We have investigated the initiation and propagation of filiform corrosion on coated aluminium surfaces was using in situ FTIR microspectroscopy and the scanning Kelvin probe. During the initiation partially hydrolysed aluminium chloride was formed in a defect on the coated surface. The conditions in the defect resembled the environment that is found in growing pits with low pH and high chloride concentration close to the coating interface. This weakened the adhesion of the organic coating and initiated the formation of a filament. The movement of the active head was followed with in situ FTIR microspectroscopy in humid air using the characteristic IR band around 2500 cm-1 from Al(H2O)6 3+. This band is present in spectra from partially hydrolysed aluminium chloride, which was found in the head during propagation of the filament. The absorption of water in the hygroscopic corrosion products in the head of the filament was followed by measuring the changes in the intensity of the Al(H2O)6 3+ band during variation in the relative humidity.