David, have you worked with Corten? That's a version of steel that is made to rust and have the iron oxide become the protectant from additional rust. U.S.Steel developed it back in the 1950's I believe. Babcock and Wilcox supplied the steel extrusions of it for the K-Mart building in Troy, MI. USS built some bridges out of it but I think that they found that salt and calcium cloride was nastier, over time, than they figured. I have no idea if it's still made, but if you wanted a couple of hundred tons, they might be able to find the recipe.