The effects of temperature, relative humidity and wet dry transition on initiation and propagation of filiform corrosion on coated aluminium alloys AA6016 have been studied. The aluminium surfaces were tested in both milled and grinded conditions with chromium, phosphate and titanium-zirconium based pretreatment. A full paint system used for automotive applications (ED coat + top-coat) and an electrodeposited coat (ED coat) were investigated for the different combinations of mechanical finish, surface pretreatment and coating system. In the temperature range between 5 to 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 also temperature. Filiform corrosion was the highest in the range 75 to 95 % relative humidity and a distinct maximum was observed at 85% R.H. for some coating systems. However, filiform corrosion propagated at humidity down to 40-50% R.H. 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. © 2003 by NACE International.