Boranni wheel corrosion

My friend Joe has just got back from a week away to Brittany on his Guzzi. Last Thursday just before boarding the return ferry he got a puncture in his rear tyre. Over this past weekend he set about replacing the inner tube and found this corrosion on the inside of the rim. As can be seen from the pics the outside of

the wheel looks in pretty good condition which makes the inside condition all the more surprising. I have to say that I have had quite a number of Boranni wheels pass through my hands over the past 20+ years and have never seen one like this before