Originally Posted by due ruote

Interesting. I lifted this from the wiki entry:

Aluminium-based alloys may be sensitive to intergranular corrosion if there are layers of materials acting as anodes between the aluminium-rich crystals. High strength aluminium alloys, especially when extruded or otherwise subjected to high degree of working, can undergo exfoliation corrosion, where the corrosion products build up between the flat, elongated grains and separate them, resulting in lifting or leafing effect and often propagating from edges of the material through its entire structure. [2] Intergranular corrosion is a concern especially for alloys with high content of copper.

I have a Stronglight 93 crankset on my 1973 Raleigh that shows signs (not nearly as severe, thank goodness) of the same thing. I'm the original owner and the bike has not been subjected to any unusually harsh treatment. So I wonder if there was some questionable metal used on some of these. Otoh, after 38 years that part doesn't really owe me anything.