Metal Release Mechanisms for Passive Stainless Steel in Citric Acid at Weakly Acidic pH

Metal release investigations from stainless steel into citric acid (CA) solutions at near-neutral pH are relevant for food applications, cleaning, and passivation. This study investigated metal release from abraded stainless steel grade AISI 304 into 5 g/L CA at pH 3.1, 4.8, and 6.4 at 40°C, as compared to a control solution (10 mM KNO3). Polyacrylic acid (PAA) was used as a model solution with and without separation from the stainless steel surface by a membrane. No significant difference was found for the released amounts of Fe and Mn between CA, PAA, and KNO3 solutions at pH 3.1, suggesting other mechanisms than complexation. At pH 4.8 and 6.4, a significantly higher release was found for CA and PAA solutions compared with KNO3 solution, but not for PAA solution when PAA molecules could not reach the stainless steel surface due to membrane separation, implying a dominant complexation-induced metal release mechanism that requires adsorption and/or close vicinity of the complexing agent to the surface. Cr was enriched in the surface oxide (surface passivation) in complexing solutions and the release of Cr was most dependent on complexation by CA at pH 4.8 and 6.4.