<p>The mechanism of the inhibition of pitting corrosion in Type 304 stainless steels by NaNO₃ is investigated in 0.1 M NaCl. The addition of 10 mM NaNO₃ inhibits stable pit initiation at CaS and MnS inclusions under potentiodynamic anodic polarization. However, NH₄Cl and NaNO₂ do not improve the pitting corrosion resistance. In the initial steps of pitting corrosion at MnS in 0.1 M NaCl, no effect is observed on the MnS dissolution and trenching at the MnS/steel boundary; however, NaNO₃ is found to inhibit the active dissolution of the steel matrix in 0.5 M HCl, which is intended to simulate the interior of pits. This inhibitory effect on active dissolution is also observed in 0.5 M HCl–10 µM Na₂S₂O₃, where elemental sulfur, a dissolution product of MnS, is generated. In 0.5 M HCl, two active dissolution peaks appear. The current density at the first peak at a lower potential is found to decrease with the addition of NaNO₃. This behavior is compared with active dissolution in solutions in which the pH and the Cl⁻ concentration were systematically varied, and the mechanism of pitting corrosion inhibition by NaNO₃ is discussed.</p>