Aluminium alloys AA 3005, AA 3103 and AA 6063 were anodised in sulphuric acid under different conditions whereby a set of porous layers with varying thicknesses was produced. The influence of porous layer thickness on the filiform corrosion behaviour was examined by means of the Lockheed test. The filiform corrosion behaviour as a function of time was investigated. It appeared that the alloys showed similar filiform corrosion behaviour since for all alloys filiform corrosion was reduced considerably by increasing the porous layer thickness. The number of filaments N and the average filiform corroded surface R increase in time until a saturation level has been reached. However, the saturation level appeared to be different for each combination of alloy and porous layer thickness. The results indicate that the mechanism for filiform corrosion is the same for all three aluminium alloys.

The inhibiting effect of unsealed anodic oxide layers on filiform corrosion (FFC) was studied. This was done by means of impedance measurements, polarisation measurements and accelerated corrosion tests (Lockheed test). The electrochemical measurements indicate that both the porous layer and the barrier layer contribute to an enhanced corrosion resistance. In the Lockheed test, the specimens with a porous layer, did not show FFC. The specimens with a barrier layer revealed first a decrease of the number of filaments but after a certain point it increased. In contrast to this, the corroded area of the filaments showed a gradual decrease with increasing barrier layer thickness.

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