Influence of Oxidizing Ion Concentration on the Corrosion Resistance of Type 304L Stainless Steel in Nitric Acid Medium

The influence of the oxidizing ions Fe³⁺ and Cr⁶⁺ with two different concentrations on the corrosion resistance of Type 304L (UNS S30403) stainless steel in 3 M and 6 M nitric acid (HNO₃) medium has been determined. The corrosion resistance and passive film behavior of Type 304L stainless steel in the presence of oxidizing ions were evaluated using electrochemical potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and x-ray photoelectron spectroscopy (XPS) techniques. The corrosion resistance was observed to decrease with an increase in acid concentration and oxidizing ions. The concentrations of oxidizing ions were found to have a profound effect on the corrosion potential and passive current density when compared to that in nitric acid without oxidizing ions. XPS results revealed a significant reduction of chromium(III) oxide (Cr₂O₃) on the passive film as the concentration of oxidizing ions in nitric acid increases. An oxide layer thickness of ~13 nm enriched with iron oxide could be estimated for the passivated sample in the nitric acid medium with higher oxidizing ion concentration compared to 1 nm thick oxide film in nitric acid without oxidizing ions. Both XPS and EIS analyses confirmed the detrimental effect of oxidizing species on the passive film leading to increased corrosion with an increase in nitric acid and oxidizing ion concentration. The corrosion results could be correlated with microstructural observations.