INHIBITION OF THE CORROSION OF MILD STEEL AND ALUMINIUM IN ACIDIC MEDIA BY SOME PURINES
INHIBITION OF THE CORROSION OF MILD STEEL AND ALUMINIUM IN ACIDIC MEDIA BY SOME PURINES
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Date
2014-03
Authors
MOMOH-YAHAYA, HABIBAT
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Abstract
Studies on the corrosion inhibition potentials of adenine (AD), guanine (GU), hypoxanthine (HYP) and xanthine (XN) on mild steel and aluminium in 0.1 M solutions of HCl, H2SO4 and H3PO4 were carried out using electrochemical impedance spectroscopy (EIS), potentiodynamic polarisation and gravimetric techniques. The study was carried out at 303 and 333 K respectively. The results obtained revealed that AD, GU, HYP and XN had moderate inhibitive effects on corrosion of mild steel in the three acid media in the decreasing order: AD> HYP> GU> XN in HCl; HYP> AD> GU> XN in H2SO4 and GU> AD> HYP> XN in H3PO4 at 303 K. The results obtained also showed that AD, GU, HYP and XN inhibited the corrosion of aluminium in HCl in the order: GU> HYP> AD> XN. It was observed that only HYP inhibited the corrosion of aluminium in H2SO4 solution while none of the purines inhibited the corrosion of aluminium in H3PO4 solution. Potentiodynamic polarisation studies showed that the purines surpressed both the anodic and cathodic half reactions of the corrosion processes, thereby acting as mixed inhibitors, while impedance data indicated that inhibition was achieved via adsorption of the inhibitor species on the mild steel and aluminium surfaces. Synergistic effects slightly increased the efficiency of the inhibitors in the presence of iodide ions. Among several adsorption isotherms assessed, Langmuir adsorption isotherm was found to best describe the adsorption behaviour of the inhibitors on the metal surfaces. According to Langmuir isotherm, calculated values of free energy of adsorption, ΔGads°, for the corrosion of mild steel in the presence of AD, GU, HYP and XN are as follows: -22.30 to -25.78 kJ mol-1 and 5.38 to -28.33 kJ mol-1 in HCl at 303 and 333 K respectively; -3.19 to -27.52 kJ mol-1 and -11.12 to -21.19 kJ mol-1 in H2SO4 at 303 and 333 K respectively; -6.70 to -27.52 kJ mol-1 and -18.39 to
-30.25 kJ mol-1 in H3PO4 at 303 and 333 K respectively. Calculated ΔGads° values for the corrosion of aluminium in the presence of AD, GU, HYP and XN are as follows:
-10.12 to -24.04 kJ mol-1 and -24.19 to 30.25 kJ mol-1 in HCl at 303 and 333 K respectively; -10.12 kJ mol-1 and -11.12 kJ mol-1 for HYP in H2SO4 at 303 and 333 K
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respectively. Calculated activation energy (Ea) values for the corrosion of mild steel in the presence of AD, GU, HYP and XN and the blank are as follows: 49.02 to 60.81 kJ mol-1 and 49.02 kJ mol-1 (blank) in HCl; 18.99 to 33.90 kJ mol-1 and
34.86 kJ mol-1 (blank) in H2SO4; 27.79 to 30.94 kJ mol-1 and 29.87 kJ mol-1 (blank) in H3PO4. Calculated Ea values for the corrosion of aluminium in the presence of AD, GU, HYP and XN and the blank are as follows: 61.51 to 88.98 kJ mol-1 and 52.13 kJ mol-1 (blank) in HCl; 54.27 to 72.19 kJ mol-1 and 50.54 kJ mol-1 (blank) respectively in H2SO4; 55.05 to 56.99 kJ mol-1 and 52.08 kJ mol-1 (blank) in H3PO4. Calculated values of heats of adsorption, ΔQads, of AD, GU, HYP and XN on mild steel surfaces are as follows: -3.86 to -10.58 kJ mol-1 respectively in HCl; 2.31 to 10.47 kJ mol-1 in H2SO4; 0.66 kJ mol-1, 0.40 to 9.27 kJ mol-1 in H3PO4. Calculated ΔQads values for the adsorption of AD, GU, HYP and XN on aluminium surfaces are as follow: -8.86 to -24.37 kJ mol-1 in HCl. The negative ΔGads values obtained in the study indicate the spontaneity of the adsorption processes while the calculated values of activation energy (Ea) for the adsorption of the purines on the metal surfaces shows that the adsorption mechanism may not be purely physisorption. The inhibition mechanisms, estimated from the temperature dependence of inhibition efficiency as well from activation energy (Ea) and heat of adsorption (ΔQads) parameters show that the purines functioned via mixed inhibition mechanism which was confirmed by polarization curves. The SEM micrographs, FTIR and impedance spectra of the metals in the presence of the inhibitors as well as molecular dynamics modeling of the adsorption of the single molecules on the metal surface confirmed the presence of protective layers over the mild steel and aluminium surfaces thereby providing evidence for the inhibition of the corrosion of the metals.
Description
A DISSERTATION SUBMITTED TO THE SCHOOL OF POSTGRADUATE STUDIES, AHMADU BELLO UNIVERSITY, ZARIA, IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE AWARD OF DOCTOR OF PHILOSOPHY (Ph.D) IN PHYSICAL CHEMISTRY
DEPARTMENT OF CHEMISTRY,
FACULTY OF SCIENCE,
AHMADU BELLO UNIVERSITY, ZARIA
NIGERIA
MARCH, 2014
Keywords
INHIBITION,, CORROSION,, MILD STEEL,, ALUMINIUM,, ACIDIC,, MEDIA,, SOME PURINES