Electrochemical studies in HNO3–N2O4 mixtures: corrosion of stainless steel in HNO3-N2O4 mixtures and the effect of inhibitors

Abstract

Electrochemical studies demonstrate that a rhodium wire can be used as a reference electrode in HNO₃–N₂O₄ mixtures. The electrode reaction NO₂⁺+ e^–⇌½N₂O₄ occurs on the rhodium surface. Platinum can also be used as a reference electrode, although its performance is slightly inferior to rhodium. Electrochemical studies using this reference system indicate that stainless steels corrode in HNO₃–N₂O₄ mixtures by a mechanism involving transpassive breakdown. This breakdown is prevented by cathodic polarisation or by addition of fluoride or phosphorus pentafluoride. The results are consistent with fluoride functioning as an anodic film-forming inhibitor. Sulphuric acid also functions as an inhibitor but its performance is less satisfactory than that of fluoride.