Alloys density values

The major dehciency of the oxygen electrode reaction is its low exchange current density (about 10 A/cm on a smooth surface) in acid electrolytes on even the best-known electrocatalyst (a platinum-chromium alloy). This value is about six orders of magnitude lower than that for the hydrogen electrode reaction in the same electrolyte. The reaction is about three orders of magnitude faster on smooth platinum or nickel oxide surfaces in an alkaline medium as compared to acid. The... 

Critical passivation current densities are calculated from the anodic polarization curve shown in Fig. E4.7. The passivation current density values calculated using mixed potenial theory are 89A/cm, 9.4 x 10 A/cm, and 4.5 X 10 ° h/axi for alloys A, B, and C, respectively. 

A list of density values for common commercial titanium alloys is given in Table 3. 

TABLE 3—Room temperature density values for some commercial titanium alloys. 

Ni substrate is observed only at certain current-density values. On a Pt substrate, one observes noticeable depolarization with two pronounced waves that seem to correspond to the formation of alloys with different... 

Here tg denotes the time to breakaway (in h), Cq the initial A1 content in (wt.%), Cb the critical A1 content at which breakaway occurs (in wt.%), p the alloy density (in g cm ), h, e, the initial foil or specimen thickness (in cm), k and n the kinetics constants defined in Eq. 22.1. In the calculations it was assumed that the critical A1 content in Eq. 22.2 is independent of temperature and/or specimen thickness. The value of Cb was set to 0.3 wt.% as found for thin-foil Fe-Cr-Al alloys after oxidation at 1200°C e.g. in [5, 6]. It should be mentioned that, in former studies, indications were found that the Cb value depends on temperature. However, to the best of our knowledge, no extensive experimental data are available yet. In the diagram all the lifetimes are given in a normalised way, setting the obtained result at 1050°C for the short cycle (15/5 s) to 100%. 

Sn-Pb and various Pb-free alloys. Depending upon the alloy, the density varies between 7 and 10 g/cm. In these references, the density values for binary alloys were reported to decrease linearly with increasing temperature. For example, from the experiments of Schwaneke et al. [37], the empirical fits for density in grams per cubic centimeters obtained for pure Sn and 60.4Sn/Pb (wt.%) were 7.16-6.3(10 " )7 and 8.59-7.3(10 )T , respectively, where Tis temperature in degrees Celsius. 

Nearly all experimental eoexistenee eurves, whether from liquid-gas equilibrium, liquid mixtures, order-disorder in alloys, or in ferromagnetie materials, are far from parabolie, and more nearly eubie, even far below the eritieal temperature. This was known for fluid systems, at least to some experimentalists, more than one hundred years ago. Versehaflfelt (1900), from a eareflil analysis of data (pressure-volume and densities) on isopentane, eoneluded that the best fit was with p = 0.34 and 8 = 4.26, far from the elassieal values. Van Laar apparently rejeeted this eonelusion, believing that, at least very elose to the eritieal temperature, the eoexistenee eurve must beeome parabolie. Even earlier, van der Waals, who had derived a elassieal theory of eapillarity with a surfaee-tension exponent of 3/2, found (1893)... 

National Institute of Standards and Technology (NIST). The NIST is the source of many of the standards used in chemical and physical analyses in the United States and throughout the world. The standards prepared and distributed by the NIST are used to caUbrate measurement systems and to provide a central basis for uniformity and accuracy of measurement. At present, over 1200 Standard Reference Materials (SRMs) are available and are described by the NIST (15). Included are many steels, nonferrous alloys, high purity metals, primary standards for use in volumetric analysis, microchemical standards, clinical laboratory standards, biological material certified for trace elements, environmental standards, trace element standards, ion-activity standards (for pH and ion-selective electrodes), freezing and melting point standards, colorimetry standards, optical standards, radioactivity standards, particle-size standards, and density standards. Certificates are issued with the standard reference materials showing values for the parameters that have been determined. 

Table 3 also lists the density p of these alloys, which was measured by the Achemedes principle using pure ethanol as immersion fluid. To monitor the accuracy of the measurements, the density of 99.999% pure Al shots was also measured and it was found to be within 0.4% from its accepted value. 

The outstanding resistance to corrosion exhibited by the high-silicon alloys is believed to be due to the development of a corrosion-resistant film containing a large proportion of silica. The full protective value of the film does not develop for most applications until at least 14-25% silicon is present in the alloy (Fig. 3.61). Increase in the content of silicon above 14-5% does not have a dramatic effect upon the corrosion resistance of the alloy (Fig. 3.61), although the further increase in film density is of service if the casting is to be exposed to solutions containing halide ions, especially hydrochloric acid. 

The above data relate to very pure iron samples with low dislocation densities. In real steels the trapping effects result in much lower apparent diffusivities, which are dependent on the metallurgical state of the steel, as well as its chemical composition. Typical values for the apparent diffusion coefficient of hydrogen in high-strength alloy steel at room temperature are in the region of 10" mVs. 

Because these variables have a very pronounced effect on the current density required to produce and also maintain passivity, it is necessary to know the exact operating conditions of the electrolyte before designing a system of anodic protection. In the paper and pulp industry a current of 4(KX) A was required for 3 min to passivate the steel surfaces after passivation with thiosulphates etc. in the black liquor the current was reduced to 2 7(X) A for 12 min and then only 600 A was necessary for the remainder of the process . From an economic aspect, it is normal, in the first instance, to consider anodically protecting a cheap metal or alloy, such as mild steel. If this is not satisfactory, the alloying of mild steel with a small percentage of a more passive metal, such as chromium, molybdenum or nickel, may decrease both the critical and passivation current densities to a sufficiently low value. It is fortunate that the effect of these alloying additions can be determined by laboratory experiments before application on an industrial scale is undertaken. 

For (AGh 0), these two regimes are approximately balanced, and a maximum in the exchange current density is predicted. This is the optimal value of the descriptor, and it immediately suggests that a reasonable goal for a computational, combinatorial electrocatalyst search is to find alloys with AGh values close to zero. 

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