Corrosion liquid metal

Liquid-Metal Corrosion Liquid metals can also cause corrosion failures. The most damaging are liqmd metals which penetrate the metal along grain boundaries to cause catastrophic failure. Examples include mercury attack on aluminum alloys and attack of stainless steels by molten zinc or aluminum. A fairly common problem occurs when galvanized-structural-steel attachments are welded to stainless piping or eqmpment. In such cases it is mandatoty to remove the galvanizing completely from the area which will be heated above 260°C (500°F). 

Testing procedures for liquid-metal corrosion are given in Chapter 19. 

Mass-transfer deposits can lead to blockages in non-isothermal circulating systems, cis in the case of liquid-metal corrosion. In fused salts, the effect can be reduced by keeping contamination of the melt by metal ions to a minimum e.g. by eliminating oxidising impurities or by maintaining reducing conditions over the melt . 

The principal variables affecting liquid-metal corrosion are ... 

Most interest has centred on solutions of oxygen in liquid sodium since this element, more than any other, renders the liquid metal corrosive. 

By now, comprehensive studies have been performed and reliable industrial experience has been gained on material corrosion in sodium. Corrosion intensity in sodium is significantly lower than that in water or lead-based coolants [5.9], In sodium, as well as in the other liquid metals, corrosion rate depends on many factors (temperature level, coolant velocity, impurity content, temperature difference, time, etc.). When evaluating corrosion rate, the major part of researchers took into account only the most contributing factors. Empirical equations for corrosion rate were most commonly derived for 316 steel at the coolant velocity of > 4 m/s and oxygen content of < 10 ppm. The most reliable results were obtained in [5.10, 5.11] for corrosion rate K, mg/cm h, that can be expressed as follows ... 

Liquid metal corrosion is of considerable interest in nuclear technology as reactor coolants and, in the case of uranium, dissolved in liquid bismuth as reactor fuel. Next to neutron capture cross section and melting point, corrosiveness or containability is perhaps the most important criterion limiting the use of liquid metals for such applications. Alloys of tin, zinc, aluminum, and magnesium, for example, are ruled out primarily by their corrosiveness. A major incentive for the use of liquid metal coolants instead of water is the achievement of higher reactor temperatures. Corrosion usually sets the maximum temperature limit in such reactors. 

In contrast to aqueous corrosion typically involving loss of electrons from the dissolving metal, liquid metal corrosion is generally considered to proceed by simple solution mechanisms. The principal variables affecting corrosion in a liquid metal system are temperature or temperature range or cycling, elements present, area-to-volume ratio, purity, flow velocity, surface condition, and microstructure. In reactor applications, the neutron flux may be an additional factor. In combination, these variables produce enough complexity so that in the present state of the art, it is rarely possible to make confident predictions about the performance of a previously untried systan. Empirical tests are usually required. 

Liu] Liu, X., Barbero, E., Xu, J., Burris, M., Chang, K.-M., Sikka, V., Liquid Metal Corrosion of 316L, Fe Fe3Al, and FeCrSi in Molten Zn-Al Baths , Metall. Mater. Trans. A, 36A(8), 2049-2058 (2005) (Experimental, Interface Phenomena, Phase Diagram, Phase Relations, 36)... 

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