After two years of use and nearly 2000 kilometers behind it, the front wheel of my Ninebot ES2 started to emit a rasping noise, all while the battery life was dropping increasingly.
When I opened it up, I noticed that water had seeped through the case, rusting the whole thing up. I figured I could probably save everything except for the bearings, which were definitely shot.
Rather than ordering new bearings, I decided to use the case (with magnets) and bearings from a broken motor I already had. It was pretty much immaculate, apart from refusing to drive on one of the phases.
In case I need it in the future, I might go down to the nitty-gritty and try to fix it, and if that happens I'll order a new pair of bearings. For now though, I'm happy with just one functioning motor!
Since I didn't want to risk having the same thing happen all over again, I let the stator sit indoors for about a week or so to dry out completely.
Meanwhile, I fitted the bearing that was loose by first freezing it for an hour to make it shrink in size, after which it just slid right in place.
When I figured the stator was entirely dry, I put the motor back together. I made sure there were no irregularities causing gaps between the main piece of aluminium and the cover plate which could cause water to leak in, and screwed it back together.
I took a video of it running again on the bench, but it somehow disappeared and now I've put the scooter back together. I can tell you though, it feels like brand new again! The bad sounds and vibrations that I had gotten used to are completely gone, and I can ride nearly double the distance before needing to charge. So all in all I'm very happy with the result!
First things first, I used this guide to disassemble the motor: https://www.youtube.com/watch?v=ZH6RM_SdRlw. It can actually be quite scary before you get the hang of it. In addition to screws, the magnets lining the inside of the case keeps the two halves together and if you're not careful as you pry them apart they may slam shut again, quite forcefully. In other words, don't keep your fingers (or any other parts of your body) between them until they are completely separated. Use tools for prying!
As a first attempt to attack the rust, I soaked the stator in 5-56 (WD-40 equivalent I guess) and let it sit for a a few hours. This did practically nothing, which shouldn't have come as a surprise.
In my second attempt, I mixed some citric acid with tap water and soaked the stator in it for two days. It shouldn't really take that long, but my solution was quite diluted since I just had a few grams of the acid at hand so the reaction was slower.
This seemed to do the trick. All the rust went away easily with the help of some steel wool, and I washed away all the corrosion and acid with tap water. After that, I soaked the whole thing in 90% ethanol to help drive the water out, and then let it sit in the sun for a few hours to dry. My only worry was whether the built-in sensors and their PCB had survived the acid treatment, but they did just fine.