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Tween-40 as corrosion inhibitor for cold rolled steel in sulphuric acid: Weight loss study, electrochemical characterization, and AFM
Applied Surface Science 252 (2005) 1254–1265 www.elsevier.com/locate/apsusc
Tween-40 as corrosion inhibitor for cold rolled steel in sulphuric acid: Weight loss study, electrochemical characterization, and AFM Xianghong Li, Guannan Mu * Department of Chemistry, Yunnan University, Kunming 650091, PR China Received 16 December 2004; accepted 9 February 2005 Available online 20 April 2005
Abstract The inhibition action of a non-ionic surfactant of tween-40 on the corrosion of cold rolled steel (CRS) in 0.5–7.0 M sulphuric acid (H2SO4) was studied by weight loss and potentiodynamic polarization methods. Atomic force microscope (AFM) provided the surface conditions. The inhibition efficiency increases with the tween-40 concentration, while decreases with the sulphuric acid concentration. The adsorption of inhibitor on the cold rolled steel surface obeys the Langmuir adsorption isotherm equation. Effect of immersion time was studied and discussed. The effect of temperature on the corrosion behavior of cold rolled steel was also studied at four temperatures ranging from 30 to 60 8C, the thermodynamic parameters such as adsorption heat, adsorption free energy and adsorption entropy were calculated. A kinetic study of cold rolled steel in uninhibited and inhibited acid was also discussed. The kinetic parameters such as apparent activation energy, pre-exponential factor, rate constant, and reaction constant were calculated for the reactions of corrosion. The inhibition effect is satisfactorily explained by both thermodynamic and kinetic parameters. Polarization curves show that tween-40 is a cathodic-type inhibitor in sulphuric acid. The results obtained from weight loss and potentiodynamic polarization are in good agreement, and the tween-40 inhibition action could also be evidenced by surface AFM images. # 2005 Elsevier B.V. All rights reserved. Keywords: Tween-40; Corrosion inhibitor; Cold rolled steel; AFM; Sulphuric acid; Adsorption
1. Introduction Corrosion is a fundamental process playing an important role in economics and safety, particularly for metals. The use of inhibitors is one of the most * Corresponding author. E-mail address: [email protected] (G. Mu).
practical methods for protection against corrosion, especially in acidic media [1]. Most well-known acid inhibitors are organic compounds containing nitrogen, sulphur, and oxygen atoms. Among them, the surfactant inhibitor has many advantages such as high inhibition efficiency, low price, low toxicity, and easy production [2–7]. Ionic surfactants have been used for the corrosion inhibition of iron [8–15], copper
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.02.118
X. Li, G. Mu / Applied Surface Science 252 (2005) 1254–1265
[16–19], aluminum [20–23], and other metals [24,25] in different corroding media. The adsorption of the surfactant on the metal surface can markedly change the corrosion-resisting property of the metal [26,27], and so the study of the relations between the adsorption and corrosion inhibition is of great importance. Non-ionic surfactants have shown a high inhibition efficiency for iron corrosion in both HCl [28,29] and H2SO4 [30] solutions. However, higher attentions have been revealed for the study of the inhibitive effects of non-ionic surfactants in HCl medium, lower attentions in H2SO4 medium, especially in a wide concentration range of H2SO4 solution. On the other hand, as a nonionic surfactant, tween-40 was rarely studied as an inhibitor for cold rolled steel in sulphuric acid. For these reasons, based on the proceeding papers, the objective of the present work is to investigate the inhibition action of tween-40 in 0.5–7.0 M H2SO4 at 30–60 8C, so as to study inhibitive mechanism of tween-40 for cold rolled steel in sulphuric acid.
2. Experimental method 2.1. Materials The chemical composition of the cold rolled steel (CRS) is listed in Table 1. 2.2. Inhibitor Tween-40 was obtained from Shanghai Chemical Reagent Company of China. Fig. 1 shows the molecular structure of the tween-40. It is obvious that tween-40 is a O-heterocyclic compound. The main functional group is hydroxyl. The molecular weight of tween-40 is also high because of a number of units of CH2CH2O. Table 1 Chemical composition (wt.%) of cold rolled steel
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Fig. 1. Chemical molecular structure of tween-40.
2.3. Solutions The aggressive solutions, 0.5–7.0 M H2SO4 were prepared by dilution of analytical grade 98% H2SO4 with distilled water. The concentration range of inhibitor used was 5–100 mg l1. 2.4. Gravimetric measurements Three parallel cold rolled steel sheets of 2.5 cm 2.0 cm 0.06 cm were abraded with emery paper (grade 320-500-800) and then washed with distilled water and acetone. After weighing accurately, the specimens were immersed in 250 ml beaker, which contained 250 ml sulphuric acid with and without addition of different concentrations of tween-40. All the aggressive acid solutions were open to air. After 6 h, the specimens were taken out, washed, dried, and weighed accurately. The average weight loss of three parallel CRS sheets could be obtained. Then the tests were repeated at different temperatures. The inhibition efficiency (IE) of tween-40 on the corrosion of CRS was calculated as follows [31]: IE% ¼
W0 W 100 W0
(1)
where W0 and W are the values of the average weight loss without and with addition of the inhibitor, respectively. 2.5. Polarization measurements
Element
wt.%
C Mn P S Si Al Fe
0.07 0.3 0.022 0.01 0.01 0.03 Rest
Polarization experiments were carried out in a conventional three-electrode cell with a platinum counter electrode (CE) and a saturated calomel electrode (SCE) coupled to a fine Luggin capillary as the reference electrode. The working electrode (WE) was in the form of a square cut from CRS embedded in epoxy resin of polytetrafluoroethylene
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(PTFE) so that the flat surface was the only surface in the electrode. The working surface area was 1.0 cm 1.0 cm. Before measurement, the electrode was immersed in test solution at natural potential for 2 h until a steady state was reached. All polarization curves were recorded by a PARSTAT 2263 potentiostat at 30 8C. The potential increased with a speed of 30 mV min1 and started from potential of 250 to +250 mV versus SCE. IE% was defined as: IE% ¼
Icorr IcorrðinhÞ 100 Icorr
(2)
where Icorr and Icorr(inh) are the uninhibited and inhibited corrosion current density values, respectively, determined by extrapolation of Tafel lines to the corrosion potential. 2.6. Atomic force microscope (AFM) The CRS specimens of size 1.0 cm 1.0 cm 0.06 cm were abraded with emery paper (grade 320500-800) and gave a mirror surface, then washed with distilled water and acetone. After immersion in 1.0 M H2SO4 without and with addition of 100 mg l1 tween-40 at 30 8C for 6 h, the specimen was cleaned with distilled water, dried with a cold air blaster, and then used for a Japan instrument model SPA-400 SPM Unit atomic force microscope examinations.
Fig. 2. Relationship between weight loss (W) and concentration of tween-40 (C) in 1.0 M H2SO4.
3.2. Effect of inhibitor concentration and temperature on inhibition efficiency The values of inhibition efficiencies obtained from the weight loss for different inhibitor concentrations in 1.0 M H2SO4 were given in Fig. 3. The results showed that inhibition efficiency increased as the concentration of inhibitor increased from 5 to 100 mg l1.
3. Experimental results and discussion 3.1. Effect of tween-40 on the weight loss The weight loss curves of cold rolled steel with the addition of tween-40 in 1.0 M H2SO4 at various temperature are shown in Fig. 2. The curves in Fig. 2 show that the weight loss values (g) of CRS in 1.0 M H2SO4 solution containing tween-40 decrease as the concentration of the inhibitor increase, i.e. the corrosion inhibition strengthen with the non-ionic surfactant concentration. This trend, it may result from the fact that adsorption amount and the coverage of surfactant on the CRS increases with the inhibitor concentration thus the CRS surface is efficiently separated from the medium [20,32].
Fig. 3. Relationship between inhibition efficiency (IE) and concentration of tween-40 (C) in 1.0 M H2SO4.
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The maximum inhibition efficiency was 91%. The inhibition was estimated to be 81% at 30 8C even at very low concentration (20 mg l1), and at 50 mg l1 concentration its protection was >80%. These better performances can be explained as follows: Fig. 1 shows that the molecular weight of tween-40 is high, thus tween-40 can relatively easily adsorb on the CRS surface by van der Waals force. In addition, the main hydrophilic part CHO(CH2CH2O)nCH2CH2OH of tween-40 attacks the CRS surface while the main hydrophobic part CHCH2OCO(CH2)14CH3 extends to the solution face. In addition, tween-40 may chemisorb at steel/solution interface via hydrogen bond between the OH groups in tween-40 molecules and water molecules adsorbed on the surface. When tween-40 adsorbed on metal surface, coordinate bond may be formed by partial transference of electrons from the polar atom (O atom) of tween-40 to the metal surface. Fig. 3 also gives that the inhibition efficiencies decreased with the experimental temperature in general, which indicated that the higher temperatures might cause desorption of tween-40 from the steel surface. It should be noted that the inhibition efficiencies at 40 8C are higher that those at 30 8C when the inhibitor concentrations are higher than 40 mg l1, and the maximum inhibition efficiency is found at 50 8C when tween-40 concentration is 100 mg l1, which also indicates that the adsorption of tween-40 is not merely a physical or a chemical adsorption but a comprehensive adsorption [33].
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Table 2 Parameters of the linear regression between C/u and C Temperature (8C)
Linear regression coefficient (r)
K (l mg1)
Slope
30 40 50 60
0.9999 0.9985 0.9873 0.9986
1.0024 0.1691 0.1160 0.0791
0.9912 0.9312 0.9135 0.9343
From the values of surface coverage, the linear regressions between C/u and C were calculated by the computer, and the parameters are listed in Table 2. Fig. 4 shows the relationship between C/u and C at 30 8C. These results show that all the linear correlation coefficients (r) are almost equal to 1 and all the slopes are very close to 1, which indicates the adsorption of inhibitor onto steel surface accords with the Langmuir adsorption isotherm. The result indicated that there was no interactions among the adsorbed species [36,37]. Table 2 also gives that the adsorptive equilibrium constant (K) values (l mg1) decrease with increasing temperature, which indicates that it is easily adsorbed strongly onto the steel surface for the inhibitor at lower temperatures. But when the temperature is relatively higher, the adsorbed inhibitors tend to desorption.
3.3. Adsorption isotherm Assuming the adsorption of tween-40 on the CRS surface obeys Langmuir adsorption isothermal equation [20,34]: C 1 ¼ þC u K
(3)
where C is the concentration of inhibitor, K the adsorptive equilibrium constant and u is the surface coverage. u was calculated by the Sekine and Hirakawa’s method [35]: u¼
W0 W W0 Wm
where Wm is the smallest weight loss.
(4) Fig. 4. The relationship between C/u and C at 30 8C.
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heat (DH0) under experimental condition [20,38]. The standard adsorption free energy (DG0) was obtained according to [39]: 1 DG0 exp K¼ 55:5 RT
(6)
where 55.5 is the concentration of water in solution expressed in mol l1. Thus, the adsorption constant K in l mg1 should change into l mol1. Because tween40 is also a polymer, it is difficult to know the exact molecular weight. Assuming the molecular weight of tween-40 is Mw in g mol1. The changed adsorption constant in l mol1 was listed in Table 3. Then the standard adsorption entropy (DS0) can be obtained by the thermodynamic basic equation: Fig. 5. The relationship between ln K and 1/T.
DS0 ¼
DH 0 DG0 T
(7)
3.4. Thermodynamic parameters Thermodynamic parameters are important to study the inhibitive mechanism. The adsorption heat could be calculated according to the Van’t Hoff equation [20,34]: ln K ¼
DH þ constant RT
(5)
where DH and K are the adsorption heat and adsorptive equilibrium constant, respectively. It should be noted that DH/R is the slope of the straight line ln K 1/T according to Eq. (5) and the molecular weight of inhibitor is also a positive constant, so the value of adsorption heat value does not change with the unit of inhibitor concentration. To obtain the adsorption heat, the regression between ln K and 1/T was dealt with. Fig. 5 is the straight line ln K 1/T. The adsorption heat (DH) can be approximately regarded as the standard adsorption
All the calculated thermodynamic parameters are listed in Table 3. The standard adsorption heat (76.15 kJ mol1) in Table 3 shows that a comprehensive adsorption (physical and chemical adsorption) might occur [34]. The negative values of DH0 also show that the adsorption of inhibitor is an exothermic process [40], which indicates that IEs decrease with increasing the temperature. The negative values of DG0 suggest that the adsorption of inhibitor molecule onto steel surface is a spontaneous process. As for the value of DS0 in Table 3, because the molecular weight (Mw) of tween-40 is about 1181–1357 g mol1 [41], the sign of DS0 is negative. The negative values of DS0 mean that the process of adsorption is accompanied by a decrease in entropy. It might be explained as follows: before the adsorption of tween-40 onto the steel surface, the chaotic degree of steel surface is high, but when inhibitor molecules were orderly adsorbed onto the steel surface, as a result, a decrease in entropy.
Table 3 The thermodynamic parameters of adsorption of tween-40 on the steel surface Temperature (8C)
K (l mol1)
DG0(kJ mol1)
DH0 (kJ mol1)
DS0 (J mol1 K1)
30 40 50 60
1.0024 103 Mw 0.1691 103 Mw 0.1160 103 Mw 0.0791 103 Mw
27.78 ln Mw 23.81 ln Mw 23.56 ln Mw 23.23 ln Mw
76.15 76.15 76.15 76.15
159.56 + 3.30 ln Mw 167.14 + 3.19 ln Mw 162.74 + 3.09 ln Mw 158.85 + 3.01 ln Mw
Mw: the molecular weight of tween-40, 1181–1357 g mol1.
X. Li, G. Mu / Applied Surface Science 252 (2005) 1254–1265 Table 4 Parameters of the regression between ln n and 1/T
3.5. Apparent activation energy (Ea) and pre-exponential factor (A) It has been reported by a number of authors [23,28,30,33,34] that for the acid corrosion of steel, the natural logarithm of the corrosion rate (g m2 h1) is a linear function with 1/T (following Arrhenius equation): ln n ¼
Ea þ ln A RT
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(8)
where Ea represents the apparent activation energy, R the gas constant, T the temperature, A the pre-exponential factor, and n is the corrosion rate. It can be calculated by the relation:
C (mg l1)
Ea (kJ mol1)
A (g m2 h1)
Linear regression coefficient (r)
0 5 10 20 30 40 50 70 100
54.95 71.79 75.40 80.46 77.64 77.23 74.61 68.63 60.42
9.20 1010 2.88 1013 8.86 1013 5.03 1014 1.49 1014 1.08 1014 3.58 1013 3.01 1012 1.13 1011
0.9999 0.9993 0.9951 0.9946 0.9979 0.9723 0.9720 0.9399 0.8724
where W is the weight loss, S the area of the rectangular steel, and t is the corrosion time (6 h). The regression between ln n and 1/T was calculated by computer, and Arrhenius plots of ln n versus 1/T for the blank and different concentrations of the non-ionic surfactant were shown in Fig. 6. All the parameters were calculated and given in Table 4. It was clear that the values of Ea in the presence of the tween-40 are higher than those in the uninhibited
acid solution. These results are according with the reported studies [28,42]. The increase of Ea in the presence of the inhibitor indicates the physical adsorption or weak chemical bonding between the tween-40 molecules and the steel surface [43]. According to Eq. (8), it can be seen that the lower pre-exponential factor A and the higher Ea lead to the lower corrosion rate (n). For the present study, the values of A in the presence of tween-40 are higher than those of in the absence of tween-40. Therefore, the decrease in steel corrosion rate is mostly decided by the apparent activation energy. The relationship of Ea and ln A with the inhibitor concentration is shown in Fig. 7. It was clear that both Ea and ln A increased in
Fig. 6. Arrhenius plots related to the corrosion rate of cold rolled steel for various concentrations of tween-40 in 1.0 M H2SO4 ((*) blank, (*) 5 mg l1, (~) 10 mg l1 and (~) 50 mg l1).
Fig. 7. The relationship of Ea and ln A with the concentration of tween-40 in 1.0 M H2SO4.
n¼
W St
(9)
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Table 5 Effect of immersion time on the corrosion rate of cold rolled steel in 1.0 M H2SO4 at 30 8C Immersion time (h)
1 3 6 12 24 48 72 96
Corrosion rate, n (g m2 h1) Blank
50 mg l1 tween-40
100 mg l1 tween-40
30.90 32.67 31.43 29.38 29.18 29.25 26.52 26.18
13.36 7.92 6.36 5.73 3.43 2.87 5.65 7.01
12.08 7.11 6.15 4.21 2.51 2.14 1.41 1.30
the presence of tween-40. When the inhibitor concentrations reached 20 mg l1, the values reach maximum, and then begin to decrease. Fig. 7 clearly shows that there is a ‘‘peak-like’’ value for this study. That is to say, the apparent activation energy acts as a function of inhibitor concentration [34,38]. 3.6. Effect of immersion time on inhibition efficiency Effect of immersion time on corrosion inhibition of tween-40 at concentrations of 50 and 100 mg l1 on the corrosion of CRS in 1.0 M H2SO4 at 30 8C was studied. Table 5 shows the corrosion rates obtained in the absence and presence of tween-40 act as a function of immersion time. In the absence of tween-40, the corrosion rate does not change obviously with the immersion time, and the corrosion rate decreases slightly after 72 h. However, the corrosion rate decreased obviously in the presence of the inhibitor, which may be ascribed to the heteroatom (oxygen atom) of the inhibitor molecule [44,45]. It should be noted that the corrosion rate began to increase after 48 h when the tween-40 concentration was 50 mg l1. Fig. 8 shows the effect of changing immersion time (1–96 h) at 30 8C on the inhibition efficiency of tween-40 at 50 and 100 mg l1. It can be seen from Fig. 8 the inhibition efficiency is higher than 55% when the immersion time is only 1 h, which indicates the adsorption rate of tween-40 adsorb on the steel surface is relatively high. Fig. 8 shows that tween-40 inhibits the corrosion of CRS for all immersion time at both concentrations. At 100 mg l1 increasing immersion time resulted in increasing IE, and the
Fig. 8. Effect of change immersion time on IE of tween-40 for the steel at 30 8C in 1.0 M H2SO4.
maximum IE was 95% at 96 h. The high inhibition efficiency with longer immersion time can be attributed to the formation of a protective film is time-dependent on the CRS surface. It has been stated that stable, two-dimensional layers of inhibitor molecules are formed on metal surfaces after longer immersion time [46]. At 50 mg l1, increasing immersion time resulted in increasing IE, and the maximum IE was 92% at 48 h, but after 48 h, IE began to decrease slightly. 3.7. Effect of sulphuric acid concentration on inhibition efficiency Fig. 9 shows the effect of changing H2SO4 concentration (0.5–7.0 M) at 30 8C on the inhibition efficiency of tween-40 at 50 and 100 mg l1. In both cases, increasing acid concentration resulted in decreasing IE, and the minimum IE was 43% in 6.0 M H2SO4 at 50 mg l1. Fig. 9 also clearly shows the changed degree of IE with H2SO4 concentration can be divided into three stages. The IE decreased obviously with the acid concentration in 0.5–1.5 M, then IE reached did not decrease obviously with the acid concentration in 2.0–6.0 M. But when the acid concentration was higher than 6.0 M, IE began to increase slightly with the acid concentration (6.0–7.0 M).
X. Li, G. Mu / Applied Surface Science 252 (2005) 1254–1265
Fig. 9. Effect of change in H2SO4 concentration on IE of tween-40 for the steel at 30 8C.
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Fig. 10. Variation of ln n with the concentration of sulphuric acid (C) at 30 8C.
3.8. Rate constant and reaction constant Assuming the corrosion rate against the molar concentration of acid concentration obeys the expression proposed by Mathur and Vasudevan [47]: ln n ¼ ln k þ BC
(10)
where k is the rate constant, B the reaction constant, and C is the molar concentration of H2SO4. Fig. 10 shows the curves of ln n versus C in different conditions. Fig. 10 shows that the graph of ln n C in uninhibited H2SO4 is a straight line. However, a break point appears at 2.0 M H2SO4 after adding tween-40. The straight lines show that the kinetic parameters could be calculated by Eq. (10), and listed in Table 6. Eq. (10) shows that k can be regarded as a commencing rate at zero acid concentration, so k means the ability of corrosion of H2SO4 for CRS [47– 49]. Table 6 clearly shows that k decreases obviously after adding tween-40 in H2SO4 solution, which indicates tween-40 is a good inhibitor in H2SO4. Furthermore, the values of k in 0.5–2.0 M H2SO4 inhibited solution are smaller than those in 2.0–7.0 M H2SO4 inhibited solution, which indicates that the values of IE in 0.5–2.0 M H2SO4 are more superior to those in 2.0–7.0 M H2SO4 (Fig. 9). According to Eq. (10), B is the slope of the line ln n C, so B indicated the changed extent of n with the acid
concentration. The values of B in inhibited H2SO4 are higher than that in uninhibited H2SO4, which indicates that the changed extent of n with C in inhibited H2SO4 is bigger than that in uninhibited H2SO4. Namely, the IE decreases obviously with the acid concentration (Fig. 9). In addition, the values of B in 0.5–2.0 M H2SO4 inhibited solution are higher than those in 2.0– 7.0 M H2SO4 inhibited solution, which indicates that the changed degree of IE in 0.5–2.0 M H2SO4 are greater than those in 2.0–7.0 M H2SO4 (Fig. 9). Also, it could be seen from Table 6 that the values of k at 50 mg l1 are lower than those at 100 mg l1, while Fig. 9 shows that IE at 50 mg l1 are lower than that at 100 mg l1. Thus, it seems the results of Table 5 are contract with those of Fig. 9. In fact, according to Eq. (10), both k and B might affect the steel corrosion Table 6 Calculated values of kinetic parameter for the corrosion of CRS in H2SO4 containing tween-40 at 30 8C C (mg l1)
0 50 100
B (g m2 h1 M1)
k (g m2 h1)
0.5–2.0 M H2SO4
2.0–7.0 M H2SO4
0.5–2.0 M H2SO4
2.0–7.0 M H2SO4
0.38 1.02 0.98
0.38 0.41 0.37
23.67 2.29 2.30
23.67 8.42 8.60
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Fig. 11. Potentiodynamic polarization curves for cold rolled steel in 1.0 M H2SO4 contain in different concentrations of tween-40 at 30 8C.
at a certain acid concentration. Generally speaking, the influence of k on the steel corrosion was bigger than that of B on the steel corrosion. However, if the values of inhibition efficiency are too close, then B might also another important factor to determine the steel corrosion. For the present study, although the values of k at 50 mg l1 are lower than those at 100 mg l1, the values B at 50 mg l1 are higher than those at 100 mg l1, and the variance of B is bigger than that of k. As a result, the IE at 50 mg l1 is lower than that at 100 mg l1. 3.9. Polarization studies Both anodic and cathodic polarization curves for cold rolled steel in H2SO4 at various concentrations of tween-40 are shown in Fig. 11. It is clear that the presence of inhibitor causes a markedly decrease in the corrosion rate, i.e. shifts the cathodic curves to more negative potentials. The inhibitor has a significant
effect on the rate of hydrogen evolution reaction. On the other hand, the non-ionic surfactant has slight effect on the anodic curves. It may be concluded that tween-40 acts as a cathodic-type inhibitor. The values of corrosion current densities (Icorr), corrosion potential (Ecorr), the cathodic Tafel slope (bc), anodic Tafel slope (ba), and the inhibition efficiency (IE) as functions of tween-40 concentration were calculated from the curves (Fig. 11) and given in Table 7. Table 7 reveals that the corrosion current decreases obviously after adding tween-40 in 1.0 M H2SO4, and IE increases with the inhibitor concentration. The corrosion potential (Ecorr) do not change obviously when tween-40 concentration is lower (10 and 50 mg l1), while the corrosion potential (Ecorr) value shifts to negative potential when the concentration is 100 mg l1. The cathodic Tafel slopes changed upon addition of increasing inhibitor concentration, while the anodic Tafel slopes did not changed obviously upon addition of tween-40. Therefore, tween-40 can be arranged as cathodic-type inhibitor in H2SO4. From Table 7, it can be concluded that inhibition efficiencies obtained from weight loss, electrochemical polarization curves are in good agreement. 3.10. Atomic force microscope surface examination The atomic force microscope provides a powerful means of characterizing the microstructure [50–53]. The three-dimensional AFM images of cold rolled steel surface in 1.0 M H2SO4 are shown Fig. 12. Fig. 13 illustrates the corresponding two-dimensional AFM images. As can be seen from Fig. 12(a), that the corrosion of CRS steel samples in the absence of inhibitor appeared to be relatively uniform in general and some parts to a low-mound-like structure. The image is quite different from the pitting corrosion image [52] or the corrosion in HClO4–CH3COOH
Table 7 Potentiodynamic polarization parameters for the corrosion of cold rolled steel in 1.0 M H2SO4 containing different contentrations of tween-40 at 30 8C C (mg l1)
Ecorr (mV)
Icorr (mA cm2)
bc (mV dec1)
ba (mV dec1)
IE (%)
0 10 50 100
434 437 438 446
1019.0 265.1 240.2 213.8
137 129 127 118
64 57 61 62
– 73.9 76.4 79.1
X. Li, G. Mu / Applied Surface Science 252 (2005) 1254–1265
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Fig. 12. AFM three-dimensional images of the cold rolled steel (CRS) surface in 1.0 M H2SO4: (a) in the absence of tween-40 and (b) in the presence of 100 mg l1 tween-40.
Fig. 13. AFM two-dimensional images of the cold rolled steel (CRS) surface in 1.0 M H2SO4: (a) in the absence of tween-40 and (b) in the presence of 100 mg l1 tween-40.
solution [54]. However, in the presence of 100 mg l1 tween-40, the surface becomes more flat and closely. Fig. 13 clearly shows the corrosion degree of CRS decreases in the presence of tween-40. Fig. 13(b) shows the spherical or bread-like particles appear the surface, which do not exist in the Fig. 13(a). There-
fore, it may be concluded that these particles are the adsorption film of the inhibitor, which efficiently inhibits the corrosion of CRS. Fig. 14 shows the CRS surface topography in 1.0 M H2SO4. Fig. 15 illustrates the height profiles, which is made along the line marked in corresponding Fig. 14.
Fig. 14. AFM images of the cold rolled steel (CRS) surface topography in 1.0 M H2SO4: (a) in the absence of tween-40 and (b) in the presence of 100 mg l1 tween-40.
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Fig. 15. Height profiles of the CRS surface in 1.0 M H2SO4: (a) in the absence of tween-40 and (b) in the presence of 100 mg l1 tween-40.
It can be seen from Fig. 14(a) that the micrograph of CRS surface after immersion in uninhibited 1.0 H2SO4 shows the main characteristic of uniform corrosion in acidic media [55]. Fig. 14(b) shows that some spherical particles decorating the steel surface and the retention of surface grinding marks, which indicate a macroseopically thin film. Fig. 15(a) shows that the surface roughness of the CRS in uninhibited 1.0 H2SO4 is about 174.61 nm, while in the presence of tween-40, the roughness decreases to 98.59 nm (Fig. 15(b)). Thus, the roughness is consistent with the results shown in Figs. 12 and 13.
apparent activation energy (Ea) and pre-exponential factor (A) also increase after adding tween-40 in sulphuric acid solution. The inhibitive mechanism is mostly decided by the apparent activation energy. 4. Tween-40 acts as a cathodic-type inhibitor in 1.0 M H2SO4. The weight loss and polarization curves are in good agreement. 5. The introduction of tween-40 into 1.0 M H2SO4 solution results in the formation of a film on the CRS surface, which causes the decrease the steel surface roughness and effectively protects steel from corrosion.
4. Conclusion Acknowledgement 1. Tween-40 acts as a good inhibitor for the corrosion of cold rolled steel in 1.0 M H2SO4. The inhibition efficiency values increase with the inhibitor concentration and the immersion time, but decrease with the acid concentration and the temperature. 2. The adsorption of the tween-40 on the cold rolled steel surface obeys the Langmuir adsorption isotherm. The adsorption process is a spontaneous and exothermic process accompanied by a decrease in entropy. 3. The rate constant (k) decreases obviously after adding tween-40 in 0.5–7.0 M H2SO4 solution, while the reaction constant (B) increases, especially in 0.5–2.0 M H2SO4 The values of both
This work was carried out in the frame of a research project funded by the Chinese National Science Foundation (Grant No. 50261004).
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[31] G.N. Mu, T.P. Zhao, M. Liu, T. Gu, Corrosion 52 (1996) 853. [32] A.K. Maayta, N.A.F. Al-Rawashdeh, Corros. Sci. 46 (2004) 1129. [33] B.A. Abd-El-Nabey, E. Khamis, M.Sh. Ramadan, A. El-Gindy, Corrosion 52 (1996) 671. [34] L.B. Tang, G.N. Mu, G.H. Liu, Corros. Sci. 45 (2003) 2251. [35] I. Sekine, Y. Hirakawa, Corrosion 42 (1986) 272. [36] A.Y. El-Etre, Corros. Sci. 43 (2001) 1031. [37] F. Bentiss, M. Lagrene´e, M. Traisnel, J.C. Hornez, Corrosion 55 (1999) 968. [38] G.N. Mu, X.M. Li, F. Li, Mater. Chem. Phys. 86 (2004) 59. [39] E. Khamis, Corrosion 46 (1990) 476. [40] M.K. Gomma, M.H. Wahdan, Mater. Chem. Phys. 39 (1995) 209. [41] Committee of Chinese Industrial Engineering, Dictionary of Chemistry and Chemical Technology, Chemical Industrial Engineering Press, Beijing, 2003, p. 1283. [42] E.S. Ferreira, C. Giacomelli, F.C. Giacomelli, A. Spinelli, Mater. Chem. Phys. 83 (2004) 129. [43] A. Popova, E. Sokolova, S. Raicheva, M. Christov, Corros. Sci. 45 (2003) 33. [44] A.M. Al-Mayouf, A.K. Al-Ameery, A.A. Al-Suhybani, Corrosion 57 (2001) 614. [45] A.M. Al-Mayouf, A.A. Al-Suhybani, A.K. Al-Ameery, Desalination 116 (1998) 25. [46] S.L. Granese, B.M. Rosales, C. Oviedo, J.O. Zerbino, Corros. Sci. 33 (1992) 1439. [47] P.B. Mathur, T. Vasudevan, Corrosion 38 (1982) 171. [48] J.G. Yin, L. Wang, V. Otieno-olego, D.P. Schweinsberg, Corros. Sci. 37 (1995) 975. [49] L. Wang, G.J. Yin, J.G. Yin, Corros. Sci. 43 (2001) 197. [50] A.A. Gewirth, B.K. Niece, Chem. Rev. 97 (1997) 1129. [51] I.C. Oppenherm, D. Trevor, C.E.D. Chidsey, P.L. Trevor, K. Sieradzki, Science 254 (1991) 688. [52] J. Li, D. Lampner, Colloids Surf. A 154 (1999) 227. [53] H.H. Teng, P.M. Dove, C.A. Orme, J.J. De Yoreo, Science 282 (1998) 724. [54] C.A. Huang, W. Lin, S.C. Lin, Corros. Sci. 45 (2003) 2627. [55] J.E. Qu, X.P. Guo, The 13th China Inhibitor Conference, Kunming, 2004, p. 7 (in Chinese).
Tween-40 as corrosion inhibitor for cold rolled steel in sulphuric acid: Weight loss study, electrochemical characterization, and AFM Xianghong Li, Guannan Mu * Department of Chemistry, Yunnan University, Kunming 650091, PR China Received 16 December 2004; accepted 9 February 2005 Available online 20 April 2005
Abstract The inhibition action of a non-ionic surfactant of tween-40 on the corrosion of cold rolled steel (CRS) in 0.5–7.0 M sulphuric acid (H2SO4) was studied by weight loss and potentiodynamic polarization methods. Atomic force microscope (AFM) provided the surface conditions. The inhibition efficiency increases with the tween-40 concentration, while decreases with the sulphuric acid concentration. The adsorption of inhibitor on the cold rolled steel surface obeys the Langmuir adsorption isotherm equation. Effect of immersion time was studied and discussed. The effect of temperature on the corrosion behavior of cold rolled steel was also studied at four temperatures ranging from 30 to 60 8C, the thermodynamic parameters such as adsorption heat, adsorption free energy and adsorption entropy were calculated. A kinetic study of cold rolled steel in uninhibited and inhibited acid was also discussed. The kinetic parameters such as apparent activation energy, pre-exponential factor, rate constant, and reaction constant were calculated for the reactions of corrosion. The inhibition effect is satisfactorily explained by both thermodynamic and kinetic parameters. Polarization curves show that tween-40 is a cathodic-type inhibitor in sulphuric acid. The results obtained from weight loss and potentiodynamic polarization are in good agreement, and the tween-40 inhibition action could also be evidenced by surface AFM images. # 2005 Elsevier B.V. All rights reserved. Keywords: Tween-40; Corrosion inhibitor; Cold rolled steel; AFM; Sulphuric acid; Adsorption
1. Introduction Corrosion is a fundamental process playing an important role in economics and safety, particularly for metals. The use of inhibitors is one of the most * Corresponding author. E-mail address: [email protected] (G. Mu).
practical methods for protection against corrosion, especially in acidic media [1]. Most well-known acid inhibitors are organic compounds containing nitrogen, sulphur, and oxygen atoms. Among them, the surfactant inhibitor has many advantages such as high inhibition efficiency, low price, low toxicity, and easy production [2–7]. Ionic surfactants have been used for the corrosion inhibition of iron [8–15], copper
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.02.118
X. Li, G. Mu / Applied Surface Science 252 (2005) 1254–1265
[16–19], aluminum [20–23], and other metals [24,25] in different corroding media. The adsorption of the surfactant on the metal surface can markedly change the corrosion-resisting property of the metal [26,27], and so the study of the relations between the adsorption and corrosion inhibition is of great importance. Non-ionic surfactants have shown a high inhibition efficiency for iron corrosion in both HCl [28,29] and H2SO4 [30] solutions. However, higher attentions have been revealed for the study of the inhibitive effects of non-ionic surfactants in HCl medium, lower attentions in H2SO4 medium, especially in a wide concentration range of H2SO4 solution. On the other hand, as a nonionic surfactant, tween-40 was rarely studied as an inhibitor for cold rolled steel in sulphuric acid. For these reasons, based on the proceeding papers, the objective of the present work is to investigate the inhibition action of tween-40 in 0.5–7.0 M H2SO4 at 30–60 8C, so as to study inhibitive mechanism of tween-40 for cold rolled steel in sulphuric acid.
2. Experimental method 2.1. Materials The chemical composition of the cold rolled steel (CRS) is listed in Table 1. 2.2. Inhibitor Tween-40 was obtained from Shanghai Chemical Reagent Company of China. Fig. 1 shows the molecular structure of the tween-40. It is obvious that tween-40 is a O-heterocyclic compound. The main functional group is hydroxyl. The molecular weight of tween-40 is also high because of a number of units of CH2CH2O. Table 1 Chemical composition (wt.%) of cold rolled steel
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Fig. 1. Chemical molecular structure of tween-40.
2.3. Solutions The aggressive solutions, 0.5–7.0 M H2SO4 were prepared by dilution of analytical grade 98% H2SO4 with distilled water. The concentration range of inhibitor used was 5–100 mg l1. 2.4. Gravimetric measurements Three parallel cold rolled steel sheets of 2.5 cm 2.0 cm 0.06 cm were abraded with emery paper (grade 320-500-800) and then washed with distilled water and acetone. After weighing accurately, the specimens were immersed in 250 ml beaker, which contained 250 ml sulphuric acid with and without addition of different concentrations of tween-40. All the aggressive acid solutions were open to air. After 6 h, the specimens were taken out, washed, dried, and weighed accurately. The average weight loss of three parallel CRS sheets could be obtained. Then the tests were repeated at different temperatures. The inhibition efficiency (IE) of tween-40 on the corrosion of CRS was calculated as follows [31]: IE% ¼
W0 W 100 W0
(1)
where W0 and W are the values of the average weight loss without and with addition of the inhibitor, respectively. 2.5. Polarization measurements
Element
wt.%
C Mn P S Si Al Fe
0.07 0.3 0.022 0.01 0.01 0.03 Rest
Polarization experiments were carried out in a conventional three-electrode cell with a platinum counter electrode (CE) and a saturated calomel electrode (SCE) coupled to a fine Luggin capillary as the reference electrode. The working electrode (WE) was in the form of a square cut from CRS embedded in epoxy resin of polytetrafluoroethylene
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(PTFE) so that the flat surface was the only surface in the electrode. The working surface area was 1.0 cm 1.0 cm. Before measurement, the electrode was immersed in test solution at natural potential for 2 h until a steady state was reached. All polarization curves were recorded by a PARSTAT 2263 potentiostat at 30 8C. The potential increased with a speed of 30 mV min1 and started from potential of 250 to +250 mV versus SCE. IE% was defined as: IE% ¼
Icorr IcorrðinhÞ 100 Icorr
(2)
where Icorr and Icorr(inh) are the uninhibited and inhibited corrosion current density values, respectively, determined by extrapolation of Tafel lines to the corrosion potential. 2.6. Atomic force microscope (AFM) The CRS specimens of size 1.0 cm 1.0 cm 0.06 cm were abraded with emery paper (grade 320500-800) and gave a mirror surface, then washed with distilled water and acetone. After immersion in 1.0 M H2SO4 without and with addition of 100 mg l1 tween-40 at 30 8C for 6 h, the specimen was cleaned with distilled water, dried with a cold air blaster, and then used for a Japan instrument model SPA-400 SPM Unit atomic force microscope examinations.
Fig. 2. Relationship between weight loss (W) and concentration of tween-40 (C) in 1.0 M H2SO4.
3.2. Effect of inhibitor concentration and temperature on inhibition efficiency The values of inhibition efficiencies obtained from the weight loss for different inhibitor concentrations in 1.0 M H2SO4 were given in Fig. 3. The results showed that inhibition efficiency increased as the concentration of inhibitor increased from 5 to 100 mg l1.
3. Experimental results and discussion 3.1. Effect of tween-40 on the weight loss The weight loss curves of cold rolled steel with the addition of tween-40 in 1.0 M H2SO4 at various temperature are shown in Fig. 2. The curves in Fig. 2 show that the weight loss values (g) of CRS in 1.0 M H2SO4 solution containing tween-40 decrease as the concentration of the inhibitor increase, i.e. the corrosion inhibition strengthen with the non-ionic surfactant concentration. This trend, it may result from the fact that adsorption amount and the coverage of surfactant on the CRS increases with the inhibitor concentration thus the CRS surface is efficiently separated from the medium [20,32].
Fig. 3. Relationship between inhibition efficiency (IE) and concentration of tween-40 (C) in 1.0 M H2SO4.
X. Li, G. Mu / Applied Surface Science 252 (2005) 1254–1265
The maximum inhibition efficiency was 91%. The inhibition was estimated to be 81% at 30 8C even at very low concentration (20 mg l1), and at 50 mg l1 concentration its protection was >80%. These better performances can be explained as follows: Fig. 1 shows that the molecular weight of tween-40 is high, thus tween-40 can relatively easily adsorb on the CRS surface by van der Waals force. In addition, the main hydrophilic part CHO(CH2CH2O)nCH2CH2OH of tween-40 attacks the CRS surface while the main hydrophobic part CHCH2OCO(CH2)14CH3 extends to the solution face. In addition, tween-40 may chemisorb at steel/solution interface via hydrogen bond between the OH groups in tween-40 molecules and water molecules adsorbed on the surface. When tween-40 adsorbed on metal surface, coordinate bond may be formed by partial transference of electrons from the polar atom (O atom) of tween-40 to the metal surface. Fig. 3 also gives that the inhibition efficiencies decreased with the experimental temperature in general, which indicated that the higher temperatures might cause desorption of tween-40 from the steel surface. It should be noted that the inhibition efficiencies at 40 8C are higher that those at 30 8C when the inhibitor concentrations are higher than 40 mg l1, and the maximum inhibition efficiency is found at 50 8C when tween-40 concentration is 100 mg l1, which also indicates that the adsorption of tween-40 is not merely a physical or a chemical adsorption but a comprehensive adsorption [33].
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Table 2 Parameters of the linear regression between C/u and C Temperature (8C)
Linear regression coefficient (r)
K (l mg1)
Slope
30 40 50 60
0.9999 0.9985 0.9873 0.9986
1.0024 0.1691 0.1160 0.0791
0.9912 0.9312 0.9135 0.9343
From the values of surface coverage, the linear regressions between C/u and C were calculated by the computer, and the parameters are listed in Table 2. Fig. 4 shows the relationship between C/u and C at 30 8C. These results show that all the linear correlation coefficients (r) are almost equal to 1 and all the slopes are very close to 1, which indicates the adsorption of inhibitor onto steel surface accords with the Langmuir adsorption isotherm. The result indicated that there was no interactions among the adsorbed species [36,37]. Table 2 also gives that the adsorptive equilibrium constant (K) values (l mg1) decrease with increasing temperature, which indicates that it is easily adsorbed strongly onto the steel surface for the inhibitor at lower temperatures. But when the temperature is relatively higher, the adsorbed inhibitors tend to desorption.
3.3. Adsorption isotherm Assuming the adsorption of tween-40 on the CRS surface obeys Langmuir adsorption isothermal equation [20,34]: C 1 ¼ þC u K
(3)
where C is the concentration of inhibitor, K the adsorptive equilibrium constant and u is the surface coverage. u was calculated by the Sekine and Hirakawa’s method [35]: u¼
W0 W W0 Wm
where Wm is the smallest weight loss.
(4) Fig. 4. The relationship between C/u and C at 30 8C.
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heat (DH0) under experimental condition [20,38]. The standard adsorption free energy (DG0) was obtained according to [39]: 1 DG0 exp K¼ 55:5 RT
(6)
where 55.5 is the concentration of water in solution expressed in mol l1. Thus, the adsorption constant K in l mg1 should change into l mol1. Because tween40 is also a polymer, it is difficult to know the exact molecular weight. Assuming the molecular weight of tween-40 is Mw in g mol1. The changed adsorption constant in l mol1 was listed in Table 3. Then the standard adsorption entropy (DS0) can be obtained by the thermodynamic basic equation: Fig. 5. The relationship between ln K and 1/T.
DS0 ¼
DH 0 DG0 T
(7)
3.4. Thermodynamic parameters Thermodynamic parameters are important to study the inhibitive mechanism. The adsorption heat could be calculated according to the Van’t Hoff equation [20,34]: ln K ¼
DH þ constant RT
(5)
where DH and K are the adsorption heat and adsorptive equilibrium constant, respectively. It should be noted that DH/R is the slope of the straight line ln K 1/T according to Eq. (5) and the molecular weight of inhibitor is also a positive constant, so the value of adsorption heat value does not change with the unit of inhibitor concentration. To obtain the adsorption heat, the regression between ln K and 1/T was dealt with. Fig. 5 is the straight line ln K 1/T. The adsorption heat (DH) can be approximately regarded as the standard adsorption
All the calculated thermodynamic parameters are listed in Table 3. The standard adsorption heat (76.15 kJ mol1) in Table 3 shows that a comprehensive adsorption (physical and chemical adsorption) might occur [34]. The negative values of DH0 also show that the adsorption of inhibitor is an exothermic process [40], which indicates that IEs decrease with increasing the temperature. The negative values of DG0 suggest that the adsorption of inhibitor molecule onto steel surface is a spontaneous process. As for the value of DS0 in Table 3, because the molecular weight (Mw) of tween-40 is about 1181–1357 g mol1 [41], the sign of DS0 is negative. The negative values of DS0 mean that the process of adsorption is accompanied by a decrease in entropy. It might be explained as follows: before the adsorption of tween-40 onto the steel surface, the chaotic degree of steel surface is high, but when inhibitor molecules were orderly adsorbed onto the steel surface, as a result, a decrease in entropy.
Table 3 The thermodynamic parameters of adsorption of tween-40 on the steel surface Temperature (8C)
K (l mol1)
DG0(kJ mol1)
DH0 (kJ mol1)
DS0 (J mol1 K1)
30 40 50 60
1.0024 103 Mw 0.1691 103 Mw 0.1160 103 Mw 0.0791 103 Mw
27.78 ln Mw 23.81 ln Mw 23.56 ln Mw 23.23 ln Mw
76.15 76.15 76.15 76.15
159.56 + 3.30 ln Mw 167.14 + 3.19 ln Mw 162.74 + 3.09 ln Mw 158.85 + 3.01 ln Mw
Mw: the molecular weight of tween-40, 1181–1357 g mol1.
X. Li, G. Mu / Applied Surface Science 252 (2005) 1254–1265 Table 4 Parameters of the regression between ln n and 1/T
3.5. Apparent activation energy (Ea) and pre-exponential factor (A) It has been reported by a number of authors [23,28,30,33,34] that for the acid corrosion of steel, the natural logarithm of the corrosion rate (g m2 h1) is a linear function with 1/T (following Arrhenius equation): ln n ¼
Ea þ ln A RT
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(8)
where Ea represents the apparent activation energy, R the gas constant, T the temperature, A the pre-exponential factor, and n is the corrosion rate. It can be calculated by the relation:
C (mg l1)
Ea (kJ mol1)
A (g m2 h1)
Linear regression coefficient (r)
0 5 10 20 30 40 50 70 100
54.95 71.79 75.40 80.46 77.64 77.23 74.61 68.63 60.42
9.20 1010 2.88 1013 8.86 1013 5.03 1014 1.49 1014 1.08 1014 3.58 1013 3.01 1012 1.13 1011
0.9999 0.9993 0.9951 0.9946 0.9979 0.9723 0.9720 0.9399 0.8724
where W is the weight loss, S the area of the rectangular steel, and t is the corrosion time (6 h). The regression between ln n and 1/T was calculated by computer, and Arrhenius plots of ln n versus 1/T for the blank and different concentrations of the non-ionic surfactant were shown in Fig. 6. All the parameters were calculated and given in Table 4. It was clear that the values of Ea in the presence of the tween-40 are higher than those in the uninhibited
acid solution. These results are according with the reported studies [28,42]. The increase of Ea in the presence of the inhibitor indicates the physical adsorption or weak chemical bonding between the tween-40 molecules and the steel surface [43]. According to Eq. (8), it can be seen that the lower pre-exponential factor A and the higher Ea lead to the lower corrosion rate (n). For the present study, the values of A in the presence of tween-40 are higher than those of in the absence of tween-40. Therefore, the decrease in steel corrosion rate is mostly decided by the apparent activation energy. The relationship of Ea and ln A with the inhibitor concentration is shown in Fig. 7. It was clear that both Ea and ln A increased in
Fig. 6. Arrhenius plots related to the corrosion rate of cold rolled steel for various concentrations of tween-40 in 1.0 M H2SO4 ((*) blank, (*) 5 mg l1, (~) 10 mg l1 and (~) 50 mg l1).
Fig. 7. The relationship of Ea and ln A with the concentration of tween-40 in 1.0 M H2SO4.
n¼
W St
(9)
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Table 5 Effect of immersion time on the corrosion rate of cold rolled steel in 1.0 M H2SO4 at 30 8C Immersion time (h)
1 3 6 12 24 48 72 96
Corrosion rate, n (g m2 h1) Blank
50 mg l1 tween-40
100 mg l1 tween-40
30.90 32.67 31.43 29.38 29.18 29.25 26.52 26.18
13.36 7.92 6.36 5.73 3.43 2.87 5.65 7.01
12.08 7.11 6.15 4.21 2.51 2.14 1.41 1.30
the presence of tween-40. When the inhibitor concentrations reached 20 mg l1, the values reach maximum, and then begin to decrease. Fig. 7 clearly shows that there is a ‘‘peak-like’’ value for this study. That is to say, the apparent activation energy acts as a function of inhibitor concentration [34,38]. 3.6. Effect of immersion time on inhibition efficiency Effect of immersion time on corrosion inhibition of tween-40 at concentrations of 50 and 100 mg l1 on the corrosion of CRS in 1.0 M H2SO4 at 30 8C was studied. Table 5 shows the corrosion rates obtained in the absence and presence of tween-40 act as a function of immersion time. In the absence of tween-40, the corrosion rate does not change obviously with the immersion time, and the corrosion rate decreases slightly after 72 h. However, the corrosion rate decreased obviously in the presence of the inhibitor, which may be ascribed to the heteroatom (oxygen atom) of the inhibitor molecule [44,45]. It should be noted that the corrosion rate began to increase after 48 h when the tween-40 concentration was 50 mg l1. Fig. 8 shows the effect of changing immersion time (1–96 h) at 30 8C on the inhibition efficiency of tween-40 at 50 and 100 mg l1. It can be seen from Fig. 8 the inhibition efficiency is higher than 55% when the immersion time is only 1 h, which indicates the adsorption rate of tween-40 adsorb on the steel surface is relatively high. Fig. 8 shows that tween-40 inhibits the corrosion of CRS for all immersion time at both concentrations. At 100 mg l1 increasing immersion time resulted in increasing IE, and the
Fig. 8. Effect of change immersion time on IE of tween-40 for the steel at 30 8C in 1.0 M H2SO4.
maximum IE was 95% at 96 h. The high inhibition efficiency with longer immersion time can be attributed to the formation of a protective film is time-dependent on the CRS surface. It has been stated that stable, two-dimensional layers of inhibitor molecules are formed on metal surfaces after longer immersion time [46]. At 50 mg l1, increasing immersion time resulted in increasing IE, and the maximum IE was 92% at 48 h, but after 48 h, IE began to decrease slightly. 3.7. Effect of sulphuric acid concentration on inhibition efficiency Fig. 9 shows the effect of changing H2SO4 concentration (0.5–7.0 M) at 30 8C on the inhibition efficiency of tween-40 at 50 and 100 mg l1. In both cases, increasing acid concentration resulted in decreasing IE, and the minimum IE was 43% in 6.0 M H2SO4 at 50 mg l1. Fig. 9 also clearly shows the changed degree of IE with H2SO4 concentration can be divided into three stages. The IE decreased obviously with the acid concentration in 0.5–1.5 M, then IE reached did not decrease obviously with the acid concentration in 2.0–6.0 M. But when the acid concentration was higher than 6.0 M, IE began to increase slightly with the acid concentration (6.0–7.0 M).
X. Li, G. Mu / Applied Surface Science 252 (2005) 1254–1265
Fig. 9. Effect of change in H2SO4 concentration on IE of tween-40 for the steel at 30 8C.
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Fig. 10. Variation of ln n with the concentration of sulphuric acid (C) at 30 8C.
3.8. Rate constant and reaction constant Assuming the corrosion rate against the molar concentration of acid concentration obeys the expression proposed by Mathur and Vasudevan [47]: ln n ¼ ln k þ BC
(10)
where k is the rate constant, B the reaction constant, and C is the molar concentration of H2SO4. Fig. 10 shows the curves of ln n versus C in different conditions. Fig. 10 shows that the graph of ln n C in uninhibited H2SO4 is a straight line. However, a break point appears at 2.0 M H2SO4 after adding tween-40. The straight lines show that the kinetic parameters could be calculated by Eq. (10), and listed in Table 6. Eq. (10) shows that k can be regarded as a commencing rate at zero acid concentration, so k means the ability of corrosion of H2SO4 for CRS [47– 49]. Table 6 clearly shows that k decreases obviously after adding tween-40 in H2SO4 solution, which indicates tween-40 is a good inhibitor in H2SO4. Furthermore, the values of k in 0.5–2.0 M H2SO4 inhibited solution are smaller than those in 2.0–7.0 M H2SO4 inhibited solution, which indicates that the values of IE in 0.5–2.0 M H2SO4 are more superior to those in 2.0–7.0 M H2SO4 (Fig. 9). According to Eq. (10), B is the slope of the line ln n C, so B indicated the changed extent of n with the acid
concentration. The values of B in inhibited H2SO4 are higher than that in uninhibited H2SO4, which indicates that the changed extent of n with C in inhibited H2SO4 is bigger than that in uninhibited H2SO4. Namely, the IE decreases obviously with the acid concentration (Fig. 9). In addition, the values of B in 0.5–2.0 M H2SO4 inhibited solution are higher than those in 2.0– 7.0 M H2SO4 inhibited solution, which indicates that the changed degree of IE in 0.5–2.0 M H2SO4 are greater than those in 2.0–7.0 M H2SO4 (Fig. 9). Also, it could be seen from Table 6 that the values of k at 50 mg l1 are lower than those at 100 mg l1, while Fig. 9 shows that IE at 50 mg l1 are lower than that at 100 mg l1. Thus, it seems the results of Table 5 are contract with those of Fig. 9. In fact, according to Eq. (10), both k and B might affect the steel corrosion Table 6 Calculated values of kinetic parameter for the corrosion of CRS in H2SO4 containing tween-40 at 30 8C C (mg l1)
0 50 100
B (g m2 h1 M1)
k (g m2 h1)
0.5–2.0 M H2SO4
2.0–7.0 M H2SO4
0.5–2.0 M H2SO4
2.0–7.0 M H2SO4
0.38 1.02 0.98
0.38 0.41 0.37
23.67 2.29 2.30
23.67 8.42 8.60
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Fig. 11. Potentiodynamic polarization curves for cold rolled steel in 1.0 M H2SO4 contain in different concentrations of tween-40 at 30 8C.
at a certain acid concentration. Generally speaking, the influence of k on the steel corrosion was bigger than that of B on the steel corrosion. However, if the values of inhibition efficiency are too close, then B might also another important factor to determine the steel corrosion. For the present study, although the values of k at 50 mg l1 are lower than those at 100 mg l1, the values B at 50 mg l1 are higher than those at 100 mg l1, and the variance of B is bigger than that of k. As a result, the IE at 50 mg l1 is lower than that at 100 mg l1. 3.9. Polarization studies Both anodic and cathodic polarization curves for cold rolled steel in H2SO4 at various concentrations of tween-40 are shown in Fig. 11. It is clear that the presence of inhibitor causes a markedly decrease in the corrosion rate, i.e. shifts the cathodic curves to more negative potentials. The inhibitor has a significant
effect on the rate of hydrogen evolution reaction. On the other hand, the non-ionic surfactant has slight effect on the anodic curves. It may be concluded that tween-40 acts as a cathodic-type inhibitor. The values of corrosion current densities (Icorr), corrosion potential (Ecorr), the cathodic Tafel slope (bc), anodic Tafel slope (ba), and the inhibition efficiency (IE) as functions of tween-40 concentration were calculated from the curves (Fig. 11) and given in Table 7. Table 7 reveals that the corrosion current decreases obviously after adding tween-40 in 1.0 M H2SO4, and IE increases with the inhibitor concentration. The corrosion potential (Ecorr) do not change obviously when tween-40 concentration is lower (10 and 50 mg l1), while the corrosion potential (Ecorr) value shifts to negative potential when the concentration is 100 mg l1. The cathodic Tafel slopes changed upon addition of increasing inhibitor concentration, while the anodic Tafel slopes did not changed obviously upon addition of tween-40. Therefore, tween-40 can be arranged as cathodic-type inhibitor in H2SO4. From Table 7, it can be concluded that inhibition efficiencies obtained from weight loss, electrochemical polarization curves are in good agreement. 3.10. Atomic force microscope surface examination The atomic force microscope provides a powerful means of characterizing the microstructure [50–53]. The three-dimensional AFM images of cold rolled steel surface in 1.0 M H2SO4 are shown Fig. 12. Fig. 13 illustrates the corresponding two-dimensional AFM images. As can be seen from Fig. 12(a), that the corrosion of CRS steel samples in the absence of inhibitor appeared to be relatively uniform in general and some parts to a low-mound-like structure. The image is quite different from the pitting corrosion image [52] or the corrosion in HClO4–CH3COOH
Table 7 Potentiodynamic polarization parameters for the corrosion of cold rolled steel in 1.0 M H2SO4 containing different contentrations of tween-40 at 30 8C C (mg l1)
Ecorr (mV)
Icorr (mA cm2)
bc (mV dec1)
ba (mV dec1)
IE (%)
0 10 50 100
434 437 438 446
1019.0 265.1 240.2 213.8
137 129 127 118
64 57 61 62
– 73.9 76.4 79.1
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Fig. 12. AFM three-dimensional images of the cold rolled steel (CRS) surface in 1.0 M H2SO4: (a) in the absence of tween-40 and (b) in the presence of 100 mg l1 tween-40.
Fig. 13. AFM two-dimensional images of the cold rolled steel (CRS) surface in 1.0 M H2SO4: (a) in the absence of tween-40 and (b) in the presence of 100 mg l1 tween-40.
solution [54]. However, in the presence of 100 mg l1 tween-40, the surface becomes more flat and closely. Fig. 13 clearly shows the corrosion degree of CRS decreases in the presence of tween-40. Fig. 13(b) shows the spherical or bread-like particles appear the surface, which do not exist in the Fig. 13(a). There-
fore, it may be concluded that these particles are the adsorption film of the inhibitor, which efficiently inhibits the corrosion of CRS. Fig. 14 shows the CRS surface topography in 1.0 M H2SO4. Fig. 15 illustrates the height profiles, which is made along the line marked in corresponding Fig. 14.
Fig. 14. AFM images of the cold rolled steel (CRS) surface topography in 1.0 M H2SO4: (a) in the absence of tween-40 and (b) in the presence of 100 mg l1 tween-40.
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Fig. 15. Height profiles of the CRS surface in 1.0 M H2SO4: (a) in the absence of tween-40 and (b) in the presence of 100 mg l1 tween-40.
It can be seen from Fig. 14(a) that the micrograph of CRS surface after immersion in uninhibited 1.0 H2SO4 shows the main characteristic of uniform corrosion in acidic media [55]. Fig. 14(b) shows that some spherical particles decorating the steel surface and the retention of surface grinding marks, which indicate a macroseopically thin film. Fig. 15(a) shows that the surface roughness of the CRS in uninhibited 1.0 H2SO4 is about 174.61 nm, while in the presence of tween-40, the roughness decreases to 98.59 nm (Fig. 15(b)). Thus, the roughness is consistent with the results shown in Figs. 12 and 13.
apparent activation energy (Ea) and pre-exponential factor (A) also increase after adding tween-40 in sulphuric acid solution. The inhibitive mechanism is mostly decided by the apparent activation energy. 4. Tween-40 acts as a cathodic-type inhibitor in 1.0 M H2SO4. The weight loss and polarization curves are in good agreement. 5. The introduction of tween-40 into 1.0 M H2SO4 solution results in the formation of a film on the CRS surface, which causes the decrease the steel surface roughness and effectively protects steel from corrosion.
4. Conclusion Acknowledgement 1. Tween-40 acts as a good inhibitor for the corrosion of cold rolled steel in 1.0 M H2SO4. The inhibition efficiency values increase with the inhibitor concentration and the immersion time, but decrease with the acid concentration and the temperature. 2. The adsorption of the tween-40 on the cold rolled steel surface obeys the Langmuir adsorption isotherm. The adsorption process is a spontaneous and exothermic process accompanied by a decrease in entropy. 3. The rate constant (k) decreases obviously after adding tween-40 in 0.5–7.0 M H2SO4 solution, while the reaction constant (B) increases, especially in 0.5–2.0 M H2SO4 The values of both
This work was carried out in the frame of a research project funded by the Chinese National Science Foundation (Grant No. 50261004).
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