I replaced the impeller this morning. It had been in for eight years but had only done a few hundred hours.
The impeller was visually indistinguishable from a new one, so it will go into the spares container.
Interestingly, the pump cover plate ( and also the plate behind the impeller ) had the profile of a couple of impeller vanes etched into it to the depth
of about .015".
It looks like it has been stopped at that position for a while and corrosion
has taken place. I had rydlyme in it for a couple of hours last week (see another post) but I doubt that would have caused it.
Was able to flip the cover plate but can do little about the plate behind.
Will see how it goes.
Thanks again for all your input.