ISSN: 0973-4945; CODEN ECJHAO E-Journal of Chemistry http://www.ejchem.net 2012, 9(4), 2213-2225 Inhibiting Properties of Morpholine as Corrosion Inhibitor for Mild Steel in 2N Sulphuric Acid and Phosphoric Acid Medium K. JAYANTHI1, M. SIVARAJU2, AND K. KANNAN3* 1Department of Chemistry, Queen Mary's College, Chennai 600004, Tamilnadu, India [email protected] 2Department of Chemistry, Muthayammal Technical Campus, Rasipuram 637408, Tamilnadu, India [email protected] *Department of Chemistry, Government College of Engineering, Salem 636 011, India [email protected] Received 11 October 2011; Accepted 25 December 2011 Abstract: The inhibition effect of morpholine on the corrosion of mild steel in 2N sulphuric acid and phosphoric acid has been studied by mass loss and polarization techniques between 302K and 333K.The inhibition efficiency increased with increase in concentration. The corrosion rate increased with increase in temperature and decreased with increase in concentration of inhibitor compared to blank. The adsorption of inhibitor on the mild steel surface has been found to obey Temkin’s adsorption isotherm. Potentiostatic polarization results reveal that morpholine act as mixed type inhibitor. The values of activation energy (Ea), free energy of adsorption (ΔGads), enthalpy of adsorption (ΔH), and entropy of adsorption (ΔS) were also calculated. Keywords: Mild Steel; Sulphuric acid; phosphoric acid; Corrosion inhibition; Temkin’s adsorption isotherm; Potentiostatic polarization; Morpholine. Introduction Phosphoric acid is a major chemical product, which has many important uses, especially in the production of fertilizers. Most of the acid is produced from phosphate rock by wet process. Generally nickel–base alloys and stainless steel are frequently used in many parts of the wet process and a considerable quantity of data has been published about the resistance of these materials to corrosion by phosphoric acid solution 1-4. Most of the previous studies were focused on the inhibition of stainless steel or chromium-nickel steel in hydrochloric acid or sulphuric acid solutions using organic compounds containing nitrogen, sulphur and oxygen atoms as corrosion inhibitors5, 6. Organic inhibitors are widely used in various industries. Among them, heterocyclic compounds comprise a potential class of inhibitors. There is wide consideration in the literature regarding corrosion inhibition studies by nitrogen containing heterocyclies 7-10. Heterocyclic compounds containing nitrogen and sulphur atoms are of particular 2214 K. KANNAN importance as they often provide excellent inhibition compared with compounds containing only nitrogen or sulphur11-12. The corrosion inhibiting property of these compounds is attributed to their molecular structure. These compounds contain electrons and heteroatom, which induce greater adsorption of the inhibition molecules onto the mild steel surface. So, in this investigation, the corrosion of mild steel in 2N phosphoric acid and 2N sulphuric acid solutions in the absence and presence of morpholine at 302 K to 333K has been studied by mass loss method and polarization techniques. It is aimed to predict the corrosion rate, inhibition efficiency on mild steel corrosion and the thermodynamic feasibility of inhibition via surface coverage on mild steel by adsorbed morpholine at various temperatures. The adsorption characteristic of morpholine was studied in order to access the adsorption isotherm(s). Experimental Methods Mass loss measurement Mild steel specimens were cut to size of 5 cm x 1 cm from the mild steel sheets having the following percentage composition as shown below. The surface of specimens were polished with emery papers ranging from 110 to 410 grades and degreased with trichloroethylene specimens were dried and stored in vacuum desiccators containing siligagel and then initially weighed in an electronic balance. After that the specimens were suspended with the help of PTFE, threads and glass rod in 100ml beaker containing acid in the presence and absence of inhibitors. The specimens were removed after 4 hours exposure period, washed with water to remove any corrosion products and finally washed with acetone. After that they were dried and reweighed. Mass loss measurements were carried out in 2N phosphoric acid and 2N sulphuric acid with Morpholine in the concentration range of 1 % to 5% as inhibitors and the temperature between 302 K and 333 K for an immersion period of 4 hours. All the solutions were prepared with AR grade chemicals in double distilled water. Mass loss measurements were performed as per ASTM method described previously 13-15. Composition of mild steel Element Fe Ni Mo Cr S P Si Mn C Composition 99.686 0.013 0.015 0.043 0.014 0.009 0.007 0.196 0.017 (%) Potentiostatic Polarization measurements Polarization measurements were carried out in a conventional three-electrode cell. Mild steel strips coated with lacquer except for an exposed area of 1 cm2 were used as the working electrode. The saturated calomel electrode and the platinum foil were used as reference and counter electrodes respectively. The potentiostatic polarization measurement was carried out using BAS – 100 a model instrument. The potential of the test electrode was measured with respect to SCE and platinum electrode was used as auxiliary electrode and the experiment were carried out at 302K to333K. Inhibiting Properties of Morpholine as Corrosion Inhibitor 2215 Table 1 Corrosion parameters of Morpholine on mild steel in 2N sulphuric acid and 2N phosphoric acid by weight loss method. Conc. 2N Sulphuric acid 2N Phosphoric acid Temp. of (K) Inhibitor CR SC CR SC IE (%) IE (%) (mmpy) () (mmpy) () (%) Blank 63.7496 ------- ------ 15.6031 ------ ------ 1 34.4753 0.4592 45.92 15.1573 0.9029 90.29 2 29.4229 0.5385 53.85 12.2595 0.9212 92.12 302 3 28.0112 0.5606 56.06 08.1730 0.9476 94.76 4 25.7822 0.5956 59.56 06.5384 0.9581 95.81 5 20.0611 0.6853 68.53 04.9038 0.9686 96.86 Blank 114.8682 ------- ------- 53.7934 ------ ----- 1 35.6641 0.6895 68.95 27.1196 0.4959 49.59 2 30.5374 0.7342 73.42 21.8443 0.5939 59.39 313 3 22.1415 0.8072 80.72 19.3924 0.6395 63.95 4 17.5349 0.8474 84.74 14.7858 0.7251 72.51 5 15.0087 0.8693 86.93 07.3557 0.8633 86.33 Blank 143.6223 ------- ------- 72.8142 ------ ------ 1 60.7033 0.5773 57.73 52.4559 0.2776 27.76 2 46.2148 0.6782 67.82 38.2646 0.4745 47.45 323 3 36.7786 0.7439 74.39 31.5776 0.5663 56.63 4 32.6178 0.7729 77.29 21.9185 0.7040 70.40 5 28.3826 0.8024 80.24 16.5689 0.7724 77.24 Blank 300.9160 ------- ------ 89.0860 ------- ------- 1 158.7053 0.4726 47.26 68.4305 0.2316 23.16 333 2 140.9476 0.5316 53.16 52.0844 0.4078 40.78 3 98.4478 0.6728 67.28 38.7847 0.5646 56.46 4 85.7425 0.7151 71.51 32.1501 0.6305 63.05 5 64.8641 0.7844 78.44 22.5129 0.7472 74.72 Results and Discussion Mass loss Studies Table 1 shows the value of inhibition efficiency [IE%] surface coverage (θ) and corrosion rate obtained at different concentration of the inhibitors in 1N hydrochloric acid solutions for an immersion period of 3 hours. From the mass value, the inhibition efficiency [IE%] and surface coverage (θ) were calculated using the following equation 16-18. 2216 K. KANNAN Inhibitor efficiency (I %) = Wb – Wi ×100 [1] Wb Surface coverage (θ) = Wb – Wi [2] Wb Where Wu and Wi are the corrosion rates for mild steel in the absence and presence of inhibitor respectively at the same temperature. It could be seen from the table that the addition of inhibitor to the acid has reduced the corrosion rate. The inhibition efficiency increased with increase in concentration of inhibitors and increased with temperature from 302 K to 313 K and then decreased in 2N sulphuric acid. The inhibition efficiency increased with increase in concentration of inhibitors and decreased with temperature from 313 K to 333 K in 2N phosphoric acid .The values of the corrosion rate and inhibition efficiency of the inhibitors are known to depend on the molecular structure of the inhibitors. The maximum inhibition efficiency of morpholine was found to be 86.93% and 96.86% in 2N sulphuric acid and 2N phosphoric acid with 5% of inhibitor concentration respectively at 313K and 302K and then decreased. Inhibition efficiency of morpholine in 2N phosphoric acid solution was found to be greater than 2Nsulphuric acid solution because in 2N phosphoric acid solution phosphate ions probably form a bridge between the protanated molecules of inhibitors and facilitate the adsorption of inhibitor molecules, while in 2Nsulphuric acid this bridge is not formed due to Low charge to mass ratio, as such that adsorption of molecules on the surface of the metal is mostly physical and gives low inhibition efficiency 19- 20 Thermodynamic Consideration Table 2 shows that the calculated values of activation energy (Ea) and free energy of absorption (ΔGads) for mild steel corrosion in 2N sulphuric acid and 2N phosphoric acid with and without inhibitors at 313K to 333K. Energy of activation (Ea) was calculated from the slopes of plots of log p versus 1/T in Fig. 1&2 and also calculated from Arrhenius equation 21-22. Log P2/P1 = Ea/2.303 R [1/T1-1/T2] [3] where P1 and P2 are the corrosion rates at temperatures T1 and T2 respectively. Ea value was found to be 41.73KJ/mole and 21.86 in 2N sulphuric acid and 2N phosphoric acid at 313K to 333 K .The Ea values for 2N sulphuric acid and 2N phosphoric acid containing inhibitors are found to be higher than that of without inhibitors.