Abstract
The surface morphology and chemistry of oxide scales formed on select chromia-forming and alumina-forming ferritic steels have been studied after exposure to a dual atmosphere of hydrogen and air. Localized Fe-rich oxide nodules with surface whiskers/platelets form at the onset of corrosion. The initiation and growth of localized nodules and breakdown of passivation are attributed to the presence of hydrogen, inclusion of iron oxide in the passivating scale, and subsequent growth of iron-rich oxide due to the establishment of redox (H2-H2O) atmosphere and modification of oxide defect chemistry.
Highlights
Ferritic steels are cost effective, commercially available in high volume, and offer corrosion resistance and matched coefficients of thermal expansion with the adjacent ceramic cell components.[1,4,6,7]. These steels are often alloyed with a significant amount of chromium, which promotes the development of a chromia (Cr2O3) scale which passivate against corrosion, acting as a dense diffusion barrier to prevent further oxidation.[6,8]
The two chromiaforming steels (AISI 444 and ZMG232G10®) develop a uniform, thin chromium-based oxide scale. This was determined from the diffraction pattern for AISI 444 exposed to dry air, which indicates one low-intensity peak corresponding to Cr2O3 (Fig. 4)
Exposure temperature.—In this study, it is shown that ZMG232G10® undergoes higher surface area coverage of iron oxide nodules/scale at 500 °C compared to 600 °C, while at 700 °C the steel grows a protective mixed Cr2O3/MnCr2O4 oxide scale
Summary
The difference in steel oxidation behavior is assigned to the presence of a hydrogen-containing gas that can diffuse into the metal, due to hydrogen solubility and the establishment of a hydrogen concentration gradient through the metal.[6,20] Hydrogen has been shown to affect bulk properties of metals and alloys by influencing pore formation,[21] causing embrittlement,[22] and changing mechanical behavior.[23] Since dual atmosphere corrosion occurs in the presence of hydrogen, it is postulated that hydrogen affects properties of oxide scales on metals This concept has been discussed in SOFC and electrochemistry literature with an emphasis on surface oxide chemistry and morphology.[13,15,20,24,25,26,27,28,29] literature remains largely unpopulated on experimental evidence as to the exact role of hydrogen on influencing rapid iron oxidation and nodular growth. This effective positive defect would increase the metal vacancy concentration, which could enable more pathways for outward metal ion diffusion.[20,29,30]
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