Metal alloys surface modification

Intermetallic Coatings for Electrical Insulation and Corrosion Resistance on High Temperature Alloys, Surface Modification Technologies IX, eds., TS. Sudarshan, W. Reitz, and J.J. Stiglich, The Minerals, Metals and Materials Society, Warrendale PA, 1996, pp.151-166. 

One promising extension of this approach Is surface modification by additives and their Influence on reaction kinetics. Catalyst activity and stability under process conditions can be dramatically affected by Impurities In the feed streams ( ). Impurities (promoters) are often added to the feed Intentionally In order to selectively enhance a particular reaction channel (.9) as well as to Increase the catalyst s resistance to poisons. The selectivity and/or poison tolerance of a catalyst can often times be Improved by alloying with other metals (8,10). Although the effects of Impurities or of alloying are well recognized In catalyst formulation and utilization, little Is known about the fundamental mechanisms by which these surface modifications alter catalytic chemistry. 

It was shown in the preceding text that even in the simplest systems many different chemisorbed particles originate on the surface during the catalytic reaction. In principle most of them can interact with each other and probably with gaseous reaction components as well. As a consequence, any catalytic reaction represents a system of simultaneous reactions, and the problem is how to influence the course of a particular reaction—in other words, it is essentially the selectivity problem. Thus in catalysis by metals, probably the modification of the surface properties (by forming the alloys, stable surface complexes, or by the addition of promotors, etc.) seems to be the most promising direction of the further fundamental research. 

Newly developed alloys have improved properties in many aspects over traditional compositions for interconnect applications. The remaining issues that were discussed in the previous sections, however, require further materials modification and optimization for satisfactory durability and lifetime performance. One approach that has proven to be effective is surface modification of metallic interconnects by application of a protection layer to improve surface and electrical stability, to modify compatibility with adjacent components, and also to mitigate or prevent Cr volatility. It is particularly important on the cathode side due to the oxidizing environment and the susceptibility of SOFC cathodes to chromium poisoning. 

Using this method, homogeneous alloys, segregated alloys, layered bi-metallics, and decorated particles are all readily accessible. An obvious advantage of the precursor concept over the conventional salt-impregnation method is that both the size and the composition of the colloidal metal precursors may be tailored independent of the support. Further, the metal particle surface may be modified by lipophilic or hydrophilic protective shells and coated by intermediate layers, e.g., of oxide. The modification of the precursor by dopants is also possible. 

This chapter will provide an overview of oxidation and corrosion behavior of candidate oxidation-resistant alloys under SOFC operating conditions and discuss surface modifications for improved stability and performance of metallic interconnects. 

The third part treats surface modification by underpotential deposition (UPD) of metals. Physical nature, thermodynamics, structural aspects, kinetics, as well as surface alloy formation are discussed. Experimental support is given based on classical electrochemical investigations as well as on some recent results from modern in situ surface analytical studies including atomic imaging by in situ STM and AFM. 

The nature of the corrosion scale is very important in deciding the corrosion resistance of the component. Surface modification or alloying alters the composition and complexity of the corrosion scales, which prevents further corrosion and outward diffusion of alloy components. Vossen et al. have shown the complex scale formation in the case of stainless steel and nickel-based alloys in comparison to pure metals.These scales are understood to cause transport problems to the outward diffusion of alloy components and thereby decrease the dissolution of the steel components. Therefore, any improvement to the corrosion resistant properties of the current collector has to come by way... 

Surface Modifications of Ferrous Metals and Alloys Induced by Potential and Thermal Perturbations... 

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