Michihisa Fukumoto

Formation of a Rhenium-Base Coating on a Nb-Base Alloy

To protect Nb–5Mo–15W alloy against high-temperature oxidation a novel coating was developed involving electroplating of a Re–Ni film, followed by pack cementation with Cr and Al. The coating consisted of a duplex-layer structure, an inner σ (Re–Cr) or Re(Cr) layer and an outer α Cr(Al) or β NiAl layer. The Re–Ni film containing more than 70u2009at.%Re, developed in the present investigation, is more useful than the conventional low Re–Ni film. The inner σ (Re–Cr) and Re(Cr) layer acts as a diffusion barrier between the Nb–5Mo–15W alloy substrate and the outer α-Cr(Al) or β-NiAl layer, which forms a protective α-Al2O3 scale. The coated Nb–5Mo–15W alloy was oxidized in air at 1373u2009K for up to 360u2009ks, showing very good oxidation resistance.

Preparation of Ni Aluminide/Ni Bilayer Coating on Nb–W Alloys by Molten Salt Electrodeposition and Oxidation Resistance

Coatings consisting of Ni aluminide/Ni bilayer on Nb–W and Nb–W–Mo alloys were prepared by the electrodeposition of Al using a molten salt electrolyte after the Ni electrodeposition. The coating specimen with a thin Ni layer showed a vigorous oxidation during the initial period. On the contrary, the coating specimen with a thick Ni layer showed a high oxidation resistance. For the coating specimen with a thick Ni layer, a surface layer consisting of NiAl was formed after the oxidation test. For this coating specimen, further, intermediate layers consisting of Ni–Nb alloys were formed by the diffusion of the Ni layer and the alloy substrate during the oxidation test. These intermediate layers contributed to the maintenance of a high concentration of Al in the surface layer and the suppression of spallation of the surface layers, leading to the improvement in the oxidation resistance of the coating.

Analysis of Water Vapor Oxidation of Fe and Fe–Cr Alloys by Measuring the Partial Pressures of Hydrogen and Oxygen

The oxidation behaviors of the Fe, Fe–15mass%Cr and Fe–30mass%Cr alloys were investigated at 1173xa0K in Ar–H2O and Ar–O2–H2O atmospheres using an oxygen sensor and a hydrogen sensor installed at the outlet of an experimental reactor. In Ar–H2O, the changes in the hydrogen partial pressure were continuously measured using the hydrogen sensor, and the amounts of generated hydrogen and resultant consumed oxygen were calculated. The calculated gross mass gain was in close agreement with that gravimetrically obtained after the experiment. In Ar–O2–H2O, the amount of oxygen consumed from the ambient oxygen gas in the atmosphere was calculated from the change in the oxygen partial pressure measured using the oxygen sensor. In this case, the gross mass gain was analyzed as the sum of the amount of oxygen consumed from the ambient oxygen and that supplied from the steam dissolution. The oxidation patterns were classified into three types: (1) water vapor oxidation (in Ar–H2O), (2) water vapor oxidation and oxygen oxidation (in Ar–500xa0ppmO2–H2O except for Fe–30Cr), and (3) oxygen oxidation (in Ar–1%O2–H2O). It was demonstrated that a gas monitoring method using a combination of hydrogen and oxygen sensors is useful for investigating the water vapor oxidation in an atmosphere containing water vapor and oxygen.

Formation of the β-NiAl Containing Hf by the Simultaneous Electrodeposition of Al and Hf using a Molten-Salt and the Cyclic Oxidation Behavior

The formation of a coating layer consisting of a Ni aluminide containing Hf on a Ni substrate was attempted by the electrodeposition of Al and Hf. Especially, the effects of the HfF4 concentration in the electrolytic bath, electrodeposition temperature and potential on the type of Ni aluminide phase and the morphology of the Hf-rich precipitates in the Ni aluminide were investigated. The cyclic oxidation resistance of the Ni covered with the coating was then evaluated. Al and Hf were deposited by molten salt electrolysis. For the sample treated with the Al and Hf deposition at 1023xa0K, a Ni2Al3 layer was formed, and no Hf in the layer was observed. On the other hand, for the sample treated with the Al and Hf deposition at 1173xa0K and −1.25 to −1.8xa0V, a NiAl layer containing Hf was formed. It was found that a thick NiAl layer was formed at 1173xa0K and −1.5xa0V. The cyclic oxidation test result showed that the oxidation rate of the untreated sample, the sample coated with a Ni2Al3 layer without Hf and the sample coated with a thin NiAl layer, were high. However, for the sample coated with a NiAl layer containing a small amount of Hf, the mass gain rate was low. On the sample after the cyclic oxidation test, a scale consisting of α-Al2O3 and HfO2 adhering to the substrate was formed.

Formation of Ni Aluminide Layer Containing La by Molten-Salt Electrodeposition and Cyclic-Oxidation Resistance

In order to improve the cyclic-oxidation resistance of Ni and SUS304 steel, the coating of a Ni aluminide containing La on both samples were carried out by the electrodeposition of Al and La in a molten salt. The electrodepositions of Al and La were conducted using a potentiostatic polarization method at 1023 K in an equimolar NaCl-KCl melt containing 3.5 mol%AlF3, and that containing 4 mol%La2O3-24 mol%NH4Cl, or 3.5 mol%LaF3. Observation and analysis of the cross-section of the specimen after polarization showed that a deposited layer consisting of a thick inner Ni aluminide layer and a thin outer layer containing a large amount of La was formed on both samples. The cyclic-oxidation resistance of the Ni specimen covered with the deposit layer having the La layer as the outer layer was higher than those of the untreated Ni specimen and the Ni specimen coated with the deposit layer without La. The stainless steel covered with the Ni aluminide showed a high cyclic-oxidation resistance in the atmosphere containing water vapor at 1273 K in comparison with untreated stainless steel, without depending on the existence of La in the deposited layer.

Preparation of Highly Oxidation-Resistant Surface by Molten Salt Electrodeposition

In order to prepare a highly oxidation-resistant surface for TiAl and SUS 304 stainless steel, the molten salt electrodeposition of Al or Y on these metals was carried out. The electrodeposition was conduced using a potentiostatic polarization method at constant potentials in an equimolar NaCl-KCl melt containing AlF3 or YF3 at 1023 K. After the Al electrodeposition, homogenous deposit layers were formed on the TiAl and the stainless steel. The deposited layer formed on the TiAl consisted of TiAl3. The deposited layer formed on the stainless steel consisted of some Fe aluminides. The TiAl and the stainless steel covered by the electrodeposited layers were far more resistant than the bare TiAl and stainless steel to high temperature oxidation. The Y electrodeposition on the stainless steel induced the deposition of Y particles on the stainless steel. The cyclic-oxidation resistance of the electrodeposited stainless steel was remarkably improved as compared to the untreated stainless steel.

Investigation of High-Temperature Oxidation in Steam for Ni–Al Alloys Using the Combination of a Hydrogen Sensor and an Oxygen Pump–Sensor

Abstrac tThe oxidation rates of Ni–Al alloys in an atmosphere of Ar–12 vol.% H2O were evaluated by a hydrogen sensor and oxygen pump–sensor installed in the latter stage of an oxidizing furnace. Steam oxidation behavior was also investigated. The oxygen pump–sensor was evaluated using Ar–1000xa0ppm H2 gas. It was found that the H2 gas concentration could be accurately measured as an oxidation current, such thatxa0the hydrogen concentration could be measured by the oxygen pump–sensor, and the oxidation rate could be calculated using this hydrogen concentration. From an isothermal oxidation experiment at 1523xa0K for 14.4xa0ks, the oxidation rates measured by the hydrogen sensor and the oxygen pump–sensor were almost the same values in each sample. In addition, the mass gain curve was obtained for each sample based on the result of the time change in the oxidation rate. Furthermore, from a cycle oxidation experiment in which the temperature was varied between 1173 and 1523xa0K it was found that for the Ni–11 at.% Cr–10 at.%Al and Ni–50 at.%Al alloys, during the temperature increase process of the first cycle, a sharp decrease in the oxidation rate was observed around 1373xa0K. This was considered to be due to the transformation from θ- to α-Al2O3 since the scales formed on both alloys consisted of Al2O3. However, during the temperature increase process of the second cycle, this decrease was not observed in both alloys. Both the hydrogen sensor and the oxygen pump–sensor method could measure small changes in the oxidation rate during the steam oxidation process, so it became clear that the change in the oxidation rate accompanying the phase transformation of the oxide scale could be determined.

High Temperature Corrosion Resistance of Siliconized Stainless Steel under Continuous Deposition of Salt

The surface alloying of Si into SUS304 austenitic stainless steel was carried out by a halide-activated pack-cementation method. By this treatment, the silicon diffusion layer containing about 13 at.% Si was formed. The high temperature corrosion resistance of this specimen was evaluated under the continuous deposition of salt. The result of the corrosion test showed that the oxidation mass gain of the siliconized stainless steel was lower than that of non-treated stainless steel. It was found from the observation of the cross-section of the specimen after the corrosion test that a thin scale was formed on the silicon diffusion layer and silicon oxide was formed as an inner layer of the scale. A mechanism of the oxidation suppression for the siliconized steel under the continuous deposition of salt was investigated by the oxidation test of pure silicon, iron, chromium or nickel powder mixed with equimolar NaCl-KCl. As a result, it was found that the high corrosion resistance of the siliconized steel was attributable to the fact that the silicon oxide formed on the silicon diffusion layer was inert to the chemical reaction with the NaCl-KCl salt.

Formation of Ni Aluminide Coating Containing Reactive Elements by Simultaneous Electrodeposition and Cyclic Oxidation Resistance

A Ni aluminide layer containing Zr or Hf was formed on a Ni specimen by the simultaneous electrodeposition of Al and Zr or Al and Hf using a molten-salt bath. When the simultaneous electrodeposition of Al and Zr was carried out using molten NaCl-KCl containing 3.5 mol%AlF3 and 0.05 mol%ZrF4, the electrodeposited layers were formed in the order of Ni2Al3, NiAl3 and Al from the Ni substrate side. The ZrAl3 particles were uniformly formed in the surface region of the NiAl3 and Al layers. On the other hand, when the simultaneous electrodeposition of Al and Hf was carried out using molten NaCl-KCl containing 3.5 mol%AlF3 and 0.05 mol%HfF4, the electrodeposited layer consisted of Ni2Al3 as the inner layer and NiAl3 as the outer layer were formed with HfAl3 particles uniformly formed in the surface region of the NiAl3 layer. For the sample treated with the simultaneous electrodeposition of Al and Zr, no significant change in the mass gain was observed during the cyclic-oxidation test at 1423 K, suggesting that the sample had a high cyclic-oxidation resistance. Similarly, the sample treated by the simultaneous electrodeposition of Al and Hf had a high cyclic-oxidation resistance. An adhesive scale, having localized inward penetrations consisting of Al2O3 containing ZrO2 or HfO2, was formed on the samples having the high cyclic-oxidation resistance.