Hydrogen diffusivity

Hafnium begins to react with nitrogen at about 900°C to form a surface nitride film, and reacts rapidly with hydrogen at about 700°C to form hydrides (qv). The hydrogen diffuses rapidly and converts the bulk metal into the brittle hydride. 

By indirect embrittlement (reaction by-product, atomic hydrogen diffusing into the lattice of the steel)... 

Compounds such as hydrogen sulfide and cyanides are the most common metal surface poisoners occurring in process units subject to aqueous-phase hydrogen attack. In many process units, these compounds can be effectively eliminated and hydrogen diffusion stopped by adding ammonium polysulfides and oxygen to the process streams which converts the compounds to polysulfides and thiocyanates, provided the pH is kept on the alkaline side. 

Fig. 16. Hydrogen diffusion coefficient as a function of inverse temperature.

In another case, acidic water was used to clean the inside of the water Jacket that surrounded a glass-lined vessel. Some hydrogen diffused through the wall of the vessel and developed sufficient pressure to crack the glass lining. 

The remarkable solubility of hydrogen in palladium discovered by T. Graham following the observation of hydrogen diffusion through red-hot platinum and iron by H. St. C. Deville and L. Troost, 1863. 

As a small interstitial atom, hydrogen diffuses rapidly in iron, the diffusion rate being of a similar order to that of solutes in aqueous solution. 

In permeation measurements the first signs of hydrogen diffusing through 1 mm steel membranes can be observed in a few minutes. The practical measurement of diffusion parameters tends to be rather unreproducibie. 

If the acid contains certain impurities such as arsenic, the arsenic raises the overvoltage for the hydrogen evolution reaction. Consequently, the amount of atomic hydrogen diffusing into the steel, and the brittleness, increase. 

Moreover, in the case of hydride intervention, still a further factor, namely the kinetics of hydrogen diffusion into the metal, influences also the overall kinetics by removing a reactant from a reaction zone. In order to compare the velocity of reaction of hydrogen, catalyzed by palladium, with the velocity of the same reaction proceeding on the palladium hydride catalyst, it might be necessary to conduct the kinetic investigations under conditions when no hydride formation is possible and also when a specially prepared hydride is present in the system from the very beginning. 

The simultaneous analysis of orthophosphate, glycerol phosphates, and inositol phosphates has been achieved by spectrophotometric analysis of the molybdovanadate complexes. Also, a sensitive and selective chemiluminescent molecular emission method for the estimation of phosphorus and sulphur is described, which is based on passing solutions into a cool, reducing, nitrogen-hydrogen diffusion flame. For organic compounds it was usually necessary to prepare test solutions by an oxygen-flask combustion technique. 

W. Geller and T. Sun, Influence of Alloy Additions on Hydrogen Diffusion in Iron and Contribution to the System Iron-Hydrogen, Arch. Eisenhuttenw 21, pp 423-430,1950. 

Diffusion. Hydrogen diffuses through air much more rapidly than other gaseous fuels. With a diffusion coefficient in air of. 61 cm2/s, the rapid dispersion rate of hydrogen is its greatest safety asset. 

Knapton, A.G., Palladium alloys for hydrogen diffusion membranes—A review of high permeability materials, Plat. Met. Rev., 21,44-50,1977. 

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