Pitting Inhibition by Surfactants

Surfactants have been tried as inhibitors of pitting corrosion of various materials such as stainless steels and aluminum alloys. In order to understand the structure-performance relationship in pitting inhibition, surfactants of different charged headgroups, i.e. , anionic sodium dodecylsulfate (SDS), cationic dodecyltrimethylammonium chloride (DTAC), and nonionic Triton-X-100 (Triton), were investigated as pitting corrosion inhibitors of 304 stainless steel in neutral NaCl solutions. SDS was found to increase the pitting resistance of 304 stainless steel, while neither DTAC nor Triton showed any inhibition effect, indicating that the negative headgroup is preferred in pitting inhibition of stainless steels. To identify the inhibition mechanism, adsorption of surfactants on 304 stainless steel particles was directly measured using the batch adsorption technique. All three surfactants tested adsorbed significantly on the stainless steel, suggesting that the adsorption alone is not sufficient for pitting inhibition. Results obtained for the electrophoretic mobility of the particles in surfactant solutions showed that only SDS made the surface of the stainless steel more negative. Based on these results the inhibition effect of the surfactants is explained in terms of competitive adsorption and electrostatic interactions between surfactants and chloride ions.