Liquid metals corrosion layer

For low fluid velocities, the partial reactions of dissolution and precipitation at the solid—liquid interface are sufficiently fast and the global dissolution reaction is thus at equilibrium. The slowest phenomenon controls the global corrosion rate the diffusion of dissolved elements in the liquid metal boundary layer. It is the mass transfer control process. In that case, increasing the fluid velocity leads to an increase in the corrosion rate. 

Chemical reaction This involves the formation of distinct compounds by reaction between the solid metal and the fused metal or salt. If such compounds form an adherent, continuous layer at the interface they tend to inhibit continuation of the reaction. If, however, they are non-adherent or soluble in the molten phase, no protection will be offered. In some instances, the compounds form in the matrix of the alloy, for example as grain-boundary intermetallic compound, and result in harmful liquid metal embrittlement (LME) although no corrosion loss can be observed. 

Static test results may be evaluated by measurement of change of mass or section thickness, but metallographic and X-ray examination to determine the nature and extent of attack are of greater value because difficulty can be encountered in removing adherent layers of solidified corrodent from the surface of the specimen on completion of the exposure, particularly where irregular attack has occurred. Changes in the corrodent, ascertained by chemical analysis, are often of considerable value also. In view of the low solubility of many construction materials in liquid metals and salts, changes in mass or section thickness should be evaluated cautiously. A limited volume of liquid metal could become saturated early in the test and the reaction would thus be stifled when only a small corrosion loss... 

Surface coating with another metal with better resistance against corrosion. The metal layer can be plated. Zinc plating is the most important example. But other methods are available to prepare a metal layer, e.g., mechanical plating, by dipping the parts into a liquid metal bath or spraying the metal. 

As with metals the corrosion of ceramics can take place by one or a combination of mechanisms. In general, a corrodent will attack a ceramic and form a corrosion product. Whether the reaction product is a gas or a solid will determine if the product remains on the surface or is fugitive. Reaction products may be gas, liquid, solid, or any combination thereof. If the reaction product formed is a solid it may form a protective layer against additional corrosion. When the reaction product is a combination of a solid and a liquid, the reaction layer may be removed. 

Alloying refers to the formation of reaction products on the containment material, when atoms other than impurities or interstitials of the liquid metal and containment material react. This effect can sometimes be used to produce a corrosion-resistant layer, separating the liquid metal from the containment (for example, aluminum added to molten lithium contained by steel). Lastly, liquid metal can attack ceramics by reduction reactions. Removal of the nonmetallic element from such solids by the melt will clearly destroy their structural integrity. Molten lithium poses a high risk for reducing ceramic materials (oxides). 

Corrosion is, as already explained in Section 5.3.2, the material damage caused by chemical or electrochemical reaction with a surrounding medium. The corrosive media can be a gas or a liquid. Electrochemical corrosion is common in metal alloys and metal compounds in which liquid media play a role. Here, an electrolyte acts on a metal and usually produces a top layer. In the case of water on steel, this top layer is rust. 

When the fluid velocity increases, diffusion in the boundary layer increases, it no longer controls the corrosion rate which becomes controlled by the interface reaction rate between the solid material and the liquid metal. It is the activation-controlled process. The corrosion rate no longer depends on the fluid velocity. 

As discussed previously, the mitigation of stmctural materials corrosion and its mechanical properties degradation can be done through the control of the oxygen potential in the liquid metal, resulting in the growth of a native oxide layer on the steel... 

Schematic of the three situations that are used for the assessment of metal chloride evaporation, (a) Vapor pressure of the gaseous metal chloride above solid or liquid metal chloride in a closed system (value used in Equation 13.23 and for determining T ). (b) Equilibrium partial pressure of pMe Cl in a closed system containing O2 and CI2 as gas phase and Me, Me Oj, and Me,.Cly as solid or liquid phases (value used for the establishment of the "static" quasi-stability diagram), (c) Mej.Cly transport rate in the gas boundary layer (metal consumption rate) as criterion for the amount of chlorine-induced corrosion in an open system with gas flow across the surface (value used for the establishment of the "dynamic" quasi-stability diagram).

Whenever corrosion resistance results from the accumulation of layers of insoluble corrosion products on the metallic surface, the effect of high velocity may be either to prevent their normal formation or to remove them after they have been formed. Either effect allows corrosion to proceed unhindered. This occurs frequently in smaU-diameter tubes or pipes through which corrosive liquids may be circulated at high velocities (e.g., condenser and evaporator tubes), in the vicinity of oends in pipe hnes, and on propellers, agitators, and cen-trifiig pumps. Similar effects are associated with cavitation and impingement corrosion. 

Sodium and potassium are restricted because they react with sulfur at elevated temperatures to corrode metals by hot corrosion or sulfurization. The hot-corrision mechanism is not fully understood however, it can be discussed in general terms. It is believed that the deposition of alkali sulfates (Na2S04) on the blade reduces the protective oxide layer. Corrosion results from the continual forming and removing of the oxide layer. Also, oxidation of the blades occurs when liquid vanadium is deposited on the blade. Fortunately, lead is not encountered very often. Its presence is primarily from contamination by leaded fuel or as a result of some refinery practice. Presently, there is no fuel treatment to counteract the presence of lead. 

The recovery of petroleum from sandstone and the release of kerogen from oil shale and tar sands both depend strongly on the microstmcture and surface properties of these porous media. The interfacial properties of complex liquid agents—mixtures of polymers and surfactants—are critical to viscosity control in tertiary oil recovery and to the comminution of minerals and coal. The corrosion and wear of mechanical parts are influenced by the composition and stmcture of metal surfaces, as well as by the interaction of lubricants with these surfaces. Microstmcture and surface properties are vitally important to both the performance of electrodes in electrochemical processes and the effectiveness of catalysts. Advances in synthetic chemistry are opening the door to the design of zeolites and layered compounds with tightly specified properties to provide the desired catalytic activity and separation selectivity. 

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