The electrochemical behavior of austenitic stainless steel with different degrees of sensitization in the transpassive potential region in 1 M H₂SO₄ containing chloride

This paper investigates electrochemical methods to estimate the degree of sensitization (DOS) for as-rolled and solution-treated austenitic stainless steels (AISI 304). The change in DOS was introduced by heat treatment of various time periods at 700 °C, and the electrochemical experiments were conducted at 27 °C in 1 M H₂SO₄ + 0.2 wt.% NaCl. The results show that there is no obvious difference in the anodic polarization curves of the specimens with the differences in DOS. However, the DOS of specimens can be clearly differentiated with AC impedance response in a particular transpassive potential region. In the transpassive potential region of the anodic polarization curve, as overpotential is increased beyond the breakdown potential, three types of anodic dissolution, passive dissolution, grain-boundary attack, and pitting can be characterized. The DOS can only be correctly evaluated with AC impedance response in the middle of the transpassive potential region, where anodic dissolution is of grain-boundary attack type, from 1.05 to 1.1 V. The chromium-depleted zone was preferentially attacked and an intergranular corrosion was found for the serious sensitized specimen after AC impedance test. The healing effect was observed in as-rolled specimens as the specimens were heated more than 72 h, but the effect was not found in the solution-treated specimens sensitized for up to 480 h at 700 °C.