RESULTS: impedance measurements - 2

• KSCN produces the inductive loop of the spectra of the sensitised materials in the reactivation region.This is demonstrated considering the same test in a 0.5 M sulfuric acid solution. In this conditions it is evident a remarkable increase of the impedance of the system due to the presence of the intact passive layer and the impedance modulus nearly match those of the spectra obtained in the passive region. The inductive behaviour reflects the adsorption of sulfur ions, which use their pair ion to bond the metal. In acidic media, hydrogen ions can react to produce sulfidric acid (cathodic reaction) that depolarise the anodic dissolution of iron at the grain boundary not protected by the passive layer due to the chromium depletion. The overall reaction can be written as follows:

M-Sads + 2H3O+ + Fe(gb) ? M---SH2 + H2O + Fe(gb)2+ (M=Cr/Fe/Ni)

Nyquist plot at (-0.2) V/SCE (reactivation region) in 0.5 M H2SO4 for conventional AISI 304 SS sensitised for 5hs at 650°C.

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