A CORROSION-PROTECTIVE FILM FORMED ON A CARBON-STEEL BY AN ORGANIC PHOSPHONATE

Surface treatments have been carried out on a carbon steel in solution s containing different phosphonates. The compounds were dissolved in a n ethanol/water mixture (80%/20%). Corrosion protection afforded by la urylphosphonic acid (LPA), ethyllaurylphosphonate (ELF; also called la uryl phosphonic acid monoethylester) and diethyllaurylphosphonate (DEL P) has been studied by steady-state current-voltage curves and electro chemical impedance measurements with a rotating disc electrode. Corros ion protection was only obtained for the ethyllaurylphosphonate (ELP), which is able to form a relatively thick film on the carbon steel sur face. Thus, analysis of the variations of electrolyte resistance throu gh the pores and defects of the film against concentration and immersi on time in the solution containing the monomer allowed us to find the optimum parameters for treatment giving the best protection (24 h of i mmersion and 5% ELP concentration). Electron-probe microanalysis corro borates that the film is thick and porous. Infra-red spectroscopy indi cates that the film was formed by reaction of the organic phosphonate with the steel surface to produce a metal salt. (C) 1997 Elsevier Scie nce Ltd.