A brief introduction: Hello, my name is Zack and I am the TechShop liaison for the OpenROV team. TechShop has taken an interest in following this story and I have been charged with supporting the team with the tools and community available at TechShop in San Francisco.
A topic of debate is keeping your brushless motors working after repeated and possibly prolonged exposure to seawater. A post-dive cleaning with freshwater then WD-40 will get you fairly far, though it is fairly far from a long term solution. I tried three other methods over the weekend to see if they had any merit.
Silicone Mold Release Spray
Spraying the coils with liquid silicone will coat them in a water resistant silicone gel. This stuff is very difficult to remove even with solvents.
Silicone Conformal Coating
This liquid is commonly used to protect electronics and is resistant humidity. It hardens after application and is able to penetrate into the coils. Once hardened it forms a permanent coating.
This was, at first, going to be a control for the experiment. But I soon realized the merits of not caring. These motors are fairly inexpensive and frankly, my dear reader, replaceable.
All three methods were subjected to a solo cup filled with tap water and three packets of In-And-Out to-go salt--only the finest. The coatings were applied to some old DC motor rotors I had lying around that exhibited similar attributes to the motors we will be using for OpenROV models. I then left for a weekend of waiting in Memorial Day traffic. Upon my return I washed and wiped the motors to remove any rust that had accumulated on the magnets and snapped a picture.
From left to right,
Not Caring, Silicone Conformal Coating, Silicone Spray
As you can see, the "Rhett" method motor had a little corrosion on the magnets but nothing to stop the next dive. The conformal coated magnets actually accumulated a little rust, surprisingly, but copper coils were pretty well encapsulated--a result that could mean better performance underwater. The silicone spray fared the best against rust, showing few signs of build up.
Silicone spray is an excellent water repellent. Coupled with regular cleaning and re-coating, this could get you much further than simply not caring.
Another issue to contend with is that there is a possible drop in power of the motors as they operate underwater due to a short-circuiting effect between the coils. It is my next goal to test some methods for sealing them without compromising durability or performance. I will probably use the following methods: polyurethane sealant, epoxy, and marine vinyl-ester resin. Any suggestions would be appreciated.