The origins of the second anodic current peak in polarization curves of AISI 430 (UNS S43000) stainless steel in deaerated 0.1 M sulfuric acid (H2SO4) have been investigated by potentiodynamic polarization and atomic emission spectroelectrochemistry (AESEC). The elemental dissolution rates of Fe, Cr, Ni, Cu, and Mn were measured in real time during linear potential sweep voltammetry, revealing the formation and dissolution of a copper-rich corrosion product layer. The deposition and subsequent dissolution of this copper layer is proposed to be the main cause of the second anodic current peak. The negative current loop observed after the first anodic peak is attributed to the cathodic reduction of H+ ion on the copper-enriched layer. Other factors such as the oxidation of the adsorbed H atoms on the copper-enriched layer and the presence of chromium-impoverished grain boundaries are shown to have a negligible effect on the second anodic current peak.

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International audienceThe depassivation of UNS S32304, S32202, and S32101 lean duplex stainless steels (DSS) in 26 wt% sodium chloride (NaCl) was investigated by potentiodynamic polarization to understand the selective dissolution processes in localized corrosion phenomena. Results showed that the depassivation of the three grades proceeds in two stages. First, the austenite is the only phase depassivated within the range of no more than 0.3 pH units, providing a single corrosion peak on the polarization curves of the DSS. Subsequently, the ferrite is also depassivated and provides a second corrosion peak at a more cathodic potential. Results are discussed in terms of the partitioning of alloying elements

The effect of grain size on the anodic dissolution of lean duplex UNS S32202 dual-phase austenitic-ferritic stainless steel was evaluated. Grain coarsening was achieved by heat treatment, and grain size and grain boundary densities determined by automatic image analysis after etching. Potentiodynamic electrochemical testing in acidic chloride medium allowed isolating the anodic dissolution behavior of the crystallographic phases of the material. A relationship between grain boundary density (for grain sizes in the micrometer range) and dissolution rate has been found, showing that reducing grain size enhances active corrosion rates in environments that promote active behavior. This leads to new possibilities of industrial adjustment of the corrosion behavior of duplex stainless steels via grain size control.

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