Weathering steels are a family of ASTM specifications, as well as several trademarked specifications exhibiting similar corrosion-resistant properties.  The primary trademarked product is Cor-ten®. The selection of a particular ASTM weathering steel specification is dependent on the type of product desired ASTM A606-4 covers steel sheet and coil and ASTM A847 is for weathering steel tube and pipe. ASTM A871-65 also cover plate, but with a 65ksi minimum yield strength. The corrosion-retarding effect of the protective layer is produced by the particular distribution and concentration of alloying elements within it. The naturally-developed patina protecting the surface regenerates continuously when subjected to the influence of the weather. In other words, the steel is allowed to rust in order to form the ‘protective’ coating. This protective coating guards against future corrosion.