Oxidation Behavior Of Cr Research Articles

Oxidation behavior of Cr(III) during thermal treatment of chromium hydroxide in the presence of alkali and alkaline earth metal chlorides.

The oxidation behavior of Cr(III) during the thermal treatment of chromium hydroxide in the presence of alkali and alkaline earth metal chlorides (NaCl, KCl, MgCl2, and CaCl2) was investigated. The amounts of Cr(III) oxidized at various temperatures and heating times were determined, and the Cr-containing species in the residues were characterized. During the transformation of chromium hydroxide to Cr2O3 at 300 °C approximately 5% of the Cr(III) was oxidized to form intermediate compounds containing Cr(VI) (i.e., CrO3), but these intermediates were reduced to Cr2O3 when the temperature was above 400 °C. Alkali and alkaline earth metals significantly promoted the oxidation of Cr(III) during the thermal drying process. Two pathways were involved in the influences the alkali and alkaline earth metals had on the formation of Cr(VI). In pathway I, the alkali and alkaline earth metals were found to act as electron transfer agents and to interfere with the dehydration process, causing more intermediate Cr(VI)-containing compounds (which were identified as being CrO3 and Cr5O12) to be formed. The reduction of intermediate compounds to Cr2O3 was also found to be hindered in pathway I. In pathway II, the alkali and alkaline earth metals were found to contribute to the oxidation of Cr(III) to form chromates. The results showed that the presence of alkali and alkaline earth metals significantly increases the degree to which Cr(III) is oxidized during the thermal drying of chromium-containing sludge.

Effect of Al on High-Temperature Oxidation of Cr–W Alloys

The effect of Al on the high temperature oxidation behavior of Cr–10 wt.%W alloy was investigated using a cyclical oxidation test at 1,000 °C in dry air. First, Al was added into the Cr–W alloy as an alloying element up to 8 weight percent. Although alloying with Al reduced the spalling, it did not eliminate it. Secondly, Al was applied to the surface using an aluminizing process. Forming an Al–Cr layer on the Cr–W alloy reduced oxidation rate significantly and eliminated spalling completely.

Oxidation Behavior of Cr2N, CrNbN, and CrTaN Phase Mixtures Formed on Nitrided Cr and Laves-Reinforced Cr Alloys

A series of single-phase Cr(X), single phase Cr 2X Laves phase, and two-phase Cr(X) + Cr2 X alloys (X = Nb or Ta) were thermally nitrided for 24 hr at 1100°C in N2--4H2 and then oxidized for 2 hr at 1100°C in air. The Cr(X) phase nitrided to form Cr2N while the Cr2X phases nitrided to form a complex local mixture of Cr2N/Cr and CrNbN/CrTaN, Cr3Nb3N/Cr3Ta3N phases depending on the depth in the nitrided zone. The Ta only slightly increased the isothermal oxidation rate of nitrided Cr(Ta) and Cr2Ta-reinforced Cr alloys, compared with nitrided, unalloyed Cr. Further, the nitrided two-phase alloys Cr--9.5Ta and Cr--20Ta exhibited improved Cr2O3 scale adherence relative to nitrided unalloyed Cr and Cr--1Ta. In contrast, Nb was detrimental to the oxidation resistance of the nitrided Cr(Nb) and Cr2Nb-reinforced Cr alloys, resulting in the formation of nonprotective Cr--Nb oxides rather than continuous Cr2O3. A phenomenological explanation for these effects based on phase chemistry and microstructural distribution is presented. Implications of these results for understanding the oxidation behavior of developmental high-temperature, Laves-strengthened Cr alloys, as well as possible applications as oxidation and wear-resistant coatings are discussed.

The Effect of Oxygen Flow Rate in HVOF Spraying on Oxidation Behavior of Cr<SUB><B>3</B></SUB>C<SUB><B>2</B></SUB> (7 mass%NiCr) Coatings

This study was performed to investigate the influence of the oxygen flow rate in HVOF spraying on the oxidation behavior of Cr 3 C 2 (7 mass%NiCr) coatings. The oxide was formed on the coatings during HVOF spraying regardless of the oxygen flow rate. The oxide observed at the coatings was Cr 2 O 3 , but chromium oxides of other forms were not observed. Cr 3 C 2 (7 mass%NiCr) coatings were stable in a temperature range up to 1073 K for 50 h. However, the oxidation behavior of the coatings at 1273 K showed great difference from that at temperature up to 1073 K and the oxygen flow rate had a significant influence on the oxidation behavior of the coatings. The surface of the coatings sprayed with 229 and 243 I/min-oxygen flow rate became porous by the aggressive evaporation of CO, CO 2 gases and the oxide clusters were partially formed after isothermal oxidation at 1273 K for 50h. The oxide clusters were complex oxides consisted of Ni and Cr, whereas the porous surface was composed of only Cr 2 O 3 . TG analysis revealed that the mass loss by intensive evaporation of gaseous oxide, in the case of the coatings sprayed with 229 I/min-oxygen flow rate, began after the oxidation time of approximately 42 h at 1273 K. The apparent activation energy (Q) for oxidation varied from 89.04 to 99.96 kJ mol -1 with increasing oxygen flow rate.

The influence of grain-boundary segregation of Y in Cr2O3 on the oxidation of Cr metal

The oxidation behavior at 900°C of pure Cr and Cr implanted with 2×1016 Y ions/cm2 was studied. The kinetics of oxidation were measured thermogravimetrically and manometrically. The mechanisms of oxide growth were studied using18O-tracer oxidation experiments, and the composition and microstructure of the oxide scales were characterized by TEM and STEM. Segregation of Y cations at Cr2O3 grain boundaries was found to be the critical factor governing changes in the oxidation behavior of Cr upon the addition of Y. In the absence of Y, pure Cr oxidized by the outward diffusion of cations via grain boundaries in the Cr2O3 scale. When Y was present at high concentration in the scale, as when Cr implanted with 2×1010 Y ions/cm2 was oxidized, anion diffusion predominated. It is concluded that strain-induced segregation of Y at grain boundaries in the oxide reduced the cation flux along the grain boundaries. The rate of oxidation was reduced because the grain-boundary diffusivity of cations became lower than the grain-boundary diffusivity of the anions, which then controlled the rate of oxidation. Changes in the relative rates of Cr3+ and O2− transport, as well as a solute-drag effect exerted by Y on the oxide grain boundaries, resulted in changes in the microstructure of the oxide.