| Abstract: | Investigation into pitting corrosion of passive iron in sulphuric acid containing chloride ions Pitting corrosion of metallic materials is generally connected with presence of a surface layer giving rise to a local differentiation of the electrochemical behaviour of the metal surface. The pitting corrosion by halogen ions on passive metals is investigated using passive iron in chloride ion-containing sulphuric acid as the model system. Quantitative data are presented concerning the mechanism and kinetics of the individual processes giving rise to pitting corrosion in a chloride ion concentration range covering three powers of ten, and in the whole potential range of iron passivity, from the Flade potential to the transpassive breakthrough potential. Pit formation normally follows a linear kinetic law, the rate depending in particular from the chloride ion concentration and from the thickness of the passive layer. The growth of pit diameters follows a linear kinetic law, too; the dissolution current density in the pits depends from the chloride ion concentration. Comparative investigations carried out on active iron, and potential distribution as measured in the pits show that the metal is active in the pits, too. The heterogeneous mixed electrode condition — active pit/passive metal surface — is stabilised by resistance polarisation. The investigations so far do not permit any statement concerning the specific effect of the chloride ions. |
