Extracts of sweet prayer leaf as corrosion inhibitors of mild steel in acidic medium: effects of extractants’ properties

Corrosion inhibitors are substances added to metallic systems to prevent degradation of the metallic surfaces and changes in their properties in corrosive environments. In a continual search for novel, eco-friendly and effective corrosion inhibitors for mild steel in acidic environment, ethanol (TDLEE) and n-hexane (TDLHE) extracts of Sweet Prayer {Thaumatococcus daniellii (Benn.) Benth.} leaves were explored for corrosion studies. Electrochemical (electrochemical impedance spectroscopy and Potentiodynamic polarisation) and gravimetric techniques as well as adsorption and surface analyses (scanning electron microscopy and Fourier transform infrared spectroscopy) were employed to probe the corrosion process. Electrochemical experiments were conducted at 298 K using 0 (blank), 2 × 103, 4 × 103, 6 × 103, 8 × 103 and 10 × 103 ppm of the extracts while gravimetric experiments were carried out using the same range of concentration at different temperature values of 298, 303, 308, 313, 318, 323 K in a thermostatic water bath for 3–96 h. For TDLEE, increase in experimental temperature reduced the inhibition efficiency, however, inhibition efficiency of TDLHE increased as the temperature increased. The equilibrium corrosion data of the TDLEE and TDLHE were well described by Langmuir and Freundlich models, respectively. The values of Gibbs free energies obtained at all experimental temperatures ranged between − 10.64 kJ/mol and − 0.2248 kJ/mol for TDLEE and between − 11.14 kJ/mol and − 0.1259 kJ/mol for TDLHE. These observations suggest that corrosion inhibition process occurred spontaneously. Results indicated that the ethanol extract yielded a higher quantity of phytochemicals (active components) than n-hexane. Ethanol extract (TDLEE) showed better corrosion inhibitive potential than the n-hexane extract (TDLHE). This suggests that phytochemicals, which can act as corrosion inhibitors, are present in higher quantity in the polar fraction of the plant’s extract than in the low polar fraction.