Weathering Steel or COR-TEN steel weathers to a nice coppery brown color, then stops rusting for the most part. John welded up this planter box for a garden in Rockridge, CA this morning.
corten steel planter box
Isba supervises
Welder and two Landscape Architects
deedsdesign 
That’s just the kind of thing they go for in Rockridge, although maybe weld some craftsman touches next time. Rockridge is full of people like that.