Apr 19, 2021 · Certain grades are proprietary, such as COR-TEN A or COR-TEN B.Patinax weathering steels, which are another group of proprietary grades. All these proprietary levels are similar to ASTM classifications A 242 and A 588. Corten by Gnee Steel ,0,5 months 1,5 months 6 months 2 years 5 years 22 years 40 years The patina