Investigation of filiform corrosion on coated aluminum alloys by FTIR microspectroscopy and scanning Kelvin probe

Fourier transform infrared microspectroscopy has been used to provide a better understanding of filiform corrosion processes on coated aluminum alloys. Results from scanning Kelvin probe microanalysis are also presented. The investigations were performed on untreated or chromium-free pretreated AA6016 covered with an organic coating. An active head with a Volta potential approximately 400 mV lower than that of the tail of the filament was identified. The head was filled with a partially hydrolyzed aluminum chloride solution, containing monomeric and polymeric cations of aluminum, which are highly soluble and hygroscopic, and contain monomeric and polymeric cations of aluminum. The aluminum ions produced by the anodic reaction in the front of the head are hydrolyzed by the hydroxyl ions generated by the cathodic oxygen reduction, which occurs at the back of the head and the tail. Complete neutralization of the aluminum chloride solution in the back of the head and in the tail leads to the formation of an aluminum hydroxide gel. During the precipitation process, carbonate ions are also incorporated in the gel. A carbonate containing the mineral, dawsonite, was formed in the scratch. (C) 2002 The Electrochemical Society.