Eco-friendly corrosion inhibitors:: the inhibitive action of Delonix Regia extract for the corrosion of aluminium in acidic media

Purpose - To investigate the inhibitive effect of Delonix regia extracts to reduce the corrosion rate of aluminium in acidic media. The study was a trial to find a low cost and environmentally safe inhibitor to reduce the corrosion rate of aluminium. Design/methodology/approach - The inhibition efficiency was evaluated using the hydrogen evolution technique at 30 degrees C. The mechanism of adsorption inhibition and type of adsorption isotherm was characterised from trends of inhibition efficiency and kinetic data. Findings - Delonix regia extracts inhibited the corrosion of aluminium in hydrochloric acid solutions. The inhibition efficiency increased with increasing concentration of the inhibitor but decreased with increase in exposure time. The acid extracts (hydrochloric acid seeds extract (HSE) and hydrochloric acid leaf extract (HLE)) were found to be more effective than the ethanolic extracts (ethanol seeds extract (ASE) and ethanol leaves extract (ALE)) and the inhibition followed the order: HSE (93.6 per cent) > HLE (83.5 per cent) > ASE (63.9 per cent) > ALE (60.4 per cent). The low negative values of Delta G(ad): - 20.14 kJ mol(-1) for HSE, - 18.08 kJ mol(-1) for HLE, -15.96 kJ mol(-1) for ASE and -15.12 kJ mol(-1) for ALE, as calculated from the Langmuir isotherm, indicated that the inhibitor molecules adsorbed onto aluminium by a physiosorption-based mechanism. A first-order type of reaction mechanism was obtained from the kinetic treatment of the H-2 gas evolution data. Research limitations/implications - Further investigations involving electrochemical studies such as polarization method should shed further light on the mechanistic aspects of the corrosion inhibition. Originality/value - This paper provides new information on the possible application of Delonix regia as an environmentally friendly corrosion inhibitor under the specified conditions. This environmentally friendly inhibitor could find possible applications in metal surface anodizing and surface coatings.