Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 2(7), 1-6, July (2012) Res.J.Chem.Sci. International Science Congress Association 1 ADS, ALS, AHDS and ADDBS Surfactants as Corrosion Inhibitors for Carbon Steel in acidic Solution Kumar Harish and SunitaMaterial Science Laboratory, Department of Chemistry, Ch. Devi Lal University, Sirsa, Haryana, INDIA Department of Chemistry, Singhania University, Rajasthan, INDIA Available online at: www.isca.in Received 21st February 2012, revised 24th March 2012, accepted 28th March 2012Abstract The role of some surfactants towards corrosion control of carbon steel in 1.0 M HCl has been investigated using weight loss and galvanostatic polarization techniques. Surfactants investigated are Ammonium decyl sulphate (ADS), ammonium laruryl sulphate (ALS), ammonium hexadecyl sulphate (AHDS) and ammonium dodecyl benzene sulfonate (ADDBS). Results showed that the inhibition occurs through adsorption of the inhibitor molecules on the metal surface. The corrosion inhibition efficiency was found to increase with surfactant concentration and decreased with increasing temperature which is due to the fact, that the rate of corrosion of carbon steel is higher than the rate of adsorption of surfactant molecules. The inhibiting action of surfactants are considerably enhanced by the addition of KI, due to the increase of the surface coverage and therefore indicate the joint adsorption of surfactants and iodide ions. Thermodynamic parameters for adsorption and activation processes were also determined. Galvanostatic polarization data indicated that surfcatants act as mixed-type inhibitors. Keywords: Corrosion inhibitors, surfactant, carbon steel, HCl solution. Introduction Carbon steel has remarkable economic and attractive materials for engineering applications owing to its low cost, easy availability and high mechanical strength. The interest of the materials arises from their importance in recent civilization. Inhibition of metal corrosion by organic compounds is a result of adsorption of organic molecules or ions at the metal surface forming a protective layer. This layer reduces or prevents corrosion of the metal. The extent of adsorption depends on the nature of the metal, the metal surface condition, the mode of adsorption, the chemical structure of the inhibitor, and the type of corrosion media. To prevent the attack of acid, it is very important to add a corrosion inhibitor to decrease the rate of metal dissolution in acidic solutions2-6. Thus, many studies concerning the inhibition of carbon steel corrosion using organic substances are conducted in acidic and basic solutions7-10. The present study aimed to investigate the efficiency of some surfactants as corrosion inhibitors for carbon steel in acidic media. An attempt was also made to clarify the effects of concentration and temperature on the corrosion inhibition efficiency of all the four investigated surfactant as corrosion inhibitors for carbon steel. The measurements of corrosion rate and percentage corrosion inhibition efficiency of different surfactant towards corrosion of carbon steel by weight loss and electrochemical polarization technique were performed in 1.0 M HCl without and with the presence of surfactants in the concentration range (10 to 100 ppm). The name and molecular structures of all the four surfactants as corrosion inhibitor for carbon steel in acidic medium are given in scheme no. 1. Material and Methods Carbon steel used for the investigations was in the form of sheet (0.25 mm thick) and had the following composition (w/w): C = 0.14, Si = 0.03, Mn = 0.32, S =0.05, P = 0.20, Ni = 0.01, Cu = 0.01, Cr = 0.01% and balance Fe. All the chemicals used were of AR grade and the solutions were prepared using triple distilled water. Duplicate or in some cases triplicate experiments have been performed to verify the reproducibility and consistency of the experimental data. For weight loss measurements, carbon steel specimens of 3 × 1.5 cm size were cut from the carbon steel sheet. All the metal specimens were mechanically polished successively with the help of emery papers of grades 100, 200, 300, 400 and 600 µ and then thoroughly cleaned with plenty of triple distilled water and then with acetone. The specimens were dried with hot air blower and stored in desiccators over silica gel11-13. After recording the initial weights of carbon steel specimens on a Mettler Toledo, Japan AB 135-S/FACT, single pan analytical balance, (with a precision of 0.01mg), they were immersed in tilted position in 250 ml beakers each having 200 ml of acidic solution as corroding medium with or without the inhibitor. Experiments were carried out in an electronically controlled air thermostat maintained at a constant temperature of 30, 40 and Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 2(7), 1-6, July (2012) Res.J.Chem.SciInternational Science Congress Association 2 50C with in an accuracy of ± 0.1C. After exposing the specimens for 24 hours, the specimens were taken out from the beaker and washed initially under the running tap water. CH (CH O S OOO NH + (a) Ammoium Decyl sulphate (ADS) CH (CH11 O S OOO NH + (b) Ammonium Laruryl sulphate (ALS) CH (CH15 O S OOO NH + (c) Ammonium Hexadecyl sulphate (AHDS) CH (CH11 NH + SO (d) Ammonium Dodecyl benzene sulfonate (ADDBS) Scheme-1 Molecular formula and structure of four surfactantsLoosely adhering corrosion products were removed with the help of rubber cork and the specimen was again washed thoroughly with triple distilled water and dried with hot air blower and then weighed again. Corrosion rate in mils per year (mpy) and percentage corrosion inhibition efficiency were calculated using the following equations14-15. Corrosion rate in mils per year (mpy) and percentage corrosion inhibition efficiency were calculated by weight loss technique using the following equations. Corrosion rate (mpy) = 534 W DAT ´ (1) Where = weight loss (mg), = density of carbon steel (gm/cm), = area of specimen (sq. inch), = exposure time (hours). The degree of surface coverage ( q ) of the investigated surfactant compounds were calculated from the following equation: InhFree   =-   D \n (2) Where, Free W D and Inh W D are weight losses of metal per unit area in absence and presence of inhibitor at given time period and temperature, respectively. Linear polarization resistance measurements were carried out potentiostatically by scanning through a potential range of 14 mV above and below the OCP value in steps of 2 mV. Experiments were carried out in absence and presence of inhibitors at their different concentrations and at a constant temperature of 30, 40 and 50C. The resulting current is plotted against the potential and slope of the line is measured. The corrosion current density Corr i , is related to the corrosion rate by the equation, Corrosion rate (C.R.) (mpy) = . 0.1288.. Corr IEqWt D ´´ (3) Where Equation Wt. = Gram equivalent weight of metal/alloy, D = Density of metal (gm/cm), Corr i = Corrosion current density (A/cm). The interaction of inhibitor molecules can be described by introducing of an parameter, , obtained from the surface coverage values (q) of the anion, cation and both. Synergism parameter, , was calculated using the following equation. S q = 12 ' 12 1 q q + + - (4) where ( ) ( ) 121212 qqqqq =+- 1 q , surface coverage by anion, 2 q , surface coverage by cation and ' 12 q + , surface coverage by both i.e. anion and the cation. Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 2(7), 1-6, July (2012) Res.J.Chem.SciInternational Science Congress Association 3 Results and DiscussionPercentage corrosion inhibition efficiency of all the four surfactant at different concentrations for the corrosion study of carbon steel after exposure to 1.0 M HCl at 30C have been tabulated in table 1. It is observed from the table 1 that the weight loss decreased, percentage corrosion inhibition efficiency increased and therefore the corrosion inhibition strengthened with increase in inhibitor concentration from 10 to 100 ppm. This trend may result from the fact that adsorption and surface coverage increases with the increase in concentration of surfactant molecules. The linear variation of weight loss with time in uninhibited and inhibited 1.0 M HCl indicates the absence of insoluble surface films during corrosion. It is clear from the tables 1 that percentage corrosion inhibition efficiency increases with increase in inhibitor concentration. The percentage corrosion inhibition efficiency for different investigated anionic surfactants as corrosion inhibitor increases in the following order d � c � b � a. Table 2 gives the values of percentage corrosion inhibition efficiency for different concentrations of all the four surfactant molecules and 1.0 × 10-2 M KI. The addition of KI improves the percentage corrosion inhibition efficiency of the investigated surfactants molecules significantly. The synergistic effect between surfactants molecules and KI is due to interactions between chemisorbed I ions and surfactants molecules. The stabilization of adsorbed organic cations on the surface, which may be exhibited by electrostatic interactions with I ions, leads to higher surface coverage and greater corrosion inhibition. Table 3 shows synergism parameters for different concentrations of all the four investigated surfactant (a to d) with addition of 1.0 ×10-2 M KI for the corrosion of carbon steel after 24 hours of immersion in1.0 M HCl at 30C. It is observed from the table 3 that synergism parameters for all the four investigated surfactant increase with increase in concentration of surfactant molecules from 10 to 100 ppm. Among the four investigated surfactants molecules, order of synergism parameters is a b c d. Table 4 shows typical current-potential polarization curves of carbon steel in 1.0 M HCl in the absence and presence of different concentrations of surfactant molecules. Values of associated electrochemical parameters such as current density Corr i ), corrosion potential ( Corr E ), Tafel slopes, and corrosion inhibition efficiency (per cent) as function of surfactant d concentrations in 1.0 M HCl are shown in table 4. It is evident from table 4 that the Tafel constants did not change significantly. It is clear from the Tafel plots that the surfactant d was mixed type because it enhances both the anodic and cathodic polarization but the anode is more polarized when an external current was applied. Addition of surfactant compounds increased both cathodic and anodic over voltage and caused mainly parallel displacement to the more negative and positive respectively. The corrosion current density ( Corr i ) decreases with increase in the concentration of surfactant compounds, which indicates that the presence of these surfactant molecule retards the dissolution of carbon steel in 1.0 M HCl and the degree of inhibition depends on the concentration of surfactant molecule. The values of corrosion potential ( Corr E ) values shifted to less negative values on increasing the concentration of surfactant molecule from 10 to 100 ppm. The effect of temperature on the corrosion behavior of carbon steel was studied by weight loss methods at different temperatures in the range 30-50 C in the absence and presence of different concentrations of compound (d) in 1.0 M HCl media and percentage corrosion inhibition efficiency of compound (d) is shown in table 5. It is obvious from the table 5 that percentage corrosion inhibition efficiency increases with increase in the concentration of compound (d) and decreases with increase of temperature. The calculated values of the apparent activation energy, * a E , activation enthalpies, * H D and activation entropies, * S D are shown in table 6. These values indicate that the presence of the additives increases the activation energy, * a E and the activation enthalpy * H D , and decreases the activation entropy, * S D for the corrosion process. The addition of inhibitors modified the values of * a E . This may be attributed to the adsorption of inhibitors on carbon steel surface and this adsorption make an energy barrier and this energy barrier of corrosion reaction increases with increase in concentration of the surfactant molecule, and hence percentage corrosion inhibition efficiency increases. This suggested that the process is activation controlled16-17. Since, the activation energy value of 62.36 kJ mol-1 (table 6) for HCl–inhibitor systems support the fact that the surfactants are physically adsorbed on carbon steel surface. According to these reports, * a E due to chemical adsorption (�80 kJ mol-1) is considerably larger than due to physical adsorption ( 80 kJ mol-1) the increase in the activation enthalpy ( * H D ) in presence of the inhibitors implies that the addition of the surfactants to the acid solution, increases the height of the energy barrier of the corrosion reaction to an extent depends on the type and concentration of the surfactant molecules. Mechanism of corrosion inhibition of all the four investigated surfactants molecules can be explained on the basis of the molecular size and the number of adsorption centers. Organic surfactant compound (d) should be the most effective inhibitors, inspite of it has two active adsorption centers. The highest inhibition efficiency of the compound (d) may be due to: i. higher molecular size and ii. benzene ring contributes p electrons to the adsorption centers and hence increases the electron density on the adsorption centers. Compounds a b c in percentage corrosion inhibition efficiency and this is agrees with their molecular size. Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 2(7), 1-6, July (2012) Res.J.Chem.SciInternational Science Congress Association 4 Table-1 Percentage corrosion inhibition efficiency at different concentrations of surfactant for the corrosion of carbon steel after exposure to 1.0 M HCl at 30C Conc. of Surfactant (ppm) Percentage corrosion inhibition efficiency of Surfactants ( a ) ( b ) ( c ) ( d ) 10 70.3 73.5 79.7 81.3 30 74.5 78.6 82.4 85.4 50 80.2 81.7 86.6 88.3 70 86.7 89.2 90.6 94.6 90 88.2 92.5 96.7 96.5 100 90.3 95.6 96.1 97.4 Table-2 Percentage corrosion inhibition efficiency at different concentrations of the four investigated surfactants (a to d) with addition of 1.0 ×10-2 M KI for the corrosion of carbon steel after 24 hours of immersion in 1.0 M HCl at 30C Conc. of Surfactant (ppm) Percentage corrosion inhibition efficiency of Surfactants ( a ) ( b ) ( c ) ( d ) 10 77.7 80.3 83.6 85.2 30 83.2 84.5 86.4 88.3 50 86.8 88.4 88.4 91.4 70 89.7 92.3 93.7 96.7 90 92.6 95.1 96.1 97.8 100 95.2 96.9 97.7 98.9 Table-3 Synergism parameter ( S q ) for different concentrations of investigated surfactant (a to d) with addition of 1.0 ×10-2 M KI for the corrosion of carbon steel after 24 hours of immersion in1.0 M HCl at 30C Conc. of surfactant (ppm) Synergism parameter ( S q ) ( a ) ( b ) ( c ) ( d ) 10 1.011 1.082 1.128 1.161 30 1.023 1.088 1.134 1.179 50 1.032 1.094 1.147 1.199 70 1.038 1.098 1.156 1.223 90 1.047 1.107 1.158 1.242 100 1.068 1.115 1.168 1.283 Table-4 The effect of concentrations of compound (d) on the free corrosion potential (Ecorr), corrosion current density (icorr), Tafel slopes ( a b and c b ), Percentage corrosion inhibition efficiency (%age CIE) and degree of surface coverage (q) for the corrosion of carbon steel in 1.0 M HCl at 30C Conc. of surfactant (d) (ppm) -Ecorr, mV icorr, (µA /cm 2 ) - c b (mV/ dec) a b (mV/ dec) q %age CIE Blank 942 62.0 217 89 - - 10 930 34.0 204 108 0.788 82.2 30 926 30.8 211 110 0.817 87.7 50 920 27.1 216 116 0.823 91.5 70 915 24.2 220 120 0.845 93.9 90 908 20.3 224 123 0.857 94.8 100 903 18.6 227 126 0.894 98.1 Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 2(7), 1-6, July (2012) Res.J.Chem.SciInternational Science Congress Association 5 Table-5 The effect of temperature on the percentage corrosion inhibition efficiency of surfactant (d) at different concentrations and temperatures Conc. of surfactant (d) (ppm) 30 0 C 40 0 C 50 0 C 10 81.3 75.3 68.5 30 85.4 78.6 72.4 50 88.3 85.1 76.8 70 94.6 88.4 80.1 90 96.5 90.1 83.4 100 97.4 92.3 84.8 Table-6 Activation parameters of the corrosion of carbon steel in 1.0M HCl in absence and presence of different concentrations of compound (d) Conc. of surfactant (d) (ppm) * a E ( ) 1 kJmol - * H D ( ) 1 kJmol - * S -D ( ) 11 JmolK-- 10 54.2846.12112.42 30 57.14 48.24 107.23 50 60.05 52.29 103.15 70 61.12 55.24 101.05 90 61.05 55.12 98.35 100 62.36 56.06 96.30 ConclusionAll the four investigated surfactant molecules act as corrosion inhibitors for the carbon steel in hydrochloric acid solution. 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