Macro and Micro Scale Corrosion of 316L Stainless Steel: A Case Study of a Hole Plate

A study of the macro and micro scale corrosion in a stainless steel hole plate from a chemical reactor is presented in this paper. The plate, which served as a support for 7 tons of copper scrap for producing copper sulphate, catastrophically collapsed after 15 months of operation. The study included an extensive analysis of the hole plate material as well as of the corroded areas. The investigation involved the following experimental procedures and testing techniques: visual inspection, optical microscopy, scanning electron microscopy, chemical analysis and hardness testing. The plate was fabricated from an AISI 316L austenitic stainless steel. At the macro scale level, the plate showed extensive corrosion damage at the hole walls, with the origin located in pits at half the thickness of the plate. Manufacturing operations led to the formation of nucleation sites for corrosion pits in these areas. At the micro level, the plate microstructure contained a relatively large amount of delta ferrite, which in the presence of the corrosive media facilitated the formation of micro galvanic pairs with the austenite phase. Corrosion occurred at the ferrite/austenite interface, and was enhanced by the high chloride content in the reactor process waters. The massive corrosion damage throughout the plate compromised its mechanical strength and caused its catastrophic collapse.