H. Habazaki

Film formation and detachment during anodizing of Al–Mg alloys

Anodic oxidation of binary Al–3 wt.% Mg and Al–5 wt.% Mg alloys has been undertaken in ammonium pentaborate electrolyte to develop barrier-type anodic films at relatively high current efficiency. During anodizing, aluminium and magnesium are oxidized at the alloy/film interface, entering the film as Al3 and Mg2 ions. Such behaviour is anticipated for an alloying element with a Gibbs free energy of oxide formation per equivalent less than that of alumina; further, the greater outward migration rate of Mg2 ions relative to that of Al3 ions in anodic alumina is expected from consideration of the respective single metal–oxygen bond energies. However, unexpectedly, with continued anodizing, the essentially alumina film detaches from the alloy surface, which is followed by growth of new film on the exposed substrate. The film detachment from the alloy is associated with void formation, considered to result from the significantly reduced Pilling–Bedworth ratio for formation of anodic MgO compared with that for anodic alumina.

Copper enrichment in Al-Cu alloys due to electropolishing and anodic oxidation

The average thickness and composition of the copper-enriched alloy layer that is present at the alloy/oxide interface during anodic oxidation of an eleetropolished Al-0.9 al.% Cu alloy at a constant current density of 50 A m−2 have been determined by Rutherford backscattering spectroscopy and transmission electron microscopy. The copper-enriched layer, of about 2 nm thickness, has an average composition of Al-40 at.% Cu and contains about 5.4×1015 Cu atoms cm−2. The average composition and thickness of the layer do not change significantly during anodizing from 10 to 200 V- The essentially steady-state, copper-enriched layer is established mainly by the prior electropolishing of the alloy. As a consequence of the pre-enrichment of copper, both aluminium and copper atoms are oxidized immediately at the alloy/film interface on subsequent anodizing. Owing to the importance of copper enrichment in Al-Cu alloys to the oxidation of copper atoms, alloy pre-trealment has an important role in determining the initial oxidation behaviour.

Evidence of oxygen bubbles formed within anodic films on aluminium-copper alloys

Abstract The composition of the barrier anodic film formed at 50 A m−2 to 200 V on solution-treated Al-l at.% Cu alloy in ammonium pentaborate electrolyte at 293 K has been investigated by Rutherfo...

Gel formation and the efficiency of anodic film growth on aluminium

Abstract The development of gel layers during anodizing, and their influence on the efficiency of growth of anodic films, has been examined for formation of films at constant current density on aluminium in saturated potassium antimonate electrolyte and 0.1 M sodium molybdate, sodium silicate and sodium tungstate electrolytes. The gels are produced immediately above the growing anodic films by the reaction of H + ions, generated at the film/gel interface, with the electrolyte anions to form uniform layers of hydrated oxide. The gels can enhance the efficiency of film growth by reducing, or eliminating, field-assisted ejection of Al 3+ ions from the film to the electrolyte. The thicknesses of the gel layers increase at constant rates with thickening of the anodic films, although the efficiencies of gel formation per se are relatively low. The thickest gels are obtained following anodizing in antimonate electrolyte possibly reflecting a more favorable rate of gel formation, relative to its rate of dissolution, than that for other gel layers. The gel layers may shrink and crack on drying, most noticeably for the gels formed in antimonate and tungstate electrolytes. The gel layers formed in molybdate electrolyte appear, by comparison, highly resistant to cracking.

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.

Impurity distributions in barrier anodic films on aluminium : a GDOES depth profiling study

The impurity distributions in the barrier anodic films formed on aluminium in a wide variety of electrolytes have been investigated by glow discharge optical emission spectroscopy (GDOES) depth profiling. The depth profiles obtained were compared, wherever available, with those obtained by other techniques. It was found that GDOES profiling is an extremely powerful and reliable technique for depth profiling analysis of thin, non-conducting alumina films. Surface charging is insignificant and the sputtering rate of the film is kept constant throughout the analysis, giving rise to excellent depth resolution which is comparable to, or better than, secondary ion mass spectrometry (SIMS) depth profiling analysis. Further sensitivity is also high, given the amount of impurity species being detected successfully. Thus, GDOES is expected to play a great role in depth profiling analysis of non-conducting anodic alumina films or other films where SIMS depth profiling is only of limited use due to ion beam bombardment-induced sample surface charging which has significant influence on the sputtering rate of the films.

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.

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.

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.