This part of ISO 4628 describes a method for assessing the amount of filiform corrosion developed from a scribed mark by measuring the length of the longest filament L and the most frequent length M of the filaments. Pictorial examples provided in Annex A of this part of ISO 4628 illustrate different ratings for the length of the longest filament L and the most frequent length M of the filaments. A comparison of the test panels with the 12 pictures in Annex A does not supersede the obligatory numerical assessment (method 1 or 2).