A Nanoscale Study of Thermally Grown Chromia on High-Cr Ferritic Steels and Associated Oxidation Mechanisms

Abstract

Fe-22Cr-0.5Mn based ferritic steels are used as interconnect materials for solid oxide fuel/electrolysis cells. Four steel samples, including the commercial steel Crofer 22 H, were oxidized at 800 °C in a model Ar-4%H2−4%H2O atmosphere simulating the fuel side of the cells and investigated by atom probe tomography (APT) in conjunction with electron microscopy and thermogravimetry. All steels form an oxide scale mainly consisting of MnCr2O4 spinel on top of Cr2O3. APT revealed segregation of minor alloying constituents (Nb and Ti) to chromia grain boundaries and highlighted their effect on mass transport through the chromia scale. Relationships between segregation activity of individual elements (in terms of Gibbsian interfacial excess), oxide scale microstructure and alloy oxidation rate have been established based on the APT results. Comparison of segregation activities revealed that vacancies formation due to Wagner-Hauffe doping with aliovalent Ti and Nb impurities cannot be solely responsible for faster oxidation, assuming alteration of the grain boundary structure and associated changes of their mass transport properties. Controlled Si addition to the alloy (about 0.4 at%) suppresses the detrimental effect of Nb on the oxidation resistance but results in formation of a thin, although still discontinuous, SiO2 layer at the metal-oxide interface.

Grain-boundary segregation of magnesium in doped cuprous oxide and impact on electrical transport properties

“…2 b slice 3, suggesting an Mg segregation into grain boundaries. Atomic density variations in the 3D volume reconstruction (or on the detector) can be ascribed to ion trajectory aberrations (deflection of the ions) and associated local magnification effects 20 , 25 , 30 – 32 . These typically originate from local disturbances in the electric field distribution, which often result from dissimilarities in the local curvature of the APT tip developed around microstructural features such as grain boundaries, interfaces, precipitates etc.…”

High Temperature Oxidation Behavior of Conventional and Nb-Modified MAR-M246 Ni-Based Superalloy

“…Nb also was shown recently to increase the chromia scale growth on ferritic steels, although the mechanism of the Nb-effect seems to be different to that of Ti involving structural changes at the chromia grain boundaries. [32] Increased chromia scale growth by Ti and Nb suppresses alumina scale formation and promotes internal oxidation and nitridation as was shown by Jalowicka et al [31] and Wollgarten et al [33] Nitridation beneath fast-growing chromia scales is a wellknown effect related to nitrogen penetration via molecular transport via scale defects such as pores and microcracks. [34] Thick, fast growing oxide scales have typically a greater tendency to spallation during cyclic oxidation compared to thin slow-growing oxidation products.…”

A Tracer Study on sCO2 Corrosion with Multiple Oxygen-Bearing Impurities

“…The Ti occupation results in a positive effective charge (Ti 4+ replacing Cr 3+ ), which needs to be compensated either by the formation of chromium vacancies or electrons [62,63]. In a previous study, where the oxidation mechanism of Ti-containing FeCr alloys was studied, Ti was found within the chromia scale at the grain boundaries with high enough concentrations to generate bulk vacancies via the Wagner-Hauffe doping [64,65]. The formation of cation vacancies contributes to the ionic mass transport, which results in accelerated outward cation diffusion along chromia grain boundaries and eventually, in increased oxidation rate as can be seen in Fig.…”