Grain boundaries, diffusion through metals along

A critical problem with adhesion layers arises from grain boundary diffusion. Deposited films tend to be polycrystalline and granular. The electrochemistry of the adhesion film is frequently much less desirable than the electrochemistry of the primary film. Moreover, minute contamination of the primary metal film surface by adhesion components can dramatically degrade the electron transfer properties (e.g., electrochemical reversibility, as evidenced by cyclic voltammetric peak potential separation) of the film [58], Thus it is essential that the adhesion layer is not exposed to solution. While the rate of diffusion of adhesion metals through the bulk of the primary layer is quite slow, grain boundary diffusion along the surfaces of grains is much faster. In many cases, the adhesion layer can seriously compromise the performance of the electrode. This is particularly a problem for chromium underlayers. Recently a codeposited Ti/W adhesion layer has been recommended as an alternative to chromium, with reportedly better adhesion and fewer interferences than Cr. A procedure was also described to recondition these electrodes to minimize interference by adhesion layer metals [58]. 

Hydrogen transport in a metal can occur by lattice diffusion, dislocation motion, or short-circuit diffusion along grain boundaries. Hydrogen can diffuse rapidly through the lattice in many metals, particularly those with a body-centered cubic (bcc) structure such as a-iron, ferritic steels, and )3-titanium alloys. However, grain boundary diffusion and dislocation... 

Special databases have been assembled dedicated to the field of corrosion of heat exchanger materials (steels and nickel-based alloys) under liquid salt layers and by direct gas diffusion through the bulk and along grain boundaries in the metal. These databases expand the scope of the standard databases for alloys (SGTE) and also for salts and oxides (FACT). The new databases have been successfully applied in the generation of classical thermodynamic one- and two-dimensional mappings but also in the kinetic models developed in the OPTICORR project. 

These are depicted schematically in Figure 18.4 in the case of metal A deposited on metal B. Bulk diffusion, as noted above, is the transfer of B into A or A into B through the crystal lattice. This is characterized by the coefficient D in the figure. Defect path diffusion is the migration along lattice defects such as grain boundaries, characterized by the coefficient D in the figure. Ordered A B, possible phases are indicated between the metals. Finally, Kirkendall void porosity is indicated and will be expected to be present if the interdiffusion rates from one metal to the other are not equal in both directions. 

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