Transpassive elemental dissolution of a silicon-rich austenitic stainless steel in acidic media investigated through atomic emission spectroelectrochemistry
Résumé
Because of their good resistance to corrosion in acidic media, austenitic stainless steels are largely used for equipment of the spent nuclear fuel reprocessing plants. Still, they might be exposed to an intergranular type of corrosion. The intergranular corrosion appears in the transpassive domain of the steel, for example when the chemical environment becomes sufficiently oxidizing. However, when few wt.% of silicon are added to a 18Cr-10Ni stainless steel, the alloy loses its sensitivity to intergranular attack. This work compares the transpassive corrosion behavior of two similar stainless steel that are differentiated by the addition of silicon in their composition. Using the in-situ atomic emission spectroelectrochemistry (AESEC), chronoamperometries were performed in the transpassive domain of each steel. During the potentiostatic transpassive dissolution of each sample, an inductive coupled plasma atomic emission spectrometer connected to a three electrode flow cell enabled to quantify each element's dissolution rate. Then, scanning electron micrographs were taken to evaluate the surface's morphology after the experiment. As expected, the steel without silicon displayed both a selective dissolution behavior according to AESEC measurements and a locally corroded surface, very sensitive to the flow rate, according to SEM micrographs. On the other hand, the silicon-rich stainless steel displayed a non-selective dissolution behavior and a homogeneous surface morphology. Transmission electron microscopy was then used in addition to this work, to understand better the dissolution mechanisms in which the silicon is involved from its metallurgical characteristics in the bulk and in the passive layer. Comparing the total current measured and the elemental dissolution rates, the valence of each element dissolving was determined. In this work, in-situ and ex-situ techniques lead to the suggestion of a transpassive dissolution mechanism for each stainless steel as a function of silicon enrichment.