I mean you need a lot. In the water there is some of the molecules that break in O+ and OH- ions =, but that quantity is minimal. With the help of the electric current that travels in the motor (and water around it). The amount of O+ is getting a bit higher. That is one of the reasons why I run the motor on low voltage. It reduce the effect the electric current as on the division of the water molecules.
Anyways. Still been 5 years I do it and no sign of rust at all.