Fluorescence Detection of Fe(II) Senses Mechanistic Differences in Pitting Corrosion between 3D-Printed and Wrought Stainless Steel 304

Additive manufacturing of stainless steel allows for automated, on-demand fabrication of complex metal materials. Additive manufacturing produces microstructures distinct from conventional wrought processing, which can alter the corrosion behavior of stainless steels. Previous studies of additively manufactured stainless steel 316 (SS316) have associated microscale molybdenum segregation at subgrain boundaries with enhanced pit growth. Stainless steel 304 (SS304) is another broadly applied stainless steel which contains no molybdenum. The corrosion properties of SS304 have had limited study in an additively manufactured form. Here, a combination of product-specific fluorescence sensing of Fe(II) with spatially, temporally, and reaction product-averaged electrochemical corrosion methods and ex situ imaging is used to reveal the mechanistic differences between the corrosion of additively manufactured and wrought SS304. The results indicate that the passive film on additively manufactured SS304 is resistant to breakdown, but pits grow quickly in an acidic, Fe(II)-rich local environment, while the passive film formed on wrought SS304 is easier to break down but repassivates quickly at local metastable pits. Overall, Fe(II)-sensitive fluorescence complements conventional electrochemical measurements by providing corrosion product-specific information that can distinguish the local corrosion chemical environment obscured by ensemble-averaged responses, providing mechanistic insight into the corrosion of additively manufactured metals.