Mat, you don't quite understand the parameters of how "corrosive" fits in this manner. Chlorate fouling, Perchlorate fouling also present in Pyrodex(17% of the charge), but not apparently in T-7, when combined with moisture from the air, or perhaps just from the wet patch, is acidic in nature, an acid that has an affinity to iron molecules. It dissolves them, forming cratered pits having excavated sides, not normal rusting as with black powder.
Black powder fouling is not corrosive in the same sense, but it is more of an oxidizer in that it forms rust with iron molecules, normal rust - if the humidity is above about 35%. If the humidity is under 35%, black powder fouling becomes inert.