It may be just the impeller but the cooling
passages as previous posters suggest can be an issue.
Its even worse on the smaller motors, Our 2hp yammer struggles to get rid of the mass of eggs & bay jellys in summer. Almost want to fit a filter & flow alarm
if it wasnt a major.
if you pull apart is use loctite 242 or equivalent on every bolt as they seize in the alloy as it corrodes. It seems counter-inuititive to use loctite but I find it stops the corrosion
. You could use anti-sieze but then you run the risk of a bolt coming loose.
Only thing ( frm memory ) I use anti-seize on is exhaust