Liquid metals dissolution rate

Because forced-convection loops are costly to construct, it is now the usual practice to operate the loops as permanent testing facilities, with corrosion specimens cycled in and out of the facility. Test specimens of various materials are generally placed in the hot leg, and the effect of the flowing liquid on the specimens is determined from changes in weight, dimensions, composition, mechanical properties, and microstructure. Such an approach yields data on maximum corrosion rates as a function of temperature and liquid metal flow rate. Any attempt to elucidate corrosion mechanisms, however, is hampered by the inability to interrelate dissolution and deposition processes. 

Kassner used a rotating disc, for which the hydrodynamic conditions are well defined, to study the dissolution kinetics of Type 304 stainless steel in liquid Bi-Sn eutectic. He established a temperature and velocity dependence of the dissolution rate that was consistent with liquid diffusion control with a transition to reaction control at 860 C when the speed of the disc was increased. The rotating disc technique has also been used to investigate the corrosion stability of both alloy and stainless steels in molten iron sulphide and a copper/65% calcium melt at 1220 C . The dissolution rate of the steels tested was two orders of magnitude higher in the molten sulphide than in the metal melt. 

Different views exist as to the reasons for selective dissolution of the asperities. According to older concepts, convection of the liquid is hindered in the solution layers filling recesses hence, reaction products will accumulate there and raise the concentration and viscosity in these layers. Both factors tend to lower a metal s anodic dissolution rate relative to that at raised points. According to other concepts, a surface condition close to passive arises during electropolishing. In this case, the conditions for passivation of the metal at raised points differ from those in recesses. 

If a binary or multicomponent alloy is undergoing the liquid-metal attack, then its dissolution can be either selective or non-selective. In the former case, the more soluble component dissolves at a higher rate. Hence, the solid phase becomes depleted, while the liquid enriched in this... 

In contrast to the solubility, the values of the dissolution-rate constants of different metals and alloys in liquid aluminium are very close. At least, they are of the same order of magnitude, namely 10 5 m s1, although the solubility values may differ by two orders of magnitude or more (see Table 5.2). This is also typical of dissolution of other solid substances in liquids.299 301... 

Table 5.8. Values of the dissolution-rate constant, k (xlO 5 m s ), of transition metals in liquid aluminium at an angular speed of the disc rotation of 25 rad s 1 303 The mean relative error of their determination is around 5 %...

This subchapter has shown that metal dissolution processes are important to numerous aspects of metal plating, however, very few concerted studies have been made in this area. An understanding of dissolution rates and processes, together with information on the stability of oxide films in ionic liquids, is essential for the development of successful metal finishing processes. 

The application of air-electrolyte mixtures as a working medium for ECM enables one to raise the localization of metal dissolution in places with the smallest gaps and, thus, to enhance the accuracy of electrochemical reproduction of TE on the WP. To achieve the highest efficiency of this method, several conditions should be fulfilled. The air-electrolyte mixture should be formed in the immediate vicinity of the gap inlet and the flow rate should be adequately high. A ratio between gas and liquid amounts from 3.0 to 3.5 is considered to be most preferable. Rigid stabilization of all the process parameters is required. The design and size of all parts of the mixing device and the values of inlet and outlet pressures are important. To avoid a considerable decline in productivity, the main metal stock should be removed in the air-free... 

The wear rate on liquid lubrication by electrolytes is defined only by metal dissolution and intensified friction, and can be varied in response to the corrosion rate under static conditions to a value that characterizes the mechanical failure of the metal. 

If the release of ions (dissolution) is the only reaction, and the metal is not connected to an external circuit, a surplus of electrons is produced in the metal. It becomes more negative, and the dissolution rate is reduced. At the same time the concentration of positive metal ions in the liquid increases, causing an increasing... 

Microstreaming, shock waves, and liquid microjets in the vicinity of solid surfaces lead to very efficient cleaning. This effect has been used in industry for more than forty years. Insoluble layers of inorganic salts, polymers, or liquids can be removed by the ultrasonic cleaning effect. In heterogeneous systems such a clean reactive surface leads to improved dissolution rates of metals in acids and enhanced reaction rates. Chemical reactions giving insoluble products are freed from these mass-transport-limiting layers and react rapidly. 

The simplest corrosion reaction that can occur in a liquid metal environment is direct dissolution the release of atoms from the containment (or test coupons) into the liquid metal, driven solely by solubility relationships, and independent of impurity elements. The solution reaction is governed by the rate controlling step in the dissolution reaction. The net rate, J, at which an elemental species can enter solution may be described by the expression... 

Under dissolution-controlled conditions, corrosion by liquid metals should increase with increasing temp)erature. For example, assuming all other factors affecting corrosion are fixed, the corrosion rate-temperature relationship can be expected to follow the classical Arrhenius expression, log k exp(-0/RT), where k is corrosion rate, Q is the activation energy, R is the gas constant, and T is the absolute temperature. This is shown graphically in Fig. 4 for type 316 stainless steel in sodium. In this case, the corrosion rate can be related directly to mass loss, which can be expressed m terms of wall thinning. 

In systems where a liquid metal is used as the working fluid (e.g., the Rankine-cycle), liquid is converted to vapor in one part of the system and vapor to liquid in another part. The distillation effects of the vaporization process result in extremely pure condensing vapor that may be able to dissolve and transport container material. As opposed to an aU-liquid system, where the liquid is always partially saturated with container material constituents, dissolution in the condenser region can continue undiminished the dissolution rate will depend on the condensation rate and temperature. In contrast, hquid in the evaporator section will ultimately become supersaturated with respect to container material constituents, so that the heated sections of a liquid metal boiUng system will be subject to deposition rather than corrosion [S/]. 

As noted above, the mass transfer kinetics of temperature gradient loops are usually described with reference to dissolution in the hot leg. It is possible to quantitatively study the dissolution step using the rotating cylinder technique. Unlike loop studies, this technique allows one to study dissolution in a system where the hydrodynamic conditions are fully defined. Experimentally, solid cylinders of the test material are rotated at various speeds in an isothermal liquid-metal bath. Changes in the concentration of solid in the liquid and changes in the cylinder radius are determined as a function of time. With these data it is possible to determine the mass transfer coefficient and the rate-controlling step for dissolution. 

As already mentioned in previous sections, corrosion by liquid metals can proceed by various ways dissolution, oxidation, carburization, formation of intermetallic compounds, etc. The nature of the corrosion process will firstly depend on the thermodynamic data of the liquid metal/sohd material system considered. These thermodynamic data give the equilibrium state of the system and thus the products susceptible to form. Of course, kinetics data are also essential to determine whether the products predicted by thermodynamics will indeed form and in that case at what rate they are going to form. These two aspects will be treated in the following paragraphs. 

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