| Electrochemical techniques have been applied to study on the pitting behavior of ion-nitrided 304L(304LSS) and 316L(316LSS) stainless steels at 25℃ in 0.5M H₂SO₄, 0.1M HCl, 0.5M H₂SO₄+ 0.1M HCl and 0.5M H₂SO₄+ 0.5M HCl solutions. Ion-nitriding was carried out at 550℃ for 10, 20, and 30hrs under pressure of 6 torr. The structure and thickness of nitrided layer and the corrosion morphology of nitrided surface were investigated using optical microscopy, SEM, TEM and EDX. Nitriding at 550℃ produced a regular surface morphology with the spherical nitrides. The ε-phase of compound layer thickened as nitriding time increased, resultting in the corrosion resistance. Electrochemical measurements showed that the corrosion potential, passive film breakdown potential and repassivation potential of ion-nitrided austenitic stainless steels decreased with increasing nitriding tune but the critical anodic current density at active region and the current density at passive region increased. The passive film stability of the 10hr nitrided 316LSS was higher than that of the nitrided 304LSS, whereas the rates of pit nucleation and growth were lower. When nitrided at 550℃ for 30hrs, larger pits were observed in the of 304LSS case rather than the 316LSS one. |
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