This issue has been the topic of an ongoing discussion on the CorvairCenter forum:
There appears to be an issue with corrosion (sometimes not obvious) leading to fatigue cracking and catastrophic failure of the magnesium fans. A known source of corrosion is sulfuric acid mist from the battery. That is why all Corvairs originally came from the factory with vented battery caps with rubber vent hoses leading out of the engine compartment. These were usually discarded long ago with a battery replacement. The originals a very scarce so the current solution is to use a sealed gel or AGM battery.
I notice your fan looks to have been painted. If it was sandblasted prior to painting, there may be embedded silica particles contributing to hidden corrosion. Our recommendation is to use soda, plastic media, or other inert soft material if a fan needs to be blast cleaned.
The original Corvair magnesium fans were not painted but were given a medium to dark gray chromate conversion coating.