Potential for Application of Special Stainless Steels for Chemical Process Equipments

The most predominant material of construction (MOC) for process equipments in chemical process industries (CPI) is plain carbon steel only. This is because of its lowest cost, easy availability, adequate thermal and mechanical properties, ease of fabrication etc. The only limitation is its low corrosion resistance. When corrosion resistance becomes the deciding factor, for overall resistance to general uniform corrosion, the common austenitic stainless steel Type 304(L) SS, 18/8(L) SS, is used. This has limitation with respect to uniform corrosion in reducing acids and to pitting corrosion, crevice corrosion and stress corrosion cracking in chloride containing media. In such situations, Type 316(L) SS, 18/8/2.5Mo/(L), shows somewhat better performance. These are called Standard Austenitic Stainless Steels. Even 316(L) SS shows unsatisfactory corrosion behavior in strong reducing acids, in concentrated chloride solutions at elevated temperatures and in fluids flowing with high velocities. In such cases, if the process conditions cannot be made milder with respect to corrosion phenomena, then there is a strong potential for upgrading the MOCs to Special Stainless Steels. These special stainless steels are either high nickel high molybdenum (~25 % Ni and ~6 % Mo) Super Austenitic Stainless Steels or low nickel (4 to 7 %) Duplex Stainless Steels. With respect to cost, the former are highly expensive than 316(L) SS because of widely varying amounts of nickel and of the presence of high molybdenum and the latter are marginally comparable with 316L SS. It is a common experience in industry that the usage of such special stainless steels is felt necessary only after facing severe corrosion problems in equipments made of standard stainless steels. The purpose of this paper is to present a few case studies of severe corrosion problems analyzed by the author resulting in recommendation of special stainless steel as the corrosion preventive step. The following case studies would be presented. Fertilizer Industry, Ammonia plant, Synthesis Gas Coolers Petrochemical Plant, Proprietary Amine manufacturing Reactor in the presence of Orthophosphoric Acid Petrochemical Plant, Proprietary Amine manufacturing process, Ammonia recovery column. Petrochemical Plant, Reboiler Tubes processing concentrated acetic acid.