Synergistic action of n-dodecylamine withn-capric acid on the corrosion of steel in sulphuric acid solution

Abstract

Studies were performed on the effect of mixtures of n-capric acid (Ac) with n-dodecylamine(Am) on the corrosion rate of mild steel in IN H 2SO 4 at 25°C. The concentration of Ac was changed within the range from 3·10 −5 to 1·5 10 −4 mole/1, and that of Am from 1·10 −5 to 2·10 −4 mole/1. The maximum increase in the inhibitive efficiency of Ac by additions of Am was observed at concentrations close to that of the saturated solution of Ac in 1N H 2SO 4. It was found that Ac and Am mixtures were adsorbed at the metal surface following the adsorption isotherm of Frumkin. This may be written: θ s 1 − θ s exp ⁡ f m i x θ s = K c A m c A c where θ 5—coverage degree by both substances, ƒ mix—parameter connected with changes of the adsorption energy with coverage, K—equilibrium constant of the adsorption reaction, c Am, c Ac—concentrations of Am and Ac respectively. The standard free energy of adsorption of the mixture of Ac with Am is approximately equal tothe sum of the standard free energies of adsorption of these compounds taken separately, and fin,: is the arithmetical mean of the respective single values. This suggests that the ratio of the number of Ac to Am molecules in the adsorbed layer is 1 : 1. Most probably, micellae having such a composition (AcAm) already exist within the sulphuric acid solution.