A. Kawashima

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.

The corrosion behavior of amorphous Fe-Cr-Mo-P-C and Fe-Cr-W-P-C alloys in 6 M HCl solution

Abstract Polarization measurement shows that the additions of molybdenum and tungsten to amorphous Fe-8Cr-13P-7C and Fe-15Cr-13P-7C alloys significantly decrease the active dissolution current and passivation potential in 6 M HCl solution. XPS and electrochemical investigations reveal that molybdenum and tungsten prevent dissolution of chromium from the air-formed films during potentiostatic passivation, although small amounts of iron dissolve from the air-formed films and alloys. The tungsten addition is more effective than the molybdenum addition. This difference seems to result from the fact that molybdenum dissolves actively at low potentials in the active region of the alloys, while tungsten does not dissolve at potentials lower than 100 mV(SCE). When an excess amount of molybdenum or tungsten is added, the passivity becomes unstable and the passive film is less enriched in chromic ions, because of transpassive dissolution of molybdenum and tungsten.

Passivity and its breakdown on sputter-deposited amorphous Al-early transition metal alloys in 1 M HCl at 30°C

Abstract The sputter-deposition method is successfully used for preparing amorphous AlTi, AlZr, AlNb, AlTa, AlMo and AlW alloys in wide composition ranges. Their corrosion behavior was investigated in 1 M HCl at 30°C. The results clearly reveal the difference in the alloying elements. When valve metals are alloyed, the passive films consist of a cation mixture of aluminum and other valve metals, their concentrations being not greatly different from the alloy compositions. These alloys are spontaneously passive. The corrosion rates of these alloys except for the AlTi alloys are considerably lower than that of aluminum metal. The corrosion rate of the AlTi alloys are almost the same as that of the aluminum metal regardless of the titanium content, because titanium is in the active state at the open circuit potential of the AlTi alloys. The AlTi, AlZr, AlNb and AlTa alloys suffer pitting by anodic polarization, but their pitting potentials are 350–1850 mV higher than the pitting potential of the aluminum metal. Alloying with molybdenum and tungsten decreases the corrosion rate of aluminum and raises remarkably the open circuit potential mostly due to decrease in the overpotential of hydrogen evolution. Anodic polarization of the AlMo alloys results in active dissolution possibly due to dissolution of molybdenum as molybdate, while that of the AlW alloys leads to coloration due to the formation of less protective corrosion product film consisting exclusively of the tungsten cation.

Hydrogen Electrode Reaction and Hydrogen Embrittlement of Mild Steel in Hydrogen Sulfide Solutions

Abstract Effects of strain rate, applied potential, pH, temperature, and hydrogen sulfide concentration on sulfide cracking of mild steel were examined at constant strain rates by means of an Instron-type machine. Fractographic observations were also carried out on a scanning electron microscope. Kinetic studies were also made to elucidate the hydrogen electrode reaction mechanism for mild steel and the effect of hydrogen sulfide on the reaction. Amount of hydrogen absorbed in mild steels stressed at a constant strain rate was measured and relationship of hydrogen embrittlement of mild steel with hydrogen electrode reaction and hydrogen absorption in hydrogen sulfide solutions was investigated. Fracture surfaces were transgranular quasi-cleavage and nucleation sites of cracks appeared to be carbides in the pearlite, inclusions and/or grain boundaries. The elementary reactions of hydrogen electrode reaction in acidic solutions are proton discharge and recombination of adsorbed hydrogen atoms and the rate d...

An XPS study of anodic behavior of amorphous nickel-phosphorus alloys containing chromium, molybdenum or tungsten in 1 m CHl ☆ ☆☆

Abstract The surfaces of amorphous Ni-18P, Ni-IOCr-20P, Ni-9Mo-19P and Ni-5W-18P alloys immersed or anodically polarized in 1 M HCl solution were analyzed in connection with their corrosion and anodic behavior. All alloys were more corrosion-resistant than crystalline nickel metal because of formation of phosphate-containing surface films on the Ni-18P, Ni-9Mo-19P and Ni-5W-18P alloys and because of spontaneous passivation due to formation of passive hydrated chromium oxyhydroxide film on the Ni-10Cr-20P alloy. The latter alloy was stable up to the transpassive region of chromium although intrusion of phosphate in the film was responsible for the higher passive current density in comparison to the amorphous Fe-Cr-13P-7C alloy of the same chromium content. The formation of thick porous phosphate films containing nickel, and molybdenum or tungsten by anodic polarization was not effective in passivating the Ni-18P, Ni-9Mo-19P and Ni-5W-18P alloys, and they suffered pitting corrosion by anodic polarization.

The corrosion behavior of sputter-deposited amorphous titanium-chromium alloys in 1 M and 6 M HCl solutions

Abstract The preparation of binary Ti-Cr alloys was achieved by magnetron sputtering using a titanium target with small chromium disks on it. The alloys containing 37–73 at% Cr were amorphous. Amorphous Ti-Cr alloys were spontaneously passive showing significantly lower corrosion rates in comparison with those of titanium and chromium metals in 1 M and 6 M HCl solutions. XPS analysis revealed that air-formed films and passive films on the Ti-Cr alloys were composed of a chromium-titanium oxyhydroxide rather than a mixture of chromium and titanium oxyhydroxides. The passive chromium-titanium oxyhydroxide film was more protective, stable and resistant against depassivation in comparison with passive films on chromium and titanium.

The corrosion behavior of amorphous nickel base alloys in a hot concentrated phosphoric acid

Abstract The corrosion behavior of more than 50 melt-spun alloys was examined in an 87 wt% H 3 PO 4 solution at 433 K. Ni-(30–50)Ta alloys possess the highest corrosion resistance due to the formation of a hydrated tantalum(V) oxyhydroxide film containing a small amount of phosphate. Corrosion rates of Ni-(1–10)Ta-P, Ni-(15–20)Cr-P and Ni-(10–20)Mo-P alloys are comparable to that of crystalline tantalum metal. The corrosion resistance of the former two alloy families is based on the formation of tantalum(V) phosphate and chromium(III) phosphate, respectively. The surface film on the Ni-Mo-P alloys consists of molybdenum(VI) and Ni(II) oxyhydroxides and phosphates, and nickel oxyhydroxide seems more protective than molybdenum oxyhydroxide.

Experimental evidence for the critical size of heterogeneity areas for pitting corrosion of Cr-Zr alloys in 6 M HCl

Abstract Sputter-deposited amorphous Cr-60Zr and Cr-67Zr alloys are spontaneously passive in 6 M HCl but suffer pitting by anodic polarization. The change in pitting susceptibility by the introduction of heterogeneity to the homogeneous amorphous alloys by crystallization heat treatment is studied. Heat treatment results in two-stage crystallization: (i) precipitation of hcp zirconium ranging in average size from 8 to 20 nm and (ii) massive transformation from amorphous phase to intermetallic compound, Cr2Zr. The heat treatment gives rise to the ennoblement of pitting potential in spite of the formation of nanocrystalline phases. This is due to the fact that the formation of the hcp zirconium phase leads to an increase in the chromium content of the matrix phase which is able to form thin, protective chromium-rich passive films covering the entire heterogenous alloy surface. When the average size of the less corrosion-resistant hcp zirconium precipitates exceeds a critical size, 20 nm, the protective chromium-rich passive films cannot completely cover the precipitates and the pitting resistance decreases.

Stress corrosion cracking of amorphous iron base alloys

Abstract The stress corrosion cracking behaviour at room temperature of amorphous Fe-Cr-Ni-P-C alloys subjected to constant strain rates was studied in some acidic solutions containing Cl− ions. Hydrogen embrittlement of the alloys occurred in the potential region lower than −300 mV relative to the corrosion potential in acidic solutions regardless of Cl− concentration. In the passive potential region no embrittlement was observed during tests in neutral NaCl solutions and in acidic solutions with low concentrations of Cl− ions. Only when tensile stress was applied to the specimen in relatively strong acidic solutions containing a certain amount of Cl− ions, fracture stress decreased in this potential region. The lowering of the fracture stress can also be attributed to hydrogen embrittlement.

Global CO2 recycling : novel materials and prospect for prevention of global warming and abundant energy supply

Abstract CO2 emissions which induce global warming, increase with the growth of the economic activity. It is, therefore, impossible to decrease emissions only by energy savings and by improvements of the energy efficiency. Global CO2 recycling can solve this problem and supply abundant renewable energy. Global CO2 recycling consists of three districts: (i) in deserts, all necessary electricities are generated by solar cells; (ii) on coasts close to the deserts, the electricity is used for production of H2 by seawater electrolysis, H2 is converted to CH4 by the reaction with CO2 and liquefied CH4 is transported to energy consuming districts; (iii) at energy consuming district, after CH4 is used as a fuel, CO2 is recovered, liquefied and transported to the coasts close to the deserts. A CO2 recycling plant for substantiation of our idea has been built on the roof of our Institute (IMR) in 1996, using key materials tailored by us. The key materials necessary for global CO2 recycling are the anode and cathode for seawater electrolysis and the catalyst for CO2 methanation. Since the quantities of CO2 to be converted far exceed an industrial level, the system must be very simple and the rate of conversion must be very fast. These requirements are satisfied in our global CO2 recycling system. When global CO2 recycling is conducted on a large scale, the energies and costs required to form liquefied CH4 in our global CO2 recycling system are almost the same as those for production of LNG from natural gas wells. A project for field experimenting the global CO2 recycling using pilot plants in Egypt has been planned in cooperation with Egyptian scientists, engineers and industries.