Rapid Diffusion Paths Research Articles

High temperature corrosion of nickel and dilute nickel-based alloys in (SO 2O 2)/SO 3 mixtures

In this paper, surveys are given of the reaction behaviour of high purity nickel, commercial quality nickel and dilute nickel-based alloys containing alloying additions of silicon, manganese and/or iron in (SO 2O 2)/SO 3 at 700°C. High purity nickel reacts rapidly in gas mixtures with sufficiently high SO 3 pressures to form NiSO 4 on the scale surface (the sulphate mechanism). The scale then primarily consists of NiO plus Ni 3± x S 2. The sulfide forms continuous channels through the scale; these serve as diffusion paths for rapid outward diffusion of nickel and thereby sustain the rapid reaction. The dilute nickel alloys and the commercial quality nickel (in which the main impurities are silicon, manganese and iron) exhibit greatly improved corrosion resistance in O 2-rich gas mixtures (e.g. O 2-4%SO 2); the sulphate is still formed on the scale surfaces and the scales also contain sulphides. It is concluded that the positive effects of impurities and alloying additions are that they partially segregate at the surface of the NiO grains and thereby affect the interfacial properties of the oxide grains and the sulphide. The ability of the sulphide to wet the oxide grains is reduced, the sulphide no longer forms continuous channels and the reaction rates are decreased. The wetting of different oxides (NiO, CoO, feldspar and Cr 2O 3) by Ni 3S 2 has been studied, and the results are in qualitative accord with the proposed mechanism.

Laser surface treatment and its influence on the development of healing Cr2O3 scales on nickel-chromium alloys

The influence of laser surface treatment on the isothermal oxidation of Ni-10%Cr and Ni-15%Cr at 1025°C in oxygen at 1 atm pressure has been studied. Particular emphasis has been placed on the progressive establishment of a Cr2O3 healing layer, which is facilitated by rapid-diffusion paths for chromium to the surface from the bulk alloy. For nonlaser treated alloys, such paths are alloy grain boundaries. A partial Cr2O3 layer forms initially in localized sites at, and immediately adjacent to, these boundaries and progresses into the alloy grains in a stepwise manner following lateral diffusion of chromium from the grain boundaries, thereby developing a contoured configuration. For Ni-15%Cr, there is sufficient chromium in the bulk alloy grains to sustain the eventual development of a self-healing layer parallel to the surface. For Ni-10%Cr, this is not the case and complete development of the healing layer results entirely from the stepwise progression from the grain boundaries. Establishment of the healing layer on laser-glazed surfaces is facilitated by additional rapid-diffusion paths, particularly retained alloy grain boundaries, retained alloy twins, a laser-induced microstructure and solidification artifacts (such as ripples). The relative importance of these features is discussed in relation to the oxidation behavior.

Further observations on creep enhanced by a liquid phase in porous potassium chloride

The enhancement of creep by the introduction of aqueous solutions into the porosity of nonfully dense, cold compacted potassium chloride powder has been characterized in room temperature experiments. This phenomenon was first investigated by Pharr and Ashby, who concluded that the mechanism of creep is diffusionally controlled, with the liquid acting as a rapid diffusion path. Current results show that at high stresses, a second mechanism becomes important. This conclusion is based on differences in the stress exponent for creep, the strain at the onset of tertiary creep, and the volumetric strain behavior at low and high stresses. The mechanism is shown to involve grain boundary sliding accommodated by the opening of intergranular cracks and voids.

Internal Phase Changes in Dense Si3N4 Associated with High‐Temperature Oxidation

The effects of oxidation at 1400°C for 100 h on both surface and internal composition of commercial and laboratory hot‐pressed Si3N4 with MgO or ZrO2 additives as well as chemically vapor deposited (CVD) Si3N4 were studied using X‐ray diffraction. Samples were also compared to the same temperature treatments in Ar. The results indicate the grain boundaries act as rapid diffusion paths for the transport of oxygen.

The Effect of Oxide Grain Structure on the High-Temperature Oxidation of Fe, Ni and Cr

AbstractInvestigations have shown that the oxidation rate of metals and alloys is strongly influenced by the oxide grain size. This paper summarizes some of our data on the oxidation of Fe, Ni, Cr, and some alloys and relates the observed oxide grain structures to the oxidation kinetics and transport processes. Fine-grained polycrystalline oxide thickens most rapidly because the many oxide grain boundaries act as preferential paths for rapid diffusion. In contrast, monocrystalline oxide such as forms on some orientations of Ni and Cr thickens very slowly and shows the highest activation energy. Oxide grain size is found to depend on such factors as metal orientation, chemical pretreatment of the surface, cold work, and oxidation procedure. Resume Plusieurs programmes de recherche ont montre que le taux d'oxydation des metaux et alliages depend de la grosseur des grains des oxydes produits. Cet article presente certains des resultats obtenus en oxydant du Ni, du Cr et certains alliages; un rapport entre la...

Phosphorus Diffusion in Partially Crystallized Films of SiO2

The diffusion of phosphorus into thin films on Si was studied to assess the significance of an anomalous, rapid diffusion mode. Diffusions were performed over a wide range of temperature and time using neutron‐activated, red phosphorus diffusion sources. It was found that diffusion anneals in P vapors catalyzed a localized amorphous‐to‐crystalline transformation in the initially amorphous films, creating rapid diffusion paths at the interfaces between the crystalline islands and the surrounding amorphous matrix. The shape of the penetration profiles confirmed that interfaces contributed to the flux of P through the film. Both rapid and normal diffusion coefficients were determined and are discussed. Crystallization with subsequent rapid diffusion may be a contributing factor to the failure of diffusion masks.

Mechanism of oxide adherence on Fe-25Cr-4Al (Y or Sc) alloys

Oxidized alloys of Fe-25Cr-4Al, some containing small additions of elements (Yttrium or Scandium) which strongly promote oxide adherence, have been subjected to extensive structural studies by scanning, replication, and transmission electron microscopy as well as other techniques. The morphological details of the Al2O3 scales, which develop on these alloys during high temperature oxidation, and the structural changes at the oxide-substrate interfaces are discussed with respect to oxidation kinetics and oxide adherence. The oxide scales grow into the alloys at rates consistent with the diffusion of oxygen down grain boundaries of the fine-grained scales. Yttrium and expecially scandium additions tend to increase oxidation rates by providing rapid diffusion paths in the form of yttria and scandia stringers. More importantly, the morphological data led to the conclusion that the spallation of nonadherent scale is caused by distributions of small voids at the oxide-substrate interface. Consistent with this conclusion, it is found that the oxide scale is tenaciously adherent when voids are absent, as is the case when yttrium or scandium is added to the alloy. It is maintained that these alloy additions promote oxide adherence by forming vacancy complexes with excess vacancies or by providing internal oxide boundaries for the condensation of excess vacancies, which would have otherwise condensed at the oxide-substrate interface and formed the voids responsible for oxide spallation. The role of vacancy sinks is further confirmed by showing that oxide adherence is promoted in Fe-25Cr-4Al, in the absence of yttrium of scandium, by a distribution of Al2O3 particles. This result indicates that particles, which have been used in engineering alloys for strengthening purposes, can also promote oxide adherence. The morphological data are also discussed with respect to other mechanisms that had been proposed for oxide adherence.

The influence of sulfides on the oxidation behavior of nickel-base alloys

The oxidation of presulfidized chromium, Ni−Cr, and Ni−Al alloys, and complex nickel base alloys was studied at 1000°C in 1.0 atm of oxygen. Sulfur-rich surface layers were produced in the pretreatment by using H2S−H2 mixtures. Presulfidized chromium oxidized at a rate similar to that of sulfur-free chromium. The oxidation rate of presulfidized Ni−Cr alloys was affected by sulfur only when liquid nickel sulfide was present which accelerated the oxidation rate by creating rapid diffusion paths through the Cr2O3 scale. The oxidation behavior of presulfidized Ni−Al alloys, with aluminum contents sufficient for the formation of a protective Al2O3 layer in the sulfur-free condition, was influenced by sulfur only when aluminum sulfide was formed in the presulfidation treatment which caused the Al2O3 scale to be porous. The oxidation behavior of nickel-base alloys containing both chromium and aluminum was insensitive to the presence of sulfides when the concentration of aluminum in the alloy was such that a protective Al2O3 scale was formed during oxidation of the sulfur-free alloy and aluminum sulfide was not formed in the presulfidizing treatment.