EVALUATION OF MECHANICAL AND CORROSION PROPERTIES OF MILD STEEL IN SOME ENVIRONMENTS INHIBITED BY AFRICAN LOCUST BEANS TREE

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Department of Engineering

ABSTRACT
Corrosion of mild steel is a big problem in most of our industries causing huge losses of millions of dollars in maintenance. Synthetic corrosion inhibitors are carcinogenic and not biodegradable hence the urgent need for organic inhibitors.Shaded leaves and pods from African locust beans tree (Parkia biglobosa) constitute agricultural waste which dirty our environment. In investigating ways to tackle these problems, four extracts(from pods, leaves, stembark and root) of African locust beans tree (Parkia biglobosa) were used as corrosion inhibitors on mild steel immersed in 0.5M H2SO4 and 0.5M NaCl. The corrosion experiment is based on weight loss method and during the experiment, time, concentration of the inhibitors and temperature were varied; the results showed that inhibition efficiencies of all the extracts when compared to blanks(controls) increase with concentration of the inhibitors in both media at room temperature.In acidic medium,stembark extract has the highest inhibition efficiency of 86.02%, whereas in salt medium, root extract has maximum corrosion inhibition efficiency of 60.57%. Inhibition efficiencies in acidic and salt media decrease with increase in temperature. Corrosion rates at room temperature in absence and presence of extracts in both media decrease with increase in time. Activation energies of corrosion (Ea) increases with increase in inhibitors concentration in both media. Enthalpy and entropy change of corrosion values ( ie ΔHa and ΔSa) in both media are positive and negative respectively. Langmuir isotherm is found to fit the experimental datas obtained from acid and salt media better than Temkin isotherm. The overwhelmly negative values of Gibb?s free energy of adsorption (ΔGads) in the media shows the spontaneous nature of the adsorption process and the formation of stable protective films. Scanning electron microscopy (SEM) analysis reveals that corrosion inhibition of mild steel actually took place at the optimum concentrations ( 0.5%v/v ) of the extracts. Mechanical properties investigated are more deteriorated in salt medium than in acidic medium, however, they are both significantly improved at 0.5%v/v stembark extract in acidic medium and 0.5%v/v root extract in salt medium.