Pitting-corrosion inhibition in stainless steel by NaNO3: mechanistic insights on sulfide dissolution, depassivation, and active dissolution

Abstract The mechanism of the inhibition of pitting corrosion in Type 304 stainless steels by NaNO 3 is investigated in 0.1 M NaCl. The addition of 10 mM NaNO 3 inhibits stable pit initiation at CaS and MnS inclusions under potentiodynamic anodic polarization. However, NH 4 Cl and NaNO 2 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, NaNO 3 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 Na 2 S 2 O 3 , 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 NaNO 3 . 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 NaNO 3 is discussed.