Tarnishing rate

In contrast to the case for copper, for silver a reduction of the tarnish rate by circa 100-fold has been observed as tarnish layers increase (Graedel et al., 1985), thus indicating that to a degree, the tarnish layers themselves impede further tarnishing. It is known that major silver tarnish can occur at concentrations of circa 100pptv and it has been shown that in effect it is the sum of OCS and H2S along with artifact reactivity, which determines the tarnish rate (Ankersmit, Tennent and Watts, 2005). 

The constant Ic can be calculated from the flux equations (5-13), with the condition of electroneutrality being used to eliminate the diffusion potential < >. The calculation is performed just as in the derivation of the rational rate constant for spinel formation in section 6.2.1. According to eq. (6-22), Tc kv, where n is the increase in volume of the product layer following the passage of one ionic equivalent, k is the rational tarnishing rate constant as introduced by Wagner [12]. It is equal to the flux in equivalents per unit area per unit time for a unit product layer thickness. By the method outlined above, k may be calculated as ... 

The Ag" ions will then be removed through the Agl electrolyte in order to maintain the sulphur activity at its potentiostatically fixed value. At the same time, a corresponding electric current flows through the external circuit. This current can easily be measured. In this way, a very precise determination of the tarnishing rate can be made (1 mA 1 sec corresponds to 1/2 10 moles ). The situation is schematically illustrated in Fig. 9-7. 

Fig. 9-7. Schematic diagram of a solid state galvanic cell to measure the tarnishing rate constant for NiS.

In addition to single-phase samples, electrochemical methods may also be employed to investigate reactions of mixed conducting materials with other phases. If the reaction rates are determined by bulk diffusion, the parabolic tarnishing rate constant k, results from a comparison of the parabolic tarnishing rate law and Tick s first law. By integration of this... 

The integral may be calculated from the stoiehiometry dependence of the chemical diffusion coefficient, y is the average stoichiometric number of the compound AyB. " Equation (9.36) allows the determination of the tarnishing rate constant as a function of all combinations of activities of the components at both sides of the sample. 

The kinetics of reactions in which a new phase is formed may be complicated by the interference of that phase with the ease of access of the reactants to each other. This is the situation in corrosion and tarnishing reactions. Thus in the corrosion of a metal by oxygen the increasingly thick coating of oxide that builds up may offer more and more impedance to the reaction. Typical rate expressions are the logarithmic law,... 

The corrosion behavior of plutonium metal has been summarized (60,61). a-Plutonium oxidizes very slowly in dry air, typically <10 mm/yr. The rate is accelerated by water vapor. Thus, a bright metal surface tarnishes rapidly in normal environments and a powdery surface soon forms. Eventually green PUO2 [12059-95-9] covers the surface. Plutonium is similar to uranium with respect to corrosion characteristics. The stabilization of 5-Pu confers substantial corrosion resistance to Pu in the same way that stabilization of y-U yields a more corrosion-resistant metal. The reaction of Pu metal with Hquid water produces both oxides and oxide-hydrides (62). The reaction with water vapor above 100°C also produces oxides and hydride (63). 

The most common form of corrosion is uniform corrosion, in which the entire metal surface degrades at a near uniform rate (1 3). Often the surface is covered by the corrosion products. The msting of iron (qv) in a humid atmosphere or the tarnishing of copper (qv) or silver alloys in sulfur-containing environments are examples (see also SiLVERAND SILVER ALLOYS). High temperature, or dry, oxidation, is also usually uniform in character. Uniform corrosion, the most visible form of corrosion, is the least insidious because the weight lost by metal dissolution can be monitored and predicted. 

The terms hot corrosion or dry corrosion are normally taken to apply to the reactions taking place between metals and gases at temperatures above 100 C i.e. temperatures at which the presence of liquid water is unusual. The obvious cases of wet corrosion at temperatures above 100 C, i.e. in pressurised boilers or autoclaves, are not considered here. In practice, of course, common metals and alloys used at temperatures above normal do not suffer appreciable attack in the atmosphere until the temperature is considerably above 100 C. Thus iron and low-alloy steels form only the thinnest of interference oxide films at about 200 C, copper shows the first evidence of tarnishing at about 180 C, and while aluminium forms a thin oxide film at room temperature, the rate of growth is extremely slow even near the melting point. 

Apart from the question of scale, the nature of the clean surface has a pronounced influence on the rate of boiling. Thus Bonilla and PERRY(79) boiled ethanol at atmospheric pressure and a temperature difference of 23 deg K, and found that the heat flux at atmospheric pressure was 850 kW/m2 for polished copper, 450 for gold plate, and 370 for fresh chromium plate, and only 140 for old chromium plate. This wide fluctuation means that caTe must be taken in anticipating the heat flux, since the high values that may be obtained initially may not persist in practice because of tarnishing of the surface. 

Electrode reactions are analogous to the growth of tarnishing (corrosion) layers (Weppner and Huggins, 1977). Assuming that bulk transport is the rate determining step, the growth rate of the reaction product is inversely proportional to the instantaneous thickness L... 

We conclude that the (practical) rate constant k for -type tarnishing layers is essentially independent of the applied oxygen pressure in the ambient atmosphere. 

Tarnishing reactions. A film of tarnish is formed on the surface of a metal by diffusion of dissolved gas A through the film from an exterior gaseous phase. The reaction rate of gas with metal is assumed to be large,... 

There are two main effects of temperature the first is that temperature affects and effects the rate of any chemical reaction, so the higher the temperature, the higher the rate of reaction (e.g., a tarnish or damage reaction) the second is that temperature directly affects the RH because air can hold different amounts of water vapor at different temperatures. Very much a prerequisite of reliable RH control is good temperature control, ft is also often the case that historically, guideline values in many museums have tended to be predicated on human comfort. 

Frequently these systems are described as chrome plating but it is nickel rather than chromium that gives the primary resistance to attack. When it was introduced in the twenties the original purpose of the layer of chromium was to prevent tarnishing of the nickel, but it has been found since that the physical nature of this layer can influence significantly the rate at which the underlying nickel deteriorates. 

From available evidence, the British oligarchy rates the Jacobs group s political usefulness as an intermediaiy for control over organized crime sufficiently important to make major efforts to rebuild the Jacobs tarnished image. The prize that motivates Britain in this regard is the Buffalo-based Marine... 

FOLLOWING A SHORT introduction dealing with the relationship between diffusion process and field transport phenomena in tarnishing layers on metals and alloys, the mechanism of oxidation of iron is discussed. Epitaxy plays an important role on the gradient of the concentration of lattice defects and, therefore, on the validity of the parabolic rate law. Classical examples of metal oxidation with a parabolic rate law are presented and the various reasons for the deviation observed are elucidated on the systems Iron in CO/CO2 and CU2O in <>2. In addition, the oxidation of alloys with interrupted oxide-metal interfaces is treated. Finally, attention is focussed on the difficulties in explaining the low temperature-oxidation mechanism. 

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