Hi KS, thanks for responding. Nice useful pictures too. That treatment that you put on the rotors and magnets should've been adequate, but evidently it was not enough protection.
What I think is happening is this; it seems that the coating that the manufacturers are putting on the magnets, as good as it is, it is not adhering well to the sintered iron-neo-boron mix.
But I suspect that the main problem could be a difference in the rate of expansion of the two materials (the sintered iron mix and the copper-nickel plating) with changing temperatures. If the iron mix expands at a faster rate than the copper-nickel plating, the plating will crack. Once this happens, water comes in contact with the sintered mix and corrosion starts. Even the water vapor in the air could be enough to start corrosion when in contact with the unprotected magnet.
I'm not an expert on this, so this is speculation on my part.
I did a bit of research earlier and found a discussion about rusty magnets on this forum from 2006 in which Les Vincent (Flux) R.I.P., and others, took part and had some interesting suggestions. I understand that Flux used to work fixing some damaged rotors, among many other things.
I'm not sure if I am allowed to post links since I don't have enough posts yet, but here it is:
https://www.fieldlines.com/index.php/topic,138006.msg913546.html#msg913546After reading all that, I'm beginning to think that the best protection for new magnets would be encapsulating the magnets in a good quality epoxy. That way, even if the copper-nickel plating gets cracked or damaged, it would still be protected by the layer of epoxy.
There are many different types of epoxies. Hopefully someone with good knowledge of epoxies reading this thread could recommend something appropriate for this application.
Another thing is isolation between the steel rotor and the magnets. That has been discussed before and was mentioned earlier by Jason. That could be accomplished by a thin sheet of vinyl or Polycarbonate, (Lexan-Plexiglas)
I found a website that sells clear rigid vinyl sheets in thickness of - .01, .015, .02, .03. Prices are from 6 to 15 $US for a 21 x 51-inch piece.
Another website sells Clear Polycarbonate, .031" thick X 24" X 48" for around US$22.
The tricky part would be; how to encapsulate the magnets in epoxy during construction? Hard to do but not impossible.
It is late here, I'm tired.
Ed