Localized Corrosion of CUSTOM 450 and AM 350 Stainless Steels in H₂SO₄ and NaCl Solutions

Precipitation-hardenable stainless steels (PHSSs) are widely used in various applications in the aerospace industry. PHSSs are used in various parts that need to combine mechanical properties with corrosion resistance when aircrafts are in corrosive environments. This study aimed to analyze the corrosion kinetics of CUSTOM 450 and AM 350 stainless steels that were passivated in acid baths for a period of 120 min at temperatures of 25 and 50 °C and then immersed in solutions containing 1 wt. % sulfuric acid (H₂SO₄) and 5 wt. % sodium chloride (NaCl). The electrochemical technique used was cyclic potentiodynamic polarization (CPP) based on ASTM G61-86. Microstructural characterization was performed via optical microscopy (OM) and scanning electron microscopy (SEM). The results revealed that the potentiodynamic polarization curves have two distinct corrosion mechanisms. The immersion of PHSSs in the H₂SO₄ solution produces an activation-passivation reaction. The NaCl solution has pseudopassivation (unstable passivation film). The values of the icorr current densities in the solutions of sodium chloride and sulfuric acids are between 10^-3 and 10^-5 mA/cm²; the stainless steels are susceptible to localized pitting corrosion in both test solutions, with positive hysteresis in the CPP.