Effect of Climatic Parameters on Filiform Corrosion of Coated Aluminum Alloys

The effects of temperature, relative humidity, and wet-dry transition on initiation and propagation of filiform corrosion on coated aluminum alloy 6016 (AA6016 [UNS A96016]) have been studied. Corrosion products formed in the filament were analyzed using Fourier transform infrared (FTIR)-microspectroscopy. The aluminum surfaces were tested in both milled finish and grinded conditions with chromium, phosphate, and titanium-zirconium-based pretreatment. An electrodeposited coat (ED coat) and a full paint system used for automotive applications (ED coat + topcoat) were investigated for the different combinations of mechanical finish, surface pretreatment, and coating system. In the temperature range between 5°C and 50°C, filiform corrosion, or underfilm corrosion in general, increased significantly. The effect of relative humidity and wet-dry cycling, on the other hand, seems to be strongly influenced by parameters like pretreatment, coating system, and temperature. Filiform corrosion was the highest in the range from 75% to 95% relative humidity (RH), and a distinct maximum was observed at 85% RH for some coating systems. However, filiform corrosion propagated at humidity down to 40% to 50% RH. For specimens with chromate- and phosphate-based surface pretreatments, filiform corrosion was lower after exposure to tests with wet-dry cycles. The samples with titanium-zirconium-based pretreatments, on the other hand, had a very poor filiform corrosion resistance in the cyclic test compared to exposures at constant relative humidity.