INFLUENCE OF CORROSION PRODUCTS ON LOCALIZED CORROSION OF CARBON STEEL IN NATURAL SEAWATER

摘要

Whatever its origin,a localized corrosion process involves cathodic and anodic zones where the kinetics of the various electrochemical processes differ.As a consequence,the composition of the corrosion product layer may be different in both types of zone,which in turn may influence the corrosion process.This question was investigated in the frame of the Aduscor research program of the Technological Research Institute Jules Verne of Nantes(France).Carbon steel coupons were immersed in natural seawater for 6 to 8 years and the resulting corrosion product layers were thoroughly characterized by X-ray diffraction and Raman micro-spectroscopy.In anodic zones,the layer was composed of an inner black stratum of magnetite,iron sulfide and sulfate green rust(GR_(SO4))covered by an outer stratum of Fe(III)oxyhydroxides.In cathodic zones,only traces of FeOOH compounds were found and the inner black stratum was mainly covered by aragonite.The main components of the black inner stratum were the same in both types of zone but their proportions differed.The higher was the cathodic character of a zone,the higher was the Fe3O4 to GR_(SO4)abundance ratio.Various pretreatments were applied to steel coupons to induce strong galvanic effects.In strongly anodic zones the formation of magnetite could be completely avoided.In strongly cathodic zones,the inner black stratum contained a small proportion of GR_(SO4),i.e.was mainly composed of FeS and Fe3O4.Because FeS and Fe3O4 are electronic conductors,the reduction of dissolved oxygen can take place on the outer surface of the black inner stratum.In contrast,the anodic zones are covered by thicker layers of insulating compounds that hinder the transport of O2.The localized corrosion process is then self-sustaining as the induced modifications of the corrosion product layer favor its persistence.