Local detection of dissolved sulfur species from inclusions in stainless steel using Ag microelectrode

The voltammetric response of silver was used to detect hydrosulfides produced from MnS inclusions in stainless steel (SS) 304 at different dissolution paths. Cyclic voltammetry was performed in aqueous solutions containing various sulfur species, such as sulfate, sulfite, thiosulfate, and hydrosulfide. Ag showed redox peaks only in hydrosulfide. An insulated Ag wire with an exposed end was inserted into a microelectrochemical cell that characterized a single MnS inclusion (20 ∼ 30 μm diam) on SS 304 dissolved over a range of potentials, at rest potential in acidic and neutral solutions, +0.2 and +0.4 V_SCE in neutral solution. In an acidic 0.1 M Na₂SO₄ (pH 2) at the rest potential, the voltammetric response by Ag indicated the presence of hydrosulfides. In a neutral 0.1 M Na₂SO₄, at applied potentials of +0.2 and +0.4 V_SCE, no voltammetric response by Ag was obtained. The measurements support a mechanism of MnS inclusion dissolution via two reaction paths. At low potentials in acidic conditions, the inclusions were found to dissolve chemically to form HS^-, while at higher potentials and neutral pH, the inclusions were found to dissolve electrochemically.