Hmmm... I've never used it on carbon steel. Cool, I'll have to check it out. If it does apply a protective layer, I would think it does so by pre-corroding the top layers, similar to the galvanic coating on galvanized steel pipes and fittings.

I'd have to look more into it to know how durable the coating is, once it is scratched or worn, it would of course be useless.

Edit: you also have to check the permeability of the corrosion product coating. If it's too permeable it's also significantly less useful.