Chromium grain boundary diffusion

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]. 

In principle it is also possible to eliminate the effects of sensitisation by prolonged heat treatment within the critical temperature range to allow diffusion of chromium from the grain interiors to level out and eliminate the region of chromium depletion adjacent to the grain boundaries. In practice, however, the times involved (many hundreds of hours) are too long. 

Under most practical carburization conditions for steels the reaction rate is under mixed control of surface reaction and diffusion. Whereas the carburization of simple steels, for instance, is straightforward, and hardness is achieved by the formation of high-carbon martensite on heat treatment, the presence of carbon in stainless steels and superalloys results in the formation of carbides based on chromium and other alloying elements. Excessive carburization can result in the removal from soluhon of protective elements such as chromium. This can seriously reduce the corrosion resistance of the component, particularly at grain boundaries. 

Self-diffusion and rates of oxidation [49] creep and sintering are dependent on the presence of ionic defects in Cr203. The self-diffusion of oxygen is much slower than that of chromium. However, recent data [50, 51] are 4-7 orders of magnitude lower than previously pubKshed data, which were dominated by dislocation and grain boundary effects. The new data, obtained for single crystals, show a dependence on pOi corresponding to the reaction... 

Porous metallic gas diffusion electrodes are used in these fuel cells. The anode consists of a nickel alloy with 2% of chromium. Chromium that is added prevents recrystallization and sintering of the porous nickel though it works as an electrode. This action is based on chromium forming a thin layer of chromium oxide at the nickel grain boundaries, which interferes with the surface diffusion of the nickel atoms. 

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