Silver(II)-Mediated Surface Degradation and Passivation of Stainless Steel 316, Graphite, and Titanium in Concentrated Nitric Acid

Abstract Electrochemically generated Ag(II) in concentrated nitric acid is a powerful oxidizing medium for mediated electrochemical oxidation, but its practical use requires construction materials that can tolerate highly aggressive redox conditions without excessive degradation or contamination of the process stream. Here, the material-specific responses of stainless steel 316, graphite, and titanium were evaluated in Ag(II)/8 M HNO3 electrolytes generated from 0.01 to 0.5 M AgNO3 using an H-cell configuration. Material performance was assessed by mass change measurements, surface profilometry, scanning electron microscopy, Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. SS316 showed concentration-dependent degradation. Graphite exhibited a competing response, with mass gain associated with surface oxidation and Ag incorporation, followed by cracking, crumbling, and particle release under the most aggressive exposure condition. In contrast, titanium showed minimal mass loss and little morphological change, indicating that its TiO2 passive layer remained stable in Ag(II)/HNO3. These results show that material compatibility in Ag(II)-mediated electrochemical oxidation is governed by material-specific surface chemistry and identify titanium as the most robust candidate among the tested materials, while SS316 and graphite require more restricted use depending on mediator concentration and component function.