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Molybdate and tungstate as corrosion inhibitors for cold rolling steel in hydrochloric acid solution
Corrosion Science 48 (2006) 445–459 www.elsevier.com/locate/corsci
Molybdate and tungstate as corrosion inhibitors for cold rolling steel in hydrochloric acid solution Guannan Mu *, Xianghong Li, Qing Qu, Jun Zhou Department of Chemistry, Yunnan University, Kunming, Yunnan 650091, People’s Republic of China Received 28 June 2004; accepted 20 January 2005 Available online 8 April 2005
Abstract The inhibition effects of molybdate and tungstate on the corrosion of cold rolling steel (CRS) in hydrochloric acid solution (0.1–0.5 M) were investigated by weight loss and electrochemistry methods. The results reveal that both molybdate and tungstate are very good inhibitors with little concentration. The adsorption of inhibitors on the CRS surface basically obeys the Langmuir adsorption isotherm equation. The effect of temperature on the corrosion behavior of CRS was also studied at 25 C and 35 C, the thermodynamic parameters such as adsorption heat (DH0) and adsorption free energy (DG0) were calculated. In the same conditions, a comparative study of corrosion inhibition of molybdate and tungstate indicated that molybdate was the better inhibitor in 0.1 M HCl. However, the value of percentage inhibition efficiency (IE) was dependent on the concentration of inhibitors in 0.2–0.5 M HCl. It seemed that molybdate did not have the strong inhibitive effect compared to tungstate with relatively small concentration of inhibitors, but molybdate was a better inhibitor over a wide concentration range of inhibitors. A kinetic study of cold rolling steel in uninhibited and inhibited acid was also discussed. Various parameters such as rate constant k and the kinetic parameter B were calculated for the reactions of corrosion. Polarization curves showed that both molybdate and tungstate are mixed-type inhibitors in acidic media. 2005 Elsevier Ltd. All rights reserved.
*
Corresponding author. Tel.: +86 871 503 3948. E-mail address: [email protected] (G. Mu).
0010-938X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2005.01.013
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Keywords: Molybdate; Tungstate; Corrosion inhibitor; Cold rolling steel; Hydrochloric acid; Polarization curves; Adsorption
1. Introduction The cost of the inorganic inhibitors is low, but most of them are toxic such as chromate, mercuride, nitrite, arsenate etc. [1]. In 1939, molybdate as a corrosion inhibitor was reported [2,3]. Robertson [4] first studied the mechanism of the inhibitive effect of molybdate on the corrosion of carbon steel in neutral solution. Pryor and Cohen [5] extended RobertsonÕs work to study the inhibitive mechanism of molybdate. Because molybdate is only a little poisonous or non-poisonous [6–8], molybdate-based treatments for open recirculating cooling systems have become popular since the early 1980s as an alternative to the toxic and ecologically unacceptable chromate-based inhibitors [9,10]. Obviously, most molybdate-based inhibitors were almost used in neutral or approximate neutral media [11–23], and the effect of inhibition was obvious when the concentration of molybdate was very high (thousands of mg/L) [24]. The toxicity of tungstate is also very low [25,51]. The corrosion inhibition of tungstate was more explored, but most of the studies were also in neutral or approximate neutral solutions [4,26–31]. However, the corrosion inhibition of molybdate and tungstate in strong acidic media was rarely studied [32–34], and most of the literature reported the effect of molybdate on stress corrosion cracking of stainless steel in strong acidic media [33,34]. Hydrochloric acid is widely used in the pickling process of metals. Inhibitors are used to prevent metal dissolution as well as acid consumption [35,36]. Because of the acid fog, the concentration of HCl and the ambient temperature could not be high. The acid consumption should also decrease for cost-effective protection. For these reasons, based on the proceeding papers, the objective of the present work is to investigate the corrosion inhibition by molybdate and tungstate in dilute HCl (0.1–0.5 M), so as to further study the inhibitive mechanism of molybdate and tungstate for cold rolling steel (CRS) in strong acidic media.
2. Experimental method 2.1. Materials Cold rolling steel (CRS) strips, containing 0.01% C, 0.50% Mn, 0.025% P, 0.025% S and remainder iron, were used for gravimetric and electrochemical measurements. Sodium molybdate (Na2MoO4), sodium tungstate (Na2WO4) and hydrochloric acid (HCl) used were of analytical-reagent grade. All solutions were prepared from distilled water.
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2.2. Gravimetric measurements The concentration range of HCl was varied from 0.1 M to 0.5 M. Three parallel cold rolling steel (CRS) sheets of 2.5 cm · 2 cm · 0.05 cm were abraded with emery paper (grade 320-500-800) and then washed with distilled water and acetone, dried at room temperature. After weighing accurately, the specimens were immersed in 250 ml 0.1– 0.5 M HCl with and without the addition of different concentrations of inhibitors for 12 h. The CRS sheets were then taken out, washed with distilled water and acetone, dried and weighed accurately. The average weight loss of three CRS sheets could be obtained. Runs were also done at 25 C and 35 C. The inhibition efficiency (IE) of molybdate and tungstate on the corrosion of CRS was calculated as follows [37]: IE% ¼
W0W 100 W0
ð1Þ
where W0 and W are the values of the average weight loss without and with addition of the inhibitors, respectively. 2.3. Electrochemical measurements Electrochemical measurements were conducted in a conventional three-electrode cylindrical glass cell at ambient temperature with a platinum counter electrode (CE) and a saturated calomel electrode (SCE) as the reference electrode. The working electrode (WE) was in the form of a square cut from CRS embedded in epoxy resin of polytetrafluoroethylene (PTFE) so that the flat surface was the only surface in the electrolyte. The working surface area was 1.0 cm · 1.0 cm. The polarization curves were recorded by using a PARSTAT2263 potentiostat. The potential increased with a speed of 30 mV/min and started from potential of 250 mV to +250 mV vs. SCE for 0.2 M HCl at 25 C. Before the electrochemical measurements, the CRS samples were abraded by different grits of emery paper (grade 320-500-800) to give a mirror surface, then cleaned with acetone, washed with distilled water and finally dried. IE% was defined as IE% ¼
I corr I corrðinhÞ 100 I corr
ð2Þ
where Icorr(inh) and Icorr are the corrosion current density values with and without inhibitor, respectively, determined by extrapolation of Tafel lines to the corrosion potential. 3. Experimental results and discussion 3.1. Gravimetric measurements 2 3.1.1. The adsorption and corrosion inhibition of MoO2 4 and WO4 on CRS surface Figs. 1–5 show the variation of IE with concentration of molybdate and tungstate for CRS in 0.1–0.5 M HCl at 25 C and 35 C. Both molybdate and tungstate
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G. Mu et al. / Corrosion Science 48 (2006) 445–459 100
80
IE(%)
60
MoO42-(25oC) WO42-(25oC)
40
MoO42-(35oC) WO42-(35oC)
20
0
0
50
100
150
200
250
300
C (mg/L)
Fig. 1. Corrosion inhibition of molybdate and tungstate for CRS in 0.1 M HCl.
100
80
IE(%)
60
MoO42-(25oC) WO42-(25oC)
40
MoO42-(35oC) WO42-(35oC)
20
0
0
50
100
150
200
250
300
C (mg/L)
Fig. 2. Corrosion inhibition of molybdate and tungstate for CRS in 0.2 M HCl.
inhibited the corrosion of CRS with relatively small concentration. The maximum IE was approximately 97% with molybdate concentration ranging from 80 mg/L to 300 mg/L in 0.2 M HCl at 35 C. The figures also show that the inhibition efficiency increases with the concentration of the inhibitors, but when the concentration reaches approximately 100 mg/L in 0.1–0.3 M HCl and 200 mg/L in 0.4 M and 0.5 M HCl, the IEs reach certain values and do not change obviously. Assuming the increase of the inhibition was caused by the adsorption of inhibitors on the CRS surface and obeys Langmuir adsorption isothermal equation: C 1 ¼ þC h K
ð3Þ
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100
80
IE(%)
60
MoO42-(25oC) WO42-(25oC)
40
MoO42-(35oC) WO42-(35oC)
20
0
0
50
100
150
200
250
300
C (mg/L)
Fig. 3. Corrosion inhibition of molybdate and tungstate for CRS in 0.3 M HCl.
100
80
IE(%)
60
MoO42-(25oC) WO42-(25oC)
40
MoO42-(35oC) WO42-(35oC)
20
0
0
50
100
150
200
250
300
C (mg/L)
Fig. 4. Corrosion inhibition of molybdate and tungstate for CRS in 0.4 M HCl.
where C is the concentration of inhibitor, K is the adsorptive equilibrium constant and h is the surface coverage. h was calculated from the following relation [37–39]: h¼
W0W W0Wm
ð4Þ
W and W0 are respectively the weight loss of CRS with and without the addition of the inhibitor in hydrochloric acid solution and Wm is the smallest weight loss. The linear regression between C/h and C was calculated by the computer. The result indicated that linear correlation coefficients (r) were all 0.9999. According to VanÕt Hoff equation: ln K ¼
DH þ constant RT
ð5Þ
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80
IE(%)
60
MoO42-(25oC) 40
WO42-(25oC) MoO42-(35oC) WO42-(35oC)
20
0
0
50
100
150
200
250
300
C (mg/L)
Fig. 5. Corrosion inhibition of molybdate and tungstate for CRS in 0.5 M HCl.
Then, the adsorption heat can be calculated approximately as follows: RT 2 T 1 K2 DH ¼ ln T2 T1 K1
ð6Þ
Because the experiment was done at the standard pressure (101,325 Pa) and the solution concentration was so low that it was close to standard condition, the adsorption heat can be approximately regarded as the standard adsorption heat DH0. The standard adsorption free energy DG0 was obtained according to [40]: 1 DG0 exp K¼ ð7Þ 55:5 RT The results were listed in Tables 1–4. The correlation coefficients and slopes of the straight lines C/h–C of molybdate and tungstate in 0.1–0.3 M HCl and in 0.4 M and 0.5 M HCl at 25 C approach 1. This illustrates that the adsorption of inhibitors on the CRS surface obeys Langmuir adsorption isothermal equation. The large positive values of adsorptive equilibrium constant (K) and the large negative values of free energy of adsorption (DG0) in 0.1 M and 0.2 M HCl indicated that inhibitors were strongly adsorbed on the CRS surface [41,42], so the inhibition was very good even at low concentration (see Figs. 1 and 2). The adsorption equilibrium constant (K) Table 1 Adsorption parameters of MoO2 4 on CRS surface at 25 C CHCl (M)
r
K
Slope
DH0 (kJ/mol)
DG0 (kJ/mol)
Maximum IE (%)
0.1 0.2 0.3 0.4 0.5
0.9999 0.9999 0.9999 0.9999 0.9999
0.721 0.274 0.185 0.061 0.044
1.001 0.990 0.999 0.986 0.984
35.91 60.19 102.4 63.07 50.29
9.146 6.753 5.785 3.035 2.268
88.60 92.08 91.07 88.39 85.06
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Table 2 Adsorption parameters of MoO2 4 on CRS surface at 35 C CHCl (M)
r
K
Slope
DH0 (kJ/mol)
DG0 (kJ/mol)
Maximum IE (%)
0.1 0.2 0.3 0.4 0.5
0.9999 0.9999 0.9999 0.9999 0.9999
0.450 0.125 0.048 0.016 0.013
0.994 0.978 0.998 0.763 0.778
35.91 60.19 102.4 63.07 50.29
8.248 4.961 2.543 1.021 0.657
93.28 97.14 94.65 93.29 88.99
Table 3 Adsorption parameters of WO2 4 on CRS surface at 25 C CHCl (M)
r
K
Slope
DH0 (kJ/mol)
DG0 (kJ/mol)
Maximum IE (%)
0.1 0.2 0.3 0.4 0.5
0.9999 0.9999 0.9999 0.9999 0.9999
0.204 0.279 0.587 0.131 0.053
0.991 1.012 0.991 0.997 0.996
64.01 175.0 93.87 85.58 4.059
6.008 6.799 8.637 4.926 2.697
79.18 81.56 83.06 81.17 71.87
Table 4 Adsorption parameters of WO2 4 on CRS surface at 35 C CHCl (M)
r
K
Slope
DH0 (kJ/mol)
DG0 (kJ/mol)
Maximum IE (%)
0.1 0.2 0.3 0.4 0.5
0.9999 0.9999 0.9999 0.9999 0.9999
0.472 2.767 0.171 0.428 0.050
0.996 1.012 0.991 0.931 0.934
64.01 175.0 93.87 85.58 4.059
8.367 12.89 5.778 2.221 2.652
84.26 86.92 87.27 85.08 79.02
decreased with the concentration of HCl, which indicated that it was difficult to adsorb on the CRS surface for the inhibitors. Therefore, molybdate and tungstate enhanced the corrosion resistance in 0.4 M and 0.5 M HCl (see Figs. 4 and 5). It was also reported that the inhibition of separate molybdate in 1 M HCl was not good, and the IE was only about 36% even at relatively high concentration (200 mg/L) [32]. These results indicated that the concentration of HCl plays an important role in the inhibition efficiency. The negative values of adsorption heat (DH0) of molybdate in 0.1–0.5 M HCl showed that the process of adsorption was exothermal, so the IEs may decrease with temperature. However, the positive values of adsorption heat (DH0) of tungstate in 0.1 M and 0.2 M HCl indicated that the IEs increased with the temperature (Figs. 1 and 2). It should be noted that although the linear correlation coefficients approach 1, the slopes of the straight lines in 0.4 M and 0.5 M HCl at 35 C deviate from 1, especially the slopes are only about 0.76 for molybdate. It shows that the adsorption of inhibitors on CRS surface does not obey the Langmuir adsorption isothermal equation. Eq. (4) shows that the surface coverage only lies in the weight loss. In fact, there
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are some other factors to affect the weight loss besides the surface coverage, such as the state of CRS surface, corrosion current densities, the interaction between ions and so on. So Eq. (4) can only express the apparent coverage. Thus h can be corrected with correction factor H from Eq. (4) and put into Eq. (3) [37,39]: C 1 ¼ þC Hh K Eq. (8) can be changed into the following relationship:
ð8Þ
C H ¼ þ HC h K
ð9Þ
Eq. (8) illustrates that only Hh can show the real coverage of the adsorption material on the metallic surface. The correction factor H can be deemed as the correlation coefficient between the surface coverage and the weight loss. The value of H shows the degree of fitting Langmuir adsorption isothermal equation for the system. The value of H approaches 1, the adsorption more conforms to Langmuir adsorption isothermal equation. This supposition is more reasonable. Fig. 6 shows that figures C/h–C are still straight lines after the correction, but Eq. (9) shows that the adsorption equilibrium constant (K) and the linear slope should be H/intercept and H, respectively. The corrected adsorption equilibrium constant K was calculated again according to Eq. (9). All of the adsorption parameters were calculated again by using aforementioned equations concerned. The results were listed in Table 5. Table 5 300 250
C/θ
200 150 100 50 0
0
50
100
150
200
250
C (mg/L) 2 2 Fig. 6. C/h–C curves at 35 C. (d)—MoO2 4 (in 0.5 M HCl), (s)—WO4 (in 0.5 M HCl), (m)—MoO4 (in 0.4 M HCl) and (n)—WO2 (in 0.4 M HCl). 4
Table 5 2 Adsorption parameters of MoO2 4 and WO4 on the CRS surface at 35 C by using corrected equation Inhibitor
CHCl (M)
r
K
H
DH0 (kJ/mol)
DG0 (kJ/mol)
Maximum IE (%)
MoO2 4 MoO2 4 WO2 4 2 WO4
0.4 0.5 0.4 0.5
0.9999 0.9999 0.9999 0.9999
0.0204 0.0181 0.0399 0.0474
0.763 0.778 0.931 0.934
79.47 63.24 91.02 9.254
0.3293 0.0171 2.039 2.478
93.29 88.99 85.08 79.02
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shows that the correction factors are smaller than 1, which indicates that the values of the apparent coverage got from the weight loss directly are bigger than those of the real coverage. 2 3.1.2. A comparative study of corrosion inhibition of WO2 4 and MoO4 for CRS Figs. 1–5 also show the results of the comparison of corrosion inhibition between 2 2 2 MoO2 4 and WO4 . Fig. 1 shows that the IE follows the order: MoO4 > WO4 in 0.1 M HCl, and the phenomena is obvious when the concentration of inhibitors is higher than 50 mg/L. The results can be explained as follows: it is well known that 2 in acidic solution MoO2 4 and WO4 condense into various polymolybdate and polytungstate ions [43], respectively. However, the structure of polytungate is more complicated than that of polymolybdate [43]. Thus, the polymolybdate ions can relatively easily adsorb on the CRS surface. Figs. 2–4 show that the IEs are dependent on the concentration of inhibitors in 0.2–0.4 M HCl, when the concentration of inhibitors is lower than certain degrees (20 mg/L in 0.2 M HCl, 40 mg/L in 0.3 M HCl, 60 mg/L in 0.4 M HCl), the IE 2 was in the order: MoO2 4 < WO4 ; however, when the concentration of inhibitors 2 is higher than these certain degrees, the IE is in the order: MoO2 4 >> WO4 . These results may be explained as follows: when the concentration of inhibitor is very low, + WO2 4 reacts with H to form the following species: 3 6 Hþ þ WO2 4 ! ½HW6 O20 n ! ½H2 W12 O40
ð10Þ
6 WO2 ! ½W10 O32 4 ! ½W6 O19 2 4 ! ½W7 O24
ð11Þ
2Hþ þ 6H2 O þ 12½W7 O24 6 ! 7½W12 O42 H2 10
ð12Þ
5 þ 7Hþ þ 6WO2 4 ! HW6 O21 þ 3H
ð13Þ
These polyanions can easily adsorb on the CRS surface [43]; in addition, WO2 4 could also react with Fe2+ to produce the precipitate of FeWO4 and then cover on the CRS surface. On the contrary, the molybdate condenses into a series of polymolybdate: 2
7½MoO4
þ 8Hþ ! ½Mo7 O24
6
þ 4H2 O
ð14Þ
7½Mo7 O24 6 þ 20Hþ ! Mo8 O4 26 þ 4H2 O
ð15Þ
These slight ions cannot easily adsorb on the CRS surface to form dense adsorption layer [10], which is not so effective to protect the CRS from corrosion. When the concentration of inhibitors is high, the surplus molybdate plays an 2+ important role in the inhibition. Firstly, MoO2 to form 4 can also react with Fe a protective film [44]: 2þ Fe2þ þ MoO2 MoO2 4 ! ½Fe 4
ð16Þ
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G. Mu et al. / Corrosion Science 48 (2006) 445–459 At the same time, MoO2 4 can also react with the Cl [45]: þ MoO2 4 þ 4H þ 2Cl ! MoO2 Cl2 þ 2H2 O
ð17Þ
MoO2 4 can also act the following reactions: ½MoO4 2 þ Hþ ! ½MoO3 ðOHÞ
ð18Þ
2½MoOðOHÞ5 ! ½ðOHÞ4 OMo–O–MoOðOHÞ4
2
þ H2 O
ð19Þ
These ions were of the ability to coordinate [43], could react with the blank orbit (d) of steel to form a complex, then adsorb on the CRS surface to further inhibit the corrosion. Thus, corrosion resistance was strengthened greatly. Fig. 5 shows that the inhibition of molybdate was superior to that of tungstate at 25 C in 0.5 M HCl. The order of IE depends on the concentration of inhibitors at 35 C. When the concentration of inhibitors was lower than 180 mg/L, the IE was in 2 the order: MoO2 4 < WO4 ; however, when the concentration of inhibitors is higher 2 than 180 mg/L, the IE follows the order: MoO2 4 > WO4 . This phenomenon probably may be attributed to the interaction between the adsorbed species. 3.1.3. A kinetic study of CRS in uninhibited and inhibited acid Assuming the corrosion rate (v) against the molar concentration of HCl (CHCl) obeys the expression proposed by Mathur and Vasudevan [46]: ð20Þ
ln v ¼ ln k þ BC HCl
where k is the rate constant, B is a constant for the reaction and CHCl is the molar concentration of acid, and v (g/m2 h) is the corrosion rate. v can be calculated by the relation [47]: v¼
W At
ð21Þ
where W is the weight loss, A is the area of the rectangular CRS, and t is corrosion time (12 h). Figs. 7 and 8 are the curves of ln v–CHCl in different conditions; they 2 1.6
ln v (g/m2 h)
1.2 0.8 0.4 0 —0.4 —0.8 —1.2 0
0.1
0.2
0.3
0.4
0.5
0.6
CHCl (M) Fig. 7. Variation of ln v with concentration of HCl at 25 C. (r)—blank, (m)—MoO2 4 (200 mg/L), (d)— 2 2 MoO2 4 (20 mg/L), (n)—WO4 (200 mg/L) and (s)—WO4 (20 mg/L).
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2.8 2.4
ln v (g/m2 h)
2 1.6 1.2 0.8 0.4 0 —0.4 —0.8 —1.2
0
0.1
0.2
0.3
0.4
0.5
0.6
CHCl (M) Fig. 8. Variation of ln v with concentration of HCl at 35 C. (r)—blank, (m)—MoO2 4 (200 mg/L), (d)— 2 2 MoO2 4 (20 mg/L), (n)—WO4 (200 mg/L) and (s)—WO4 (20 mg/L).
show that the graphs of ln v–CHCl in uninhibited acid solution are straight lines. The state of active surface does not change obviously because of the slow reaction in dilute HCl acid (0.1–0.5 M) [48–50]. Therefore, the corrosion of CRS does not change. However, a break point appears at 0.2 M HCl with the inhibited solution because of the complexity of the system with addition inhibitors. In 0.2–0.5 M inhibited acid solutions, the straight lines of ln v–CHCl showed that the kinetic parameters could be calculated by Eq. (20). Calculated kinetic parameters are listed in Table 6. According to Eq. (21), when CHCl ! 0, ln v = ln k, that is ð22Þ
k ¼ vCHCl !0 It can be concluded that ln (v2/v1) = B(CHCl(2) CHCl(1)), (CHCl(2) CHCl(1)) = 1, the following equation can be deduced: v2 B ¼ ln v1
so
when
ð23Þ
Eq. (22) shows that k can be regarded as a commencing rate at zero acid concentration, so k means the ability of corrosion for CRS by HCl [46,48]. Table 6 shows that k decreases obviously after adding inhibitors in hydrochloric acid solution. The Table 6 2 Calculated values of kinetic parameters for the corrosion of CRS in HCl containing MoO2 4 or WO4 Inhibitor
B (g m2 h1 M1)
k (g m2 h1)
Concentration
25 C
35 C
25 C
35 C
None MoO2 4 (20 mg/L) WO2 4 (20 mg/L) MoO2 4 (200 mg/L) WO2 4 (200 mg/L)
0.8816 2.6245 2.8650 2.7065 1.2454
1.0433 2.4440 3.9917 6.3017 3.3109
4.6313 1.1281 0.7194 0.3011 0.7873
9.6398 3.6550 1.2179 0.1207 0.6757
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increasing k with the temperature in uninhibited acid solutions indicated that the corrosion ability was strengthened at high temperature. Eq. (23) shows that, B can be regarded as the ln (v2/v1) when the concentration difference of HCl is 1, so B indicated the changed extent of v with the concentration of HCl. B at 25 C is smaller than that at 35 C except for the HCl solution with addition 20 mg/L MoO2 4 , which indicates the changed extent is small at 25 C. When the concentration of inhibitors is 200 mg/L, k (MoO2 4 ) is smaller than k (WO2 4 ). This means that the inhibition of molybdate is more superior to tungstate. The values of B in inhibited HCl are bigger than that in uninhibited HCl. To explain these phenomena, it might be important to consider the abilities of inhibition in different concentration of HCl are different with the same addition of inhibitor. Namely, the IEs decreased obviously with the concentration of acid. 3.2. Electrochemical measurements Fig. 9 shows the effect of molybdate and tungstate concentration on the anodic and cathodic polarization curves of CRS in 0.2 M HCl solution at 25 C. Fig. 9 reveals that the presence of increasing concentrations of inhibitors causes a markedly decrease in the corrosion rate i.e. shifts the anodic curves to more positive potentials and the cathodic curves to more negative potentials. This may be ascribed to adsorption of inhibitors over the corroded surface. 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 molybdate and tungstate concentration, were calculated from the curves of Fig. 9 and given in Table 7. Table 7
—0.2 —0.3
E (V/SCE)
—0.4 —0.5 —0.6
—0.7 —0.8 —6
—5
—4
—3
—2
—1
log (µA/cm2)
Fig. 9. Polarization curves for CRS in 0.2 M HCl containing different concentrations of MoO2 and 4 2 2 2 WO2 4 at 25 C. (h)—blank, (m)—MoO4 (200 mg/L), (d)—MoO4 (20 mg/L), (n)—WO4 (200 mg/L) and (s)—WO2 4 (20 mg/L).
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Table 7 Polarization parameters in the corrosion of CRS in 0.2 M HCl containing different concentration of inhibitors at 25 C Concentration
Ecorr (mV/SCE)
Icorr (lA/cm2)
bc (mV/dec)
ba (mV/dec)
IE (%)
Blank MoO2 4 (20 mg/L) WO2 4 (20 mg/L) MoO2 4 (200 mg/L) WO2 4 (200 mg/L)
485 473 481 454 473
517 222 165 83 140
150 160 158 180 166
66 43 45 46 46
– 57.1 68.1 83.9 72.9
reveals that the corrosion potential is slightly shifted to noble direction as the inhibitors concentrations are increased. Moreover, the corrosion current decreases markedly and the IE increases with the concentration of inhibitors. Table 7 also reveals that both anodic and cathodic Tafel slopes changed upon addition of increasing concentrations of inhibitors. This change in anodic and cathodic Tafel slopes in presence of inhibitors indicates that the inhibitors affect both anodic and cathodic reactions. Therefore, molybdate and tungstate can be arranged as mixed-type inhibitors in strong acidic media. These results are quite different from those in neutral media where molybdate and tungstate exhibited as anodic inhibitors [15,19,28,30]. Furthermore, results obtained from gravimetric and electrochemical measurements were in good agreement. However, IE% obtained by the electrochemical method was less than determined by the gravimetrical method. The difference between both methods could be attributed to the different experimental conditions of two methods. 4. Conclusions 1. Molybdate and tungstate act as good inhibitors for the corrosion of cold rolling steel in 0.1–0.5 M HCl, but best performances are seen in the case of 0.2 M HCl at 35 C. The concentration of HCl plays an important role in the inhibition efficiency. 2. The adsorption of molybdate and tungstate on the CRS surface basically obeys the Langmuir adsorption isotherm. However, the corrected equation by correction factor H can explain the results that the correlation coefficients of the straight line C/h–C approach 1, but the slopes deviate from 1. 3. Both molybdate and tungstate act as mixed inhibitors in strong acidic media. 4. The weight loss studies shows the corrosion of cold rolling steel in 0.1–0.5 M uninhibited HCl and in 0.2–0.5 M inhibited HCl support the kinetic equation proposed by Mathur and Vasudevan. The rate constant k obviously decreased after adding molybdate and tungstate. Acknowledgement 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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Molybdate and tungstate as corrosion inhibitors for cold rolling steel in hydrochloric acid solution Guannan Mu *, Xianghong Li, Qing Qu, Jun Zhou Department of Chemistry, Yunnan University, Kunming, Yunnan 650091, People’s Republic of China Received 28 June 2004; accepted 20 January 2005 Available online 8 April 2005
Abstract The inhibition effects of molybdate and tungstate on the corrosion of cold rolling steel (CRS) in hydrochloric acid solution (0.1–0.5 M) were investigated by weight loss and electrochemistry methods. The results reveal that both molybdate and tungstate are very good inhibitors with little concentration. The adsorption of inhibitors on the CRS surface basically obeys the Langmuir adsorption isotherm equation. The effect of temperature on the corrosion behavior of CRS was also studied at 25 C and 35 C, the thermodynamic parameters such as adsorption heat (DH0) and adsorption free energy (DG0) were calculated. In the same conditions, a comparative study of corrosion inhibition of molybdate and tungstate indicated that molybdate was the better inhibitor in 0.1 M HCl. However, the value of percentage inhibition efficiency (IE) was dependent on the concentration of inhibitors in 0.2–0.5 M HCl. It seemed that molybdate did not have the strong inhibitive effect compared to tungstate with relatively small concentration of inhibitors, but molybdate was a better inhibitor over a wide concentration range of inhibitors. A kinetic study of cold rolling steel in uninhibited and inhibited acid was also discussed. Various parameters such as rate constant k and the kinetic parameter B were calculated for the reactions of corrosion. Polarization curves showed that both molybdate and tungstate are mixed-type inhibitors in acidic media. 2005 Elsevier Ltd. All rights reserved.
*
Corresponding author. Tel.: +86 871 503 3948. E-mail address: [email protected] (G. Mu).
0010-938X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2005.01.013
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Keywords: Molybdate; Tungstate; Corrosion inhibitor; Cold rolling steel; Hydrochloric acid; Polarization curves; Adsorption
1. Introduction The cost of the inorganic inhibitors is low, but most of them are toxic such as chromate, mercuride, nitrite, arsenate etc. [1]. In 1939, molybdate as a corrosion inhibitor was reported [2,3]. Robertson [4] first studied the mechanism of the inhibitive effect of molybdate on the corrosion of carbon steel in neutral solution. Pryor and Cohen [5] extended RobertsonÕs work to study the inhibitive mechanism of molybdate. Because molybdate is only a little poisonous or non-poisonous [6–8], molybdate-based treatments for open recirculating cooling systems have become popular since the early 1980s as an alternative to the toxic and ecologically unacceptable chromate-based inhibitors [9,10]. Obviously, most molybdate-based inhibitors were almost used in neutral or approximate neutral media [11–23], and the effect of inhibition was obvious when the concentration of molybdate was very high (thousands of mg/L) [24]. The toxicity of tungstate is also very low [25,51]. The corrosion inhibition of tungstate was more explored, but most of the studies were also in neutral or approximate neutral solutions [4,26–31]. However, the corrosion inhibition of molybdate and tungstate in strong acidic media was rarely studied [32–34], and most of the literature reported the effect of molybdate on stress corrosion cracking of stainless steel in strong acidic media [33,34]. Hydrochloric acid is widely used in the pickling process of metals. Inhibitors are used to prevent metal dissolution as well as acid consumption [35,36]. Because of the acid fog, the concentration of HCl and the ambient temperature could not be high. The acid consumption should also decrease for cost-effective protection. For these reasons, based on the proceeding papers, the objective of the present work is to investigate the corrosion inhibition by molybdate and tungstate in dilute HCl (0.1–0.5 M), so as to further study the inhibitive mechanism of molybdate and tungstate for cold rolling steel (CRS) in strong acidic media.
2. Experimental method 2.1. Materials Cold rolling steel (CRS) strips, containing 0.01% C, 0.50% Mn, 0.025% P, 0.025% S and remainder iron, were used for gravimetric and electrochemical measurements. Sodium molybdate (Na2MoO4), sodium tungstate (Na2WO4) and hydrochloric acid (HCl) used were of analytical-reagent grade. All solutions were prepared from distilled water.
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2.2. Gravimetric measurements The concentration range of HCl was varied from 0.1 M to 0.5 M. Three parallel cold rolling steel (CRS) sheets of 2.5 cm · 2 cm · 0.05 cm were abraded with emery paper (grade 320-500-800) and then washed with distilled water and acetone, dried at room temperature. After weighing accurately, the specimens were immersed in 250 ml 0.1– 0.5 M HCl with and without the addition of different concentrations of inhibitors for 12 h. The CRS sheets were then taken out, washed with distilled water and acetone, dried and weighed accurately. The average weight loss of three CRS sheets could be obtained. Runs were also done at 25 C and 35 C. The inhibition efficiency (IE) of molybdate and tungstate on the corrosion of CRS was calculated as follows [37]: IE% ¼
W0W 100 W0
ð1Þ
where W0 and W are the values of the average weight loss without and with addition of the inhibitors, respectively. 2.3. Electrochemical measurements Electrochemical measurements were conducted in a conventional three-electrode cylindrical glass cell at ambient temperature with a platinum counter electrode (CE) and a saturated calomel electrode (SCE) as the reference electrode. The working electrode (WE) was in the form of a square cut from CRS embedded in epoxy resin of polytetrafluoroethylene (PTFE) so that the flat surface was the only surface in the electrolyte. The working surface area was 1.0 cm · 1.0 cm. The polarization curves were recorded by using a PARSTAT2263 potentiostat. The potential increased with a speed of 30 mV/min and started from potential of 250 mV to +250 mV vs. SCE for 0.2 M HCl at 25 C. Before the electrochemical measurements, the CRS samples were abraded by different grits of emery paper (grade 320-500-800) to give a mirror surface, then cleaned with acetone, washed with distilled water and finally dried. IE% was defined as IE% ¼
I corr I corrðinhÞ 100 I corr
ð2Þ
where Icorr(inh) and Icorr are the corrosion current density values with and without inhibitor, respectively, determined by extrapolation of Tafel lines to the corrosion potential. 3. Experimental results and discussion 3.1. Gravimetric measurements 2 3.1.1. The adsorption and corrosion inhibition of MoO2 4 and WO4 on CRS surface Figs. 1–5 show the variation of IE with concentration of molybdate and tungstate for CRS in 0.1–0.5 M HCl at 25 C and 35 C. Both molybdate and tungstate
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G. Mu et al. / Corrosion Science 48 (2006) 445–459 100
80
IE(%)
60
MoO42-(25oC) WO42-(25oC)
40
MoO42-(35oC) WO42-(35oC)
20
0
0
50
100
150
200
250
300
C (mg/L)
Fig. 1. Corrosion inhibition of molybdate and tungstate for CRS in 0.1 M HCl.
100
80
IE(%)
60
MoO42-(25oC) WO42-(25oC)
40
MoO42-(35oC) WO42-(35oC)
20
0
0
50
100
150
200
250
300
C (mg/L)
Fig. 2. Corrosion inhibition of molybdate and tungstate for CRS in 0.2 M HCl.
inhibited the corrosion of CRS with relatively small concentration. The maximum IE was approximately 97% with molybdate concentration ranging from 80 mg/L to 300 mg/L in 0.2 M HCl at 35 C. The figures also show that the inhibition efficiency increases with the concentration of the inhibitors, but when the concentration reaches approximately 100 mg/L in 0.1–0.3 M HCl and 200 mg/L in 0.4 M and 0.5 M HCl, the IEs reach certain values and do not change obviously. Assuming the increase of the inhibition was caused by the adsorption of inhibitors on the CRS surface and obeys Langmuir adsorption isothermal equation: C 1 ¼ þC h K
ð3Þ
G. Mu et al. / Corrosion Science 48 (2006) 445–459
449
100
80
IE(%)
60
MoO42-(25oC) WO42-(25oC)
40
MoO42-(35oC) WO42-(35oC)
20
0
0
50
100
150
200
250
300
C (mg/L)
Fig. 3. Corrosion inhibition of molybdate and tungstate for CRS in 0.3 M HCl.
100
80
IE(%)
60
MoO42-(25oC) WO42-(25oC)
40
MoO42-(35oC) WO42-(35oC)
20
0
0
50
100
150
200
250
300
C (mg/L)
Fig. 4. Corrosion inhibition of molybdate and tungstate for CRS in 0.4 M HCl.
where C is the concentration of inhibitor, K is the adsorptive equilibrium constant and h is the surface coverage. h was calculated from the following relation [37–39]: h¼
W0W W0Wm
ð4Þ
W and W0 are respectively the weight loss of CRS with and without the addition of the inhibitor in hydrochloric acid solution and Wm is the smallest weight loss. The linear regression between C/h and C was calculated by the computer. The result indicated that linear correlation coefficients (r) were all 0.9999. According to VanÕt Hoff equation: ln K ¼
DH þ constant RT
ð5Þ
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80
IE(%)
60
MoO42-(25oC) 40
WO42-(25oC) MoO42-(35oC) WO42-(35oC)
20
0
0
50
100
150
200
250
300
C (mg/L)
Fig. 5. Corrosion inhibition of molybdate and tungstate for CRS in 0.5 M HCl.
Then, the adsorption heat can be calculated approximately as follows: RT 2 T 1 K2 DH ¼ ln T2 T1 K1
ð6Þ
Because the experiment was done at the standard pressure (101,325 Pa) and the solution concentration was so low that it was close to standard condition, the adsorption heat can be approximately regarded as the standard adsorption heat DH0. The standard adsorption free energy DG0 was obtained according to [40]: 1 DG0 exp K¼ ð7Þ 55:5 RT The results were listed in Tables 1–4. The correlation coefficients and slopes of the straight lines C/h–C of molybdate and tungstate in 0.1–0.3 M HCl and in 0.4 M and 0.5 M HCl at 25 C approach 1. This illustrates that the adsorption of inhibitors on the CRS surface obeys Langmuir adsorption isothermal equation. The large positive values of adsorptive equilibrium constant (K) and the large negative values of free energy of adsorption (DG0) in 0.1 M and 0.2 M HCl indicated that inhibitors were strongly adsorbed on the CRS surface [41,42], so the inhibition was very good even at low concentration (see Figs. 1 and 2). The adsorption equilibrium constant (K) Table 1 Adsorption parameters of MoO2 4 on CRS surface at 25 C CHCl (M)
r
K
Slope
DH0 (kJ/mol)
DG0 (kJ/mol)
Maximum IE (%)
0.1 0.2 0.3 0.4 0.5
0.9999 0.9999 0.9999 0.9999 0.9999
0.721 0.274 0.185 0.061 0.044
1.001 0.990 0.999 0.986 0.984
35.91 60.19 102.4 63.07 50.29
9.146 6.753 5.785 3.035 2.268
88.60 92.08 91.07 88.39 85.06
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Table 2 Adsorption parameters of MoO2 4 on CRS surface at 35 C CHCl (M)
r
K
Slope
DH0 (kJ/mol)
DG0 (kJ/mol)
Maximum IE (%)
0.1 0.2 0.3 0.4 0.5
0.9999 0.9999 0.9999 0.9999 0.9999
0.450 0.125 0.048 0.016 0.013
0.994 0.978 0.998 0.763 0.778
35.91 60.19 102.4 63.07 50.29
8.248 4.961 2.543 1.021 0.657
93.28 97.14 94.65 93.29 88.99
Table 3 Adsorption parameters of WO2 4 on CRS surface at 25 C CHCl (M)
r
K
Slope
DH0 (kJ/mol)
DG0 (kJ/mol)
Maximum IE (%)
0.1 0.2 0.3 0.4 0.5
0.9999 0.9999 0.9999 0.9999 0.9999
0.204 0.279 0.587 0.131 0.053
0.991 1.012 0.991 0.997 0.996
64.01 175.0 93.87 85.58 4.059
6.008 6.799 8.637 4.926 2.697
79.18 81.56 83.06 81.17 71.87
Table 4 Adsorption parameters of WO2 4 on CRS surface at 35 C CHCl (M)
r
K
Slope
DH0 (kJ/mol)
DG0 (kJ/mol)
Maximum IE (%)
0.1 0.2 0.3 0.4 0.5
0.9999 0.9999 0.9999 0.9999 0.9999
0.472 2.767 0.171 0.428 0.050
0.996 1.012 0.991 0.931 0.934
64.01 175.0 93.87 85.58 4.059
8.367 12.89 5.778 2.221 2.652
84.26 86.92 87.27 85.08 79.02
decreased with the concentration of HCl, which indicated that it was difficult to adsorb on the CRS surface for the inhibitors. Therefore, molybdate and tungstate enhanced the corrosion resistance in 0.4 M and 0.5 M HCl (see Figs. 4 and 5). It was also reported that the inhibition of separate molybdate in 1 M HCl was not good, and the IE was only about 36% even at relatively high concentration (200 mg/L) [32]. These results indicated that the concentration of HCl plays an important role in the inhibition efficiency. The negative values of adsorption heat (DH0) of molybdate in 0.1–0.5 M HCl showed that the process of adsorption was exothermal, so the IEs may decrease with temperature. However, the positive values of adsorption heat (DH0) of tungstate in 0.1 M and 0.2 M HCl indicated that the IEs increased with the temperature (Figs. 1 and 2). It should be noted that although the linear correlation coefficients approach 1, the slopes of the straight lines in 0.4 M and 0.5 M HCl at 35 C deviate from 1, especially the slopes are only about 0.76 for molybdate. It shows that the adsorption of inhibitors on CRS surface does not obey the Langmuir adsorption isothermal equation. Eq. (4) shows that the surface coverage only lies in the weight loss. In fact, there
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are some other factors to affect the weight loss besides the surface coverage, such as the state of CRS surface, corrosion current densities, the interaction between ions and so on. So Eq. (4) can only express the apparent coverage. Thus h can be corrected with correction factor H from Eq. (4) and put into Eq. (3) [37,39]: C 1 ¼ þC Hh K Eq. (8) can be changed into the following relationship:
ð8Þ
C H ¼ þ HC h K
ð9Þ
Eq. (8) illustrates that only Hh can show the real coverage of the adsorption material on the metallic surface. The correction factor H can be deemed as the correlation coefficient between the surface coverage and the weight loss. The value of H shows the degree of fitting Langmuir adsorption isothermal equation for the system. The value of H approaches 1, the adsorption more conforms to Langmuir adsorption isothermal equation. This supposition is more reasonable. Fig. 6 shows that figures C/h–C are still straight lines after the correction, but Eq. (9) shows that the adsorption equilibrium constant (K) and the linear slope should be H/intercept and H, respectively. The corrected adsorption equilibrium constant K was calculated again according to Eq. (9). All of the adsorption parameters were calculated again by using aforementioned equations concerned. The results were listed in Table 5. Table 5 300 250
C/θ
200 150 100 50 0
0
50
100
150
200
250
C (mg/L) 2 2 Fig. 6. C/h–C curves at 35 C. (d)—MoO2 4 (in 0.5 M HCl), (s)—WO4 (in 0.5 M HCl), (m)—MoO4 (in 0.4 M HCl) and (n)—WO2 (in 0.4 M HCl). 4
Table 5 2 Adsorption parameters of MoO2 4 and WO4 on the CRS surface at 35 C by using corrected equation Inhibitor
CHCl (M)
r
K
H
DH0 (kJ/mol)
DG0 (kJ/mol)
Maximum IE (%)
MoO2 4 MoO2 4 WO2 4 2 WO4
0.4 0.5 0.4 0.5
0.9999 0.9999 0.9999 0.9999
0.0204 0.0181 0.0399 0.0474
0.763 0.778 0.931 0.934
79.47 63.24 91.02 9.254
0.3293 0.0171 2.039 2.478
93.29 88.99 85.08 79.02
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453
shows that the correction factors are smaller than 1, which indicates that the values of the apparent coverage got from the weight loss directly are bigger than those of the real coverage. 2 3.1.2. A comparative study of corrosion inhibition of WO2 4 and MoO4 for CRS Figs. 1–5 also show the results of the comparison of corrosion inhibition between 2 2 2 MoO2 4 and WO4 . Fig. 1 shows that the IE follows the order: MoO4 > WO4 in 0.1 M HCl, and the phenomena is obvious when the concentration of inhibitors is higher than 50 mg/L. The results can be explained as follows: it is well known that 2 in acidic solution MoO2 4 and WO4 condense into various polymolybdate and polytungstate ions [43], respectively. However, the structure of polytungate is more complicated than that of polymolybdate [43]. Thus, the polymolybdate ions can relatively easily adsorb on the CRS surface. Figs. 2–4 show that the IEs are dependent on the concentration of inhibitors in 0.2–0.4 M HCl, when the concentration of inhibitors is lower than certain degrees (20 mg/L in 0.2 M HCl, 40 mg/L in 0.3 M HCl, 60 mg/L in 0.4 M HCl), the IE 2 was in the order: MoO2 4 < WO4 ; however, when the concentration of inhibitors 2 is higher than these certain degrees, the IE is in the order: MoO2 4 >> WO4 . These results may be explained as follows: when the concentration of inhibitor is very low, + WO2 4 reacts with H to form the following species: 3 6 Hþ þ WO2 4 ! ½HW6 O20 n ! ½H2 W12 O40
ð10Þ
6 WO2 ! ½W10 O32 4 ! ½W6 O19 2 4 ! ½W7 O24
ð11Þ
2Hþ þ 6H2 O þ 12½W7 O24 6 ! 7½W12 O42 H2 10
ð12Þ
5 þ 7Hþ þ 6WO2 4 ! HW6 O21 þ 3H
ð13Þ
These polyanions can easily adsorb on the CRS surface [43]; in addition, WO2 4 could also react with Fe2+ to produce the precipitate of FeWO4 and then cover on the CRS surface. On the contrary, the molybdate condenses into a series of polymolybdate: 2
7½MoO4
þ 8Hþ ! ½Mo7 O24
6
þ 4H2 O
ð14Þ
7½Mo7 O24 6 þ 20Hþ ! Mo8 O4 26 þ 4H2 O
ð15Þ
These slight ions cannot easily adsorb on the CRS surface to form dense adsorption layer [10], which is not so effective to protect the CRS from corrosion. When the concentration of inhibitors is high, the surplus molybdate plays an 2+ important role in the inhibition. Firstly, MoO2 to form 4 can also react with Fe a protective film [44]: 2þ Fe2þ þ MoO2 MoO2 4 ! ½Fe 4
ð16Þ
454
G. Mu et al. / Corrosion Science 48 (2006) 445–459 At the same time, MoO2 4 can also react with the Cl [45]: þ MoO2 4 þ 4H þ 2Cl ! MoO2 Cl2 þ 2H2 O
ð17Þ
MoO2 4 can also act the following reactions: ½MoO4 2 þ Hþ ! ½MoO3 ðOHÞ
ð18Þ
2½MoOðOHÞ5 ! ½ðOHÞ4 OMo–O–MoOðOHÞ4
2
þ H2 O
ð19Þ
These ions were of the ability to coordinate [43], could react with the blank orbit (d) of steel to form a complex, then adsorb on the CRS surface to further inhibit the corrosion. Thus, corrosion resistance was strengthened greatly. Fig. 5 shows that the inhibition of molybdate was superior to that of tungstate at 25 C in 0.5 M HCl. The order of IE depends on the concentration of inhibitors at 35 C. When the concentration of inhibitors was lower than 180 mg/L, the IE was in 2 the order: MoO2 4 < WO4 ; however, when the concentration of inhibitors is higher 2 than 180 mg/L, the IE follows the order: MoO2 4 > WO4 . This phenomenon probably may be attributed to the interaction between the adsorbed species. 3.1.3. A kinetic study of CRS in uninhibited and inhibited acid Assuming the corrosion rate (v) against the molar concentration of HCl (CHCl) obeys the expression proposed by Mathur and Vasudevan [46]: ð20Þ
ln v ¼ ln k þ BC HCl
where k is the rate constant, B is a constant for the reaction and CHCl is the molar concentration of acid, and v (g/m2 h) is the corrosion rate. v can be calculated by the relation [47]: v¼
W At
ð21Þ
where W is the weight loss, A is the area of the rectangular CRS, and t is corrosion time (12 h). Figs. 7 and 8 are the curves of ln v–CHCl in different conditions; they 2 1.6
ln v (g/m2 h)
1.2 0.8 0.4 0 —0.4 —0.8 —1.2 0
0.1
0.2
0.3
0.4
0.5
0.6
CHCl (M) Fig. 7. Variation of ln v with concentration of HCl at 25 C. (r)—blank, (m)—MoO2 4 (200 mg/L), (d)— 2 2 MoO2 4 (20 mg/L), (n)—WO4 (200 mg/L) and (s)—WO4 (20 mg/L).
G. Mu et al. / Corrosion Science 48 (2006) 445–459
455
2.8 2.4
ln v (g/m2 h)
2 1.6 1.2 0.8 0.4 0 —0.4 —0.8 —1.2
0
0.1
0.2
0.3
0.4
0.5
0.6
CHCl (M) Fig. 8. Variation of ln v with concentration of HCl at 35 C. (r)—blank, (m)—MoO2 4 (200 mg/L), (d)— 2 2 MoO2 4 (20 mg/L), (n)—WO4 (200 mg/L) and (s)—WO4 (20 mg/L).
show that the graphs of ln v–CHCl in uninhibited acid solution are straight lines. The state of active surface does not change obviously because of the slow reaction in dilute HCl acid (0.1–0.5 M) [48–50]. Therefore, the corrosion of CRS does not change. However, a break point appears at 0.2 M HCl with the inhibited solution because of the complexity of the system with addition inhibitors. In 0.2–0.5 M inhibited acid solutions, the straight lines of ln v–CHCl showed that the kinetic parameters could be calculated by Eq. (20). Calculated kinetic parameters are listed in Table 6. According to Eq. (21), when CHCl ! 0, ln v = ln k, that is ð22Þ
k ¼ vCHCl !0 It can be concluded that ln (v2/v1) = B(CHCl(2) CHCl(1)), (CHCl(2) CHCl(1)) = 1, the following equation can be deduced: v2 B ¼ ln v1
so
when
ð23Þ
Eq. (22) shows that k can be regarded as a commencing rate at zero acid concentration, so k means the ability of corrosion for CRS by HCl [46,48]. Table 6 shows that k decreases obviously after adding inhibitors in hydrochloric acid solution. The Table 6 2 Calculated values of kinetic parameters for the corrosion of CRS in HCl containing MoO2 4 or WO4 Inhibitor
B (g m2 h1 M1)
k (g m2 h1)
Concentration
25 C
35 C
25 C
35 C
None MoO2 4 (20 mg/L) WO2 4 (20 mg/L) MoO2 4 (200 mg/L) WO2 4 (200 mg/L)
0.8816 2.6245 2.8650 2.7065 1.2454
1.0433 2.4440 3.9917 6.3017 3.3109
4.6313 1.1281 0.7194 0.3011 0.7873
9.6398 3.6550 1.2179 0.1207 0.6757
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increasing k with the temperature in uninhibited acid solutions indicated that the corrosion ability was strengthened at high temperature. Eq. (23) shows that, B can be regarded as the ln (v2/v1) when the concentration difference of HCl is 1, so B indicated the changed extent of v with the concentration of HCl. B at 25 C is smaller than that at 35 C except for the HCl solution with addition 20 mg/L MoO2 4 , which indicates the changed extent is small at 25 C. When the concentration of inhibitors is 200 mg/L, k (MoO2 4 ) is smaller than k (WO2 4 ). This means that the inhibition of molybdate is more superior to tungstate. The values of B in inhibited HCl are bigger than that in uninhibited HCl. To explain these phenomena, it might be important to consider the abilities of inhibition in different concentration of HCl are different with the same addition of inhibitor. Namely, the IEs decreased obviously with the concentration of acid. 3.2. Electrochemical measurements Fig. 9 shows the effect of molybdate and tungstate concentration on the anodic and cathodic polarization curves of CRS in 0.2 M HCl solution at 25 C. Fig. 9 reveals that the presence of increasing concentrations of inhibitors causes a markedly decrease in the corrosion rate i.e. shifts the anodic curves to more positive potentials and the cathodic curves to more negative potentials. This may be ascribed to adsorption of inhibitors over the corroded surface. 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 molybdate and tungstate concentration, were calculated from the curves of Fig. 9 and given in Table 7. Table 7
—0.2 —0.3
E (V/SCE)
—0.4 —0.5 —0.6
—0.7 —0.8 —6
—5
—4
—3
—2
—1
log (µA/cm2)
Fig. 9. Polarization curves for CRS in 0.2 M HCl containing different concentrations of MoO2 and 4 2 2 2 WO2 4 at 25 C. (h)—blank, (m)—MoO4 (200 mg/L), (d)—MoO4 (20 mg/L), (n)—WO4 (200 mg/L) and (s)—WO2 4 (20 mg/L).
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Table 7 Polarization parameters in the corrosion of CRS in 0.2 M HCl containing different concentration of inhibitors at 25 C Concentration
Ecorr (mV/SCE)
Icorr (lA/cm2)
bc (mV/dec)
ba (mV/dec)
IE (%)
Blank MoO2 4 (20 mg/L) WO2 4 (20 mg/L) MoO2 4 (200 mg/L) WO2 4 (200 mg/L)
485 473 481 454 473
517 222 165 83 140
150 160 158 180 166
66 43 45 46 46
– 57.1 68.1 83.9 72.9
reveals that the corrosion potential is slightly shifted to noble direction as the inhibitors concentrations are increased. Moreover, the corrosion current decreases markedly and the IE increases with the concentration of inhibitors. Table 7 also reveals that both anodic and cathodic Tafel slopes changed upon addition of increasing concentrations of inhibitors. This change in anodic and cathodic Tafel slopes in presence of inhibitors indicates that the inhibitors affect both anodic and cathodic reactions. Therefore, molybdate and tungstate can be arranged as mixed-type inhibitors in strong acidic media. These results are quite different from those in neutral media where molybdate and tungstate exhibited as anodic inhibitors [15,19,28,30]. Furthermore, results obtained from gravimetric and electrochemical measurements were in good agreement. However, IE% obtained by the electrochemical method was less than determined by the gravimetrical method. The difference between both methods could be attributed to the different experimental conditions of two methods. 4. Conclusions 1. Molybdate and tungstate act as good inhibitors for the corrosion of cold rolling steel in 0.1–0.5 M HCl, but best performances are seen in the case of 0.2 M HCl at 35 C. The concentration of HCl plays an important role in the inhibition efficiency. 2. The adsorption of molybdate and tungstate on the CRS surface basically obeys the Langmuir adsorption isotherm. However, the corrected equation by correction factor H can explain the results that the correlation coefficients of the straight line C/h–C approach 1, but the slopes deviate from 1. 3. Both molybdate and tungstate act as mixed inhibitors in strong acidic media. 4. The weight loss studies shows the corrosion of cold rolling steel in 0.1–0.5 M uninhibited HCl and in 0.2–0.5 M inhibited HCl support the kinetic equation proposed by Mathur and Vasudevan. The rate constant k obviously decreased after adding molybdate and tungstate. Acknowledgement 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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