Hydrogen permeation through bilayers of Fe/electrodeposited Ni, Fe/electrodeposited Cr and Fe/passivating oxide film have been investigated by electrochemical method. The hydrogen diffusion and absorption behaviors of the electrodeposited layers are detailed in the chapters III and IV. The effect of structural defects on the hydrogen diffusivity of the passivating oxide film on Fe has been studied in the Chapter V. Moreover, the existence of structural defects within the passivating oxide film has been inferred from measured electric properties of the film by using AC inpedance technique in the Chapter VI.
In Chapter III, time lag methods have been presented and compared to describe hydrogen permeation through an Fe/electrodeposited Ni composite. The diffusivity of hydrogen in the electrodeposited Ni was determined from the time lag equations derived for the composite. The results obtained under galvanostatic boundary condition agreed well with those obtained under potentiostatic boundary condition. The diffusivity of hydrogen in the electrodeposited Ni increased with increase of the Ni electrodeposit thickness. Also the steady-state hydrogen permeation flux across the composite increased slightly and non-linearly with increasing reciprocal thickness of the electrodeposit in the range of 1 μm to 50 μm for the both boundary conditions. The behavior of hydrogen permeation through the composite is discussed as related to the barrier effect of the electrodeposit and also to the hydrogen trapping by micropores present in the electrodeposit.
In Chapter IV, hydrogen absorption of pulse-plated Cr during electroplating has been studied as a function of pulse current parameter of duty cycle. The Cr depos obtained at duty cycles of 100(direct current) to 60 % were composed of a hemispherical nodular growth with bcc lattice while those obtained at 20 % duty cycle were composed of a needle-like structure containing a mixture of bcc lattice and hexagonal hydride. Occluded hydrogen content in the Cr deposites decreased up to 60 % duty cycle, and then increased with further decrease in duty cycle to 20 %. The resulting corrosion and wear properties are discussed in terms of hydride formation and crack density of chromium deposite.
In Chapter V, hydrogen permeation through the passivating oxide film on iron has been studied as a function of applied anodic potential and input hydrogen concentration under modulating input hydrogen concentration(modulation method). Hydrogen transport through the Fe/oxide composite is regarded as a diffusion-diffusion/migration reaction from the frequency dependence of the phase shift between input and output sides of hydrogen. Hydrogen diffusivityb in the oxide is also evaluated from the measured phase shift. The hydrogen diffusivity in the oxide decreased with increasing anodic potential and decreasing input hydrogenconcentration. This behavior of hydrogen diffusivity was discussed as related to concentrations of lattice defects and hydrogen-containing species in the oxide layer. The activation energy for hydrogen transport across the oxide layer has been found to be 44.1 kJ/mol, which is very larger than the value of 15.8 kJ/mol for the lattice diffusion of iron. This difference is due probably to additional interaction of hydrogen with oxygen in the oxide layer. Also the hydrogen solubility ratio of ron metal to iron oxide at their interface was calculated together with the hydrogen diffusivity in the oxide. The hydrogen solubility in the oxide is very larger to about five orders in magnitude than that of iron due probably to electrical potential jump or drop and activity coefficient ratio of Fe metal to oxide film at the interface. Also the hydrogen diffusivity in the oxide layer obtained from modulation technique was compared to that obtained from time-lag method and the former was more less than the latter. The low hydrogen diffusivity estimated from the time lag method is explained in terms of hydrogen trapping through $H_2O$ formation within the oxide during hydrogen permeation.
In Chapter VI, the effect of hydrogen on the electric properties of the passivating oxide film on Fe such as dielectric constant, donor density and flatband potential has been studied at anodic applied potentials of 600 and 800 $mV_SHE$ by using AC impedance technique. The dielectric constant and donor density of the passivating film increased with decreasing anodic potential or with hydrogen injection into the passivating films due probably to increasing $Fe^{2+}$ and $H_2O$ concentration within the films. The flatband potential of the passivating film ranges between 105 and 151 $mV_SHE$. The results suggests that hydrogen provides electronic defects within the passivating film, which help semiconducting mechanism of the film.
