Nickel oxide diffusion coefficient

The tlrermodynamic activity of nickel in the nickel oxide layer varies from unity in contact with tire metal phase, to 10 in contact with the gaseous atmosphere at 950 K. The sulphur partial pressure as S2(g) is of the order of 10 ° in the gas phase, and about 10 in nickel sulphide in contact with nickel. It therefore appears that the process involves tire uphill pumping of sulphur across this potential gradient. This cannot occur by the counter-migration of oxygen and sulphur since the mobile species in tire oxide is the nickel ion, and the diffusion coefficient aird solubility of sulphur in the oxide are both vety low. 

The control of reaction rates by a bulk difiusion process is not usually demonstrable by microscopic observations, but support may be obtained from measurements of diffusion coefficients of appropriate species within the structure concerned. This approach has been invaluable in formulating the mechanisms of oxidation of metals, where rates of reaction have been correlated with rates of transportation of ions across barrier layers of product. Sometimes the paths by which such movements occur correspond to regions of high difi isivity, involving imperfect zones within the barrier layer, compared with normal rates of intracrystalline diffusion across more perfect regions of material [63]. Difiusion measurements have been made for ions in nickel sulfide and it was concluded that the decomposition of NiS is diffiision controlled [50]. 

Ramachandran and Smith obtained satisfactory agreement with experimental results on the reduction of nickel oxide with carbon monoxide (pore opening case) by considering the product layer diffusion coefficient as an adjustable parameter. Similarly, the model predicted pore closure and reaction die-off for the reaction of calcium oxide with sulfur dioxide, where the molar volume of calcium sulfate product is about three times that of the calcium oxide reactant. 

The diffusion coefficient of aluminium in a ferritic iron-aluminium matrix is some orders of magnitude higher than in austenitic Ni3Al. Because of the low diffusion coefficient, diffusion in nickel aluminides is slow and aluminium depletion beneath the alumina scale and formation of nonprotective nickel oxides has been observed in Ni3Al. [4]. 

A more complete test of the Wagner mechanism and treatment has been carried out by Fueki and Wagner and by Mrowec and Przybylski who derived values for the diffusion coefficient of nickel in NiO and cobalt in CoO from measurements of the parabolic oxidation rate constant. Since the parabolic rate constant can be expressed in the form of Equation (3.48),... 

Although carburization can enhance the performance of certain components, in cases such as reformer tubes in the treatment of oil and hydrocarbons, carburization of stainless steels is deleterious and life limiting. In this case internal carbides form with kinetics analogous to those for internal oxidation." Consequently, alloys that are resistant to carburization are developed, primarily by alloying with nickel to reduce the diffusion coefficient of carbon, and with silicon and aluminium, which are thought to impart some protection by the formation of impervious silica and alumina surface hlms in the low-oxygen-potential atmospheres. ... 

Small Cr contents increase the rate of reaction, but at 20% Cr, the reaction rate starts to decrease and exhibits a minimum value at 25%-30% Cr (Figure 20.62). The minimum value depends on the pressure. More chromium is needed to stabilize a protective film since the diffusion coefficient of chromium in cobalt is lower than for chromium in nickel. However, since the adhesion strength of the film on Co-Cr alloys is poorer than on Ni-Cr alloys despite the identical oxidation rate of the Cr-containing Co alloys with Cr203 protective film, the practical oxidation resistance is lower. Other alloying elements, as Figure 20.62 shows, have little influence on scale resistance. 

Lander JJ, Kem HE, Beach AL. Solubility and diffusion coefficient of carbon in nickel reactionrates of nickel-carbon alloys with barium oxide. Appl Phys Lett 1952 23 1305-9. 

The diffusion of Fe in nickel oxide was studied in air at 1000 to 1400C, using a decrease in surface activity method. The measured diffusion coefficients were expressed as ... 

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