Mechanisms of Oxide Scale Formation on Ferritic Interconnect Steel in Simulated Low and High pO2 Service Environments of Solid Oxide Fuel Cells

The Laves phase strengthened ferritic steel Crofer 22 H has recently been proposed as construction material for interconnects in solid oxide fuel cells (SOFCs). In the present study, the oxidation kinetics of Crofer 22 H at 800 °C was compared with that of a Mn-free batch during exposure in air and Ar–H₂–H₂O, the latter simulating the anode gas of a SOFC. Main emphasis was put on studying the oxidation mechanisms of Crofer 22 H during the early stages of isothermal reaction as well as during long-term discontinuous exposure. Differences in oxidation mechanisms in air and simulated anode gas (SAG) were defined. Oxidation rates for Crofer 22 H in air were found to be initially slower than in SAG, but the opposite effect was observed after longer exposure times. This effect of atmosphere composition on the oxidation rate is shown to be governed by the time dependent extent to which the inner chromia scale layer is covered by an outer Cr/Mn spinel layer. In both test gases the minor addition of silicon to Crofer 22 H resulted in internal silica formation during the very early stages of oxidation; this effect vanished after longer exposure times. Incorporation of titanium in the scale differed for the two test atmospheres because the oxygen partial pressure affects titanium solubility in chromia and especially the spinel phase.     

Effect of Nb Addition on Oxidation Mechanisms of High Cr Ferritic Steel in Ar–H2–H2O

High chromium ferritic steels are being used as construction materials for interconnects in solid oxide electrolysis cells (SOEC). Addition of niobium in the range of a few tenths of a percent is suitable for increasing the high-temperature creep strength of this type of ferritic steel. In the present work, the high-temperature isothermal oxidation behavior of a niobium containing ferritic steel at 800 °C was investigated in Ar–4%H₂–4%H₂O gas simulating the service environment in an SOEC (cathode side) and compared with that of a Nb-free counterpart alloy. Gravimetric data were correlated with the results from microstructural analyses using, among others, scanning and transmission electron microscopy as well as glow discharge optical emission spectroscopy. Atom probe tomography was used for obtaining atomic-scale insight into the segregation processes in external oxides and their interfaces. The oxidation rate was substantially higher for the Nb-containing than for the Nb-free alloy. Both alloys formed double-layered oxide scales consisting of inner chromia and outer MnCr₂O₄ spinel. Additionally, a thin layer of rutile-type Nb(Ti,Cr)O₂ oxide of 200–300 nm thickness was observed at the scale–alloy interface in the Nb-containing steel. Nb addition to the alloy led to its segregation at chromia grain boundaries which affected the diffusion of Cr and other solute species such as Ti, Mn and Si.

     

Chimney stoves modestly improved Indoor Air Quality measurements compared with traditional open fire stoves: results from a small‐scale intervention study in rural Peru

Nearly half of the world's population depends on biomass fuels to meet domestic energy needs, producing high levels of pollutants responsible for substantial morbidity and mortality. We compare carbon monoxide (CO) and particulate matter (PM_2.5) exposures and kitchen concentrations in households with study‐promoted intervention (OPTIMA‐improved stoves and control stoves) in San Marcos Province, Cajamarca Region, Peru. We determined 48‐h indoor air concentration levels of CO and PM_2.5 in 93 kitchen environments and personal exposure, after OPTIMA‐improved stoves had been installed for an average of 7 months. PM_2.5 and CO measurements did not differ significantly between OPTIMA‐improved stoves and control stoves. Although not statistically significant, a post hoc stratification of OPTIMA‐improved stoves by level of performance revealed mean PM_2.5 and CO levels of fully functional OPTIMA‐improved stoves were 28% lower (n = 20, PM_2.5, 136 μg/m³ 95% CI 54–217) and 45% lower (n = 25, CO, 3.2 ppm, 95% CI 1.5–4.9) in the kitchen environment compared with the control stoves (n = 34, PM_2.5, 189 μg/m³, 95% CI 116–261; n = 44, CO, 5.8 ppm, 95% CI 3.3–8.2). Likewise, although not statistically significant, personal exposures for OPTIMA‐improved stoves were 43% and 17% lower for PM_2.5 (n = 23) and CO (n = 25), respectively. Stove maintenance and functionality level are factors worthy of consideration for future evaluations of stove interventions.     

Berechnung der Schubspannungs-Abgleitungs-Kurven aus Druckversuchen an mittelorientierten Einkristallen

Mittelorientierte Einkristalle verformen sich im Druckversuch über ihre Länge nicht homogen. Es wird ein Rechenverfahren angegeben, in dem diese Inhomogenitäten berücksichtigt werden und Schubspannungs-Abgleitungskurven für den homogen verformten Teil der Probe errechnet werden können. Die Rechnung wird an experimentellen Ergebnissen von Fe—6 Gew.% Si überprüft.     

Development of high strength ferritic steel for interconnect application in SOFCs

High-Cr ferritic model steels containing various additions of the refractory elements Nb and/or W were studied with respect to oxidation behaviour (hot) tensile properties, creep behaviour and high-temperature electrical conductivity of the surface oxide scales. Whereas W additions of around 2 wt.% had hardly any effect on the oxidation rates at 800 and 900 °C, Nb additions of 1% led to a substantially enhanced growth rate of the protective surface oxide scale. It was found that this adverse effect can be alleviated by suitable Si additions. This is related to the incorporation of Si and Nb into Laves phase precipitates which also contribute to increased creep and hot tensile strength. The dispersion of Laves phase precipitates was greatly refined by combined additions of Nb and W. The high-temperature electrical conductivity of the surface oxide scales was similar to that of the Nb/W-free alloys. Thus the combined additions of Nb, W and Si resulted in an alloy with oxidation resistance, ASR contribution and thermal expansion comparable to the commercial alloy Crofer 22 APU, but with creep strength far greater than that of Crofer 22 APU.     

Effect of Specimen Thickness on the Oxidation Rate of High Chromium Ferritic Steels: The Significance of Intrinsic Alloy Creep Strength

Previous studies revealed that initial sample thickness affects the growth rate of oxide scales formed during 800 or 900 °C air exposure. The effect is partially related to differences in depletion of minor alloying additions such as Mn, Ti, La in thick and thin specimens. However, it has previously been proposed that the specimen thickness dependence is partially governed by differences in creep strength of thick and thin substrates. To investigate this hypothesis, discontinuous air oxidation experiments were carried out with the Laves phase strengthened ferritic steel Crofer 22 H at 800 °C. Comparing the data for solution annealed and pre-aged (500 h, 900 °C) materials it could be shown that intrinsic creep strength of the alloy substantially affects oxidation rates. The observations can qualitatively be explained by assuming the relaxation of oxide growth stresses by plastic deformation of the metallic substrate to be an important parameter affecting the kinetics of oxide scale growth.     

A new austenitic alumina forming alloy: an aluminium‐coated FeNi32Cr20

The FeCrAl alloys owe their low oxidation rate to the formation of a slow growing α‐aluminium oxide scale. Therefore they are used, for example, as a substrate material in metal‐supported automotive catalytic converters. Increasing exhaust gas temperatures mean that, in addition to the oxidation properties, high temperature mechanical properties should also be improved. Compared to the ferritic FeCrAl alloys, austenitic alloys possess the required high mechanical strength at higher temperatures. However for most commercially available materials the oxidation resistance is not sufficient due to a low aluminium content. High aluminium contents are avoided in austenitic alloys, since they cause severe workability problems, even at aluminium contents, which are below the necessary amount to get a pure alumina scale. The newly developed material Nicrofer 3220 PAl (coated FeNiCrAl) consists of an austenitic FeNi32Cr20 alloy coated with aluminium on both sides. It combines the outstanding oxidation resistance of an alumina forming FeCrAl alloy with the advantage of the high temperature strength of an austenitic alloy. Additionally the oxidation is even lower than the oxidation of the commercial grade Aluchrom YHf (FeCr20Al6)—conventional homogenous FeCrAl. Aluminium coated FeNiCrAl can easily be formed into its final shape. Prior to service, an in situ heat treatment is recommended in order to optimize the properties.     

High temperature wear testing of a Ni‐based superalloy pin on a cast iron disc

A pin‐on‐disc wear test rig is described, which allows to extract reproducible mechanical and microstructural wear data from very small sample volumes at temperatures up to 900°C. The friction and wear behavior of Alloy 80A against a cast iron is evaluated at temperatures from ambient to 800°C. The wear rate of Alloy 80A decreased with increasing temperature. This was attributed to the development of protective tribolayers, which prevented a direct contact between the two sliding partners. Energy Dispersive X‐Ray (EDX) mapping of surface wear products and Transmission Electron Microscopy (TEM) results for the evolution of subsurface microstructures are exemplarily presented for wear experiments performed for 10 min at 300°C (frequency: 20 Hz, load: 20 N, stroke: 1 mm). EDX investigations provide a good insight into material transfers and oxide layer generations during sliding wear. TEM‐micrographs revealed cell structure formation and very small nanograins directly beneath the surface.