Film formation on type-316L stainless steel as a function of potential: Probing the role of gamma-radiation

Oxide formation and conversion on Type 316L stainless steel at pH 10.6 was investigated as a function of potential under both potentiodynamic and potentiostatic conditions using a range of electrochemical and surface analysis techniques. Due to low iron oxide solubility at this pH, oxide growth on a metal surface is preferred over metal dissolution during corrosion. This enhances our ability to investigate the role that the type of oxide on stainless steel surface plays in corrosion and provides a new insight into the role of competing oxide film kinetics. Four characteristic potential regions were identified for anodic oxidation at pH 10.6. In Ox I (E < −0.5 VSCE) the anodic process is limited to the conversion of the pre-existing but defective CrIII oxide layer to a chromite-like inner layer followed by growth of a magnetite-like outer layer. In Ox II (−0.5 VSCE < E < 0.0 VSCE) additional oxidation of magnetite to γ-Fe2O3 and conversion of hydrated FeII species to γ-FeOOH can occur. In Ox III (0.0 VSCE < E < 0.3 VSCE) oxidation of the maghemite-covered Fe3O4 to γ-FeOOH leads to film fracture followed by oxidation of the exposed underlying layer. The oxide layer grows via film fracture and repassivation. In Ox IV (0.3 VSCE < E) oxidative dissolution of CrIII becomes important, resulting in a predominantly iron oxide film on the alloy. gamma-irradiation at 5.7 kGy h−1 increases the corrosion potential on stainless steel from a value that lies near the boundary between regions Ox I and Ox II to a value between regions Ox II and Ox III. Understanding the oxide formation in the four different regions can be used to predict the effect of metal exposure to ionizing radiation on its corrosion behaviour.

► Comprehensive electrochemical and surface analysis study on film formation on stainless steel.
► Study shows γ-irradiation increases the corrosion potentials on these materials.
► Anodic oxidation mechanisms for four distinct potential regions at pH 10.6 proposed
► First study to propose that competing kinetics determines the type of oxide that can be formed.