Hydrogen diffusion and trapping in iron
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. The diffusion and trapping of hydrogen in iron was investigated and results applied to a model based on Fick's second law of diffusion modified to include the rate of trapping. Experiments were performed by charging a semi-infinite electrode with hydrogen atoms at a cathodic potential. Upon completion of the charging period tc, ranging between 10 ms and 100 s, the egress of hydrogen was then measured as an anodic transient at a potential close to the corrosion potential. Anodic currents and charges were analysed according to expressions previously derived for the model. Total anodic and total cathodic charge densities were measured on a cold-worked pure polycrystalline iron electrode immersed in acetate buffer of pH 4.55 at 298K. The total anodic charge was found to be linearly dependent on the square root of the charging time up until about 50 s. Similar behaviour was observed at various cathodic potentials. However the trapping rate constant kT was too small to be measured by this method. Analysis of experimental anodic current-time transients gave a linear relationship between the observed anodic current density and the time function t-½ - (t + tc)-½ which indicates that kT is too small to be found from current transients. However the chronocoulometric estimation of kT using charge-time transients indicated that although small, kT was finite. This method overcomes problems such as those arising from imperfect boundary conditions and low signal-to-noise ratio in the region of interest that affect other methods. Mean values of kT of about 0.03 s-1 and 0.09 s-1 were ascertained by the chronocoulometric method on abraded and smoothly polished iron surfaces respectively. The difference between these results was ascribed to the occurrence of surface deformed layers which altered both the diffusion and trapping characteristics of the metal near the surface. The ratio of the total anodic charge to total cathodic charge densities was generally found to decrease as both the time and the magnitude of the cathodic potential were increased. This was accounted for by a model linking the hydrogen evolution reaction on the electrode surface to the hydrogen atom concentration just beneath the surface cs. It is envisaged that a virtually instantaneous equi1ibrium exists between adsorbed hydrogen on the electrode surface and that just within the metal lattice. Further evidence for the equilibrium ingress of adsorbed hydrogen atoms came from the behaviour of the term csDA½, where DA is the apparent diffusion coefficient of hydrogen in iron. Values of csDA½ determined from current and charge results were found to be potential dependent and plots of log10 (csDA½) versus the mass transport corrected cathodic potential were linear.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1986