Corrosion Protection by Self-Assembly

The inhibition effect and properties of adsorption layers formed by molecule series ,-diphosphono-alkane on iron surface are investigated. A significant difference is obtained in inhibition effect between compounds having alkyl chains with odd and even numbers of carbon atoms; better protection can be achieved by use of diphosphonates containing odd numbers of methylene groups. The best corrosion inhibition could be attained with 1,7-diphosphono-heptane (1,7-DPH). It is found that a certain threshold concentration of the diphosphonate (c > 4 × 10^–4 M) is necessary to ensure sufficient inhibition. The influence of solution concentration on inhibition effect can be interpreted with the layer formation properties and the surfactant nature of molecules. A significant effect of the diphosphonate is appeared on the kinetic of anodic iron dissolution process. Diphosphonates act as anodic inhibitors, the inhibitor layer formation results in a passivation of the metal surface. Well-ordered self-assembled layer with high corrosion protection effect is obtained in aqueous solution of ,-diphosphonic acids. The self-assembly process takes a longer time, few days are necessary to reach the optimal organization of adsorbed organic layer. It is found that aqueous conditions are preferred for the protective layer formation. It is supposed that with simple chemical surface modification, i.e. with immersion of iron into the solution of alkane-diphosphonates, a new surface pre-treatment could be developed.