Duplex alloys were developed around a 22% Chromium addition level, which largely defines them from subsequent super duplex alloys which were developed around a 25% Chromium addition level for higher corrosion resistance still. The reason why they are called ‘duplex’ is due to the two-phase micro-structure consisting of both austenitic and ferritic grains that give them a combination of attractive properties. In general, they are twice as strong as either austenitic or ferritic stainless steels. They achieve good toughness and ductility, somewhere between the two. Their corrosion resistance is also very good, assuming comparable levels of Chromium, Molybdenum and Nitrogen in selected compositions. One important advantage over austenitic stainless steels is their resistance to stress corrosion cracking. Yet they are significantly more cost-effective, and less prone to price variability, due to their lower nickel content.