Journal of the Chilean Chemical Society
versão On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.51 n.2 Concepción jun. 2006
J. Chil. Chem. Soc., 51, Nº 2 (2006) , pags: 849-853
INHIBITION OF MILD STEEL CORROSION IN ACID MEDIA BY N-(2-THIOPHENYL)-N/-PHENYL THIOUREA
S. DIVAKARA SHETTY1, PRAKASH SHETTY2*, AND H. V. SUDHAKER NAYAK3
1Department of Mechanical Engineering, Manipal Institute of Technology, India.
The corrosive behavior of mild steel in 0.05 M and 0.1M HCl, 0.025 M and 0.05 M H2SO4 solutions containing different concentrations of N-(2-thiophenyl)-N/-phenyl thiourea (TPTU) was investigated using potentiodynamic polarization technique. The results obtained reveal that TPTU is an efficient anodic inhibitor in the acid environment and it is more effective in reducing corrosion of mild steel in HCl than in H2SO4 media. The adsorption of the inhibitor on the mild steel surface in both acids obeys the Temkins' adsorption isotherm. The thermodynamic parameters of adsorption deduced reveal a strong interaction and spontaneous adsorption of TPTU on the mild steel surface. The influence of temperature and inhibitor concentration on the corrosion of mild steel has also been investigated.
Metals are exposed to action of acids in many different ways and for many reasons. The exposures can be most severe but in many cases, the corrosion can be controlled by means of inhibitors 1). A considerable amount of interest has been generated in the study of organic inhibitors owing to their usefulness in several industries: during the pickling of metals, cleaning of boilers, oil and gas wells acidizing, acid descaling etc 2).
Several nitrogen containing organic compounds have been used as corrosion inhibitor for metals in acid environment 1, 3). It has been shown that organic compounds containing both nitrogen and sulphur atoms are of particular interest as they give better inhibition efficiencies than those containing nitrogen or sulphur alone 4, 5). Among these, thiourea and its derivatives have been investigated extensively 6-10). These are polar molecules with sulphur atom having a permanent negative charge whereas the nitrogen atom has a positive charge. As the molecule approaches the electrode surface, the electric field of double layer increases the polarization of molecules and induces additional charges on sulphur and nitrogen atoms, a condition that enhances the adsorption of molecules 11).
Some of the derivatives of thiourea, which have been investigated earlier are allyl thiourea, tolyl thriourea, phenyl thiourea, di-orthotolyl thiourea, di-orthoxenyl thiourea, phenyl o-tolyl thiourea, N, N/-n-dibutyl thiourea, di-n-butyl thiourea, n-butyl thiourea, di-isopropyl thriourea, di-methyl thiourea, tetramethyl thiourea, N-methyl thiourea, N-N/ -dimethyl thiourea, N, N, N/, N/-tetra methyl thiourea, di-phenyl thiourea, N, N/-diethyl thiourea, syndiotolyl thiourea and n-phenyl thiourea.
Mild steel is an industrially important structural material and is corroded by many agents, of which aqueous acids are the most dangerous. The protection of steel from corrosion is the need of an hour. Already, substituted thioureas have been proved to be fairly effective in corrosion control. The selection of N-(2-thiophenyl)-N/-phenyl thiourea (TPTU) is based on the fact that substituents to thiourea increase further its electron densities for their adsorption on the metal surface. The objective of the present study is to investigate the inhibiting effect of TPTU on the corrosion of mild steel in both hydrochloric and sulphuric acid media with different levels of temperature and inhibitor concentration by using potentiodynamic polarization technique.
Mild steel specimens of chemical composition (wt %): C : 0.205; Si : 0.06; Mn : 0.55; S : 0.047; and P : 0.039 were used for the measurement of corrosion rate. The specimen was mirror polished successively with 1/0 5/0 grit emery papers, thoroughly cleaned with soap water, rinsed with distilled water and then with alcohol and dried in air. The sample was tightly fitted to one end of the Teflon holder, which exposes a polished surface area of 0.786 cm2. AR grade HCl and H2SO4 and distilled water were used to prepare 0.025 M 0.1 M solutions. N-(2-thiophyenyl)-N/-phenyl thiourea (HSC6H4NHC(S) NHC6H5) was synthesized by following the reported procedure 12). The compound was purified by recrystallization with ethanol and its purity was checked by elemental analysis, IR spectra and m.p. (161 0 C). The concentration range of inhibitor employed was 0.0001 to 0.0004 mol/L in both the acids.
The electrochemical studies were performed by using a Wenking potentiostat (LB95L) and a three electrode cell containing 400 ml of electrolyte at room temperature (RT) with and without inhibitor. The steady state open circuit potential (OCP) with respect to saturated calomel electrode was noted at the end of 25-30 min. The polarization studies were then made from -250 mV versus OCP to +250 mV versus OCP with a scan rate of 20 mV/min from the cathodic side and the corresponding steady state currents were noted. The experiments were repeated for 40, 45 and 50 0C. From the Tafel graphs of potential versus log i, corrosion current density (Icorr), corrosion potential (Ecorr) were determined. The corrosion rate (CR) and the percentage inhibition efficiency (% IE) were calculated. The results were also confirmed by Linear polarization technique.
RESULTS AND DISCUSSION
The inhibition effect of N-(2-thiophenyl)-N/-phenyl thiourea (TPTU) on the corrosion of mild steel, studied by potentiodynamic polarization technique at different temperatures in hydrochloric and sulphuric acid media is presented in Tables 1 and 2. The influence of TPTU on the polarization behavior of mild steel was noted by a drastic reduction in corrosion current density (Icorr), and a positive shift in corrosion potential (Figures 1and 2). The shift in corrosion potential (Ecorr) in the positive direction indicates that TPTU is an efficient anodic inhibitor for mild steel in both acid media. The result shows that the effectiveness of TPTU in reducing corrosion of mild steel in H2SO4 is comparatively lower than in HCl media. It is evident from the results that the corrosion rate (Tables 1 and 2) increases very rapidly with temperature in the absence of inhibitor.
The % IE of the compound increases with increase in inhibitor concentration at all temperatures (Figures 3 and 4). The increase in efficiency may be due to the blocking effect of the surface by both adsorption and film formation mechanism which decreases the effective area of attack. The inhibition estimated in 0.1 M HCl and 0.05 M H2SO4 is superior to 91% and 80% respectively (Figures 5 and 6) even at very low concentration (0.0001 mol/L). The optimum concentration for maximum efficiency of 96% in HCl and 85% in H2SO4 was found to be 0.0004 mo/L at 28 0C It is interesting to observe that the inhibition efficiency of TPTU has not changed significantly with increase in temperature from 28 to 50 0C. This indicates that TPTU can work more effectively at lower temperature and concentration of both the acids. The highest IE exhibited by the compound may be attributed to its adsorption on the metal surface through polar groups of the double bond.
The corrosion rate (CR), the percentage inhibition efficiency (% IE) and the surface coverage (q) were calculated using the following equations:
where Icorr is the current density in µA/cm2, D is the specimen density in g/cm3, eq.wt is the specimen equivalent weight in grams.
where Icorr & Icorr (inh) are the current densities in the absence and presence of inhibitor respectively.
here are many factors influencing the ability of a molecule to inhibit corrosion, viz., the morphology (shape, branching, or conformation) of the corrosion inhibitor, aromaticity and /or conjugated bonding, bonding strength to metal substrate, the type and number of bonding atoms or groups in the molecule, the ability for an inhibitor layer to become compact, or cross link, the ability of an inhibitor to complex with the metal within the metal lattice, and efficient solubility of the inhibitor (i.e., whether small concentrations of inhibitor produce a large inhibition effect). Further, the stability of the inhibitor molecule in the corroding medium may become the determining factor. For example, molecules such as thioacetamide decompose readily in an acidic environment. Other molecules, e.g., thioketones and thioaldhydes, may undergo polymerization to yield new molecular species which may or may not be good inhibitors 9). It is believed that the molecule studied in this paper is stable during the test time period, and further that it gives a large inhibition effect with lower concentration of the inhibitor.
The compound in the present study contains N and S atoms which are in the protonated form and hence it is quickly adsorbed on the metal surface and thus forming an insoluble stable film on the mild steel surface. This is usually observed by the decrease in corrosion loss which depends on the concentration of inhibitor 1,3). The bonding of adsorbed corrosion inhibitors onto the metals has been described in terms of concepts of "hard-soft acid and bases" and electrosorption valency. Inhibitive efficiencies change with the nature of substituents in the inhibitor molecules as electron densities change at functional groups. Substituents increase the inhibitive efficiency probably because of strong adsorption forces arising from increased electron density due to nucleophilic or electrophilic substitutents. The protective efficiency is also related with steric factor 13).
The fraction of surface coverage (q) shows a linear relationship with log C (Figures 7 and 8) indicating that adsorption of the compound on the mild steel surface obeys Temkin's adsorption isotherm 14). The applicability of Temkin's adsorption isotherm verifies the assumption of mono-layer adsorption on a uniform, homogeneous metal surface with an interaction in the adsorption layer 15). The values of apparent activation energy (Ea), the free energy of adsorption (DGads) and the equilibrium constant (K) were calculated by using the following equations 16):
Where r1 and r2 are the corrosion rates at temperatures T1 and T2 respectively, DT is the difference in temperature and R is the universal gas constant in Joules.
Where 55.5 is the concentration of water in the solution in mol/L. and
The higher values of Ea (Table 3) in the inhibited solutions indicate that dissolution of the metal is retarded 17). The enhancement of inhibition efficiency with increase in inhibitor concentration either could be due to the higher activation energy or to the increase in surface coverage by the inhibitor. The DGads values for the compound studied at higher temperatures were nearer to 40 kJ mol-1 indicating that inhibition is governed by chemisorption mechanism.224). The negative values of DGads as recorded in Table 3 indicated spontaneous adsorption and a strong interaction of the compound on the mild steel surface 18, 19).
The authors are grateful to Prof. Dr. B. S. Prabhu, Director, Prof. Dr. A. M. Chincholkar, Head of the Department, Mechanical & IP Engg and Dr. S. R. Girish, Head, Department of Chemistry, Manipal Institute of Technology for their guidance encouragement and help during the course of this work.
* e-mail. email@example.com.