Only a few corrosion resistant type of iron has reached the present day from ancient times. The most well known and greatest example of this is “Iron Pillar of Delhi” which was built by Kumara Gupta I in the city of Delhi, India in 400s. However, unlike stainless steel, this iron monument gets its stainless feature not from chromium, but from the high levels of phosphorus it contains. Phosphorus creates a protective surface layer, consisting of iron-oxide and phosphorus on the surface of the iron monument with appropriate weather conditions of the region and provides the resistance against corrosion.The resistance of iron-chromium alloys to corrosion was first noticed in 1821 by the French metallurgist Pierre Berthier. The technology of that period was not sufficient to process chrome and iron as it is today, therefore, could not be practiced.In the 1890s, German Hans Goldschmidt invented the Alimunothermic Method that paved the way for “carbon-free” chromium production. Since then, many studies have been done on stainless steel.The beginning of modern stainless steel is considered to be 1913 by British metallurgist Harry Brearly with the discovery while he was doing research to develop rifle barrels. In later studies, the austenitic stainless steel (304), which is described as 18-8, came to the market in about 1920’s. A great success was achieved in its applications. Since the 1930s, stainless steel has been indispensable in the industry and in our daily life. Over time, researchers who continued these developments with other alloying elements developed the stainless steel grades we know today. Duplex stainless steels were developed about 70 years ago for use in the sulphite paper industry. In the first place, duplex alloys were used to deal with the corrosion problem caused by chlorite bed coolants and other sharp chemical process fluids. With the development of gas and petrol stations in the North Sea in the 1970s, a second generation duplex stainless steel was developed. Accordingly, the addition of nitrogen at a certain rate increased saturation, weldability and chlorite corrosion resistance. The most widely used mixed composition, called UNS S31803, was standardized on duplex steel UNS S32205 in 1996.During World War II, researchers discovered stainless steels are hardened by precipitation. The first example of these non-standard stainless steel was named 17-7 pH in 1948.
