New knowledge on localized corrosion obtained from local measuring techniques

Going back in history, corrosion was originally treated as a typical local phenomenon. The theoretical background in those early days was limited and mainly based on thermodynamics. Later, electrochemical theory developed, in cluding reaction kinetics, leading to the mixed potential theory, which ele gantly explained uniform corrosion phenomena. In the last decade, many loca l measuring techniques, with different lateral resolutions, have become ava ilable also for application in the electrochemical field. Since then, exper imental evidence has been gathered, showing on a submicroscopic scale how i n many cases corrosion phenomena can now really be described as local. Quan titative data on local anodic and cathodic potential values have become ava ilable. Two examples are given in this paper. The local attack of aluminum alloys, in relation to the microstructure resulting from different heat and quench treatments, was studied with scanning electron microscopy (SEM) and atomic force microscopy (AFM) with combined scanning potential technique. The in situ measured surface potential can explain the observed pitting in detail. Filiform corrosion, a typical example of an oxygen concentration ce ll, was studied with different techniques including electrochemical impedan ce spectroscopy (EIS), SEM/EDX and potential measurements with the Kelvin a nd AFM probes, while polarization curves for typical anolyte and catholyte solutions were related to the local corrosion phenomena in the filiform hea d. (C) 2001 Published by Elsevier Science Ltd.