| Pitting corrosion is the major factor limiting the use of major industrial materials such as stainless steels and aluminum alloys, causing significant damages both in terms of material loss and resultant accidents. Surfactants have long been used as general corrosion inhibitors, but little is known for their application as pitting corrosion inhibitors. The main objectives of this study are to explore the possibilities of using surfactants as pitting inhibitors, to elucidate their inhibition mechanisms and to develop better surfactant-based pitting inhibitor packages.; Anionic surfactants N-lauroylsarcosine sodium salt (NLS) and sodium dodecylsulfate (SDS) were found to be promising pitting inhibitors, as indicated by the potentiodynamic polarization and scanning electron microscopy results. Cationic surfactant dodecyltrimethyl ammonium chloride (DTAC) and nonionic surfactant octylphenol ethylene oxide (Triton-X-100) showed no inhibition effect at any concentration tested.; Only the anionic surfactants tested are able to make the interfacial charge of the stainless steel more negative. The adsorption of surfactant was found to be necessary but not sufficient for pitting inhibition. Neither the degree of the surface activity of surfactants nor the surface hydrophobicity of the surface is relevant for the pitting inhibition. Based on the above observations, the inhibition mechanism of anionic surfactants was proposed to be the displacement of adsorbed chloride ions through electrostatic repulsion.; The pitting inhibition by anionic surfactants NLS and SDS was shown to depend on environmental factors such as temperature, chloride concentration and solution pH. With higher temperature, higher chloride concentration and lower solution pH, the inhibition of pitting corrosion by surfactants becomes more difficult.; A novel technique utilizing the solution color change as a result of pitting corrosion with the aid of pH indicator was tested to visualize localized corrosion in situ in real time. The pitting corrosion was demonstrated to be a random process and the location of the pits was unpredictable.; Higher pitting inhibition efficiencies were achieved using mixtures of anionic and cationic surfactants. Synergism with lesser magnitude was observed for the mixtures of anionic surfactant SDS with inorganic calcium ion. Antagonism occurs when mixing anionic surfactants SDS with nonionic surfactant Triton-X-100.; In sum, this study has established that surfactants do inhibit pitting corrosion. More efficient inhibitor packages have been developed through combinations of surfactants with other synergistic compounds. The knowledge will serve as a solid intellectual foundation for the industrial applications of surfactant-based pitting corrosion inhibitors. |
