Alloy films surface examination

In earlier work with pure metals, it was generally accepted that the area of films deposited at, say, 0°C was proportional to their weight (with the exception of group IB and low melting-point metals). Information was available on the surface areas of films of Ni, Pt, Pd, Rh, etc. (71), and hence absolute reaction rates could be calculated. It would be a considerable undertaking to establish similar data for alloy systems, bearing in mind that various compositions would have to be examined and also a method for preparing exact compositions would be required. However, for sintered alloy films, approximate methods can be proposed. 

The paper by Watanabe et al. is important for alloy catalysis they examined the possible enrichment of Cu in the surface layers of Cu-Ni alloys. They used Auger spectroscopy to study the escape of low- and high-energy electrons (around 100 and 700-1000 eV) and were able to make quantitative in-depth profiles, of which an example is shown in Figure 2. This enrichment was also confirmed by AES measurements for films by Benndorf et al. ... 

Nanoscopic Investigations of Dealloyed Surfaces Erom the background of competitive models of selective alloy dissolution as described above, a closer microscopic examination of this process with the ultimate objective of atomic resolution and chemical information on an atomic scale appears mandatory. Ex situ transmission electron microscopy (TEM) of thin, corroded alloy films provides lateral resolution at the nanometer scale, but suffers from poor depth resolution and from structural relaxation processes that may occur after termination of the anodic polarization and transferring the samples into high vacuum. Classical TEM investigations in this field were performed under open circuit conditions in oxidizing environments (that is, at > Eq) [51,... 

When considering zinc-aluminum alloys, the surface oxide film normally present is likely to reduce any corrosion current. The risk of bimetallic corrosion is small in atmospheric exposure trials by Noranda have been in progress since 1984 on ZA alloys coupled to other common metals. No visual effects were noted at the 5-year examination (Barmhurst and Belisle, 1992). A zinc-25% aluminum-0.05% magnesium alloy coupled to other materials and exposed on the Noranda Research Center roof showed pitting attack on the zinc-based material (but only up to 0.38 mm deep in 10 years) when joined to copper, brass, or steel, but less when joined to stainless steel or lead and least when joined to aluminum. 

The electrodeposition of Al-Zr alloys was examined in the 66.7-33.3 mol% [EMIM]1 Cl /AlCli liquid [19]. The reduction of Zr(IV), which was introduced as ZrCLj in the liquid, produces a small ill-defined cathodic wave and a small negative shift to the Al deposition wave. Voltammetric data show that the small ill-defined cathodic wave corresponds to the Zr(IV)/Zr(III) reaction. It is noted that a surface passivating film is formed on the electrode surface after this reaction, indicating that the Zr(III) is insoluble in the liquid. 

The analysis of several pure metals and binary alloys yields generally at least a duplex and in some cases a multilayer structure of the passive film, as depicted schematically in Fig. 19. These systems have been examined with surface analytical methods, mainly XPS, but also ISS in some cases. The systematic variation of the electrochemical preparation parameters gives insight to the related changes of layer composition and layer development, and support a reliable interpretation of the results. Usually the lower valent species are found in the inner part and the higher valent species in the outer part of the passive layer. It is a consequence of the applied potential which of the species is dominating. Higher valent species are formed at sufficiently positive potentials only and may suppress the contribution of the lower... 

Most descaling and passivation processes for steels were developed prior to the widespread use of electrochemical techniques. As a result, a variety of visual and chemical tests are widely used for determining the surface cleanliness. Chemical tests have also been established to verify the presence of a robust oxide film on austenitic and ferritic stainlesses (8). These methods are very simple to conduct in a manufacturing environment, but they are qualitative in nature and rely strongly on the judgment of the inspector. Outside of the laboratory, electrochemical methods have not been widely used to evaluate cleanliness of carbon and alloy steels after pickling. Nevertheless, they are well suited for this purpose and have been examined in considerable detail in laboratory studies. 

Examination of Eqs. (35) to (41) does not reveal any parameter that is obviously dependent on flow rate, provided that the applied voltage is maintained constant and vacancy condensation dominates the induction time. Thus, neither the thermodynamics of absorption of X into a surface oxygen vacancy nor the ejection of a cation from the film is expected to depend on flow velocity, nor are the events (e.g., vacancy condensation) that occur at the metal/film interface expected to be sensitive to fluid motion. Thus, the PDM predicts that the breakdown ( pitting ) potential for passive alloys that are of interest to the thermal power industry should not be sensitive to flow rate. The PDM also predicts that the induction time should be insensitive to fluid flow velocity, provided that the induction period is dominated by vacancy condensation at the metal/film interface. 

Moss et al have used AES as their source of information on surface composition in alloy catalysts in their study of ethylene hydrogenation over Ni-Pd films. Mossbauer spectroscopy has also been used by Lam and Boudart to examine " Au in Pd-Au particles on Si02. Alloying and uniform Au distributions were observed. 

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