Alloys surface stability

Erosion and Corrosion combined require special consideration. Most of the stainless steels and related corrosion-resistant alloys ow e their surface stability and low rate of corrosion to passive films that develop on the surface either prior to or during exposure to reactive fluids. If conditions change from passive to active, or if the passive film is removed and not promptly reinstated, much higher rates of corrosion may be expected. 

Chemical Reactivity - Reactivity with Water Mild liberation of heat Reactivity with Common Materials Corrosive to copper, copper alloys, aluminum alloys, galvanized surfaces Stability During Transport Stable Neutralizing Agents for Acids and Caustics Dilute with water Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

EQCM Analyses It was suggested by XPS analysis that the nonprecious metals were leached out from the alloy surface during CVs in acidic electrolyte solution. Therefore, we used an EQCM to measure the mass change at the electrode. The resolution and stability of our 10 MHz EQCM was +0.1 Hz, i.e., +0.44ngcm without any signal averaging. 

As described in the previous sections, a stable Pt skin of a few nanometers is formed on the Pt-Fe, Pt-Co, and Pt-Ni alloy surfaces after electrochemical stabilization. Figure 10.12 shows Arrhenius plots of kapp on the alloy electrodes at —0.525 V vs. E° in comparison with that of a pure Pt electrode. In the low temperature region (20-50 °C for Pt54Fe45, 20-60 °C for Pt6gCo32 and Ptg3Ni37), linear relationships between log kapp and 1 / Tare observed at all the electrodes, corresponding to the following Arrhenius equation ... 

Environmental tests have been combined with conventional electrochemical measurements by Smallen et al. [131] and by Novotny and Staud [132], The first electrochemical tests on CoCr thin-film alloys were published by Wang et al. [133]. Kobayashi et al. [134] reported electrochemical data coupled with surface analysis of anodically oxidized amorphous CoX alloys, with X = Ta, Nb, Ti or Zr. Brusic et al. [125] presented potentiodynamic polarization curves obtained on electroless CoP and sputtered Co, CoNi, CoTi, and CoCr in distilled water. The results indicate that the thin-film alloys behave similarly to the bulk materials [133], The protective film is less than 5 nm thick [127] and rich in a passivating metal oxide, such as chromium oxide [133, 134], Such an oxide forms preferentially if the Cr content in the alloy is, depending on the author, above 10% [130], 14% [131], 16% [127], or 17% [133], It is thought to stabilize the non-passivating cobalt oxides [123], Once covered by stable oxide, the alloy surface shows much higher corrosion potential and lower corrosion rate than Co, i.e. it shows more noble behavior [125]. 

With an emphasis on scale electrical conductivity (surface stability as well), a number of new alloys have been recently developed specifically for SOFC interconnect applications. The one that has received wide attention is Crofer 22 APU, an FSS developed by Quadakkers et al. [136, 137] at Julich and commercialized by Thyssen Krupp of Germany. Crofer 22 APU, which contains about 0.5% Mn, forms a unique scale, as shown in Figure 4.6, comprised of a (Mn,Cr)304 spinel top layer and a chromia sublayer [137-139], The electrical conductivity of (Mn,Cr)304 has been reported... 

The origin of the attractive interaction and its difference on Ni(l 10) and Ni(lOO) is somewhat uncertain, but it is reasonable to speculate that it arises from dipole-dipole attractions in the adlayer. Quantitative TPRS (99) and results on (110) oriented Cu/Ni alloy surfaces (100) showed that the anhydride required four nickel atoms for stabilization on the (110)... 

On the alloy surface the reaction proceeded both via the anhydride and formate intermediates (117). As the copper concentration was increased, the formate species dominated the reaction, until at 63% copper the CO/COj ratio was less than 0.1. This change was due to the decrease in the amount of anhydride formed with increasing copper and the corresponding increase in formate. Since only the anhydride decomposition produced CO, the relative amount of anhydride formed could be determined as a function of surface composition. This relationship is shown in Fig. 21 the anhydride concentration fell as the fourth power of the nickel concentration, suggesting the requirement of four nickel atoms for its stabilization. This value agreed with the earlier determination for the saturation density of anhydride intermediates on Ni(llO) (99). 

The emf of the lithium-aluminium system versus pure lithium in a Lil-KI-LiCl molten eutectic is shown in Fig. 8.2 as a function of temperature and composition. It can be seen that the emf remains constant (at about 300 mV more negative than pure lithium) in the range of stability of the /3-phase (-7-47 atoms per cent of lithium), thus implying a constant lithium activity in the alloy surface. At concentrations greater than 47 atoms per cent, the lithium activity becomes strongly composition-depen-dent. 

To quantily the metal dissolution trends, and to offer comparisons of the stability of surface Pt atoms in different environments, we reported the development and application of a computational approach based on first-principles calculations on metal slabs, using the methodologies explained in this chapter. The method allows us to evaluate the electrochemical potential shift AU (V) for the dissolution of Pt atoms in an alloy surface, relative to the potential at which the same reaction would take place on pure Pt(lll) surfaces. Recent investigations in our lab have found interesting correlations between the potential shift for the onset of surface oxidation of Pt in Pt-based alloys with respect to the same potential in pure Pt surfaces and the d-band shift of the surface atoms, reflecting the changes in the electronic structure due to alloying. The results will be published elsewhere. 

Nanoparticles have different morphologies than flat, bulk surfaces. Perez et al. have considered the activation of water and COads + OHads reactions on Pt and PtRu clusters including the effects of solvation." They found that the presence of under-coordinated Ru adatoms on the Pt cluster surfaces enhances the production of OHads from water compared to Ru alloyed into the nanoparticle surfaces. More significantly, they found that the presence of an aqueous environment simulated by up to six water molecules dramatically stabilized the transition state and products of the reactions. For example, in a gas-phase environment they calculated a water dissociation barrier of 20 kcal/mol whereas in the solvated environment the barrier was reduced to 4.5 kcal/mol on the alloy surface. The barrier for water dissociation on the Ru adatom in the aqueous environment was only 0.9 kcal/mol. Although their results are for an adatom on a near flat (111) surface, they may have significance in describing the catalytic properties of undercoordinated Ru atoms at edge and corner sites on nanoparticles. 

Alloy stability is always of concern in heterogeneous catalysis, but in electrocatalysis there are new mechanisms for destabilizing alloys, namely electrochemical dissolution or corrosion. Greeley and Norskov developed an intuitive and simple thermodynamic framework for estimating the stability of alloy surfaces in electrochemical environments. " Their scheme is essentially an extension of an atomistic thermodynamic approach that uses chemical potentials to determine stability to one that uses electrochemical potentials to determine stability. They estimate the electrochemical potentials using total energies calculated within DFT and ideal solution behavior of the ions to consider concentration and pH effects. Within this formalism they are able to estimate the dissolution potential of metals in alloys. They further compared the trends in dissolution behavior to trends in segregation behavior and... 

The activity, stability, and tolerance of supported platinum-based anode and cathode electrocatalysts in PEM fuel cells clearly depend on a large number of parameters including particle-size distribution, morphology, composition, operating potential, and temperature. Combining what is known of the surface chemical reactivity of reactants, products, and intermediates at well-characterized surfaces with studies correlating electrochemical behavior of simple and modified platinum and platinum alloy surfaces can lead to a better understanding of the electrocatalysis. Steps, defects, and alloyed components clearly influence reactivity at both gas-solid and gas-liquid interfaces and will understandably influence the electrocatalytic activity. 

---