Effect of the Nature of the Counterions of N-alkyl Quaternary Ammonium Salts on Inhibition of the Corrosion Process†

Regina Fuchs-Godec*
Fakulteta za kemijo in kemijsko tehnologijo Maribor, Smetanova 17, Maribor, Slovenija,
Tel: 02-2294-443, Fax: 02-2527-774
E-mail: fuchs@uni-mb.si

Abstract
Electrochemical measurements were performed to investigate the effectiveness of cationic surfactants of the N-alkyl quaternary ammonium salt type with different counterions and different chain lengths, as corrosion inhibitors for ferritic stainless steel type X4Cr13 in 2 M H₂SO₄ solution. Two of them were single-chained surfactants and the other two were composed of three C₈ alkyl-chains. The chosen cationic surfactants were myristyltrimethylammonium chloride (MTACl), myristyltrimethylammonium bromide (MTABr), trioctylmethylammonium chloride (TOMACl and trioctylmethylammonium bromide (TOMABr). Potentiodynamic polarisation measurements showed that these surfactants hinder both anodic and cathodic processes, i.e. they act as mixed-type inhibitors. It was found that the adsorption of the nalkyl ammonium ion in 2 M H₂SO₄ solution is in accordance with the Langmuir adsorption isotherm. Plots of log [θ/(1–θ)] vs. log c_inch yielded straight lines with a slope which drastically changed at the CMC values of used surfactants. The plot of log θ vs. log c_inh confirms šthe four-region’ reverse orientation model of adsorption, suggested by Somasundaran and Fuerstenau. In region IV, where the formation of a multilayer is in progress, it is supposed that two different multilayers formed on metal surface in the case of TOMABr and MTABr. The influence of added –CH₂ groups (chain length) on the inhibition efficiency is greater than the influence of different counterions.

Keywords: Cationic surfactants, corrosion inhibitors, counterions, sulphuric acid, critical micelle concentration.