A Study of Various Critical Pitting Potentials for Type 316 Stainless Steel in Sulfuric Acid Containing Chloride Ions

During the determination of various critical pitting potentials for AISI Type 316 stainless steel in 1.0N H₂SO₄ containing 0.5N Cl⁻ ions by potentiostatic method, it was observed that before a pit becomes self propagating, unstable pits are nucleated. The critical pitting potentials for nucleation and instantaneous pitting for unstable as well as stable pits are redefined. From the plots of potential versus rate of pit initiation and those of potential versus incubation time, linear relationships have been obtained. The intercepts of these lines as calculated by least square method represent accurate values of critical pitting potentials. The pit nucleation potential obtained by potentiodynamic or semipotentiostatic experiment relates to the nucleation of only unstable pitting. The unified mechanism of autocatalytic pitting corrosion seems to satisfactorily explain the stable and unstable pitting processes. When all the conditions necessary for the autocatalytic pitting corrosion are fulfilled, then stable pits occur, while unstable pitting is observed when any of the conditions for autocatalytic pitting are not met.