My Crappy tire bike has a three piece bottom brack with loose bearing cones. I rode the thing for three years no problem, but earlier this summer I notice my spindel was all weird. I took the BB apart and the non-drive side cone was completely corroded. I bought me another cone and put the BB back together and everything was fine.

Untill last night...

I notice my spindel was out of wack again and thought it was just a matter of tighten the cup a little. I figured, while I had all the tools out, I would do a little bit of cleaning. I took the BB apart and one of the bearings on the non-drive cone had fallen out and the cone showed signs of corrosion.

My questions are:

1) am I doing something wrong with the assemble that I could do different to avoid this issue or is corrision just a matter of course and something I'm gonna have to live with.
2) is it worth it to by a cartridge BB and if so, what information do I need in order to make the right purchase.