Impact of crystallographic structures on the corrosion resistance of AISI 304L processed by machining and cryogenic deep rolling

Grade 304L is considered one of the most widely used stainless steels in chemical and marine applications due to its good corrosion properties. Nevertheless, this type is sensitive to localized corrosion in aggressive environments containing chloride anions. Its structure can be fully austenitic, but it can also exhibit the presence of other phases within the austenite matrix. This can be related to surface modifications occurring during manufacturing processes which affect the corrosion behaviour of the material. As a matter of fact, the effects of the existing crystallographic phases the AISI 304L as-received, machined and cryogenically deep rolled samples on the corrosion behaviour synthetic seawater were investigated. Mechanisms and kinetics of phase transformations resulting from machining and cryogenic deep rolling were discussed. It was found that the as-received material had a combination of phases: austenite, 13 Vol.% of α-ferrite and 9 Vol.% of α′-martensite. After machining, the AISI 304L microstructure turned completely to austenitic. In this condition, an improved corrosion behaviour was observed where the corrosion rate decreased by 14% regarding the as-received state (3.20 µm/year). However, it was noticed that cryogenic deep rolling damaged the corrosion behaviour of the processed samples regarding the as-received and machined states. In fact, its application repetitively at deep depth of penetration led to high deformation rates resulting in a high amount of strain-induced martensite (62%), which generated a galvanic effect between austenitic and martensitic phases.