When welding Corten the precautions are similar to mild steel except that you may need a special filler material to produce the weathering steel weld. (6013s are not acceptable.) If the plate is 10mm thick or less, and the weld is a single pass (eg a fillet weld), you can use an E7018 or SG2 MIG wire.
Corten if left alone will take 2-4 years to reach it's peak patina and then should remain in that state with very little change. A. Hi. "Corten" is a trademark of US Steel for "weathering steel", but US Steel only makes it in limited shapes anymore.
Corten Steel is created by mixing up a particular combination of steels and alloys. It is this combination that creates the easy rusting properties and gives Corten Steel its recognisable look. It is produced in a sheet form which is usually only ¼ inch thick