the centers of adsorption are not thoroughly conjugated to the ring. Compound (I) is the most efficient inhibitor because of the presence of highly electron releasing p–OCH3 group (σ = –0.27) which enhances the delocalized π-electrons on the molecule and also may add an additional active center to the molecule due to its oxygen atom. Compound (II) comes after (I) in the inhibition efficiency. Because it has p–CH3 in
the phenylazo group with (σ = –0.17) lower Hammett constant and hence lower sharing electron density to the molecule. Compounds (III) and (IV) come after (I) and (II) because they have positive σ values (σH = 0 and σNO = +0.78), and hence 2 lower electron sharing to their molecules. The inhibition efficiency of these molecules is parallel to the increased order of electron withdrawing (electrophilic) character of these groups. Due to the weak dependence of the adsorption character of the reaction center on the polar effect of the substituent groups, one can explain the order of the inhibition efficiency. From the structural organic point of view, both methyl and methoxy
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groups have +R effect but the inductive effect is +I and –I, respectively. Although the methyl group has +R, but its effect is very little as a result of hyperconjugation, thus: H H
C
N
N
H+ H2C
N
N
H
In the case of methoxy group, the effect of +R is large and also the inductive effect, –I, thus:
H3CO
N
N
+ H 3CO
N
N
H3CO
N
N
H3CO
N
N
Also methoxy group may add an additional active center and this increases the inhibition efficiency in the case of methoxy group than in the case of methyl group. On the other hand, the lower efficiency of nitro derivatives as compared to methyl and methoxy derivatives may be due to i) their highest electrophilic character; ii) their reduction in acid medium and iii) the evolved heat of hydrogenation may aid the desorption of the molecules. 4. Conclusions 1. The compounds under investigation, the pyrazolone derivatives, are fairly efficient inhibitors for C-steel dissolution in 2 M HCl. 2. The adsorption of these compounds on the C-steel surface was found to obey Frumkin’s adsorption isotherm. 3. From the effect of temperature, the activation parameters for the corrosion process (E*a, ∆H* and ∆S**) were calculated.
4. Percentage inhibition (% In) increased in the presence of 1×10–3 M KI, KBr and KSCN due to the synergistic effect. 5. Galvanostatic polarization data indicate that these compounds are mixed-type inhibitors, but the cathode is more polarized than the anode when an external current is applied. 6. The order of the inhibition efficiency of all inhibitors as given by polarization measurements is in good agreement with that obtained from weight-loss measurements. This order was explained on the basis of the chemical structure and adsorption active centers of the compound.
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Pyrazolone derivatives as corrosion inhibitors for C-steel in hydrochloric acid solution A.S. Foudaa*, A.A. Al-Sarawyb, E.E. El-Katorib a
Department of Chemistry, Faculty of Science, Mansoura University, Mansoura-35516, Egypt Tel. +20 (2) 50 2245730; Fax +20 (2) 50 2246781; email: [email protected] b Department of Mathematical and Physical Science, Faculty of Engineering, Mansoura University, Mansoura-35516, Egypt
Received 25 July 2005; accepted 11 March 2006
Abstract 4-phenylazo-3-methyl-2-pyrazolon-5-one and three of its derivatives have been investigated as corrosion inhibitors for C-steel in 2 M hydrochloric acid solution using weight-loss and galvanostatic polarization techniques. The efficiency of the inhibitors increases with the increase in the inhibitor concentration but decreases with a rise in temperature. The conjoint effect of the pyrozolone derivatives and KBr, KSCN and KI has also been studied. The apparent activation energy (Ea*) and other thermodynamic parameters for the corrosion process have also been calculated. The galvanostatic polarization data indicated that the inhibitors were of mixed-type, but the cathode is more polarized than the anode. The slopes of the cathodic and anodic Tafel lines (βc and βa) are approximately constant and independent of the inhibitor concentration. The adsorption of these compounds on C-steel surface has been found to obey the Frumkin’s adsorption isotherm. The mechanism of inhibition was discussed in the light of the chemical structure of the undertaken inhibitors. Keywords: Corrosion inhibitors; C-steel; Polarization; Weight loss; Synergistic effect; HCl; Pyrazolone derivatives
1. Introduction Corrosion and corrosion inhibition of iron and iron alloys, in general, and steel, in particular, have received a great attention in different media [1– 5] with and without various types of inhibitors. The corrosion inhibition of C-steel becomes of *Corresponding author.
such interest because it is widely used as a constructional material in many industries and this is due to its excellent mechanical properties and low cost. A number of studies have recently appeared in the literature [6–10] on the topic of the corrosion inhibition of C-steel in acidic media. But little work appears to have been done on the corrosion
0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.03.519
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inhibition of steel alloys in hydrochloric acid using pyrazolone derivatives. The present work was designed to study: i) corrosion inhibition of C-steel in hydrochloric acid solutions by some pyrazolone derivatives using weight-loss and galvanostatic polarization techniques; ii) the effect of substituted groups on the inhibition efficiency; iii) the effect of temperature on the corrosion rate in order to calculate some thermodynamic parameters related to the corrosion process. 2. Experimental techniques 2.1. Materials The experiments were performed with C-steel having the chemical compositions given in Table 1. The inhibitors used in this study were selected from pyrazolone derivatives and are listed below: N
H3C C
C
N N H
where: X = = = =
N
X
CH
OCH3 CH3 H NO2
O
(I) (II) (III) (IV)
2.1.1. Preparation of inhibitors used Pyrazolone derivatives (I)–(IV) were prepared by adding 0.01 mole of sodium nitrite acidified Table 1 Chemical composition of C-steel
Element
Weight (%)
C Mn P Si Fe
0.200 0.350 0.024 0.003 The rest
with 1:1 HCl dropwise with gradual stirring to the hydrochloric acid solution of 0.01 mole of both [p-methoxy aniline, p-methyl aniline , aniline, pnitro aniline] [11,12] and kept for about 20 min in ice bath .The formed diazonium chloride solution was added gradually with vigorous stirring to a 0.01 mole cold solution of 3-methyl-2-pyrazolin-5-one in 50 ml ethyl alcohol containing 5 g sodium acetate, respectively. After dilution the azo pyrazolone compounds formed were recrystallized from ethyl alcohol and then dried in vacuum desiccators over anhydrous calcium chloride. The important IR spectra bands of studied ligands are presented and discussed. The strong band at 3271 cm–1 was assigned to the ν (NH) of the pyrazolone ring. A broad band centered in the 3440–3450 cm–1 region assigned to the intramolecular hydrogen bonding (O–H.....N). The strong band in the 1655–1665 cm–1 region and amide at 1600 cm–1 are both assigned to the free keto-enol form of the side chain and pyrazolone ring. The ν (N=N) symmetric stretching vibration appears in 1550–1540 cm–1 region. The ν (C=C) bands was observed at 1500 cm–1. Finally, ν (N–N) band was observed at 1415 cm–1. 100 ml stock solutions (10–3 M) of compounds (I–IV) were prepared by dissolving an accurately weighed quantity of each material in an appropriate volume of absolute ethanol, then the required concentrations (1×10–6–11×10–6 M) were prepared by dilution with bidistilled water. Hydrochloric acid solution was prepared by diluting the appropriate volume of the concentrated chemically pure acid (BDH grade), with bidistilled water and its concentration was checked by standard solution of Na2CO3. 100 ml stock solutions (10–3 M) of KI, KBr and KSCN (BDH grade) were prepared by dissolving an accurately weighed quantity of each material in an appropriate volume of bidistilled water. Two different techniques have been employed for studying the corrosion inhibition of C-steel by pyrazolone derivatives, these are: i) chemical
A.S. Fouda et al. / Desalination 201 (2006) 1–13
technique (weight-loss method) and ii) electrochemical technique (galvanostatic polarization method). 2.2. Chemical technique (weight-loss method) The reaction basin used in this method was a graduated glass vessel with a 6 cm inner diameter and a total volume of 250 ml. 100 ml of the test solution were employed in each experiment. Three test pieces were cut into 2×2×0.2 cm. They were mechanically polished with emery paper (a coarse paper was used initially and then progressively finer grades were employed), degreased in acetone [13], rinsed with bidistilled water and finally dried between two filter papers and weighed. The three test pieces were suspended by suitable glass hooks at the edge of the basin, and under the surface of the test solution by about 1 cm. After specified periods of time, the three test pieces were taken out of the test solution, rinsed with bidistilled water, dried as before and weighed again. The average weight loss at a certain time for each set of three samples was taken. The weight loss was recorded to nearest 0.0001 g. 2.3. Electrochemical technique (galvanostatic polarization method) Three different types of electrodes were used during polarization measurements: The working electrode was C-steel electrode, which cut from C-steel sheets, thickness 0.2 cm. The electrode was of dimensions 1 cm × 1 cm and was weld from one side to a copper wire used for electric connection. The sample was embedded in a glass tube using epoxy resin [14]. The electrode was prepared before immersion in the test solution as in the case of weight loss. Saturated calomel electrode (SCE) and a platinum coil as reference and auxiliary electrodes, respectively, were used. A constant quantity of the test solution (100 ml) was taken in the polarization cell. A time interval of about 30 min was given for the system to attain steady state. Both cathodic and anodic polarization
3
curves were recorded galvanostatically using Amel galvanostat (Model 549) and digital multimeters (Fluke-73) were used for accurate measurements of the potential and current density. All the experiments were carried out at 30±1°C by using an ultra circulating thermostat. 3. Results and discussion 3.1. Weight-loss measurements Weight-loss of C-steel was determined, at various time intervals, in the absence and presence of different concentrations of pyrazolone derivatives, compounds (I–IV). The obtained weight-loss time curves are represented in Fig. 1 for inhibitor (I), the most effective one. Similar curves were obtained for other inhibitors (not shown). The inhibition efficiency of corrosion was found to be dependent on the inhibitor concentration, the nature of the substituents and their positions in the phenyl ring, with respect to the phenylazo molecule derivatives. The curves obtained in the presence of inhibitors fall significantly below that of free acid. In all cases, the increase in the inhibitor concentration was accompanied by a decrease in weight-loss and an increase in the percentage inhibition. These results lead to the conclusion that the compounds under investigation are fairly efficient as inhibitors for C-steel dissolution in hydrochloric acid solution. Also, the degree of surface coverage (θ) by the inhibitor, calculated from Eq. (1), would increase by increasing the inhibitor concentration. θ = 1 − ( ∆Winh / ∆Wfree )
(1)
where ∆Winh and ∆Wfree are the weight losses per unit area in the presence and absence of the inhibitor, respectively. In order to get a comparative view, the variation of the percentage inhibition (In %) of the four inhibitors with their molar concentrations was calculated according to Eq. (2). The values obtained are summarized in Table 2.
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A.S. Fouda et al. / Desalination 201 (2006) 1–13
Fig. 1. Weight-loss time curves for C-steel dissolution in 2 M HCl in the absence and presence of different concentrations of compound (I) at 30ºC. Table 2 Values of % inhibition efficiencies of inhibitors for the corrosion of C-steel in 2 M HCl from weight-loss measurements at different concentrations at 30ºC
Concentration % Inhibition efficiency (M) I II III
IV
1×10–6 3×10–6 5×10–6 7×10–6 9×10–6 11×10–6
49.3 51.1 54.7 58.1 59.3 60.1
59.5 65.2 72.2 75.5 76.9 77.5
56.8 60.4 64.9 69.2 71.1 71.9
52.4 55.7 60.0 63.6 65.1 66.0
% In = θ × 100
(2)
Careful inspection of these results showed that, at the same inhibitor concentration, the order of inhibition efficiencies is as follows: I > II > III > IV The variation of surface coverage determined by weight-loss, θ, with the logarithm of the inhibitor concentration, log C, are represented in Fig. 2. These curves are of S-shape obeying the Frumkin adsorption isotherm [15] and are in good agreement with the Frumkin Eq. (3). As shown
A.S. Fouda et al. / Desalination 201 (2006) 1–13
5
Fig. 2. θ–log c curves for C-steel dissolution in 2 M HCl in the presence of different concentrations of different inhibitors from weight-loss measurements at 30ºC.
from this figure, one can conclude that the degree of surface coverage increases as the concentration of the inhibitor increases and hence, the inhibition efficiency increases. The Frumkin equation is:
[θ /1 − θ] exp ( −2aθ ) = KC
(3)
where a is a molecular interaction parameter depending on the molecular interaction in the adsorption layer and on the degree of heterogeneity of the surface, C is the molar concentration of the inhibitor, K is the equilibrium constant of the adsorption reaction. 3.1.1. Effect of temperature The effect of temperature on both corrosion
and corrosion inhibition of C-steel in 2 M HCl solution in the absence and presence of different concentrations of inhibitors (I–IV) at different temperatures ranging from 30 to 50ºC was investigated. Arrhenius plot [Eq. (4)] of logarithm of the corrosion rate, log k, with the reciprocal of absolute temperature, 1/T, in the absence and presence of 5×10–6 M of inhibitors (I)–(IV) is shown graphically in Fig. 3. ln k = B − ( Ea* / RT )
(4)
where B is a constant depends on the metal type and electrolyte, and R is the universal gas constant. From the slopes of the plots, the respective activation energies (E*a) were calculated and are
6
A.S. Fouda et al. / Desalination 201 (2006) 1–13 Table 3 Activation parameters for the dissolution of C-steel in 2 M HCl in the absence and presence of 5×10–6 M of different inhibitors Inhibitor
Ea* (KJ mol–1)
∆H* (KJ mol–1)
–∆S* (J mol–1 K–1)
Blank I II III IV
30.2 34.6 34.2 33.2 32.5
27.6 32.1 31.6 30.6 29.9
180.1 175.9 175.5 177.8 179.2
tion, on the one hand, and decreases with the rise of temperature, on the other hand. This means that these compounds are adsorbed physically on the C-steel surface. Enthalpy and entropy of activation (∆H*, ∆S*) were calculated from the transition state theory [17] (Table 3): k = ( RT / Nh ) exp ( ∆S * / R ) exp ( −∆H * / RT ) (5)
Fig. 3. log corrosion rate (k) vs.1/T for C-steel in the presence of 5×10–6 M inhibitor compounds.
tabulated in Table 3. The results show that the values of activation energy increase in the same order as the increase of the inhibition efficiency of the inhibitors. It is also indicated that the whole process is controlled by surface reaction, since the energy of the activation corrosion process is over 20 KJ mol–1 [16]. From the results of the effect of temperature, it was observed that the inhibition efficiency decreases with increasing the inhibitor concentra-
where h is the Plank constant, N is the Avogadros number. A plot of log (k/T) vs. (1/T), [Eq. (5)], gave straight lines as shown in Fig. 4 for C-steel dissolution in 2 M HCl in the absence and presence of 5×10–6 M of inhibitors (I). The order of the inhibition efficiency of the investigated compounds as gathered from the increase in E*a and ∆H* and the decrease in ∆S** values is the same as above. E*a and ∆H* of the inhibition process of C-steel in 2 M HCl in the presence of inhibitors are nearly the same (or slightly higher) as those in free 2 M HCl solution, indicating that no energy barrier is attained. These data reveal that the inhibition of the corrosion reactions is affected without changing the mechanism. The entropy of activation in the presence and absence of the inhibitor is large and negative. This implies that the activated complex in the rate-determining step represents association rather than dissociation, indicating that a decrease in disorder takes place, going from reactant to the activated complex.
A.S. Fouda et al. / Desalination 201 (2006) 1–13
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Fig. 4. log corrosion rate/T (k/T) vs.1/T for Csteel in the presence of 5×10–6 M inhibitor compounds.
3.1.2. Synergistic effect The corrosion behavior of C-steel in 2 M hydrochloric acid solution in the presence of 1×10–3 M potassium bromide, potassium thiocyanate and potassium iodide at different concentrations of inhibitors (I)–(VII) was studied. The obtained weight-loss time values are represented and summarized in Table 4. From these values, it is observed that % In of the inhibitors increases with increasing the concentration of inhibitors due to synergistic effects [18]. The synergistic effect of these
anions (Br–, SCN– and I–) has been observed [19]. This effect depends on the type of anion. From the results it was found that the order of decreasing the inhibition efficiency in the presence of these anions is KI > KSCN > KBr. The strong chemisorptions of (Br–, SCN– and I–) anions on the metal surface are responsible for the synergistic effect of bromide, thiocyanate and iodide anions in combination with cation of the inhibitor. The cation is then adsorbed by columbic attraction on the metal surface where bromide, thiocyanate and iodide
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Table 4 Inhibition efficiency of different inhibitors for C-steel in 2 M HCl containing 1×10–3 M KBr/KSCN/KI at 30ºC for an exposure period of 150 min Anions Concentration % Inhibition efficiency (M) I II III IV KBr
0.0 1×10–6 3×10–6 5×10–6 7×10–6 9×10–6 11×10–6
20.3 68.6 71.8 74.8 78.1 80.2 82.3
20.3 62.7 65.3 67.0 70.4 72.7 74.5
20.3 58.2 60.5 62.2 64.7 67.3 68.7
20.3 54.6 57.1 59.1 60.8 63.0 65.3
KSCN
0.0 1×10–6 3×10–6 5×10–6 7×10–6 9×10–6 11×10–6
28.4 76.7 78.5 81.2 83.0 85.3 86.7
28.4 68.4 70.1 72.4 73.8 75.8 77.8
28.4 63.0 64.5 66.7 68.2 70.1 71.8
28.4 59.7 61.1 62.8 64.8 67.0 68.7
KI
0.0 1×10–6 3×10–6 5×10–6 7×10–6 9×10–6 11×10–6
40.1 84.0 85.3 86.7 88.1 90.1 91.8
40.1 75.3 77.0 78.4 80.1 81.8 83.2
40.1 69.9 71.6 73.3 75.0 77.0 78.7
40.1 67.0 68.5 70.2 71.6 73.0 74.7
anions are already adsorbed by chemisorption. Stabilization of adsorbed iodide, bromide and thiocyanate anions with cations leads to a greater surface coverage and therefore greater inhibition. One can conclude that the addition of iodide, bromide and thiocyanate ions enhances the inhibition efficiency to a considerable extent, due to the increase of the surface coverage in the presence of iodide, bromide and thiocyanate ions. The order of investigated compounds remains unchanged, as before. From the previous results it is known that KI could be considered as one of the effective anions for synergistic action within the investigated inhibitors. 3.2. Galvanostatic polarization measurements Fig. 5 shows the galvanostatic polarization
curves for C-steel dissolution in 2 M HCl in the absence and presence of different concentrations of inhibitor (I) at 30ºC. Similar curves were obtained for other inhibitors (not shown). The numerical values of the variation of the corrosion current density (icorr.), the corrosion potential (Ecorr.), Tafel slopes (βa and βc), ), the degree of surface coverage (θ) and the inhibition efficiency (% In) with the concentrations of inhibitors (I)–(IV) are given in Table 5. The results indicate that: 1. The cathodic and anodic curves obtained exhibit Tafel-type behavior. Addition of pyrazolone derivatives increased both the cathodic and anodic overvoltages and the presence of pyrazolone derivatives in solution inhibits both the hydrogen evolution and the anodic dissolution processes. 2. The corrosion current density (icorr.) decreases with increasing the concentrations of the pyrazolone derivatives which indicates that these compounds act as inhibitors, and the degree of inhibition depends on the concentration and type of inhibitors present. 3. The slopes of anodic and cathodic Tafel lines (βa and βc) ), were slightly changed on increasing the concentration of the tested compounds. This indicates that there is no change of the mechanism of inhibition in the presence and absence of inhibitors. The pyrazolone derivatives are mixed-type inhibitors, but the cathode is more polarized than the anode when an external current is applied. 4. The orders of inhibition efficiency of all inhibitors at different concentrations as given by polarization measurements are listed in Table 5. The results are in good agreement with those obtained from weight-loss measurements. 3.3. Chemical structure and corrosion inhibition Variation in the structure of inhibitor molecules (I–IV) takes place through the phenylazo group. So, the inhibition efficiency will depend on this part of the molecule.
A.S. Fouda et al. / Desalination 201 (2006) 1–13
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Fig. 5. Polarization curves for the dissolution of C-steel in 2 M HCl in the presence and absence of different concentrations of compound I at 30ºC.
A skeletal representation of the proposed mode of adsorption of studied compounds is shown in Fig. 6 and it clearly indicates the active adsorption centers. This order of increased inhibition efficiency of the additives can be accounted for in terms of the polar effect [20] of the p-substituents on the phenylazo group. This behavior can be rationalized on the basis of the structure–corrosion inhibition relationship of organic compounds. Linear free energy relationship (LFER) has previously been used to correlate the inhibition efficiency of organic compounds with their Hammett substi-
tuent constant (σ) [21], the LFER or Hammett relation is given by [22,23]: log k or % In = −ρ ⋅ σ
(6)
where ρ is the reaction constant. Those substituents which attract electrons from the reaction center are assigned positive σ values and those which are electron donating have negative σ values. Thus σ is a relative measure of the electron density at the reaction center. The slope of the plot of % In or log k rate of corrosion vs. σ is ρ, and its sign indicates whether the process is
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Table 5 Electrochemical parameters for C-steel in 2 M HCl with and without different concentrations of inhibitors at 30°C Inhibitors
Concentration (M)
–Ecorr (mV)
icorr (mA cm–2)
ßa (mV dec–1)
–ßc θ (mV dec–1)
% In
I
0.00 1×10–6 3×10–6 5×10–6 7×10–6 9×10–6 11×10–6 1×10–6 3×10–6 5×10–6 7×10–6 9×10–6 11×10–6 1×10–6 3×10–6 5×10–6 7×10–6 9×10–6 11×10–6 1×10–6 3×10–6 5×10–6 7×10–6 9×10–6 11×10–6
468 458 457 457 456 455 455 460 460 459 458 458 459 462 461 461 460 459 460 463 462 461 461 462 461
179.2 77.9 68.0 57.5 51.6 47.1 44.9 83.1 74.8 68.5 62.8 57.5 55.0 91.1 83.1 77.1 72.8 68.2 65.6 96.7 91.5 86.8 83.0 78.9 76.6
62 75 77 78 80 83 85 73 75 77 79 81 83 70 73 76 78 80 80 68 72 75 77 79 80
107 122 125 128 130 131 133 120 122 125 127 128 130 116 119 122 124 126 127 114 116 119 121 122 124
— 56.5 62.1 67.9 71.2 73.7 75.0 53.6 58.3 61.8 64.9 67.9 69.3 49.2 53.6 56.9 59.4 62.0 63.4 46.1 49.0 51.6 53.7 56.0 57.3
II
III
IV
N
N
H3C N
N
— 0.57 0.62 0.68 0.71 0.74 0.75 0.54 0.58 0.62 0.65 0.68 0.69 0.49 0.54 0.57 0.59 0.62 0.63 0.46 0.49 0.52 0.54 0.56 0.57
X where: X = = = =
O
H
//////////
OCH3 (I) CH3 (II) H (III) NO2 (IV)
Fig. 6. Skeletal representation of the mode of adsorption of inhibitor compounds.
inhibited by an increase or a decrease of the electron density at the reaction center. The magnitude of ρ indicates the relative sensitivity of the inhibition process to electronic effects. The results
shown in Fig. 7 indicate that pyrazolone derivatives give a good correlation line (ρ = –11.7) which shows a weak dependence of adsorption character of the reaction center on the electron density of
A.S. Fouda et al. / Desalination 201 (2006) 1–13
11
Fig. 7. Variation of the inhibition efficiency (In %) with the substitutent constant (σ) of the substitutent group in para position on the phenyl ring of inhibitor compounds.
the ring, and electron-releasing substituents result in an increase in inhibition. The weak dependence of the adsorption character of the reaction center of pyrazolone derivatives on the electron density of the ring may be due to the fact that the centers of adsorption are not thoroughly conjugated to the ring. Compound (I) is the most efficient inhibitor because of the presence of highly electron releasing p–OCH3 group (σ = –0.27) which enhances the delocalized π-electrons on the molecule and also may add an additional active center to the molecule due to its oxygen atom. Compound (II) comes after (I) in the inhibition efficiency. Because it has p–CH3 in
the phenylazo group with (σ = –0.17) lower Hammett constant and hence lower sharing electron density to the molecule. Compounds (III) and (IV) come after (I) and (II) because they have positive σ values (σH = 0 and σNO = +0.78), and hence 2 lower electron sharing to their molecules. The inhibition efficiency of these molecules is parallel to the increased order of electron withdrawing (electrophilic) character of these groups. Due to the weak dependence of the adsorption character of the reaction center on the polar effect of the substituent groups, one can explain the order of the inhibition efficiency. From the structural organic point of view, both methyl and methoxy
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groups have +R effect but the inductive effect is +I and –I, respectively. Although the methyl group has +R, but its effect is very little as a result of hyperconjugation, thus: H H
C
N
N
H+ H2C
N
N
H
In the case of methoxy group, the effect of +R is large and also the inductive effect, –I, thus:
H3CO
N
N
+ H 3CO
N
N
H3CO
N
N
H3CO
N
N
Also methoxy group may add an additional active center and this increases the inhibition efficiency in the case of methoxy group than in the case of methyl group. On the other hand, the lower efficiency of nitro derivatives as compared to methyl and methoxy derivatives may be due to i) their highest electrophilic character; ii) their reduction in acid medium and iii) the evolved heat of hydrogenation may aid the desorption of the molecules. 4. Conclusions 1. The compounds under investigation, the pyrazolone derivatives, are fairly efficient inhibitors for C-steel dissolution in 2 M HCl. 2. The adsorption of these compounds on the C-steel surface was found to obey Frumkin’s adsorption isotherm. 3. From the effect of temperature, the activation parameters for the corrosion process (E*a, ∆H* and ∆S**) were calculated.
4. Percentage inhibition (% In) increased in the presence of 1×10–3 M KI, KBr and KSCN due to the synergistic effect. 5. Galvanostatic polarization data indicate that these compounds are mixed-type inhibitors, but the cathode is more polarized than the anode when an external current is applied. 6. The order of the inhibition efficiency of all inhibitors as given by polarization measurements is in good agreement with that obtained from weight-loss measurements. This order was explained on the basis of the chemical structure and adsorption active centers of the compound.
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