http://journalcps.com/ Communication in Physical Sciences 2018, 3(2): 27-42 Theoretical and Experimental Investigations of the Corrosion Inhibition Action of Piliostigma Thonningii Extract on Mild Steel in Acidic Medium *Paul Ocheje Ameh and Nnabuk Okon Eddy Received 05 July 2018/Accepted September 2018 /Published online: 4 December 2018 Abstract The search for green corrosion inhibitor for mild steel in solution of HCl was implemented in this work by using electrochemical polarization, gravimetric and thermometric techniques to investigate the corrosion inhibition efficiency of ethanol extract of Piliostigma thonningiil leaf. Results obtained from weight loss and thermometric measurements indicated that the inhibition efficiency of ethanol extract of Piliostigma thonningii leaf ranged from 65.12 to 79.55 % and from 73.61 to 88.16 % respectively. The ranges for inhibition efficiencies from, linear and potentiodynamic polarization measurements were 66.40 to 89.97 % and 51.29 to 85.53 % respectively Active components of the extracts that synergistically cooperated to enhance their adsorption on mild steel surface (and hence corrosion inhibition) were identified through GCMS analysis and they included 1,1,7-trimethyl-4- methylenedecahydro-1H-cyclopropa[e]azulene; hydroquinone; 3-tridecene; 3,5-bis(1,1- dimethylethyl)-phenol; Pentadecanoic acid; 1- ethenyl-1-methyl-2,4-bis(1-methylethenyl)- cyclohexane and 9-oxa-bicyclo[3,3,1]nona-3,6- dien-2-one. Results from quantum chemical calculations revealed that these compounds (apart from meeting the basic requirements for corrosion inhibition) are characterized by frontier molecular energy values that are unique for well-known corrosion inhibitors. Key Words: Mild steel, corrosion inhibition, Piliostigma thonningii leaf, experimental and theoretical considerations [*P. O. Ameh Physical Chemistry Unit, Department of Chemistry Nigeria Police Academy, Wudil, P. M B. 3474, Kano State Nigeria E-mail: amehpaul99@gmailcom N. O. Eddy Department of Pure and Applied Chemistry University of Nigeria, Nsukka, Enugu State, Nigeria Email: okon.nnabuk@unn.edu.ng 1.0 Introduction Mild steel (among metals), is widely applied in several industrial installations due to some of its unique parameters (Eddy et al., 2009a). However, the metal is highly prone to corrosion when expose to aggressive medium such as those used for acid cleaning, etching, descaling, crude oil/crude oil products transportation, transportation of fertilizers and fertilizer chemicals (Ameh and Eddy, 2014). Efforts in reducing damages due to corrosion have resulted in different control options including electroplating, cathodic/anodic protections, greasing/oiling among others. However, it has been established that the use of corrosion inhibitors is the best and unique option of retarding metal corrosion, especially those accelerated by aggressive medium (Eddy et al., 2010). Different compounds have been found to be active corrosion inhibitors ranging from some inorganic compounds, drugs, polymers, Schiff bases, molecular liquids, nano materials, plant materials and others (Awe et al., 2019; Anand and Chitra, 2020; Azzan et al., 2019;Eddy et al., 2011;Eddy et al., 2009b; Saraswat and Yadav, 2020; Odoemelam et al., 2020). However, in spite of their high corrosion inhibition efficiencies, most of them have some setbacks in meeting environmental and other requirements including biodegradability, cost, toxicity, ease of availability and eco friendliness (Eddy and Odiongenyi, 2010; Lebrini et al.,2008; Umoren et al., 2008). Therefore, several hopes have been laid on the use of extracts of living things since most of them can meet these requirements. Products of plant origin contain various organic compounds such as alkaloids, flavonoids, terpenoids, primary