The corrosion portion of the warranty doesn't even mention the frame. Just sheet metal.
From 2014 warranty booklet:
Sheet Metal Coverage
Sheet metal panels are covered
against corrosion and rust-through
as follows:
Corrosion: Body sheet metal
panels are covered against rust for
3 years or 36,000 miles, whichever
comes first.
Rust-Through: Any body sheet
metal panel that rusts through, an
actual hole in the sheet metal, is
covered for up to 6 years or
100,000 miles, whichever
comes first.
Important: Cosmetic or surface
corrosion, resulting from stone chips
or scratches in the paint, for
example, is not included in sheet
metal coverage.