Can weathering Steel be left unpainted? Are there any environments when the properties of weathering steel will prevent the steel from performing as designed, i.e. surface rust forming to encapsulate the remaining steel core.
Anywhere within about 5 miles of a sea coast , they should be coated to resist corrosion . Otherwise they are pretty good. They do slowly rust and turn reddish brown , I have heard of them being painted because important people don't like the color , but otherwise it is generally not necessary to paint them . They have tiny amounts of P , Cu , Cr , and a few other elements depending on the manufacturer.
The reason that weathering steel was developed was so that it would have better weathering characteristics than regular carbon steel. It was specifically developed so that it creates a patina of rust that is durable and keeps the remaining steel from rusting further.
When not to use it?
In certain environments, the protective patina will not form. In these locations, additional protective coatings are recommended. These can be paint or zinc coatings.
The Wikipedia page lists more specific details about locations where uncoated weathering steel is not recommended.
In general, locations that are corrosive (e.g. pollution or swimming pools) or locations that do not experience wetting and drying cycles are not good locations for weathering steel.