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Deplanque, R. (1983) Kinetics of hydrogen evolution and absorption during electrochemical reactions as determined by monitoring the volume of hydrogen bubbles. Doctoral thesis, City of London Polytechnic.
The piezo electric technique (P.E.T) devised in this department is a highly sensitive method for measuring the total volume of gas bubbles suspended in a solution. This is achieved by simply performing a static compressibility test, causing small pressure fluctuations in the volume of a solution enclosed in a rigid vessel.
Volume fluctuations may be produced by utilising the properties of piezo electric materials, which either change their shape in an applied electric field or generate electric fields when under pressure. Applying ac current to the piezo material positioned in this rigid vessel will cause a cyclic pressure fluctuation which can be measured by a second piezo crystal. On introducing a gas bubble into the system the coupling of both crystals is lessened, which can be measured as a decrease in the e.m.f. of the second crystal. This decrease in e.m.f. is a direct measure of the amount of gas present in the system.
By cathodically producing H on an electrode which does not absorb hydrogen, and by using a hydrogen saturated solution, it is possible to calibrate the output e.m.f. These results are compared with those obtained using an electrode which does absorb hydrogen and this makes it possible to calculate the quantitative amount of gas absorbed by the metal.
Experiments were carried out on a variety of metals with low and high hydrogen absorption capabilities. The metals under study included Fe, Au, Cu, Pt, Ti, Al, Al-3Mg, Al-5Mg and Duralumin. Comparative studies were performed between the P.E.T and the solid state vacuum extraction method for Ti, and between P.E.T. and the permeation method, as developed by Devanathan for Fe, and good agreement between these three methods was attained.
The diffusion coefficient for Fe was found to be DH = 7 x 10- 6 cm2 s-1 , and for Al, DH = 1.87 x 10- 9 cm2 s-l, whilst its equilibrium solubility was found to be SH = 1 x 10- 4 gH/g Al. These results were compared with values found in the literature for Fe: no values were available for the results on Al.
By introducing As into the electrolyte it was possible to show that at low concentration (2 ppm) As acts as a promoter for the h.a.r. on Ti, A1, A1-3Mg, A1-5Mg and as inhibitor in the case of Duralumin. At higher concentrations >5 ppm As acts as inhibitor for the same reaction. A mechanism for this effect is proposed.
A detailed study was made on the growth mechanism of hydrides on Ti with and without As. It was shown that As acts as an inhibitor for the formation of y-hydrides on Ti.
Proposals for extension of the work and uses in industry and research are made.
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