Over that long a time period it could be damaged just by rain - it's hard to say because corrosion tests aren't done with respect to a 1000 year operating life

Over that long a time period it could be damaged just by rain - it's hard to say because corrosion tests aren't done with respect to a 1000 year operating life in mild conditions. I'm sure an expert could figure that out, but it would require looking at the corrosion rate of the specific steel, the exposure to any kind of forces that could damage the oxide layer (e.g. scratches), and the thickness of the member.