Experimental and theoretical evaluation of new 3,3´-methylenedianiline Schiff bases as corrosion inhibitors for carbon steel in sulfuric acid

Three Schiff bases were synthesized from 3,3´-methylenedianiline and used as corrosion inhibitors for 1018 carbon steel in 0.5 M sulfuric acid.New compounds include 3-bromosalicylaldehyde (BSD), 5-nitro-salicylaldehyde (NSD) and 2-hydroxy-naphtaldehyde (HND).They were evaluated by means of gravimetric tests as well as potentiodynamic polarization curves and electrochemical impedance spectroscopy measurements.Tests were complemented by theoretical calculations using the density functional theory (DFT).The results have shown that the evaluated Schiff bases act as good mixed-type corrosion inhibitors with an inhibitor efficiency that increased with an increase in their concentration and decreased in the series NSD>HND>BSD.They are adsorbed on to the steel surface by a combination of physisorption and chemisorption, dominated by a weak, physical type of adsorption following a Temkin adsorption isotherm.The corrosion process was controlled by the adsorption/desorption of some intermediate species for inhibitor concentrations lower than 100 ppm, whereas for higher inhibitor concentrations, the process was under charge transfer control.The theoretical results obtained from the quantum chemical studies showed that parameters such as HOMO, LUMO, dipolar moment, hardness, electronegativity, electron affinity, ionization potential and electrophilicity made NSD the compound which had the best inhibitory properties, in good agreement with the experimental findings.Analysis of Fukui indices for the three molecules revealed nucleophilic and electrophilic sites responsible for electron donation and acceptance.