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]
The nitridation of chromium and Cr-Ti alloys
Reaction zones and growth kinetics were studied after exposing high-purity chromium and alloys containing 0.5, 3.0, and 5.0 wt pct Ti to 1 atm of nitrogen between 1000° and 1400°C. Outer layers of Cr2N and regions of internal nitridation, containing dispersed TiN particles, grew in a parabolic manner. An exact solution of Maak’s simplified analysis for internal oxidation provided calculations of nitrogen diffusion in the internal-nitride zone of each alloy. Extrapolation gave the relationshipD = 9.6 × 10-3 exp (-28,500/RT) cm2 sec-1 for nitrogen diffusion in high-purity chromium. Increasing titanium to 5.0 wt. pct gaveD = 2.5 × 10-3 exp (-24,000/RT) cm2 sec-1.
The Mechanical Properties and Breakdown of Surface Oxide Films at Elevated Temperatures
When metallic components operate at high temperatures in aggressive environments their useful life is governed by the presence of a protective surface film which acts as a barrier to reactants. Considerable effort has been directed toward understanding the mechanisms by which ions diffuse through these surface films, but the mechanical maintenance of the film is often a greater prerequisite for many engineering applications.