Outside gfi Outlet

I am in South Carolina and I am having a problem with an outside GFI outlet. Whenever we get a extreme amount of rain the GFI outlet goes bad, It is not getting wet, but there was a little rust on one end of a black wire running to it. The Gfi is in the midle of the run and connected to anouther gfi in the bathroom on the same level. Do i have to have a gfi outside if it is connected to the one in the bathroom. As soon as the outside one goes bad it pops the one in the bathroom. I replace the gfi outside and it is fine until we get an other extreme storm. I put a good outdoor cover on it and it does not happen often but it is still a problem.