Failed Rims Due To Tire Sealant

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Rims sometimes fail due to ammonia (caustic) in some tire sealants.

From http://velonews.competitor.com/2009/03/bikes-tech/lennard-zinn-tire-sealant-corrosion-disc-brake-spoking-patterns-and-more_89375 as of 2010/02:

[S]ome sealants [are] very basic and thus quite corrosive to untreated metal. That's why a person can have pretty positive results with newer wheels, but can experience issues once there are tool marks and other wear that penetrates the anodization.

The problem is not so frequent as the e-mail seems to suggest, but this might hold some truth [for sealants with] the typical ammonia smell [...]. Ammonia is used to stabilize natural latex and [...] can harm both tires (depending on the rubber formulation) and rims in the long run.

Corrosion is often larger than the uncorroded base metal. Thus, corrosion can act as a "wedge" that pries apart at the corrosion, exposing new metal and thus allowing new corrsion. This can cause large cracks to develop even when the bulk stress is fairly low. That appears to be a reason hard-anodized rims fail, and also appears to be the kind of crack seen in the first picture, above.