Liquid-metal corrosion temperature effect

Section V on Testing in Environments (H. Hack, Section Editor) includes chapters on outdoor and indoor atmospheres, seawater, fresh water, soils, concrete, industrial waters, industrial chemical, petroleum, high-temperature gases, organic liquids, molten salts, liquid metals, corrosion inhibitors, in-vivo, and microbiological effects. Each chapter provides a descriptive overview of the environment and factors and variables affecting corrosion rates and mechanisms. 

The time-dependence of void formation in Inconel, as observed both in thermal-convection and forced-circulation systems, indicates that the attack is initially quite rapid but that, it then decreases until a straight-line relationship exists between depth of void formation and time. This effect can 1)0 explained in terms of the corrosion reactions discussed above. The initial rapid attack found for both types of loops stems from the reaction of cliromium with impurities in the molt [reactions (13-1) and (13-2)] and with the FF4 constituent of the salt [reaction (13-3)] to establish a quasi-etiuilibrium amount of CrF2 in the salt. At this point attack proceeds linearly with time and occurs by a mass-transfer mechanism which, although it arises from a different cause, is similar to the phenomenon of temperature-gradient mass transfer observed in liquid metal corrosion. 

Whenever corrosion resistance results from the formation of layers of insoluble corrosion products on the metallic surface, the effect of high velocity may be to prevent their normal formation, to remove them after they have been formed, and/or to preclude their reformation. All metals that are protected by a film are sensitive to what is referred to as its critical velocity i.e., the velocity at which those conditions occur is referred to as the critical velocity of that chemistry/temperature/veloc-ity environmental corrosion mechanism. When the critical velocity of that specific system is exceeded, that effect allows corrosion to proceed unhindered. This occurs frequently in small-diameter tubes or pipes through which corrosive liquids may be circulated at high velocities (e.g., condenser and evaporator tubes), in the vicinity of bends in pipelines, and on propellers, agitators, and centrifugal pumps. Similar effects are associated with cavitation and mechanical erosion. 

The results of studies of copper surfaces by low-temperature adsorption isotherms may be summarized as follows. True surface areas of metallic specimens as small as 10 sq. cm. can be derived with a precision of 6% from low-temperature adsorption isotherms using vacuum microbalance techniques. This method is of special value in determining the average thickness of corrosion films formed by the reaction of gases or liquids with solids. The effect of progressive oxidation of a rough polycrystalline metal surface is to decrease the surface area to a point where the roughness factor approaches unity. 

Useful atomic and subatomic scale information on hydroxylated oxide surfaces and their interaction with aggressive ions (e.g., Cl ) can be provided by theoretical chemistry, whose application to corrosion-related issues has been developed in the context of the metal/liquid interfaces [34 9]. The application of ah initio density functional theory (DFT) and other atomistic methods to the problem of passivity breakdown is, however, limited by the complexity of the systems that must include three phases, metal(alloy)/oxide/electrolyte, then-interfaces, electric field, and temperature effects for a realistic description. Besides, the description of the oxide layer must take into account its orientation, the presence of surface defects and bulk point defects, and that of nanostructural defects that are key actors for the reactivity. Nevertheless, these methods can be applied to test mechanistic hypotheses. 

Temperature differences in a loop stem provide the driving force for mass transport of corrosion products. Although no definitive experiments have been conducted, indications are that the magnitude of the AT in sodium has no effect above a minimum value of about 100 C [691 From a consideration of heat transfer properties, in any heat transfer system at a given flow velocity, different liquid metals would require different ATs to dehver the same heat load. If the surface area and geometry are fixed by heat flux considerations, the AT and/or flow velocity, and hence mass transfer fluxes, will change for liquid metals with different heat transfer characteristics. 

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/]. 

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. 

As a general rule, assuming impurity and dissimilar metal effects are controlled and not overriding, the lower the nickel content of an alloy, the better is its corrosion resistance in liquid metals. The materials shown in Table 1 have proven to be corrosion-resistant to the specified liquid metals up to the temperature limit indicated. For additional information on materials compatibility, the reader is referred to the Liquid Metals Handbook [92,93]. 

For the gaseous-layer effects, such as entrainment and detrainment of species across the liquid interface, chemical transformations in the gas phase, the effects of solar radiation on photosensitive atmospheric reactions, and temperature effects on the gas phase, reaction kinetics are important. In the interface regime, the transfer of molecules into the liquid layer prior to their chemical interaction in the liquid layer is studied. Not only does the liquid regime receive species from the gas phase, but species from the liquid are also volatilized into the gas phase. Important variables in the liquid regime include the aqueous film thickness and its effect on the concentration of species, chemical transformations in the liquid, and reactions involving metal ions originating from the electrochemical corrosion reactions. 

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