Internal Oxidation Of Cr Research Articles

Effect of Hydrogen on the Internal Oxidation of a Pd–Cr Alloy in Dual-Atmosphere Conditions

The effect of hydrogen on oxygen permeability has been studied in a diluted Pd–Cr alloy in dual- and single- atmosphere conditions between 600 and 950 °C. The 0.3 mm thick Pd–1.5Cr foil was exposed in dry and humid air as well as in dual-atmosphere conditions, with one sample surface being exposed to air and one to hydrogen, as encountered in solid oxide fuel cells. At all temperatures, Cr oxidized internally forming internal oxidation zones which were measured in metallographic cross sections. Below 800 °C, an external layer of PdO formed on the surface decreasing the internal oxidation kinetics. No measurable effect of hydrogen on the internal oxidation of Cr in Pd has been detected.

Temperature-dependent selective oxidation processes for Ni-5Cr and Ni-4Al

The selective oxidation of Ni-5Cr and Ni-4 Al alloys is evaluated during high (800 °C) and low (420 °C) temperature exposures with the oxygen partial pressure moderated by a Ni/NiO powder buffer. Internal oxidation of Cr and Al is observed throughout the matrix and at grain boundaries at 800 °C accompanied by the ejection of Ni onto the surface for both. At 420 °C, matrix internal oxidation was eliminated and only Ni-4 Al exhibited intergranular (IG) oxidation. Surprisingly, a protective surface oxide rapidly formed for Ni-5Cr blocking IG oxidation. This is contradictory to results in 330–360 °C hydrogenated water environments where both alloys show IG oxidation.

Hot-dip galvanising behaviours of a CMnSiCr DP780 steel

ABSTRACTIn order to evaluate the galvanisability of a CMnSiCr DP780 steel, which is originally designed for manufacturing continuously annealed bare steel sheet, hot-dip galvanising experiments were conducted on a hot-dip process simulator. The process and galvanising behaviours were investigated. The results indicate that the alloying elements such as Si, Mn and Cr segregate and selectively oxidise at the surface during annealing. Besides external oxidation, internal oxidation of Cr and Mn is observed. Aluminothermic reducing reaction is effective to reduce the negative effect of surface oxides on the galvanisability of this steel and improve the coating quality. According to the measurement results and discussions, it is believed that this CMnSiCr DP780 steel could possibly be used to produce the corresponding hot-dip galvanised or galvannealed product when further increasing the Al content in Zn bath slightly, especially for galvannealed product.

Simultaneous oxidation and carburization of a Fe-9Cr alloy under different oxygen pressures at 800 °C

A model Fe-9Cr alloy was exposed to various H2CO2CH4 mixtures providing different oxygen pressures and carbon activities at 800°C. Three oxidation modes and three oxygen pressure ranges have been observed. At low pressures (Range I) only thin external Cr2O3 scales formed. An intermediate range (Range II) produced an internal precipitation of Cr2O3 and Cr carbide without an external scale. Higher pressures (Range III) produced external FeOx scales plus an internal oxidation of Cr. Two critical oxygen pressures for the transitions between the three oxidation modes were observed and also calculated in reasonable agreement with the experimental results.

Cyclic Oxidation Kinetics and Oxide Scale Morphologies Developed on the IN600 Superalloy

The oxidation behavior of the IN600 Ni–Cr–Fe superalloy was investigated in air at temperatures ranging from 750 to 950 °C, for up to 12 cycles. Oxidation kinetics and oxide scale morphologies were examined using weight gain measurements, SEM-EDS, and X-ray diffraction. The cyclic oxidation kinetic results suggested that the oxidation behavior of the IN600 alloy approximately followed a sub-parabolic rate and the scaling process was controlled by the formation of a chromia scale. At 850 °C, SEM-EDS observations indicated that the formed oxide scale was primarily composed of Cr2O3, and the internal oxidation of Cr and Ti occurred. At 950 °C, a fast initial stage with high weight gain was observed, followed by a steady-state stage with gradual weight gain. Additionally, a considerable change in the oxidation kinetic occurred. SEM-EDS observations and XRD results indicated that the external scale was relatively thick with a localized porous, preferential adherent, and a complex oxide scale was developed. This complex oxide scale consisted of an outermost thin layer composed of MnCr2O4–Cr2O3 mixed together with a small amount of isolated TiO2, an intermediate relatively thick layer, composed of Cr2O3, and an innermost discrete layer formed at the scale/alloy interface, which enriched by Ni/NiO mixed with Ti-, Al-, and Fe-oxides. Finally, only the Al alloying element was internally oxidized to form Al2O3 fingers, which create a discrete and narrow internal oxidation zone. Al oxide was observed as a dark area and primarily grows along the alloy grain boundaries in the vicinity of the inward chromia pegs.

Potential suitability of ferritic and austenitic steels as interconnect materials for solid oxide fuel cells operating at 600 °C

The oxidation behavior of a number of commercially available ferritic and austenitic steels was tested in air and in two simulated anode gases of a solid oxide fuel cell (SOFC) to evaluate the potential suitability as construction materials for interconnects in SOFC's operating at 600 °C. During air exposure all studied materials showed excellent oxidation resistance due to formation of a protective, double layered chromia/spinel surface scale even if the steel Cr content was as low as 17%. However, in the anode side gases the presence of water vapour (and possibly CO/CO 2) increased the tendency to form poorly protective Fe-base oxide scales, in combination with internal oxidation of Cr. The occurrence of this adverse effect could be suppressed not only by increased Cr contents of the alloy but also by a small alloy grain size either in the bulk of the material or in the specimen/component surface. The latter can be promoted by cold work e.g. introduced by specimen/component grinding. As high Cr contents may lead to undesired σ-phase formation and defined surface treatments of an interconnect will not be possible in all designs, the relatively low operating temperature of 600 °C, resulting in low Cr diffusivity in the alloy grains, may require the use of a fine grained interconnect material to obtain and sustain protective chromia base surface scale formation during long-term operation.

The Effect of Si Additions on the High-Temperature Oxidation of a Ternary Ni–10Cr–4Al Alloy in 1 ATM O2 AT 900–1000 °C

The oxidation of four Ni–10Cr–ySi–4Al alloys has been studied in 1 atm O2 at 900 and 1000 °C to examine the effects of various Si additions on the behavior of the ternary alloy Ni–10Cr–4Al, which during an initial stage formed external NiO scales associated with an internal oxidation of Cr + Al, later replaced by the growth of a chromia layer at the base of the scale plus an internal oxidation of Al. The addition of 2 at.% Si was able to prevent the oxidation of nickel already from the start of the test, but was insufficient to form external alumina scales at 1000 °C, while at 900 °C alumina formed only over a fraction of the alloy surface. At 1000 °C the addition of 4 at.% Si produced external chromia scales plus a region of internal oxidation of Al and Si, a scaling mode which formed over a fraction of the alloy surface in combination with alumina scales also by oxidation at 900 °C. Conversely, the presence of about 6 at.% Si produced external alumina scales over the whole sample surface at 900 °C, but only over about 60 % of the alloy surface at 1000 °C. The changes in the oxidation modes of the ternary Ni–10Cr–4Al alloy produced by Si additions have been interpreted by extending to these quaternary alloys the mechanism of the third-element effect based on the attainment of the critical volume fraction of internal oxides needed for the transition to the external oxidation of the most-reactive-alloy component, already proposed for ternary alloys.

Internal oxidation of Cr in Cu–Cr/Cu2O composite powder prepared by mechanical activation

The preparation of Cr2O3/Cu composite is discussed using mechanical activation to promote the internal oxidation of Cr during sintering of CuCr/Cu2O powder. Experiments were carried out to evaluate the processing parameters during the preparation. Temperature for a sufficient oxidation of Cr was in the temperature range of 1176–1325 K. The maximum oxygen partial pressure increased with increasing temperature in these experiments. It is shown that the mechanical activation can promote diffusion and reaction efficiently. 0.22 h is regarded as the minimum time for the internal oxidation in this process. Since the exothermic effect is huge during the internal oxidation of Cr, the heating rate and the reaction temperature must be chosen very carefully depending on Cr content. Spherical grains of Cr2O3 with a size of 2–5 μm in diameter were observed uniformly dispersed in Cu matrix and the distance between the particles was about 5–10 μm. The properties of the Cr2O3/Cu composite can be further improved by heat treatment. Therefore, mechanical activation and using Cr instead of Al in a Cr2O3/Cu composite is recommended to make the internal oxidation easier to control.

High-temperature oxidation of two-phase nanocrystalline Ag-Cr alloys in 1 atm O-2

Two nanocystalline two-phase Ag-Cr alloys prepared bq: mechanical alloying and containing approximately 30 and 50 wt.% Cr were oxidized in I atm OZ at 700 and 800 degreesC. Under all conditions, a continuous layer of chromia formed at the surface of the alloys, in spite of the very low solubility of Cr in Ag. A laver of AgCrO2 also formed externally to the chromia layer. In the case of the Ag-30 Cr alloy, some Ag particles were also present on the scale, directly in contact with the gas phase. Moreover, Cr particles dissolved in the subsurface region of the alloy, while internal oxidation of Cr, was absent. Ag-Cr alloys prepared by powder metallurgy with coarse grain si-es were able to form an irregular thin chromia layer only at a Cr content of 69 wt.%, while an alloy containing 35 wt.% Cr corroded much more rapidly than the nanocrystalline Ag-30Cr alloy. This difference in the scaling behavior is attributed to the large reduction in the alloy grain size, which favors the dissolution of the Cr-rich particles in a Cr-depleted silver matrix and thus provides a faster supply of chromium from the alloy to the scale.

浸炭焼入材の疲労特性に及ぼす不完全焼入層の影響とそれに対する平均応力と残留応力の寄与

Non-martensitic structure is formed in the surface zone of carburized steel as a result of the local reduction of hardenability caused by internal oxidation of Cr and Mn during the gas carburizing process. Plane bending fatigue tests were carried out to investigate the effect of non-martensitic structure on the fatigue behavior in relation with the mean stress and residual stress. The results obtained are as follows;(1) Non-martensitic structures formed during the gas-carburizing process lower the hardness and compressive residual stress at the surface of carburized steel.(2) Both the residual stress and mean stress give the same effect on the fatigue strength of carburized steel, whether non-martensitic structures exist or not in the surface zone.(3) The increase of fatigue strength resulting from the removal of non-martensitic structures by electro-polish is mainly caused by the increase of compressive residual stress at the surface.(4) The S-N curves for the specimens tested under high tensile mean stress and low compressive residual stress show one discontinuity but those for the specimens tested under low tensile mean stress and high compressive residual stress show no discontinuity.(5) A semicircular crack propagation region is observed on the fracture surface by means of scanning electron microscope.