Effective hydrogen absorption

Niobium like tantalum relies for its corrosion resistance on a highly adherent passive oxide film it is however not as resistant as tantalum in the more aggressive media. In no case reported in the literature is niobium inert to corrosives that attack tantalum. Niobium has not therefore been used extensively for corrosion resistant applications and little information is available on its performance in service conditions. It is more susceptible than tantalum to embrittlement by hydrogen and to corrosion by many aqueous corrodants. Although it is possible to prevent hydrogen embrittlement of niobium under some conditions by contacting it with platinum the method does not seem to be broadly effective. Niobium is attacked at room temperature by hydrofluoric acid and at 100°C by concentrated hydrochloric, sulphuric and phosphoric acids. It is embrittled by sodium hydroxide presumably as the result of hydrogen absorption and it is not suited for use with sodium sulphide. 

Finally, it is necessary to point out that although a particular method of corrosion control may be quite effective for the structure under consideration it can introduce unforeseen corrosion hazards elsewhere. Perhaps the best example is provided by cathodic protection in which stray currents (interaction) result in the corrosion of an adjacent unprotected structure or of steel-reinforcement bars embedded in concrete a further hazard is when the cathodically protected steel is fastened with high-strength steel bolts, since cathodic protection of the tatter could result in hydrogen absorption and hydrogen cracking. 

B. M. Lichstein, and D. D. Perry The Chemistry of Alane, I. Some Observations on the Effect of the Coordinating Base on the Aluminium-Hydrogen Absorption in the Infrared. Inorg. Chem. 2, 650 (1963). 

The main factors governing hydrogen embrittlement are the metal surface films that can restrict hydrogen absorption, the effect of electrical discharge machining that may... 

Said this, we can let the reader to recall Fig. 1.15, where we depicted amorphous-like phase regions at grain boundaries as the pathways open for preferential diffusion of hydrogen atoms. Apparently, an alloy can benefit from some fraction of amorphous phase to improve kinetics of hydrogen absorption, but complete amorphization of crystalline lattice lowers capacity for storing hydrogen [156]. Mechanochemical activation is therefore a complex process where kinetic and thermodynamic effects must be firstly well understood, and then optimized. 

L. Zaluski, A. Zaluska, P. Tessier, J.O. Strom-Olsen, R. Schulz, Catalytic effect of Pd on hydrogen absorption in mechanically alloyed Mg Ni, LaNij and FeTi, J. Alloys Compd. 217 (1995) 295-300. 

K.S. Jung, E.Y. Lee, K.S. Lee, Catalytic effects of metal oxide on hydrogen absorption of magnesium metal hydride, J. Alloys Compd. 421 (2006) 179-184. 

M. Hara, S. Morozumi, K. Watanabe, Effect of a magnesium depletion on the Mg-Ni-Y aUoy hydrogen absorption properties , J. Alloys Compd. 414 (2006) 207-214. 

Figure 5.26 Hydrogen absorption in the effective medium theory, (a) Hydrogen atom on a path to the surface of a metal and through the lattice (b) electron density along the path of the hydrogen atom (c) potential energy of the hydrogen atom along the path.

In order to study the effect of absorption in the nickel-hydrogen system in more detail, Beeck et al. (11) have investigated the hydrogen sorption isobars between 20°K. and room temperature. As shown in Fig. 6, the solid curves represent the isobars for increasing and decreasing temperature. With increasing temperature (the part between 20 and 80°K. will be discussed later), sorption increases fast between 80 and... 

Equation 9 indicates the addition of benzoquinone to CoH to form a new complex which cannot react further with CoH. Equation 10 defines the role of excess alkali in effecting the catalytic reduction of benzoquinone. As shown in previous examples, the hydroxo complex may then undergo the reverse aging process, leading to hydrogen absorption. The over-all result is reduction of benzoquinone to hydroquinone when limited amounts of substrate are available, and to quinhydrone when excess substrate is available. Equation 11 is an attempt to explain the lowered amount of hydrogen absorption noted when cyanocobaltate(II) is prepared in the presence of excess benzoquinone. Displacement of reduced substrate from this binuclear complex by alkali is assumed, since quinone was catalyti-cally reduced when the above procedure was carried out in the presence of added alkali. 

Fig. 3.23 Isotope effect on hydrogen absorption for the CaNij Hj system at 25...

Another possible effect of PdAu deposits on PdAu/SnOx sensors is through the formation of a Schottky barrier between PdAu and SnOx, as in the case of the Pd/CdS hydrogen sensor. If such a barrier is formed, then a depletion layer is created inside the semiconductor tin oxide. Since the Pd work function can be reduced by hydrogen absorption through dipole or hydride formation (14,15), the width of the depletion layer in tin oxide may be reduced. The reduction of the depletion layer width causes the sample resistance to decrease. Such a possibility was checked and was ruled out, because a good ohmic contact was obtained between Pd (-50 nm thick) and SnOx- It is also commonly known that gold forms an ohmic contact with tin oxide. 

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