Oxidation or Nitridation Behavior of Pure Chromium and Chromium Alloys Containing 10 mass%Ni or Fe in Atmospheric Heating
The oxidation or nitridation behavior of pure chromium and chromium alloys containing 10 mass%Ni or 10 mass%Fe in atmospheric heating was investigated in the temperature range of 1073 to 1373 K. The formation behavior of the outer oxide phase of Cr2O3 and its growth kinetics in both chromium alloys was very similar to that of pure chromium. The nitride layer with the Cr2N phase was formed beneath the oxide phase at temperatures above 1273 K in pure chromium. Marked internal nitridation with the formation of the Cr2N phase was observed in the Cr-10 mass%Fe alloy, while the formation of Cr2N was negligible in the Cr-10 mass%Ni alloy. Neither Ni nor Fe were detected inside the Cr2O3 phase, and was enriched in the subsurface layer between the oxide layer and the base alloy in a respective alloy. The Ni-enriched layer in Cr-10 mass%Ni alloy had a relatively narrow width, but a very high concentration of Ni compared with the Fe-enriched layer formed in the Cr-10 mass%Fe alloy. [doi:10.2320/matertrans.47.2540]
Application of sol–gel technique to synthesis of copper–cobalt spinel on the ferritic stainless steel used for solid oxide fuel cell interconnects
Abstract The conductive CuCo 2 O 4 spinel coating is applied on the surface of the AISI 430 ferritic stainless steel by the dip-coating sol–gel process and it is evaluated in terms of the microstructure, oxidation resistance and electrical conductivity. The results show that the CuCO 2 O 4 coating forms a double-layer scale consisting of a Cr, Fe-rich subscale and Cu–Co spinel top layer after 500 h in air at 800 °C. This scale is protective, acts as an effective barrier against Cr migration into the outer oxide layer and alleviates the cathode Cr-poisoning. The oxidation resistance is significantly enhanced by the protective coating with a parabolic rate constant of 5.8 × 10 −13  gr 2  cm −4  s −1 , meanwhile the electrical conductivity is considerably improved due to inhibited growth of resistive Cr 2 O 3 oxide scale. The area specific resistance at temperatures between 550 and 800 °C is in the range of 11.5 and 22.2 mΩ cm 2 .
Formation and stability of a nitride with the structure of beta manganese in Ni-Cr-N ternary system
A nitride with the metal-atom arrangement of β manganese, which is designated as π phase, was confirmed to exist as an equilibrium phase in the ternary Cr-Ni-N system. A Cr-40 mass pct Ni binary alloy was nitrided under a pure nitrogen atmosphere at 1523 K to prepare a Cr-40Ni-5N alloy consisting of a nickel-rich face-centered cubic (fcc) γ phase and a dichromium nitride, Cr2N, designated as ε phase. Upon aging the alloy at 1273 K, the π phase was formed through a peritectoid reaction between the γ and ε phases. A volume fraction of the π phase reached 95 pct after 3.6 × 105 s, and no more change of the volume fraction was observed, even after 3.6 × 107 s aging. The chemical composition of the π phase was determined to be Cr13Ni7N4, whose lattice parameter wasa = 0.6323 nm. The π phase became unstable above 1473 K, which explains a previous unsuccessful attempt to produce the Cr-Ni-N ternary π phase by the replacement of molybdenum in Mo12Ni8N4 by chromium at 1473 K.