INVESTIGATION OF FILIFORM CORROSION ON COATED ALUMINIUM ALLOYS BY FTIR MICROSPECTROSCOPY AND SCANNING KELVIN PROBE

摘要

A Scanning Kelvin Probe (SKP) and FTIR microspectroscopy have been used to provide a better understanding of filiform corrosion processes on coated aluminium alloys. The investigations were performed on untreated or chromium-free pretreated AA6016 covered with an organic coating. With a Volta potential lower than the tail of the filament by approximately 400 mV, an active head was identified. The head was filled with partially hydrolysed aluminium chlorides, which are highly soluble and hygroscopic, and contain monomeric and polymeric cations of aluminium. A primary cathodic site was located in the trailing edge of the head, where carbonates were incorporated to the corrosion products. The oxygen was also reduced in the tail, resulting in a pH increase and the production of an aluminium hydroxy carbonate gel. A carbonate containing the mineral, dawsonite was formed in the scratch.