Effect of anions on the efficiency of aromatic carboxylic acid corrosion inhibitors in near neutral media: Experimental investigation and theoretical modeling

The influence of the nature and concentration of electrolyte anions on the efficiency of carboxylic acid based inhibitors for steel in neutral solution was studied using anodic and cathodic polarization at a rotating disk electrode. The N-ethyl-morpholine salts of a omega-benzoyl alcanoic acid model compound and of benzoic acid were used as inhibitors. Both compounds were found to inhibit the anodic partial reaction. In the active potential region, the inhibition effect was more pronounced at low electrolyte concentration suggesting an adsorption mechanism. Both inhibitors favored passivation of the electrode, the effect being attributed to their capacity to act as a buffer in the anodic diffusion layer. For a given inhibitor concentration in different electrolytes, the stability of the passive film under anodic polarization conditions decreased in the order CIO4- > CI- > SO4--. A theoretical model has been developed that is able to describe the observed influence of anion and inhibitor concentration on the inhibition of iron in the active potential region. The inhibition effect is attributed to blocking of surface sites for anodic dissolution by the inhibitor molecule. The degree of adsorption follows a Langmuir isotherm and depends on the nature and concentration of the electrolyte anion present. The model predicts a stronger inhibition effect for low electrolyte concentration in agreement with experimental observations made with N-ethyl-morpholine salts of two aromatic carboxylic acids. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.