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Dual Function of Benzotriazole as Copper Alloy Corrosion Inhibitor and Hydrochloric Acid Flow Improver
Anees A. Khadom
Chemical Engineering Department, College of Engineering, Diyala University,
Baquba City 32001, Diyala governorate, Iraq, e-mail: [email protected]
The corrosion of copper - nickel alloy in hydrochloric acid has been investigated at different temperatures, benzotrizole concentrations and corrosive solution velocities. Weight loss technique has been used to evaluate the corrosion rate data. Results obtained have proved that benzotrizole has a dual effect by reducing both metal corrosion and flow losses. Maximum inhibition efficiency was 91.5%, while maximum drag reduction was 52.4%. Several mathematical equations are suggested successfully to represent the data with high correlation coefficients. Molecular dynamic simulations have been also performed to investigate the adsorption behavior of the inhibitor on the copper alloy surface. One of the novelties of the given work is the analogy between the corrosion process and fluid flow, as well as the investigation of the dual effect of benzotriazole on the corrosion process and the flow losses.
Keywords: fluid flow, weight loss, rotating disc electrode, acid corrosion, drag reduction.
УДК 620.197
INTRODUCTION
Industrial equipment is often exposed to acidic action during the removal of scales and deposits.
One of such acids, hydrochloric acid, may cause corrosion problems after cleaning processes. Various attempts have been made to prevent or retard the destructive effect of corrosion on metals and alloys.
Using inhibitors is one of the most practical methods for protecting metals against corrosion, especially in acidic media [1]. Among numerous inhibitors,
N-heterocyclic compounds are considered to be the most effective corrosion inhibitors [2]. Up to now, various N-heterocyclic compounds are reported as good corrosion inhibitors for steel in acidic media, such as imidazoline derivatives [3], 1,2,3-triazole derivatives [4], benzotriazole [5]. Generally,
N-heterocyclic compounds exert their inhibition by adsorption on the metal surface through N heteroatom, as well as those with triple or conjugated double bonds or aromatic rings in their molecular structures. Furthermore, the adsorption of inhibitor on metal/solution interface is influenced by the chemical structure of inhibitor, the nature and charged surface of metal, the distribution of charge over the whole inhibitor molecule and the type of aggressive media [6]. Corrosion is a very important problem but not the only one. Another problem accompanying industrial flow system is friction losses. Therefore, the aim of present paper was to study the effect of temperature and velocity on the corrosion of copper - nickel alloy in hydrochloric acid in the presence of benzotrizole as a corrosion inhibitor and flow improver
EXPERIMENTAL WORK
The corrosion behavior of copper-nickel alloys, widely used in many industrial applications, was
studied using weight loss in the absence and presence of benzotriazole (BTA) in 5% HCl solution at different temperatures (35, 45, 55 and 65oC), different velocities (100, 200 and 300 rpm) and different inhibitor concentrations (0.001, 0.01 and 0.1M). Ring shape specimens of the Cu-Ni alloy were with the following dimensions: outside diameter 2.22 cm, width 1.5 cm, and thickness 0.13 cm, exposing a surface area of about 10 cm2 to corrosive media. These specimens were fixed on a rotating shaft. Angular velocity was generated using Tecquiment Limited Nottingham, UK, system rotation with a range of 0-3000 rpm. The experimental work was done as mensioend in our privuos work [7]. Specimens were washed by a detergent and flushed by tap water followed by distilled water, degreased by ana-lar benzene and acetone, then annealed in vacuum up to 600oC for one hour and cooled under vacuum to room temperature. Before each run, specimens of Cu-Ni were abraded in sequence using emery paper of grade numbers 220, 320, 400, and 600, then washed by running tap water followed by distilled water, then dried by clean tissue, degreased with benzene, dried, degreased with acetone, dried, and finally left in desicater over silica gel. Weighing the specimen was carried out using 4 decimals digital balance and its dimensions were measured with vernier. The metal samples for weight loss runs were completely immersed in 250-cm3 solution of corro-dant contained in a conical flask. They were exposed for a period of three days at a desired temperature, acid concentration, and inhibitor concentration. Weight losses were determined in absence and presence of the inhibitor. The data are expressed as mass loss per unit time per unit area. In the given work the unit of the corrosion rate was the mass loss: g/m2-day (gmd). The chemical composition of the Cu-Ni alloy was 0.148%Sn, 0.2%Fe, 0.134%Zn,
© Anees A. Khadom, Электронная обработка материалов, 2014, 50(2), 57-70.
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Table 1. Effect of velocity, inhibitor concentration and temperature on corrosion of Cu-Ni in 5% HCl solution in absence of inhibitor
No Inhibitor concentration (M) Temperature (oC) Velocity (rpm) Corrosion Rate (gmd) %IE
1 Nil 35 100 75.503 -
2 200 93.61 -
3 300 98.23 -
4 45 100 90.98 -
5 200 188.73 -
6 300 222.35 -
7 55 100 199.54 -
8 200 250.11 -
9 300 310.91 -
10 65 100 249.31 -
11 200 308.53 -
12 300 333.67 -
13 0.001 35 100 16.91 77.6
14 200 27.71 70.4
15 300 36.05 63.3
16 45 100 25.56 71.9
17 200 70.58 62.6
18 300 98.95 55.5
19 55 100 78.42 60.7
20 200 119.31 52.3
21 300 173.79 44.1
22 100 106.95 57.1
23 65 200 169.07 45.2
24 300 228.89 31.4
25 0.01 35 100 11.09 85.3
26 200 18.81 79.9
27 300 26.81 72.7
28 45 100 19.83 78.2
29 200 49.82 73.6
30 300 66.92 69.9
31 55 100 56.27 71.8
32 200 89.29 64.3
33 300 125.91 59.5
34 65 100 84.02 66.3
35 200 140.69 54.4
36 300 173.17 48.1
37 0.1 35 100 6.41 91.5
38 200 13.66 85.4
39 300 19.44 80.2
40 45 100 11.55 87.3
41 200 34.53 81.7
42 300 54.25 75.6
43 55 100 38.31 80.8
44 200 61.27 75.5
45 300 92.96 70.1
46 100 61.58 75.3
47 65 200 108.61 64.8
48 300 162.83 51.2
0.015%Al, 0.0003%P, 0.5%Sb, 0.0583%Pb,
0.0202%Si, 0.017%S, 0.0056%As, 10%Ni, and the remainder is Cu.
RESULTS AND DISCUSSIONS
different concentrations. Corrosion rates were obtained by the following equation [8]:
CR = weight loss (g) (i)
area (m2)*time (day)
Corrosion rate data
Table 1 shows the corrosion rate data of the Cu - Ni alloy in 5% HCl acid obtained at different temperatures and acid solution velocities in the absence and presence of BTA, a corrosion inhibitor, in
From the corrosion rate, the percentage inhibition efficiency was calculated using the following equation:
IE% = CRuninibit ~ CjRnhibit x 100 (2)
nhibit
59
where CRuninhibit and CRinhibit are the corrosion rates in the absence and presence of the inhibitor, respectively. It is clear from Table 1 that the corrosion rate increased with both temperature and acid solution velocity, while it decreased with the inhibitor concentration. The data reveal that the inhibition efficiency of BTA enhances with increasing its concentration in the solution. The increase in the inhibition efficiency observed at a higher inhibitor concentration indicating that more inhibitors are adsorbed onto the metal surface, thus providing wider surface coverage. The maximum inhibitor efficiency was 91.5% at 0.1M inhibitor concentration, 100 rpm velocity and 35oC. The minimum value was 31.4% at 0.001M inhibitor concentration, 300 rpm acid solution velocity and 65oC.
tration of water in solution expressed in M. According to the Van’t Hoff equation [12]:
A H 0
ln Kd =---------— + constant. (5)
UUS R T
Thus, the adsorption enthalpy AH°ads was obtained by the linear regression between ln Kads and 1/T (Fig. 2). In the experimental temperature range, the values of the adsorption enthalpy and adsorption entropy of the inhibition process could be approximately regarded as the standard adsorption enthalpy and adsorption entropy. Likewise, the standard adsorption entropy AS°ads could be calculated from the thermodynamic basic equation:
AG0, = AH°, - TAS0d . (6)
ads ads ads x '
Effect of inhibitor concentration and adsorption studies
It is generally accepted that organic molecules inhibit corrosion by adsorption on the metal/solution interface and that the basic information on the interaction between the inhibitor and the Cu - Ni alloy surface can be provided by the adsorption isotherm [9]. Surface coverage (0 = %IE/100) is often employed to character inhibitor adsorption. Many endeavors were made to fit these 0 values to various isotherms and the best fit was found with the Langmuir adsorption isotherms (Fig. 1) [10]:
C
0
Ka
-+ C.
(3)
Where C is the concentration of inhibitor, Kads is the adsorptive equilibrium constant representing the interaction of additives with the metal surface, and 0 is the surface coverage. The obtained parameters, such as linear regression coefficient (R2) and Kads, are listed in Table 2. The results show that all the linear regression coefficients and all the slopes are close to unity, which further confirms that the adsorption of BTA in 5% HCl follows the Langmuir adsorption isotherm. It is well known that Kads depicts the interaction power between an adsorbate and an adsorbent. As seen from Table 2, the value of Kads decreased with the increase in temperature and acid solution velocity, suggesting that more efficient adsorption was obtained at a lower temperature and velocity. So the inhibition efficiency decreased with the increase in temperature and acid motion as the result of the weakening of adsorption of BTA on the Cu - Ni surface. The values of the standard free energy of adsorption were obtained from the following equation [11]:
AGl = -RT ln(55.55 Kads) (4)
where R is the gas constant, T is the absolute temperature and the value of 55.55 is the molar concen-
All the obtained thermodynamic parameters are listed in Table 2. The negative value of AH°ads also shows that the adsorption of the inhibitor is an exothermic process [13], indicating that the adsorption is strengthened with the decrease of temperature. The values of AS°ads agree with the result of other researchers [14, 15]. It is a well-known fact that adsorption is an exothermic phenomenon (AH°ads < 0) accompanied by a decrease in entropy AS°ads [16]. Commonly, the adsorption type was regarded as physisorption whose inhibition action due to the electrostatic interactions between the charged molecules and the charged metal surface when the values of AG0ads reached up to -20 kJ-mol"1, while the values around -40 kJ-mol-1 or smaller, were seen as chemisorption, which is due to the charges sharing or a transfer from the inhibitor molecules to the metal surface to form a covalent bond [17]. The value of AG0ads in the given work is around -29 kJ-mol-1; suggesting that the adsorption of the inhibitor involves two types of interaction: chemisorption and physisorption.
Effect of temperature and kinetic parameters studies
Kinetic model was employed to further explain the inhibition properties of the inhibitor. The apparent activation energy for the corrosion process is calculated from the Arrhenius equation [18, 19]:
E
ln CR = ln A--------^ (7)
RT
where Ea is the apparent activation energy, A is the pre-exponential factor, T is the absolute temperature, R is the gas constant and CR is the corrosion rate. The Arrhenius plots of ln CR vs 1/T for the blank and various concentrations of BTA at the temperatures studied and 100 rpm are shown in Fig. 3. Similar figures were also obtained for other conditions. The values of Ea and A were calculated from the
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Fig. 1. Langmuir adsorption isotherms of BTA on Cu - Ni alloy surface at different conditions. Table 2. Adsorption parameters of BTA on Cu - Ni alloy surface at different conditions
T (oC) V (rpm) R2 Slope Kads (M-1) AG°ads (kJ-mol-1) AH0ads (kJ-mol-1) AS°ads (kJ-K'1-mol'1)
35 100 0.9998 1.088 2000 -29.75 - 37.58 -0.218
200 0.9988 1.166 2000 -29.75 -0.218
300 0.9978 1.239 1250 -28.54 -0.214
45 100 0.9969 1.138 1350 -29.67 - 36.83 -0.209
200 0.9988 1.216 1250 -29.47 -0.208
300 0.9999 1.315 1111 -29.16 -0.207
55 100 0.9998 1.228 1000 -29.79 - 34.27 -0.195
200 0.9998 1.311 666 -28.68 -0.191
300 0.9999 1.409 555 -28.18 -0.190
100 0.9998 1.327 998 -28.66 -0.181
65 200 0.9998 1.112 458 -26.69 -32.33 -0.178
300 0.9988 1.301 297 -24.22 -0.172
slopes and intercepts of the Arrhenius plots, respectively. All the parameters are given in Table 3. All the linear regression coefficients (R2) are close to unity, indicating that the Cu - Ni corrosion in 5% HCl can be elucidated using the kinetic model. According to Eq. (7), the corrosion rate of the Cu - Ni alloy depends on Ea and A. As seen from Table 3, the value of A in the presence of BTA is higher than
that of the blank, so a lower corrosion rate of the Cu - Ni alloy is mostly lies on the higher apparent activation energy. A higher value of Ea was reported as physical adsorption that occurred in the first stage [20, 21]. The adsorption mode involves the change of the activation energy of the corrosion process with the addition of an inhibitor. According to Popova et al. [22], assuming that the electrochemical
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Fig. 3. Arrhenius plots for Cu - Ni alloy in 5% inhibited HCl acid at 200 rpm. Table 3. Kinetics parameters of Arrhenius equation and transition state equation
C (M) V (rpm) Arrhenius equation Transition state equation
R2 A (gmd) Ea (kJ-mol-1) R2 AHa (kJ-mol-1) ASa (J-K-1-mol-1)
Nil 100 0.9101 5.8x1008 41.55 0.9149 38.22 -86.64
0.001 0.9345 2.3x1012 66.31 0.9221 61.55 -22.68
0.01 0.9777 3.9x1012 68.35 0.9689 65.43 -13.33
0.1 0.9615 7.5x1012 73.57 0.9639 70.55 -5.84
Nil 200 0.9948 3.4x1006 28.22 0.9478 25.75 -80.99
0.001 0.9791 9.9X1011 61.42 0.9817 59.11 -21.84
0.01 0.9832 3.7x1012 66.68 0.9888 63.13 -11.83
0.1 0.9811 4.9x1012 70.83 0.9887 67.11 -11.17
Nil 300 0.9633 6.2x1005 25.35 0.9947 22.62 -78.01
0.001 0.9621 6.9x1010 59.56 0.9856 57.45 -19.49
0.01 0.9878 4.1x1011 63.65 0.9268 61.56 -8.15
0.1 0.9841 2.8x1012 66.66 0.9248 65.07 -11.17
Fig. 4. Transition state plots for Cu - Ni alloy in 5% inhibited HCl acid at 200 rpm.
corrosion is a heterogeneous reaction, the preexponential factor in the Arrhenius equation A is related to the number of active centers. There are two possibilities related to these active centers with different Ea on the metal surface: In the first case, the activation energy in the presence of inhibitor molecules is lower than that of pure acidic media
(Ea,i„h < EaHCl), a smaller number of more active sites remain uncovered, which takes part in the corrosion process [22]. In the other case, (Eainh > EaHCl) an inhibitor is adsorbed on the most active adsorption sites (having the lowest energy) and the corrosion process takes place chiefly on the active sites (having higher energy). In the experiment for the
62
given study, the values of A and Ea in the presence of inhibitor molecules are higher than those in a pure hydrochloric acid, which are consistent with the latter mentioned above. This means that the adsorbed BTA molecules block the most active sites, while the sites of a higher activation energy, which are greater in number, take part in the subsequent corrosion [21]. An alternative formulation of the Arrhenius equation is the transition state equation [23]:
CR = exp№ 1 expf-^a
Nh l R ) l RT
(8)
where h is Plank's constant and N is Avogadro's number. The values of the entropy of activation ASa and of the enthalpy of activation AHa were obtained by drawing of information from equation 8 as ln (CR/T) vs (1/T) as shown in Fig. 4. These values were also listed in Table 3. The behavior of AHa values was similar to Ea. As observed, for all cases Eact > AHact by a value approximately equal to RT. From the thermodynamic and kinetic points of view, the unimolecular reactions are characterized by the following equation [24]:
E -AH = RT. (9)
The negative values of ASa pointed to a greater order produced during the process of activation. This can be achieved by the formation of an activated complex that represents association or fixation with a consequent loss in the degree of freedom of the system during the process [25].
Effect of velocity and fluid flow studies
Table 1 shows that under various conditions, as the velocity increased, the corrosion rate also increased. The effect of flow on the corrosion rate of copper has been used in a number of instances to determine whether corrosion is under activation, diffusion, or mixed control. It was demonstrated that the linear increasing of the corrosion rate with velocity indicates that the corrosion proceeds under diffusion control, while its slightly increase indicate the mixed control process and no effect of velocity on the activation control process. Figure 5 shows the effect of velocity on the corrosion rate in the absence of an inhibitor, which suggests the mixed control corrosion process. The higher the temperature, the stronger the effect of the speed of rotation on the corrosion process. A similar behavior was observed in the presence of inhibitor. The enhancement of the fluid flow generally results in the metal total weight loss by supplying the corrosives at faster rates. Relative metal/environment motion thins the quiescent layers at the metal leading to less restriction of corrosives by diffusion process. When velocity becomes extremely high mechanical effects add to cor-
rosion and increase the damage to metals (i.e. erosion). One of the important things of present work is the analogy between corrosion process and fluid flow, and to explore the dual effect of BTA as corrosion inhibitor and flow improve material. Table 4 shows the physical properties of the corrosive solution in the presence and absence of BTA at different conditions, the Reynolds numbers, and Funning friction factor. The Reynolds number can be defined as follows:
Re = (10)
h
where p is the solution density (kg-m-3), u is the linear velocity (m-s-1), d is the sample diameter (m), and p is the dynamic solution viscosity (Pa-s). Blasius’ equation can be used to evaluate the values of the Fanning factor as:
f = 0.0791 R-025. (1 1)
Equation 11 is valid for turbulent flow and smooth pipes [26]. The rotating working cylinder electrode was assumed to be the pipe, and the angular velocity was converted to linear velocity. Flow improvement can be measured using the equation of drag reduction [27]:
%Dr = APb ~AP“ x 100% (12)
APb
where %Dr is percentage of the drag reduction, APb and APa is the pressure drop before and after addition of BTA. Pressure drop as a function of the Fanning friction factor can be written in the form [26]:
ap = 4 f (L) £f (13)
d 2
where L is the cylinder length (m). Substituting Eq. 13 in Eq. 12 and eliminating the constants from denominator and numerator, the following equation can be obtained:
%Dr = fun fin x 100% (14)
f
J un
where fun and fin are the Fanning friction factors of uninhibited and inhibited 5% HCl solutions. Table 4 and Fig. 6 show that the values of %Dr increased with a higher inhibitor concentration. This behavior is attributed to the formation of an inhibition layer that may reduce the friction between metal and solution. However, the inhibition efficiency of BTA was greater than its drug reduction. As noticed above, many variables can affected the values of the drag reduction. Therefore, the dimensional analysis was used in the given work for grouping the significant quantities into a dimensionless group in order to reduce the number of variables appearing and to make the result compact and applicable to all similar situa-
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Fig. 5. The effect of velocity on the corrosion of Cu-Ni Alloy in 5% HCl solution in absence of inhibitor.
Table 4. Physical properties of 5% inhibited HCl solution at different operating conditions, Reynolds numbers, and friction factors
BTA Temperature, oC Density, kg-m-3 Viscosity, Pa-s x 10-4 Velocity (rpm) Re x 103 f x 10-4 %Dr
35 1018.01 6.94 3.77 100.84 0
Nil 45 1014.10 5.85 4.46 96.73 0
55 1009.60 5.01 5.20 93.12 0
35 9999.51 6.89 37.36 56.81 43.66
0.001M 45 9999.43 5.85 44.09 54.58 43.57
55 9999.26 5.01 100 51.53 52.49 43.63
35 9999.42 6.50 39.63 56.05 44.41
0.01M 45 9999.33 5.51 46.83 53.76 44.47
55 9999.20 4.71 54.77 51.71 44.46
35 9998.61 3.62 71.52 48.36 52.04
0.1M 45 9998.55 3.01 85.64 46.23 52.21
55 9998.43 2.54 101.45 44.32 52.42
35 1018.01 6.94 7.56 84.83 0
Nil 45 1014.10 5.85 8.94 81.33 0
55 1009.60 5.01 10.41 78.31 0
35 9999.51 6.89 74.82 47.82 43.62
0.001M 45 9999.43 5.85 88.30 45.88 43.58
55 9999.26 5.00 200 103.20 44.13 43.64
35 9999.42 6.50 79.37 47.12 44.45
0.01M 45 9999.33 5.51 93.78 45.21 44.41
55 9999.20 4.71 109.69 43.46 44.51
35 9998.61 3.60 143.23 40.66 52.06
0.1M 45 9998.55 3.01 171.51 38.86 52.21
55 9998.43 2.54 203.17 37.25 52.43
35 1018.01 6.94 11.33 76.66 0
Nil 45 1014.10 5.85 13.39 73.52 0
55 1009.60 5.01 15.59 70.78 0
35 9999.51 6.89 112.02 43.23 43.61
0.001M 45 9999.43 5.84 132.21 41.48 43.57
55 9999.26 5.01 300 154.51 39.89 43.64
35 9999.42 6.51 118.84 42.61 44.41
0.01M 45 9999.33 5.51 140.42 40.86 44.42
55 9999.20 4.71 164.23 39.29 44.49
35 9998.61 3.60 214.45 36.75 52.06
0.1M 45 9998.55 3.01 256.79 35.13 52.21
55 9998.43 2.54 304.21 33.68 52.41
tions. The drag reduction is influenced by the physical properties of the solvent and properties of the flow. The relationship may be written as:
AP = f (d, p, p,V, C, L, e, T). (15)
By applying the dimensional analysis, the following non-dimensional relation was proposed:
% Dr = f (Re, e/ d, L/d, C, T) (16)
or
%Dr = a (Re)6 ^ [^ (C )k (T)h (17)
where a, b, c, d, k and h are constants, and e is the roughness of metal. There are many assumptions and facts to be taken into account:
64
Fig. 6. Behavior of %IE and %Dr with inhibitor concentration at 35oC and 100 rpm.
1. The working electrode is smooth, i.e. roughness term is neglected.
2. The ratio L/d is constant.
3. From Table 4, the effect of temperature on %Dr is insignificant.
Then, Eq. 17 can be reduced to the following one:
% Dr = a Reb Ck. (18)
The method of least squares was used to determine the coefficients of correlation for the Reynolds number range (3779.11 - 304205.85), which yields the following equation with 0.9236 correlation coefficient.
%Dr = 45.12 Re002 C004. (19)
Mathematical and statistical studies
Statistical analysis can be used to determine the effects of each variable on the corrosion rate of the Cu - Ni alloy. One of these analyses is a nonlinear regression of the data of Table 1. A third-order fitting equation can be used to know whether the variables affect Cu - Ni individually or there is an interaction effect. Most of probabilities can be taken into account, the main effect of each variable and the interaction effect:
C R = b0 + bT + b2C + b3V + b4TC +
+b5TV + b6 CV + b7TCV + bT2 + (20)
+b9 C2 + b10V2 + b11T3 + b12 C3 + b13V3
where b0 is constant, b1, b2, b3 are linear coefficients, b4, b5, b6, b7 are interaction coefficients, b8, b9, b10 are quadratic coefficients and b11, b12, b13 are cubic coefficients. The above model can be estimated using the Levenberg-Marquardt method, and the following equation can be obtained with 0.9795 correlation coefficient:
CR =8150092-1309T -
- 1.1x 105 C +1.4 x 105 V -
-26TC + 0.012TV - 5.4CV - (21)
- 0.043TCV + 4.1T2 +1.1 x 107C2 +
+ 810.3V2 -1289 x 10-6 T3 +
+ 9.5 x 10-7 C3 +19 x 10-6 V3.
The analysis of variance (F-test) was used to test the significance of each effect in equation 21 [28]. The calculations are given in Table 5. An estimate of the variance Sb2 is obtained by dividing the experimental error variance Sr2 by the sum of squares of
each effect ZX2, as follows: Sb
Si
I x 2
where,
2 I e2
Sr =^y~ and у = N-n is the degree of freedom. The
significance of effects may be estimated by comparing the values of the ratio (b2/Sb2) with the critical value of the F-distribution at 95% confidence level (F0 95 = 6.61). If b2/Sb2 > 6.61, then the effect is significant. Thus, according to the results shown in Table 5, it appears that the cubic and most of quadratic effects are insignificant, while the interaction effects and the main effect of each variable are the dominated ones. The best response function is then conveniently written as follows:
CR = 8150092 - 1309T - 1.1x 105 C +
+ 1.4 x 105 V - 26 TC + 0.012 TV - 5.4 CV - (22)
-0.043TCV + 4.1T2.
Another equation can be suggested to represent the corrosion rate data. This model depends on the discussion mentioned above. Instead of using multiterms model (eq. 21), a one-term model can be used. A similar model was suggested successfully in our previous studies [29, 30]. It was found out that the Cu - Ni alloy corrosion varies exponentially with
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Table 5. Analysis of variance
Effect IX2 Coefficients, b (units) Variance S 2 S2 Sb - ^X2 F-value =b2/S2 F0.95 = 6.61
ba (gmd) 815x104
T 75300 b (gmd-K-1) -1309 0.763 224x104 S*
C 0.091 b2 (gmd-M-1) -11x103 1.77x10-26 203x1012 S
V 168x104 b3(gmd-rpm-1) 14x104 9212.4 212x104 S
TC 190.15 b4 (gmd-M-1K-1) -26 0.043 157x102 S
TV 35x108 b5 (gmd-rpm-1K-1) 0.012 1.33x10-16 107x1010 S
CV 4242.42 b6 (gmd-M-1rpm-1) 5.4 0.0041 6985.35 S
TCV 887x104 b7 (gmd-rpm-1M-1K-1) -0.043 2x10-11 96x106 S
T2 177x106 b8 (gmd-K'2) 4.1 3x10-9 5.3x109 S
C2 9x10-4 b9 (gmd-M-2) 11x106 33x1012 3.559 NS
V2 11x1010 *10 (gmd-rpm-2) 810.3 39x106 3.05 NS
T3 34x1016 bn (gmd-K-3) 13x10-6 4x107 3.11 NS
C3 9x1010 b 12 (gmd-M-3) 95x10-7 2.8x1021 3.2 x10-34 NS
V3 72x1024 bn (gmd-rpm-3) 19x10-6 39x1012 4.56 NS
* S: significant effect, NS: not significant effect
the absolute value of temperature (according to the Arrhenius eq.), in the same way it is proportional to the speed of rotation and BTA concentration as follows:
CR <x exp(—1—)(—)(V) (23)
temp. C
Re-arranging equation 23 we get:
CR = b14C—b'5Vb'6 exp(bz-) (24)
where b14, b15, b16, b17 are other coefficients and T is the absolute temperature (K). This equation can be estimated using the above-mentioned estimation method producing the following equation with
0.9773, correlation coefficient:
CR = 5.6 x 107C-0066 V0576 exp(—-47—). (25)
Generally, correlation coefficient up to 0.30 indicates a weak relationship and is of uncertain validity that between 0.50 and 0.70 indicates a significant relationship and is of practical importance, while the one above 0.90 means a strong relationship [30].
Figure 7 shows the predicted corrosion rates by a third-order polynomial model and an exponential model against experimental values. Eq. 25 is easier and more practical than Eq. 22 since it deals with only one term. The coefficient b17 in Eq. 25 can be compared with the slope of the Arrhenius equation (-E/R). The activation energy value by Eq. 25 equals to 45.53 kJ/mol, within the range of the activation energy given in Table 3. Also the value of coefficient b14 is comparable with the frequency factor values.
Theoretical and quantum chemical studies
Theoretical and quantum chemical calculations have been used to explain the mechanism of corrosion inhibition, [31, 32]. Quantum chemical calculations are proven to be a very powerful tool to understand the inhibition mechanism and to emphasize the experimental data [33, 34]. Through the method of quantum chemical calculations, structural parameters such as the frontier molecular orbital (MO), the highest occupied molecular orbital (HOMO), the
66
(a)
(b)
(c)
Fig. 8. (a) Optimized BTA structure; (b) HOMO distribution; (c) LUMO distribution.
lowest unoccupied molecular orbital (LUMO), dipole moment (ц) and the fraction of electrons (AN) transfer from inhibitors to the metal surface were calculated and correlated with corrosion inhibition efficiencies. The structure of the inhibitor was optimized by the ArgusLab 4.0.1 package. The quantum chemical parameters were estimated by the PM3-SCF method. The optimized minimum energy geometrical configurations of tested compounds are given in Fig. 8a. The computed quantum chemical parameters like the energy of the highest occupied molecular orbital (EHOMO), the energy of the lowest unoccupied molecular orbital (ELUMO), HOMO-LUMO energy gap and the dipole moment are summarized in Table 6. It has been well documented in literature [35] that the higher the value of EHOMO of the inhibitor, the greater is the ease of the inhibitor to offer electrons to unoccupied d orbital of the metal atom, and the higher is the inhibition efficiency of the inhibitor. Furthermore, the lower the ELUMO, the easier is the acceptance of electrons from the metal atom to form feedback bonds. The gap between
HOMO-LUMO energy levels of molecules was another important parameter that needs to be considered. The smaller the value of AE of an inhibitor, the higher is the inhibition efficiency of that inhibitor [36]. Further higher values of dipole moment will favor the enhancement of corrosion inhibition [37]. The number of transferred electrons (AN) was also calculated according to Eq. 26 [37]
AN = XCu ~ Xinh (26)
2(hcu - ninh)
where XCu and Xinh denote the absolute electronegativity of iron and the inhibitor molecule, respectively; HCu and ninh denote the absolute hardness of iron and the inhibitor molecule, respectively. These quantities are related to the electron affinity (A) and ionization potential (I)
X = I^A 2
П =
I - A 2
(27)
67
Table 6. Quantum chemical parameters for BTA on Cu - Ni alloy
FHQMQ flumo AF Dipole AN
(eV) (eV) (eV) moment, ц(debye)
-9.438 -0.685 8.795 3.751 0.066
Table 7. Mulliken atomic charges for BTA
No Atom Charge No Atom Charge
1 C -0.0909 8 N -0.0999
2 C -0.2187 9 N 0.0916
3 C -0.1517 10 H 0.1939
4 C -0.1747 11 H 0.1996
5 C -0.1763 12 H 0.2153
6 C -0.1542 13 H 0.2079
7 N -0.0092 14 H 0.1672
In turn, I and A are related to FHOMO and FLUMO:
1 = - ^HOMO, (28)
A = - F
^ -^LUMQ-
Values of X and ^ were calculated by using the values of I and A obtained from the quantum chemical calculation. The theoretical values of XCu and ^Cu are 4.48 and 0 eV/mol, respectively [38]. The fraction of electrons transferred from the inhibitor to the iron molecule (AN) was calculated. According to other reports [39], the value of AN showed an inhibition effect as a result from electrons donation. According to Lukovits et al. [40], if AN < 3.6, then the inhibition efficiency increases with increasing electron-donating ability at the metal surface. In this study, BTA was the donor of electrons, and the copper oxide surface was the acceptor. BTA was bound to the copper oxide surface and thus formed an in-hibitive adsorption layer against corrosion. The Mul-liken charge distributions of the inhibitors are presented in Table 7. It can be readily noticed that carbon and nitrogen atoms have higher charge densities. The regions of the highest electron density are generally the potential sites for the electrophiles attacked [40, 41]. The use of the Mulliken population analysis to probe the adsorption center of inhibitors has been reported earlier [42]. Based on Mulliken charge calculations, highest electron densities were located on N and C atoms implying that those atoms were active centers, which have the strongest ability of bonding to the Cu - Ni surface. Qn the other hand, HQMQ (Fig. 8b) was mainly distributed on the area containing carbon and nitrogen atoms and this area is probably the primary site of the bonding. It was found out that those inhibitors apart from existing in the cationic form can also interact with the metal surface through the electrostatic attraction. This interaction with the metal surface with several numbers of active centers leads to the formation of a protective layer on the Cu - Ni alloy surface.
Corrosion and inhibition mechanism of Cu-Ni alloy
Anodic dissolution of copper in chloride media has been studied extensively [43-45]. The accepted anodic reaction of HCl solution is the dissolution of copper through oxidation of Cu (0) to Cu+:
Cu ^ Cu+ + e. (29)
Then Cu+ reacts with chloride ion from the solution to form CuCl
Cu + + Cl- ^ CuCl. (30)
Insoluble CuCl precipitates on the copper surface. The CuCl species has poor adhesion, is unable to produce enough protection for the copper surface, and transforms into the sparingly soluble cuprous chloride complex, CuCl-2 [46];
CuCl + Cl-^ CuCl-. (31)
It has also been reported that CuCl2 adsorbed on the surface dissolves by further oxidation [45]
CuCl-dds ^ Cu2+ + 2Cl- + e. (32)
It is also stated that the anodic dissolution of copper in the acidic chloride solution is controlled by both electro dissolution of copper and diffusion of CuCl-2 to the solution bulk [45, 46]. These conclusions agree with the results of the given work. The cathodic corrosion reaction in an aerated acidic chloride solution is:
4H+ + O2 + 4e-^ 2H2O. (33)
The total corrosion reaction of copper in acidic chloride solutions is then as follows:
2Cu + 4H+ + 4Cl + O2 ^ (34)
^ 2Cu2+ + 4Cl + 2H2O.
The inhibition mechanism depends on the effect of an inhibitor in the above equations. BTA is an organic compound consisting of benzene and triazole rings with a formula of C6H5N3 which can exist in the following tautomeric forms [47]:
BTA can exist in a protonated form (BTAH), which can be presented as below:
This last structure shows that BTAH can act as a weak acid by releasing a proton or a base by accepting a proton to one of the nitrogen lone pairs of
68
electrons. On the other hand, BTAH can utilize the lone pairs of electrons existing on nitrogen atoms to bond itself to the copper surface. In this way, a coordination compound can be formed on the copper alloy surface that can act as an inhibitor against corrosion. This makes benzotriazole one of the most efficient inhibitors for the corrosion of copper and its alloys in queous media [48, 49]. Generally, BTAH acts as an anodic or mixed-type corrosion inhibitor [50-52] via its physisorpion [53, 54] on the copper surface that follows, in most cases, the Langmuir isotherm [53, 54]. One of proposed mechanism attributed the inhibition efficiency of BTAH to the formation of a protective film of Cu(i) BTA complex on the metal surface, i.e.
Cu + [BTAH] о Cu(I)[BTA] + H++ A. (35)
There is an ample evidence that the Сцр BTA complex exists in a polymeric form which further stabilizes the film:
n(Cu(I)(BTA)) o[Cu(I)BTA] . (36)
This polymeric chain has alternating copper atoms and BTAH molecules with the following proposed structure [55]
These were primarily concerned with the equilibrium properties of the protective film and its effects on the anodic and cathodic partial reactions. Youda et al. [56] have suggested that adsorption and complex formation are in equilibrium, i.e.
n BTAHads + nCu о ^Cu^BTA] + n H + + ne~.(37)
Equation 37 reveals that by increasing the pH value, both the potential in the noble direction and the inhibitor concentration favor the formation of the protective polymeric complex, while adsorption becomes favorable in acidic media, at a lower inhibitor concentration and when the potential changes to more negative values, with the rate of adsorption of BTAH on a Cu2O surface being faster than that on a CuO surface [47]. The inhibition efficiency of BTAH in a specific medium can be explained via its chemical properties. Based on the pH of the medium, BTAH can exist in either one of three forms [56]. In strongly acidic media, BTAH can exist in a protonated form BTAH+2, while in weakly acidic, neutral, and weakly alkaline media it has the form BTAH. On the other hand, in strongly alkaline media it is BTA". Considering that Cu surface is positively charged in the solution, it can easily be con-
cluded which form is more and which is less favorable.
CONCLUSIONS
The main conclusions of the given study can be summarized as follows:
1. Measurements have demonstrated that under the chosen experimental conditions BTA offers sufficient protection against corrosion for the copper -nickel alloy in 5% HCl solutions.
2. BTA adsorbs on a metal surface and, according to the Langmuir adsorption isotherm, forms a protective monolayer.
3. Heat of adsorption values are within the range of physical and chemical adsorption.
4. Corrosion rate of the copper - nickel alloy increases with temperature, according to the Arrhenius equation, with a higher activation energy in the presence of BTA.
5. Corrosion rate of the copper - nickel alloy increases with the speed of rotation. The nonlinear increase of corrosion rate with velocity indicates that the reaction takes place according to both the mass transfer effect and the charge surface reaction.
6. Molecular dynamic simulations have been performed to investigate the adsorption behavior of BTA on the copper alloy surface.
7. This study has demonstrated that BTA has a dual effect in the Cu - Ni alloy/HCl/system. In addition to being a good corrosion inhibitor effect, it acts as a flow improver agent.
ACKNOWLEDGEMENTS
This work was supported by Baghdad University, College of Engineering, Chemical Engineering Department, which is gratefully acknowledged.
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Received 07.02.13 Accepted 10.09.13
Реферат
Объектом исследования была коррозия медноникелевого сплава, погруженного в соляную кислоту, при разных температурах, концентрациях бензотриа-зола и скоростях коррозии. Для оценки скорости коррозии использовался метод определения потерь в массе. Полученные результаты показали двойной эффект применения бензотриазола: уменьшилась как коррозия сплава, так и потери в потоке соляной кислоты. Максимальная эффективность ингибирования составила 91,5%, а максимальное уменьшение наволакивания меди было равно 52,4%. Предложен ряд уравнений, описывающих полученные экспериментальные данные с высоким коэффициентом соответствия. Динамическое молекулярное моделирование было применено при исследовании режима адсорбции ингибитора на поверхности медного сплава. Новым в данной работе является проведение аналогии между процессом коррозии и поведением потока соляной кислоты, а также исследование двойного эффекта применения бензотриазола: как ингибитора коррозии и добавки, способствующей уменьшению потерь соляной кислоты.
Ключевые слова: поток жидкости, потери в массе, вращающийся дисковой электрод, кислотная коррозия, уменьшение наволакивания меди.
