There are two ways to stop this depending on how badly your bike is exposed to water. The first solution is to replace the washers with nylon washers. if your bike does not see a lot of rain then this may be the solution for you. my bike sits outside under a bike cover. Here are the nylon washers. I got them from Ace Hardware for .11cents a piece.
This is what it looks like after replacing mine. It has seen the rain and seems to be working great.
However if you want to take this a step further you could also buy nylon bolts too. these only hold the body panels to nylon will be fine for this application.