The phase diagrams of ternary systems involving two metal components and one oxidant are considered first, the limitations to their use is discussed in relation to the high temperature oxidation of binary alloys. Kine...The phase diagrams of ternary systems involving two metal components and one oxidant are considered first, the limitations to their use is discussed in relation to the high temperature oxidation of binary alloys. Kinetic diagrams, which are useful to predict the conditions for the stability of the two mutually insoluble oxides as the external scale, are then calculated on the basis of thermodynamic and kinetic data concerning both the alloys and the oxides, assuming the validity of the parabolic rate law. A combination of the two types of diagrams provides a more detail information about the oxidation behavior of binary alloys. The calculation of the diffusion paths, which relate the oxidant pressure to the composition of the system in terms of the alloy components both in the alloy and in the scale during an initial stage of the reaction in the presence of the parabolic rate law, is finally developed.展开更多
The corrosion properties of a Co-15wt% Y alloy were studied in H_2-H_2S mixtures under a sulfur pressure of 10-3 Pa at 600-800℃ and of 10-2 Pa at 800℃ to examine the effect of Y on the resistance of pure cobalt to s...The corrosion properties of a Co-15wt% Y alloy were studied in H_2-H_2S mixtures under a sulfur pressure of 10-3 Pa at 600-800℃ and of 10-2 Pa at 800℃ to examine the effect of Y on the resistance of pure cobalt to sulfur attack at high temperatures.The alloy is nearly single-phase.containing mostly the intermetallic compound Co17Y2 plus a little amount of the solid solution of Y in cobalt.At 600-700℃ and at 800℃ under 10-2 Pa of S2 the alloy forms multi-layered scales consisting of an outer region of pure cobalt sulfide,an intermediate region of a mixture of the sulfides of the two metals and finally an innermost layer of a mixture of yttrium sulfide with metal cobalt.At 800℃ under 10-3Pa of S2,below the dissociation pressure of cobalt sulfide, the alloy forms only a single layer composed of a mixture of metallic cobalt with yttrium sulfide.Pure Y produces only the oxysulfide Y2O2S, as a result of the good stability of this compound and of the presence of some impurities in the gas mixtures used The corrosion kinetics is generally rather complex and irregular except al 800℃under 10-3 Pa of S2.The addition of yttrium always reduces the sulfidation rate of cobalt, even though the formation of a continuous protective external layer of yttrium sulfide is never achieved.The internal sulfidation of Y in Co-15% Y is not associated with a depletion of Y in the alloy.This kind of diffusionless internal attack is typical of alloys with a very small solubility of the most reactive component Y in the base metal A.which restricts severely the Y flux from the alloy towards the alloy-scale interface.展开更多
The air oxidation of Cu Ni alloys with 50% and 70% nickel (mole fraction) at 800?℃ was studied. The kinetic curves for the oxidation of the two alloys are complex and deviate from the parabolic rate law. Typical doub...The air oxidation of Cu Ni alloys with 50% and 70% nickel (mole fraction) at 800?℃ was studied. The kinetic curves for the oxidation of the two alloys are complex and deviate from the parabolic rate law. Typical double layered scales are produced, which consist of a CuO outer layer and an inner layer containing a mixture of Cu 2O and NiO with many pores. Cu 50Ni presents a small degree of internal oxidation of nickel, which is observed in many binary double phase systems, but is quite rare in single phase systems.展开更多
The corrosion of alloys based on Fe,Co and Ni containing 15 and 30 wt-% Nb was studied at 700℃ in H_2-H_2S mixtures providing 10^(-8) atm S_2.The Fe and Co-based alloys formed duplex scales with an outer layer of pur...The corrosion of alloys based on Fe,Co and Ni containing 15 and 30 wt-% Nb was studied at 700℃ in H_2-H_2S mixtures providing 10^(-8) atm S_2.The Fe and Co-based alloys formed duplex scales with an outer layer of pure base-metal sulphide and an inner layer containing both metals.The Nb addi- tions were not able to produce exclusive Nb sulphidation but yielded a significant decrease of the corrosion rates for both types of alloys with respect to the pure metals even though they were still much higher than that of pure Nb.The Ni-base alloys corroded rapidly forming a single complex scale layer,mainly due to the appearance of a liquid Ni-S solution.展开更多
基金Financial supports by the NSFC(China)under the research projects(No.50071058 and No.59725101)by the CAS(China)-CNR(Italy)under an international collaboration agreement are gratefully acknowledged.
文摘The phase diagrams of ternary systems involving two metal components and one oxidant are considered first, the limitations to their use is discussed in relation to the high temperature oxidation of binary alloys. Kinetic diagrams, which are useful to predict the conditions for the stability of the two mutually insoluble oxides as the external scale, are then calculated on the basis of thermodynamic and kinetic data concerning both the alloys and the oxides, assuming the validity of the parabolic rate law. A combination of the two types of diagrams provides a more detail information about the oxidation behavior of binary alloys. The calculation of the diffusion paths, which relate the oxidant pressure to the composition of the system in terms of the alloy components both in the alloy and in the scale during an initial stage of the reaction in the presence of the parabolic rate law, is finally developed.
文摘The corrosion properties of a Co-15wt% Y alloy were studied in H_2-H_2S mixtures under a sulfur pressure of 10-3 Pa at 600-800℃ and of 10-2 Pa at 800℃ to examine the effect of Y on the resistance of pure cobalt to sulfur attack at high temperatures.The alloy is nearly single-phase.containing mostly the intermetallic compound Co17Y2 plus a little amount of the solid solution of Y in cobalt.At 600-700℃ and at 800℃ under 10-2 Pa of S2 the alloy forms multi-layered scales consisting of an outer region of pure cobalt sulfide,an intermediate region of a mixture of the sulfides of the two metals and finally an innermost layer of a mixture of yttrium sulfide with metal cobalt.At 800℃ under 10-3Pa of S2,below the dissociation pressure of cobalt sulfide, the alloy forms only a single layer composed of a mixture of metallic cobalt with yttrium sulfide.Pure Y produces only the oxysulfide Y2O2S, as a result of the good stability of this compound and of the presence of some impurities in the gas mixtures used The corrosion kinetics is generally rather complex and irregular except al 800℃under 10-3 Pa of S2.The addition of yttrium always reduces the sulfidation rate of cobalt, even though the formation of a continuous protective external layer of yttrium sulfide is never achieved.The internal sulfidation of Y in Co-15% Y is not associated with a depletion of Y in the alloy.This kind of diffusionless internal attack is typical of alloys with a very small solubility of the most reactive component Y in the base metal A.which restricts severely the Y flux from the alloy towards the alloy-scale interface.
文摘The air oxidation of Cu Ni alloys with 50% and 70% nickel (mole fraction) at 800?℃ was studied. The kinetic curves for the oxidation of the two alloys are complex and deviate from the parabolic rate law. Typical double layered scales are produced, which consist of a CuO outer layer and an inner layer containing a mixture of Cu 2O and NiO with many pores. Cu 50Ni presents a small degree of internal oxidation of nickel, which is observed in many binary double phase systems, but is quite rare in single phase systems.
文摘The corrosion of alloys based on Fe,Co and Ni containing 15 and 30 wt-% Nb was studied at 700℃ in H_2-H_2S mixtures providing 10^(-8) atm S_2.The Fe and Co-based alloys formed duplex scales with an outer layer of pure base-metal sulphide and an inner layer containing both metals.The Nb addi- tions were not able to produce exclusive Nb sulphidation but yielded a significant decrease of the corrosion rates for both types of alloys with respect to the pure metals even though they were still much higher than that of pure Nb.The Ni-base alloys corroded rapidly forming a single complex scale layer,mainly due to the appearance of a liquid Ni-S solution.
