Corruption does not work correctly if you cast it on an already built shard. If you have the built shard selected, when cast corrupt on it, it will basically put a un-buildable death shard, on top of the already existing shard.
I've found that I can get it to work correctly, only if I cast it on an unbuilt shard, and do not have it selected when casting the corrupt spell on it. Since using this method, I've had 0 failures with the corrupt spell.
See more details here.