Influence of Oxygen Concentration of 288°C Water and Alloy Composition on the Films Formed on Fe-Ni-Cr Alloys Available to Purchase

In situ identification of the films formed on alloys of Fe-13Cr-10Ni, Fe-5Cr-10Ni, and Type 304 (UNS S30400) stainless steel immersed in high-temperature (288°C), high-purity water was performed using Raman spectroscopy, surface-enhanced Raman spectroscopy, and scanning electron microscopy. The films were a function of the alloy’s chromium concentration and corrosion potential, which was controlled by the water’s dissolved oxygen concentration. Below a critical value of the corrosion potential, the surface films were composed of M₃O₄. The critical value of potential was approximately equal to the equilibrium potential of Fe₃O₄ ⇔ Fe₂O₃ (magnetite ⇔ hematite), –0.466 V. At potentials above –0.466 V, the films consisted of M₃O₄ and an outer layer of M₂O₃. The particular modification of M₂O₃ depended on the alloy’s composition and corrosion potential. For alloys containing 5% Cr and 13% Cr, the outer layer was α-M₂O₃ at potentials just above –0.466 V and a mixture of α-M₂O₃ and γ-M₂O₃ at potentials well above –0.466 V. For Type 304 stainless steel, the outer layer was γ-M₂O₃ at lower potentials and a mixture of α-M₂O₃ and γ-M₂O₃ at higher potentials.