POTENTIALS OF EXTRACTS OF AFRICAN STAR APPLE AND COCK’S COMB LEAVES AS CORROSION INHIBITORS FORMEDIUM CARBON LOW ALLOY STEEL IN ACIDIC MEDIA

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Department of Engineering

ABSTRACT
The potentials of African Star Apple (Chrysophyllum albidium; (CA) and Cock’s Comb (Heliotropium indicum;(HI)as corrosion inhibitorsfor medium carbon low alloy steel in 1.0M H2SO4 and HCl acidic media were evaluated and compared with a known conventional standard inhibitor (Shell Ensis SX; (SE)).The plant extracts were first characterized to determine their phytochemical constituents; saponins, tannins, alkaloids and flavonoids. These constituents were found to be present. Proximate analysis and physicochemical parameters such as pH, colour, viscosity and solubility of the extracts were also evaluated. The rheological and biodegradability of plant extracts were studied, Chrysophyllum indicum showed a shear-thickening behaviour while Heliotropium indicum gave a shear-thinning behaviour, rapid biodegradation occurs in the formulated plant extracts during Organization for Economic, Cooperation and Development (OECD) marine Biochemical Oxygen Demand (BOD) tests. Ultraviolent-visible (UV-Vis) and Fourier Transform Infra-Red (FTIR) spectrometry were used to characterize the bioactive compounds present in the leave extracts. Corrosion inhibition efficiencies and ability of the plant extracts were studied by weight loss and Linear Resistance Polarization (LPR) techniques. Solutions of concentrations ranging from 100 to 500part(s) per million(ppm) of formulated and standard inhibitors were prepared. Results showedthat inhibitionefficiencies (IE %) increasedwith increase in concentration of inhibitors for gravimetricbased mass loss and electrochemical methods. For gravimetric based mass loss studies, rate of corrosion decreased progressively with increase in inhibitor concentration and exposure time. Potentiodynamic polarization curves revealed that the studied inhibitors function as mixed–type inhibitors withinhibition efficiency values of 98 % (CA) and 96% (HI and SE) respectively at concentration of 500ppm of inhibitors. Kinectic and thermodynamic studies showedthat inhibitionmechanism occurs due to the adsorption of the inhibitor molecules onto the steel surface is an endothermic process; both physical and chemical takes place on metal surface and best described by the Langmuir isotherm. Scanning Electron Microscope (SEM) analysis after corrosion test showed the appearance of smooth surfaces for inhibited solutions, which may be due to the absorption of inhibitor molecules onto the metal surface that serves as a protective layer. Mechanical properties decreased drastically for uninhibited samples but increased with application of inhibitors.