Impeller swelling and seizing?

[Reef Pilot]

What causes pump impellers to swell and then causing seizing and hard start-ups? I have had that happen to a couple pumps (different brands) so far, and then have had others running for many years without a problem. Is this just a defective impeller or some other cause, like not cleaning enough? These pumps are run underwater so can't be because of overheating. Or is it just bad luck?

[spit.fire]

Generally they swell because of heat or a defective impeller, which then causes cracking and water gets in causing it to rust and swell more

[gregzz4]

I've read it happens because some manufacturers use inferior magnet material and then coat them in an attempt to stop the iron rusting
Once SW penetrates the coating they swell

Something to do with the cost of cheap magnets vs rare earth material

[hillegom]

+1

[Reef Pilot]

Yeah, that's kind of what I figured. So just my luck, I guess...