Hiroki Habazaki

The anodic behavior of amorphous Ni-19P alloys in different amorphous states

Abstract An amorphous Ni-19P alloy prepared by rapid quenching of white heat melt showed a higher anodic dissolution current density in 1 N HCl in comparison with the same amorphous alloy prepared by rapid quenching of red heat melt. After structural relaxation these two specimens showed the same anodic polarization curve which is located between the polarization curves of as-quenched two specimens. The thermograms of these two as-quenched specimens were different from each other, showing that the difference in the amorphous states is due to the difference in structural relaxation during preparation. The difference in anodic behavior between these two as-quenched specimens seems due to the difference in the amounts of quenched-in defects. The higher current density of the relaxed specimen in comparison with the as-quenched specimen prepared by rapid quenching of the red heat melt has been interpreted in terms of introduction of chemical heterogeneity as a result of rearrangement and regroupings of atoms in the alloy during structural relaxation. The steady state current density was fairly low in the low potential region without showing a difference between two as-quenched specimens and then increased with increasing polarization potential. The difference in the quality of as-quenched alloy specimens seemed to be masked by the formation of phosphorus-covered alloy surfaces during anodic polarization at potentials lower than about 200 mV(SCE), because of a negligibly small dissolution rate constant of phosphorus in comparison with that of nickel.

Characterization of sputter-deposited Ni-Mo and Ni-W alloy electrocatalysts for hydrogen evolution in alkaline solution

Abstract Binary Ni-Mo and Ni-W alloy coatings with good adhesion to nickel substrate are successfully prepared by d.c. magnetron sputter deposition method. These alloy electrodes are found to be active hydrogen evolution electrocatalysts in 1 M NaOH solution at 30 °C. Ni-Mo alloy electrodes exhibit the highest activity, which is higher than that of smooth platinum electrode. Leaching treatment in hot concentrated caustic solution for Ni-Mo alloys significantly enhances the activity.

CO2 methanation catalysts prepared from amorphous Ni-Zr-Sm and Ni-Zr-misch metal alloy precursors

Abstract Nickel catalysts supported on nano-grained oxides have been prepared from amorphous Ni–Zr–Sm and Ni–Zr–Mm (Mm: misch metal) alloys and crystalline Ni–Sm and Ni–Mm alloys. These catalysts show higher catalytic activity for methanation of carbon dioxide than a conventionally prepared zirconia supported nickel catalyst. The catalytic activity of Ni–Zr–5 at% Sm catalysts increases with increase in nickel content, and is higher than the samarium-free Ni–Zr catalysts containing the same amount of nickel. The stabilization of tetragonal zirconia and the increase in the number of active surface nickel sites by addition of samarium to the nickel-rich catalysts leads to enhancement of catalytic activity. In the Ni–Zr–5 at% Mm catalysts, only the activity of the catalyst containing 60 at% nickel is enhanced in comparison with misch metal-free Ni–Zr catalysts. It is also found that Ni–Sm and Ni–Mm catalysts show activities as high as that of Ni–Zr catalyst, suggesting that samarium and misch metal oxides also act as good catalyst supports for methanation catalysts.

Compositional dependence of the CO2 methanation activity of Ni/ZrO2 catalysts prepared from amorphous NiZr alloy precursors

Abstract Finely grained Ni/ZrO 2 catalysts were prepared from amorphous Ni Zr alloy precursors by oxidation and subsequent reduction pretreatment, and the catalytic activity for CO 2 methanation was examined as a function of precursor alloy composition and temperature. The catalysts thus prepared produce exclusively methane, apart from water as a by-product. The conversion of CO 2 increases with temperature in the range of 373–573 K. Among the catalysts examined, the maximum methanation rate is obtained on the catalysts prepared from the amorphous alloy precursors containing 40 and 50 at% zirconium. Further, the methanation rates of all the catalysts prepared from the amorphous alloy precursors are higher than that of a 3 at% Ni/ZrO 2 catalyst prepared by wet impregnation. The number of surface nickel atoms, determined by hydrogen chemisorption, increases with zirconium content in the catalysts, while, interestingly, the turnover number decreases with increasing zirconium content. In the catalysts prepared from the amorphous alloys, two types of zirconia are present: metastable tetragonal and stable monoclinic zirconia. The former zirconia phase is present predominantly in the catalyst prepared from the Ni-30 at% Zr alloy, but the relative amount of this oxide phase, with respect to the total amounts of zirconia, gradually decreases with an increase in zirconium content of alloys. Thus, the higher turnover number of the catalysts with higher nickel content can be attributed to nickel supported on metastable tetragonal zirconia. Increasing nickel content of the precursor alloys leads to an increase in tetragonal zirconia and to a decrease in the number of surface nickel atoms on the catalysts. This is responsible for the fact that the maximum conversion appears at medium contents of zirconium in the precursor alloys.

Recent progress in corrosion-resistant metastable alloys

Tailoring new corrosion-resistant alloys has recently been performed mostly by the sputter deposition technique. This technique is suitable for forming a single-phase solid solution even when the boiling point of one component is lower than the melting points of the other components and/or when one component is immiscible with another component in the liquid state. Aluminium-refractory metal, chromium-valve metal and molybdenum-chromium-nickel alloys have been successfully prepared in a single amorphous phase. Amorphous aluminium-refractory metal alloys are corrosion resistant in 1 M HCl and chromium-valve metal alloys are spontaneously passive in 12 M HCl, showing a better corrosion resistance in comparison with the alloy components. The amorphous aluminium-refractory metal alloys also have an extraordinarily high hot corrosion resistance. Their sulphidation resistance at higher temperatures is far higher than any other known metallic materials and their oxidation resistance is comparable to chromia- or alumina-forming alloys.

Corrosion-resistant amorphous surface alloys

Abstract This is a review of laser and electron beam processing and sputter deposition for the preparation of corrosion-resistant amorphous surface alloys and of the characteristics of thus prepared surface alloys. Amorphous surface alloys with a large surface area were prepared by repetition of instantaneous melting of a very restricted volume of the surface by irradiation with a CO 2 laser or electron beam and subsequent self quenching by the cold bulk substrates. The materials consisting of the amorphous surface alloys and bulk crystalline metals are quite suitable for corrosion-resistant materials with other specific properties. Sputter deposition was used for preparation of various amorphous alloys such as Al-Ti, Al-Zr, Al-Nb, Al-Ta, Al-Cr, Al-Mo, Al-W, Cr-Ti, Cr-Zr, Cr-Nb, Cr-Ta, Cu-Nb and Cu-Ta. The corrosion resistance of amorphous aluminum alloys was very high and could be changed from that comparable to 304 stainless steels to that far exceeding corrosion-resistant nickel-base alloys by changing the alloying element and its concentration. The corrosion rates of amorphous chomium-valve metal alloys are several orders of magnitude lower than those of alloy constituting elements. The amorphous Al-Mo alloys show high resistance to both sulfidation and oxidation at high temperatures. Sputter deposition is a potential method to produce new materials with specific properties.

The corrosion behavior of amorphous NiCrP alloys in concentrated hydrofluoric acid

Abstract In order to develop alloys resistant to aggressive hydrofluoric acids, the corrosion behavior of the amorphous Ni-Cr-19P alloys in 47% HF solution was examined by corrosion tests, electrochemical measurements and XPS analyses. The corrosion rate for the Ni-19P alloy is 0.7 mm y−1 at 30°C, which is about 20% of that for Ni. This high corrosion resistance is based on the formation of an elemental phosphorus layer on the alloy surface which acts as a diffusion barrier against alloy dissolution, as has been found in HCl solutions. The addition of Cr enhances the corrosion resistance due to spontaneous passivation. The corrosion rate of amorphous Ni-Cr-19P alloys containing ≥15 at% Cris 2-7 × 10−3mmy−1. The passive film on the chromium-containing alloys consists mainly of hydrated chromium and nickel oxyhydroxide.

The effect of microcrystallites in the amorphous matrix on the corrosion behavior of melt-spun CrNiP alloys

Abstract The metallurgical structures of the melt-spun Crue5f8Niue5f8P alloys prepared by changing the ratio between eutectic Cr-13 at% P and Ni-19 at% P alloys were studied with TEM techniques. The presence of the microcrystalline phases in the amorphous matrix of Cr-27Ni-15P alloy was undetectable by X-ray diffraction, but was confirmed by TEM. Both phosphorus and chromium enhance the glass-forming ability of Cr-Ni-P alloys, but the microcrystalline bee Cr(Ni,P) and Cr(Ni) 3 P phases appear in the amorphous matrix with increasing chromium and decreasing phosphorus. The corrosion behavior is significantly sensitive to the presence of the microcrystallites in the amorphous matrix, and depassivation takes place by the presence of microcrystallites which are undetectable by X-ray diffraction. It is necessary to form a single amorphous phase for maintaining the highly corrosion-resistant passive film in a severe environment such as 6 M HCl.

The corrosion behavior of amorphous Ni-Cr-19p alloys in hydrochloric acid

Abstract The corrosion behavior of the amorphous Ni-Cr-19P alloys in 1, 6 and 12 M HCl solutions at 30°C was investigated. The amorphous Ni-19P alloy shows an almost constant corrosion rate of about 0.05 mm y −1 regardless of the concentration of hydrochloric acid. This is attributed to the formation of an elemental phosphorus layer acting as a diffusion barrier against active dissolution of the alloy. The corrosion rate of the amorphous Ni-Cr-19P alloys decreases with increase in alloy chromium content, but increases in 12 M HCl when 15 at% or more chromium is added. The formation of the elemental phosphorus layer on Ni-P and low chromium alloys is effective in decreasing the corrosion rate and in ennobling the corrosion potential with a consequent passivation. By contrast, when the content of active chromium is very high such as 15 at% or more, active dissolution of chromium in 12 M HCl solution prevents the formation of an elemental phosphorus layer, and hence no passivation by the formation of chromium enriched passive film occurs. Pitting corrosion occurred only in 12 M HCl but the pitting corrosion resistance can be improved effectively by the addition of chromium.

The Effect of Tungsten on the Corrosion Behavior of Amorphous Fe‐Cr‐W‐P‐C Alloys in 1M HCl

The active current density of Fe-8Cr-7W-13P-7C alloy is almost three orders of magnitude lower than that of the Fe-8Cr-13P-7C alloy. The passive current density in the low potential region also decreases with increasing alloy tungsten content. The surface analysis by XPS reveals that chromium and tungsten ions are concentrated in the surface film on a tungsten containing alloy in the active region. The thickness of the surface film formed on the low tungsten alloy in the active region is w-ray photoelectron spectroscopically infinite, whereas the film thickness on the Fe-8Cr-7W-13P-7C alloy in the active region is of the same order of magnitude as the thickness of the passive film. The chromium enrichment in the passive film becomes more significant with increasing tungsten content. Passivation of low tungsten alloys seems to occur by the formation of the passive film with the least enrichment of chromium ions necessary for passivation due to different dissolution rates of iron and chromium during a large amount of alloy dissolution. Passivation of tungsten containing alloys takes place through transformation of the air-formed film to the passive films as a result of preferential dissolution of a small amount of iron without dissolution of chromium ions