Absorption hydrogen

The hydrogen is, at small hydrogen to metal ratios (H M 0.1), exothermically dissolved (solid solution, a-phase) in the metal. The metal lattice expands proportionally to the hydrogen concentration by approximately 2-3 A per hydrogen atom [31]. At greater hydrogen concentrations in the host metal (H M 0.1), a strong 

The thermodynamic aspects of hydride formation from gaseous hydrogen are described by means of pressure-composition isotherms in equilibrium (AG = 0). While the solid solution and hydride phase coexist, the isotherms show a flat plateau, the length of which determines the amount of H2 stored. In the pure P-phase, the H2 pressure rises steeply vfith increase in concentration. The two-phase region ends in a critical point T, above which the transition from the a- to the P-phase is continuous. The equilibrium pressure peq as a function of temperature is related to the changes AH° and AS° of enthalpy and entropy  

As the entropy change corresponds mostly to the change from molecular hydrogen gas to dissolved solid hydrogen, it amounts approximately to the standard entropy of hydrogen (S° = 130 J mol ) and is therefore ASf —130 J mol H2 for all metal-hydrogen systems. The enthalpy term characterizes the stability of the metal hydrogen bond. The decomposition temperature for P = p° = 1 bar (usually) is 

It is well-known that most of the compounds formed between rare earth elements and 3d metals are able to absorb large quantities of hydrogen gas in a reversible way 

The location of the H atoms in various Dy2Fe14BHx alloys was studied by Ferreira et al. (1985), using results of 57Fe and 161 Dy Mossbauer spectroscopy made on these materials in conjunction with neutron diffraction results obtained on Y2Fe14BD35. 

Hydrogen absorption in Ce2Fe14B was studied in detail by D. Fruchart et al. (1987). These authors reported that the hydrogen induced increase in Curie temperature is approximately three times stronger than in the other R2Fe14B compounds and discussed this property in terms of a valence change of Ce. 

The change of the magnetic properties upon hydrogenation of compounds of the type Pr2 xR xCo14B was studied by Pourarian et al. (1988). 

High-temperature slope of the thermal expansion (a, in K 1), total spontaneous volume magnetostriction at 4.2 K (us) and relative contributions to s of the 3d and 4f sublattices ( sd and usf) in various R2Fe)4B compounds. (After Buschow and Grossinger 1987.) 

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Heat is produced during hydrogen absorption, and must be suppHed during its release. The weight of hydrogen that can be released from the hydride, about 1.5 wt %, requires approximately the same volume as Hquid hydrogen, but the hydride is much heavier. The hydrogen released from a hydride has a... 

In galvanic coupling, titanium is usually the cathode metal and consequently not attacked. The galvanic potential in flowing seawater in relation to other metals is shown in Table 10. Because titanium is a cathode metal, hydrogen absorption may be of concern, as it occurs with titanium complexed to iron (38). 

All metallic materials can suffer electrolytic corrosion. Fractures caused by cathodic hydrogen only occur when the activity of the absorbed hydrogen and the level of the tensile stress, which can be external or internal, reach a critical value. In general, critical hydrogen absorption is achieved only in the presence of promoters. However, under very severe conditions such as at very low pH or very negative potential, critical hydrogen absorption can occur. Steels with a hardness greater than HV 350 are particularly susceptible. 

With hot-dipped galvanized steel, hydrogen absorption with the formation of blisters can be observed in cathodic protection [38]. 

Decarburization results from hydrogen absorption from gas streams at elevated temperatures. In addition to hydrogen blistering, hydrogen can remove carbon from alloys. The particular mechanism depends to a large extent on the properties of other gases present. Removal of carbon causes the metal to lose strength and fail. 

Reduction of 17a-EthynyI to 17a-Ethyl °° A solution of 5 g of 17a-ethynyl-androst-5-ene-3j9,17j5-diol in 170 ml of absolute alcohol is hydrogenated at atmospheric pressure and room temperature using 0.5 g of 5 % palladium-on-charcoal catalyst. Hydrogen absorption is complete in about 8 min with the absorption of 2 moles. After removal of the catalyst by filtration, the solvent is evaporated under reduced pressure and the residue is crystallized from ethyl acetate. Three crops of 17a-ethylandrost-5-ene-3) ,17j9-diol are obtained 3.05 g, mp 197-200° 1.59 g, mp 198.6-200.6° and 0.34 g, mp 196-199° (total yield 5.02 g, 90%). A sample prepared for analysis by recrystallization from ethyl acetate melts at 200.6-202.4° [aj, —70° (diox.). 

Hydrogenation A mixture of 27.4 g (0.20 mole) of /7-aminobenzoic acid, 200 ml of water, and 2 g of the catalyst is hydrogenated at 50 psi in a Parr apparatus. At the completion of the hydrogenation (absorption of 0.6 mole of hydrogen), the mixture is filtered and concentrated under vacuum. When crystals start to form, the mixture is diluted with 200 ml of DMF and cooled in an ice bath. The crystals are collected by filtration, washed with DMF followed by methanol, and dried. About 20 g of cis- and fr[Pg.42]

Hydrogenolysis of aromatic carbonyls occurs mainly by conversion to the benzyl alcohol and its subsequent loss. If hydrogenolysis is desired, the usual catalyst is palladium 38). Hydrogenolysis is facilitated by polar solvent and by acid (55). For instance, hydrogenation of 3,3-dicarbethoxy-5,8-dimethoxy-l-tetralone (5) over 5% Pd-on-C gave 6 quantitatively 54) when hydrogen absorption ceased spontaneously. 

In the hydrogenation, 200 g of acetophenone azine, 1000 ml of EtOAc and 5 g of 10% Pd-on-C was shaken at 30-50 psig for 10 h. Hydrogen absorption had ceased. About 7.6 g of phenylethylamine, formed by cleavage of the N—N bond, was obtained as a by-product. Oxidation of hydrazines can be done catalytically. Ethyl 2-arylhydrazine carboxylates were oxidized easily by bubbling air at 25 "C through a toluene or dioxane solution in the presence of Pd or Pt (5d). 

After the hydrogen absorption ceases, the autoclave is cooled, vented, and the reaction mixture is filtered to separate the catalyst. The filtrate is then heated on a steam bath at 60-80 mm pressure to remove the dioxane. The less volatile residue consists of 1,933 parts of crude dithioglycerol, a viscous oil. 

Hydrogen absorption being completed, the solution was filtered and evaporated to one-fifth of its volume and cooled in a refrigerator. The precipitate was filtered, washed with water, and then crystallized from ligroin, thus yielding 4 g of 2-cyclohexyl-2-hydroxy-cyclohexane-... 

Edwards e/a/. carried out controlled potential, slow strain-rate tests on Zimaloy (a cobalt-chromium-molybdenum implant alloy) in Ringer s solution at 37°C and showed that hydrogen absorption may degrade the mechanical properties of the alloy. Potentials were controlled so that the tensile sample was either cathodic or anodic with respect to the metal s free corrosion potential. Hydrogen was generated on the sample surface when the specimen was cathodic, and dissolution of the sample was encouraged when the sample was anodic. The results of these controlled potential tests showed no susceptibility of this alloy to SCC at anodic potentials. 

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. 

The proposed mechanism includes the production of HCl from the pyro-hydrolysis of the metal chlorides. Similar reactions are likely for bromides and iodides. Fluorides however are relatively stable and would not be expected to hydrolyse. It was considered that this might account for the inability of fluorides to cause cracking. Hydrogen absorption by titanium alloys exposed to chloride salts at elevated temperatures has been detected and found to be proportional to the amount of moisture participating in the reaction. 

The role of the stress in embrittlement and stress-corrosion processes has been examined in some detail by employing the slow strain-rate technique . Specimens of alloy 7179-T651 tested in air or in vacuum after pre-exposure to water at 70° C or in water at various potentials at ambient temperature exhibited a reversible embrittlement in excess of that arising from testing in moist air . The embrittlement was attributed to hydrogen absorption, and recovery was thought to be due to loss of hydrogen (particularly under vacuum) or to diffusion to traps. Potentiostatic tests revealed... 

The inhibitor should not decompose during the life of the pickle nor decrease the rate of scale removal appreciably. Some highly efficient inhibitors, however, do reduce pickling speed a little. It would be expected that since the hydrogen evolution is reduced the amount of hydrogen absorption and embrittlement would also be reduced. This is not always the case thiocyanate inhibitors, for example, actually increase the absorption of hydrogen. 

Probably the major use of inhibitors in acid solutions is in pickling processes. The chief requirements of the inhibitor are that it should not decompose during the life of the pickle, not increase hydrogen absorption by the metal... 

The present Section, which provides an outline of selected relevant topics in electrochemistry, is intended primarily as an introduction to aqueous corrosion for those readers whose basic training has not involved a study of electrochemistry. The scope of electrochemistry is enormous and cannot be treated adequately here, but there are now a number of excellent books on the subject, and it is hoped that this outline will serve to stimulate further study. The topics selected are as follows a) the nature of the electrified interface between the metal and the solution, (b) adsorption, (c) transfer of charge across the interface under equilibrium and non-equilibrium conditions, d) overpotential and the rate of an electrode reaction and (e) the hydrogen evolution reaction and hydrogen absorption by ferrous alloys. For reasons of space a number of important topics, such as the electrochemistry of electrolyte solutions, have been omitted. 

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. 

All AB, alloys are very brittle and are pulverized to fine particles in the hydrid-ing-dehydriding process (see Sec. 7.2.1). Thus electrodes must be designed to accommodate fine powders as the active material. There are several methods of electrode fabrication Sakai et al [35] pulverize the alloy by subjecting it to several hydrogen absorption-desorption cycles, before coating the resulting particles with Ni by chemical plating. The powder is mixed with a Teflon dispersion to obtain a paste which is finally roller-pressed to a sheet and then hot-pressed to an expanded nickel mesh. The fabrication of a simple paste electrode suitable for laboratory studies is reported by Petrov et al. [37],... 

Fig. 10. Coefficient of H atom recombination on Ni-Cu alloy catalysts as a function of the alloy composition, at 20°C. A, on Ni-Cu foils (59), O, on Ni-Cu evaporated films af ter their previous homogenization at 400°C (65,65a) d, on Ni-Cu foils after a multiple hydrogen absorption-desorption treatment (64a).

Fig. 11. X-ray diffraction pattern of a Ni99Cul alloy partially transformed into its (3-hydride (0 NiCuH) before (a) and after (b) hydride decomposition. Arrows point to the diffraction peaks representing the rich in copper alloy phsae desegregated from the initial alloy after a multiple hydrogen absorption-desorption treatment. After Palczew-ska and Majchrzak (48).

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