Corrosion of stainless steels in nitric acid service: corrosion degradation issues and control measures

摘要

This paper highlights the corrosion degradation modes for stainless steels in applications that use nitric acid. Though the use of nitric acid grade stainless steels (NAG SS) has ensured freedom from intergranular corrosion (IGC) of welded components, other degradation issues have to be addressed to avoid corrosion related failures in plants that operate with nitric acid as the main process fluid. End grain corrosion is a major form of degradation that affects all grades of stainless steels including NAG stainless steels. Controlled solution annealing, laser surface remelting, weld overlay and welding of a material over the expose end faces have been shown to be highly effective in avoiding end grain corrosion. Other factors that are of concern are the presence of halide ions in nitric acid streams and the influence of cold work in fabricated components on the corrosion behaviour of stainless steels. Presence of highly oxidizing fission products in the process stream causes even the solution annealed stainless steel to corrode by intergranular corrosion. Therefore chemical plants using nitric acid as the process stream have to take these aspects in to consideration to avoid degradation and ensure long plant life. Implementation of these degradation control measures, related to fabrication and process chemistry, would ensure trouble free operation of these plants. Austenitic stainless steels, nitric acid grade; end grain corrosion; laser surface remelting; cold