REACTION KINETICS OF CORROSION OF MILD STEEL IN PHOSPHORIC ACID

Journal of the University Aprael of S. Chemical Yaro, Rafal Technology K. Wael, Anees and Metallurgy, A. Khadom45, 4, 010, 443-448 REACTION KINETICS OF CORROSION OF MILD STEEL IN PHOSPHORIC ACID Aprael S. Yaro, Rafal K. Wael, Anees A. Khadom Department of Chemical Engineering, College of Engineering, Baghdad University, Baghdad, Iraq E-mail: aneesdr@gmail.com Received 14 June 010 Accepted 1 November 010 ABSTRACT The effect of different temperatures and acid concentrations on the corrosion of mild steel in phosphoric acid were studied in this work. The effect of temperature was explained by application of Arrhenius equation and transition state theory, while the acid concentration effect was explained using reaction kinetic equations. The combined effect of temperature and acid concentration was modeled using a nonlinear regression method. A detail of thermodynamic parameters of activation (E, ÄH * and ÄS * ) and kinetic studies for the corrosion reaction were obtained. Nonlinear corrosion rates as a function of temperature and acid concentration were estimated with a good prediction of the corrosion rates values. The values of activation energy E and enthalpy of activation ÄH * decrease with increase of acid concentration indicating the increasing in reaction rate. Entropy of activation ÄS * tends to lower values with increasing of acid concentration which indicates that the activated complex is more orderly relative to the initial state. The observed corrosion rate values from the experimental data were in a good agreement with those predicted by the mathematical equation. Keywords: reaction kinetics, activation parameters, phosphoric acid, corrosion, mild steel. INTRODUCTION Corrosive environments have received a considerable amount of attention because of their attack on materials. One of these environments are the acid solutions which often used in industry for cleaning, descaling and pickling of steel structures, processes which are normally accompanied by considerable dissolution of the metal. The information about corrosion rate and kinetic parameters may be helpful in the corrosion control. Activation parameters for some systems can be estimated either from the Arrhenius equation (eq. 1) or from transition state theory (eq. ) [1]: E k= A Exp (1) RT * * RT H S k = exp exp Nh RT R () where k is reaction rate, A - modified frequency factor (pre-exponential factor), E - activation energy (J mol -1 ), R - gas constant (8.314 J mol -1 K), T - absolute temperature (K), ÄH * - enthalpy of activation, DS * - entropy of activation, N - Avogadro s number (6.0x10 3 molecule.mol -1 ), h - Plank s constant (6.66x10-34 J sec mol -1 ). A comparison of eq. with Arrhenius equation indicates that the energy of activation E is related to the enthalpy of activation ÄH *. The pre-exponential factor (A) is now RT S exp Nh R. Chemical kinetics is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the rate of a chemical reaction and yields information about the reaction mechanism, as well as the construc- 443

Journal of the University of Chemical Technology and Metallurgy, 45, 4, 010 tion of mathematical models that can describe the characteristics of a chemical reaction. In corrosion reactions like almost all chemical reactions, normally as the concentration of a corrosive acid media is increased, the corrosion rate likewise increases. This is primarily due to the fact that the amounts of hydrogen ions, which are the active species, are increased as the acid concentration is increased []. Corrosion rate data as a function of acid concentration can be used to show the rate dependence of hydrochloric acid concentration. The first model proposed by Mathur and Vasudevan [3] are described by the following eq. 3: r BC = (3) ke where C is the acid concentration, B - constant for the reaction studies. This model can be compared with the conventional equation of chemical reaction kinetics: r n = (4) kc where n is the order of reaction. The aim of this research is to study the effect of temperature and acid concentration using Arrhenius equation, transition state equation and reaction rate kinetic equations for the corrosion of mild steel in H 3 acid. EXPERIMENTAL Mild steel specimens were used as working electrodes throughout the study. The composition (wt.%) of the mild steel was: Fe - 98.9; C - 0.199; Si - 0.14; Mn - 0.053; Al - 0.0514; Cr - 0.009; Cu - 0.0468; Ti - 0.0089; V - 0.0076; Ni - 0.0035; Co - 0.0091; Mo - log (Corrosion Rate) (gmd) 3.4 3. 3.0.8.6.4. 0.0095 0.00305 0.00315 0.0035 0.00335 0.00300 0.00310 0.0030 0.00330 0.005 and Pb - 0.05. The exposed area of the specimens was 10.8 cm. The specimens were cleaned according to ASTM standard G1-03 [4]. The specimens were fully immersed for two hours in 50 cm 3 corrosive solutions of 0.5, 1.0, 1.5 and.0 M H 3 at 30, 40, 50 and 60 o C. RESULTS AND DISCUSSION Table 1 shows 16 runs of weight loss experimental results of mild steel corrosion in 0.5, 1.0, 1.5, and.0 M H 3 acid solutions as function of temperature. As shown in Table 1, the corrosion rate increases with increasing the acid concentration and temperature. The values of activation energies and frequency factors are evaluated using Eq. 1, by plotting 1 ln( Corr.Rate) Vs. T 1/T (K -1 ) 0.5 M H 3 PO4 1.0 1.5 Fig. 1. Arrhenius plot for the corrosion of mild steel at different concentration of H 3. as shown in Fig. 1 and these values are listed in Table. Eq. can be rearranged in the form of straight line Table 1. Effect of temperature and H 3 acid concentration on the corrosion rate (g/m d) of mild steel. H 3 conc.(m) Temperature ( C) 30 40 50 60 0.5 4.03 340 596.67 978.67 1.0 55.77 449.9 783.00 134.77 1.5 344.33 631.4 1151.75 1973.8.0 360.36 688.56 165.17 1984.83 444

Aprael S. Yaro, Rafal K. Wael, Anees A. Khadom ln (Corrosion Rate/T) (gmd/k).0 1.8 1.6 1.4 1. 1.0 0.8 0.6 0.4 0. 0.0-0. -0.4 0.0095 0.00305 0.00315 0.0035 0.00335 0.00300 0.00310 0.0030 0.00330 1/T (K -1 ) 0.5 M H 3 PO4 1.0 1.5 Fig.. Transition state plot for the corrosion of mild steel at different concentratio of H 3. ln (Corrosion rate) (gmd 7.8 7.6 7.4 7. 7.0 6.8 6.6 6.4 6. 6.0 5.8 5.6 5.4 30 o C 40 50 60 5. 0.4 0.6 0.8 1.0 1. 1.4 1.6 1.8.0. C (M) Fig. 3. Rate equation as a function of acid concentration at different temperatures. equation in order to find the values of enthalpy and entropy of activations. The rearranged equation is: * * k R S H ln = ln + (5) T Nh R RT Eq. 5 can be drawn as shown in Fig. as K 1 ln Vs. T T. The values of enthalpy of activation and entropy of activation can be evaluated from the slope and intercept, which are also given in Table. The values of DH * was 48.76 (kj mol -1 ) at 0.5 M acid concentration. This value decreases with increasing the acid concentration, which indicates that the reaction needs low energy to occur with increasing the acid concentration. This means that the energy barrier of corrosion reaction decreases as the concentration of phosphoric acid increases and activated complex or transition state complex can be formed faster with the acid concentration increasing. The positive sign of DH * reflects the endothermic nature of the steel dissolution process. The values of DS * are negative at all acid concentrations. The negative values decrease with increasing the acid concentration. The corrosion of iron in acid solutions takes place with hydrogen depolarization. The spontaneous dissolution of iron can be described by anodic dissolution reaction ++ Fe = Fe + e accompanied by the corresponding cathodic reaction H + + e = H [5]. According to Abiola [6], the corrosion of metals in acidic solutions is cathodically controlled by the hydrogen evolution reaction which occurs in two steps: H + + e H ads (6) H ads +Hads H (7) Table. Values of enthalpy of activation, entropy of activation, activation energy and frequency factor at different acid concentrations. Arrhenius model (eq. 1) Transition state model (eq. ) C (M) A, d -1 E, kj mol -1 R ÄH *, kj mol -1 * ÄS J mol -1 K -1 0.5 0.34 10 10 48.76 0.994 46.11-71.33 0.9935 1.0. 10 10 48.05 0.9998 45.40-55.78 0.9998 1.5 9.5 10 10 46.99 0.9999 43.35-43.56 0.9999.0 7. 10 10 41.13 0.997 39.47-45.81 0.9968 Average 4.8 10 10 46.3 0.9977 43.58-54.1 0.9975 R 445

Journal of the University of Chemical Technology and Metallurgy, 45, 4, 010 ln (Corrosion rate) (gmd) 7.8 7.6 7.4 7. 7.0 6.8 6.6 6.4 6. 6.0 5.8 5.6 5.4 30 o C 40 50 60 5. -0.8-0.6-0.4-0. 0.0 0. 0.4 0.6 0.8 ln C (M) Fig. 4. Conventional chemical reaction rate as a function of acid concentration at different temperatures. predicted corrosion rate (gmd) 400 00 000 1800 1600 1400 100 1000 800 600 400 00 line equation Y = 30.3704+0.966*x 0 0 00 400 600 800 1000 100 1400 1600 1800 000 00 observed corrosion rate (gmd) Fig. 5. Observed Vs predicted corrosion rates of mild steel in H 3 obtained using combined influence model. The rate-determining step for the hydrogen evolution reaction is the recombination of adsorbed hydrogen atoms to form hydrogen molecules (eq. 7). The transition state of the rate determining recombination step represents high orderly arrangement relative to initial state and hence a negative value for the entropy of activation was obtained. In other words, these indicate that the activated complex is more orderly relative to the initial state. From eq. 1, it can be seen that at given temperature, the value of corrosion rate is jointly determined by the activation energy and pre-exponential factor. Values of E vary in the same way as the values of DH *.The values of E, approximately, agree with the literature data of E for iron and steel in phosphoric acid [7, 8]. The kinetic constants can be obtained by rearranging eq. 3 and eq. 4; these equations can be rewrite in a linear form: ln r = ln k + BC (8) By plotting ln r vs. C, as shown in Fig. 3, the values of B and k can be obtained from the slopes and intercepts of these lines. The second kinetic equation can be written as: ln r = ln k + n ln C (9) and can be drawn as shown in Fig. 4. The values of n and k can also be obtained from the slopes and intercepts of these lines. Table 3 shows the values of the kinetic parameters. The changes in temperature have great effect when the rate-determining step is the activation process. In general, if the diffusion rates are doubled for a certain increase in temperature, activation process may be increased by 10-100 times depending on the magnitude of the activation energy. The values of the rate constants k increase with increasing of temperature and this is observed from both models by Jianguo et al. [9], who study the corrosion of low carbon steel in acid media at different concentrations. The value of slope B is constant up to 1.5 M acid concentration and then it reduced to a lower value for acid concentration greater than 1.5 M. The change in slope (value of B) may be due to the formation of a tightly adsorbed more protective layer of corrosion products on the metal surface at high acid concentration [3]. In this study, the values of B increase with temperature increase indicating that the mechanism of corrosion reaction is changed at different acid concentrations. The first model was more suitable to represent the corrosion reaction process of mild steel in phosphoric acid, with higher values of correlation coefficients (R ), as compared with the values obtained with the second model. This is due to high increase in corrosion rate with acid concentration increasing, thus the exponential representation of the corrosion rate data is better than the linear one. Mathur [3] states that the conventional rate eq. 4 differs from the present rate eq. 3 in the concentration term. If BC<<1, the exponential term (e BC ) can be expanded and eq. 3 can be written as: 446

Aprael S. Yaro, Rafal K. Wael, Anees A. Khadom Table 3. Kinetics parameters of the first model and second conventional reaction model T ( o C) Mathur and Vasudevan model (eq. 3) Conventional reaction model (eq. 4) k, gmd B, gmd M -1 R k, gmd n R 30 181.3 0.345 0.9835 79.8 0.367 0.911 40 70.4 0.491 0.9653 480.6 0.53 0.9735 50 468.7 0.58 0.9757 856.6 0.569 0.963 60 796.3 0.504 0.9633 1417.9 0.554 0.9480 Average 49. 0.467 0.9719 758.75 0.506 0.9515 r = k( 1+ BC) (10) Relation (8) indicates that r varies linearly with concentration C only in very low concentration of electrolyte solutions, as in conventional rate eq. 4. Hence, eq. 4 is only special case of the more general eq. 3. Also, eq. 3 appears to be more valid than the linear rate eq. 4 at high acids concentration. Combined influence of temperature and acid concentration The combined effect of temperature and acid concentration on the corrosion of mild steel in phosphoric acid can be evaluated using Arrhenius equation eq. 1 and eq. 3, since both equations are more suitable in representing the corrosion rate data than transition state theory (eq. ) and conventional equation of chemical reaction (eq. 4) depending on the values of correlation coefficients. Therefore, the combined equation can be obtained by substituting eq. 1 in eq. 3, so that: E r = Aexp( )exp(bc) (11) RT The values of A, E and B were defined previously. These values can be non-linearly estimated using Levenberg-Marquardt estimation method using STATISTICA 7 software. Eq. 11 is suitable for representing the combined effect of temperature and acid concentration on the corrosion rates with a correlation coefficient of 0.9857. The estimated equation can be written in the form of: 3 10 5.591 10 r = 1.697 exp( ) exp(0.45c) (1) T The coefficient 1.697x10 10, which appears in eq. 1 is in the same order of the frequency factor values which shown in Table, with average value of 4.8 10 10. The acid concentration coefficient (B), of value equal to 0.45, is in a good agreement with the values of B shown in Table 3, with average B values equal to 0.467. The temperature coefficient of value E R 3 1 1 = K E = kj mol 5.591 10, 46.48 is in agreement with the average values of activation energy (E=46.3 kj mol -1 ) shown in Table. The observed corrosion rate data from the experiments and the predicted corrosion rate data by eq. 1 are shown in Fig. 5, with a line slope of 0.966, indicating a good correlation between the two values. Wang et al. [10] and Morad [11] use Arrhenius equation and eq. 4 separately to evaluate the kinetic parameter for the corrosion of carbon steel in acid graphically, in which they obtained the value of k and B from the plot of ln r against C and the values E and A were calculated from a plot of ln r against 1, while in T our combination model, these parameters can be calculated using eq. 11. Ehteram and Al-Moubaraki [1] studied the corrosion behavior of mild steel in hydrochloric acid solutions. They related the corrosion rate r with the acid concentration by the following equation: log r = log k + B log C and concluded that the mild steel studied corrodes in HCl solutions with a first order reaction and the corrosion rate increases with the increase in acid concentration, with good correlation coefficient of 0.969. The estimated B values are 0.56, which is in a good agreement with the obtained results.. 447

Journal of the University of Chemical Technology and Metallurgy, 45, 4, 010 CONCLUSIONS Both Arrhenius equation and transition state theory are suitable to represent the effect of temperature on the corrosion rates of mild steel in phosphoric acid solutions. Present rate equation used in this study is more suitable than the conventional equation of chemical reactions. The combined temperature and acid concentration model is suggested using nonlinear estimation method and this model is suitable to represent the combined effect on corrosion rate data of low carbon steel in phosphoric acid. Acknowledgements This work was supported by Baghdad University, Chemical Engineering Department, which is gratefully acknowledged. REFERENCES 1. A. A. Khadom, A. S. Yaro, A. S. AlTaie, A. A. H. Kadum, Portugaliae Electrochimica Acta, 7, 009, 699-71.. A. A. Khadom, A. S. Yaro, A. H. Kadum, A. S. AlTaie and A. Y. Musa, American Journal of Applied Sciences, 6, 009, 1403-1409. 3. P.B. Mathur, T. Vasudevan, Corrosion, 38, 198, 150-160. 4. ASTM G1-3, Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens (003). 5. A.Popova, S. Veleva, S. Raicheva, Reaction kinetic and catalyst letters, 85, 005, 99-105. 6. O.K. Abiola, J. Chil. Chem. Soc., 50, 005, 685-690. 7. M. Benabdellah, R. Touzani, A. Dafali, B. Hammouti, S. El Kadiri, Materials Letters, 61, 007, 1197 104. 8. E.A. Noor, Corrosion Science, 47, 005, 33 55 9. Y. Jianguo, W. Lin, V. Otieno, D.P. Schweinsberg, Corrosion Science, 37, 1995, 975-985. 10. L. Wang, G.J. Yin and J.G. Yin, Corrosion Science, 43, 001, 1197-10. 11. M.S. Morad, Mater. Chem. Phys., 60, 1999, 188-195. 1. A. Ehteram, A.H. Al-Moubaraki, Int. J. Electrochem. Sci., 3, 008, 806-818. 448