Why do the rockers rust on top?

[rsgt]

The truck I bought has had the rockers replaced in the past. The replacements are rusted at the top like a lot of them do. I cut the top out of the rocker to look inside. The rest of the rockers are solid. I just wondered what the cause was and how to prevent it.

[turp mcspray]

They are spot welded to the floor. Moisture gets trapped between the two pieces. Anywhere you find rust, there are likely two pieces of metal, sandwiched together, creating a place for rust to grow.