Inhibitory mechanism of low-carbon steel corrosion by mimosa tannin in sulphuric acid solutions

Mimosa tannin was investigated as inhibitor of low-carbon steel sulphuric acid corrosion in concentrations from 10^–5 to 10^–1 mol L^–1, at the temperature of 298 K in the solutions of pH 1, 2 and 3. The inhibitor effectiveness increases with increase in concentration. The adsorptive behaviour of mimosa tannin in solutions of pH 1 and 2 may be approximated, both by Temkin and Frumkin type isotherms, probably due to the chemisorption of tannin molecules on the metal surface. The free energies of adsorption are in the range from –35.1 to –39.5 kJ mol^–1. At pH 3, a Freundlich type isotherm is obeyed, probably due to the physisorption of ferric-tannate that forms at this pH, both on the metal surface and in the bulk electrolyte. The free energy of adsorption at pH 3 is –11.8 kJ mol^–1. The activation energy of the iron dissolution process at pH 1 was found to be 51.4 kJ mol^–1 and decreased to 48.0 kJ mol^–1 on the addition of 1.25 × 10^–2 mol L^–1 mimosa tannin. The addition of the same amount of mimosa tannin into solutions of pH 2 and 3, increased the activation energy of iron dissolution from 15.6 to 34.3 kJ mol^–1 and from 12.0 to 19.2 kJ mol^–1, respectively.