I was watching a house building program last night ( grand designs ), and they were talking about rusty steel cladding on a house. It apparently forms a protective coat of rust as it weathers that stops any further rust but gives it that deep golden rusty finish. Has anyone come across this before? Is there a workable version a blacksmith could use?

Google Cor-ten steel.  I've used literally tons and tons of it.  It's less of a wonder material than the architects think it is, but in general it does slow down rusting at a certain point.  Final color can be all over the map from bright rust orange to almost a purple brown.  

So its not a surface finish? You can forge it like normal stock? 

 

Not a "finish" that you apply.  An oxide layer that happens with time.  Look it up.

It's forgeable, stiffer than normal mild steel, if memory serves its got copper, chromium, manganese, silicon as major alloys.  Greenish flames come out of the forge when up to temp, a little disconcerting. Weld it with approved filler materials or else expect call backs from the client, try not to put it in contact with other steels unless you want accelerated rusting.

When U.S. Steel first started pushing Cor-ten, they advertised that once the initial coating set, it would rust no further.  Got their butt sued on that from a NYC project where a skyscraper was clad in the stuff.  

Like stainless...it stains less--it is not corrosion proof 

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Corten will also stain concrete below it with rust stains; Cornell University had a building made with it that made the walkways very stained to say the least.

Weld it with approved filler materials or else expect call backs from the client, try not to put it in contact with other steels unless you want accelerated rusting.

There was some discussion about this in my class this summer, talking about a Coreten sculpture my instructor had to do repairs on in Philly. From what I remember it stops rusting when exposed to oxygen, but in places where two pieces touch and water can get in there was accelerated rusting that was causing issues. I wish I'd paid more attention to that random discussion.

I've got some arty fabricator mates that use Corten sheet for plasma cutting rusty garden decorations for middle aged rich women that like that sort of thing. I think I remember one of them saying a sheet of 3mm costs 3 times the amount of mild steel.

Rust only progresses so deep with Cor-10 the surface rust sort of seals the surface. Unfortunately it's a generally a light fluffy red rust so rubs or rinses off staining everything it touches and the rust has to progress deeper to form the "protective" layer.

It works and doesn't at the same time.

Frosty The Lucky.