Inhibition of mild steel corrosion in sulphuric acid using indigo dye and synergistic halide additives

Gravimetric method was used to study the inhibitory properties of indigo dye during corrosion of mild steel in aerated sulphuric acid solutions at 30–50 °C. The effect of addition of halide salts KCl, KBr and KI was also investigated. The corrosion rates in all systems studied increased with rise in temperature. The inhibition efficiency of indigo dye increased with concentration and synergistically increased on addition of halide salts. Temperature studies revealed increased inhibition efficiency at higher temperatures, which is suggestive of chemisorption mechanism. The inhibitor adsorption characteristics were approximated by Frumkins isotherm and Flory–Huggins isotherm. Activation energy for Fe dissolution in sulphuric acid was observed to reduce from 54.6 kJ mol−1 in the uninhibited system to 34.9 kJ mol−1 in the inhibited system.