High Temperature Oxidation And Chlorination Of Metallic Materials

Metallic materials for practical applications at high temperatures usually contain more than one phase, which are fundamental for a good performance of the alloys in service, particularly on the mechanical properties such as creep resistance. However, the microstructural parameters, such as the size of the grains and particles, the spatial distribution and the degree of interconnection of each phase were not considered, which are of great importance in affecting the scaling behavior of these materials. The present work concerns studies of the oxidation of binary single-phase Cu-Ni alloy, binary two-phase Fe-Cu alloy, as well as ternary two-phase Cu-Ni-Fe alloy, as simple model alloys, at high temperatures by a single oxidant. These alloys have been obtained by conventional casting, mechanical alloying (MA), magnetron sputtering deposition (MSD), exhibiting quite different microstructures in the overall compositions with the grain or phase sizes from 10-100 microns down to very small order of 20-40 nm.The air oxidation of two copper-base Cu-lONi alloys having the same overall composition, but prepared by different methods and presenting widely different grain sizes, has been carried out at 800-900 癈 to examine the effect of a large reduction in the grain size on the oxidation behavior of a solid-solution alloy. Both alloys formed an external scale of copper oxide (CuO+Cu2O) plus a region of internal oxidation of Nickel. However, the overall depth of internal oxidation was larger than those measured for the conventional alloy under the same conditions. The difference was attributed to the a faster transport of oxygen in the alloy and of oxygen and copper in the scales for the fine-grained materials, resulting from a decrease of the grain size both in the alloy and in the scale and from a concomitant increase in the effective diffusion coefficients of the relevant species.The oxidation behaviors of as-cast Fe-Cu alloys and their sputtered coatings with different phase sizes were examined. For the as-cast Fe-50Cu alloy, a complex mixture oxide scale formed after air oxidation. Under oxygen pressure low enough that only Fe can be oxidized, a simultaneous external and internal oxidation of Fe was still present. However, a continuous iron oxide layer formed beneath an external copper oxide layer on the Cu-25Fe coating, whereas a single external iron oxide scale layer, overlying an iron-depleted region, was present on the Fe-50Cu coating. The transition of the oxidation modes above indicates that the grain size reduction can promote the selective oxidation of the more reactive component in binary two-phase alloys effectively. The transition mechanisms of the above oxidation modes were attributed to an enlarged solubility of iron in copper provided .by the grain size reduction in the coating, and also to the presence of high density of grain boundaries both in the coating and in the oxide layer, which could act as short-circuit diffusion paths and therefore resulted in the changes of the microstructure of the initial oxide scale. Finally, it can also be found that the oxidation rate of the coating is always higher than that for the as-cast alloy with the same composition, which implies that the reduction of the alloy grain size is not always beneficial for its oxidation resistance.The oxidation of a ternary two-phase Cu-30wt%Ni-25wt%Fe alloy in air has been studied at 700-900 ℃. The oxidation kinetics is rather irregular, but always slower than those for the oxidation of pure copper at the same temperatures. The scales are composed of an outermost layer of almost pure copper oxide plus an inner region of complex nature, containing a mixture of variable composition of the oxides of the three components plus a nickel-iron spinel. A relatively thin region of internal oxidation is also present at all temperatures, containing a large volume fraction of oxide in a matrix of the copper-rich phase. The alloy remains two-phase immediately below the front of internal oxidation, so that it does not undergo any special...