Corruption is...simply put, when a sword's spirit becomes corrupted. Typically it's caused by going against why the saniwa manifested us in the first place. If we try to alter history due to personal feelings...ah well, it's not advised.

[ He won't get into the specifics of corruption but that's the gist of it. There are probably some special cases here and there...he sips his tea again...very thoughtfully... ] All sword spirits are susceptible to it. Some perhaps more than others.

...As far as I know, there's no way to reverse it once it's happened either.