Hi Darcy. My experience with this has been that the armature frame will eventually corrode if it has not been pretreated. I did some experimentation with this. When I was experimenting with a 0.5% bleach solution with which to decontaminate/disinfect the OpenROV. Exposure to salt water will probably yield the same results.
What I observed is that the armature frame corrodes but the copper winding remained unaffected. The copper winding is coated with an enamel during the manufacturing process. However, the armature frame does not appear to be treated.
I tested three motors during the experiment. Two motors were straight “out of the box” and one was treated with rustoleum crystal clear enamel paint. All were rinsed thoroughly with fresh water and sprayed with a silicone lubricant after the rinse. One of the untreated motors was exposed to distilled water, fresh water rinsed, and sprayed with lube (control). One of the untreated motors was exposed to a 0.05% bleach solution, fresh water rinsed, and sprayed with lubricant (experimental #1). And the third motor (treated was exposed to 0.05% bleach solution, fresh water rinsed, and then sprayed with lubricant (experimental #2).
On the distilled water (control) unit I noticed only light oxidation on the armature frame and magnets of the motor and no corrosion on the copper winding, no corrosion on the bearing. . On the 0.05% bleach Experimental #1) unit I had heavy corrosion on the armature and corrosion/pitting on the motor magnets but no corrosion on the copper winding, moderate corrosion on the bearing. On the Treated (experimental #2) unit which was pretreated with rustoleum enamel, I had no corrosion on any components.
Since I did not apply rustoleum to the bearings I am assuming that the fresh water rinse and spray lubricant was sufficient to protect bearing assembly.
The armature frame is probably composed of ferrous material, thus unless it is well protected it will corrode rather easily. The magnets are probably Neodymium which is not very resistant to oxidation which would account for their rapid oxidation in presence of bleach solution (oxidizer) .
My conclusion is that (clean) freshwater rinse is absolutely necessary after each exposure to either fresh or salt water. A water disbursal lubricant spray should also be reapplied after the fresh water rinse is completed. The lubricant spray should be compatible with motor & ROV components.
Pretreating motor windings with rustoleum crystal clear enamel in very thin coat(s) is effective in corrosion abatement of some components of the motors. Since bearing assemblies can not practically be coated with enamel spray, fresh water rinse and lubricant is the only “practical” measure of corrosion abatement for bearings.
These suggestions are based on preliminary analysis of my experimental data/observations.
A detailed report of experimental methodology and observations should be published on the OpenROV site by the end of next week.
Hope this helps