Synergistic corrosion inhibition of HEDTA and Iodide Ions on mild steel in acidic medium

The inhibition of corrosion of mild steel in 1 M H2SO4 solution in the presence of N-(2- Hydroxyethyl) ethylenediaminetriacetic (HEDTA), and the effect of the addition of potassium iodide to the inhibitor (HEDTA + 0.005 M KI mixtures) was studied using the weight loss method. Weight loss data reveal that the inhibition efficiency increased with an increase in the concentration of HEDTA. At 30 °C, the inhibition efficiency for HEDTA increases in the order: 0.001 M HEDTA(20.40%) < 0.0015 M HEDTA(26.53%) < 0.0020 M HEDTA(30.61%) < 0.005 M HEDTA(42.85%) while HEDTA + 0.005 M KI mixtures followed the order: 0.0010 M HEDTA + 0.005 M KI (97.96%) < 0.0015 M HEDTA + 0.005 M KI (93.88%) < 0.0020 M HEDTA + 0.005 M KI (91.84%) < 0.005 M HEDTA + 0.005 M KI (87.86%). The inhibition efficiencies in the presence of HEDTA + 0.005 M KI mixtures are higher than the corresponding HEDTA, signifying a synergistic effect between HEDTA (inhibitor) and the iodide ions. The inhibition efficiencies decrease with increase in temperature. Activation energy increased from 44.65 kJ/mol (blank) to 98.42 kJ/mol (HEDTA + KI) confirming physisorption. The thermodynamic parameters suggest spontaneous adsorption involving both physisorption and chemisorption mechanisms. The probable adsorption centres on HEDTA have been highlighted using the Mulliken atomic charges and Fukui electrophilic and Fukui nucleophilic indices. The adsorption of both HEDTA and HEDTA + 0.005 M KImixtures onto mild steel surface obeyed the Langmuir adsorption isotherm.