Grain liquid penetration along

If the major constituents of a solid alloy in contact with a liquid alloy are highly soluble in the latter without formation of compounds, progressive attack by solution is to be expected. If, on the other hand, a stable inter-metallic compound is formed, having a melting point above the temperature of reaction, a layer of this compound will form at the interface and reduce the rate of attack to a level controlled by diffusion processes in the solid state. By far the most serious attack, however, occurs in the presence of stresses, since in this case the liquid alloy, or a product of its reaction with the solid alloy, may penetrate along the grain boundaries, with resultant embrittlement and serious loss of strength. 

If the liquid phase is in direct contact with two grains of a solid, it will penetrate along the interface when i7ss> where y is interfacial tension, and indexes /, s designate liquid and solid respectively. This requirement is met in most current oxide and silicate systems. 

Such a penetration of the liquid phase along grain boundaries was observed in a number of polycrystalline systems, such as Zn-Ga, Cu-Bi, NaCl-H20. One has reason to assume that a number of geological processes, such as the transport of substances in the Earth s crust and the formation of ore and mineral deposits, are related to this phenomenon. 

When a polycrystal is immersed in a liquid which is in chemical equilibrium with the solid, the liquid can penetrate along the grain boundaries... 

He also showed that as 0 increases from zero the ability of the liquid phase to penetrate between the solid grains decreases, although up to 0 = 60° it should still be capable of penetrating along three-grain edges. When 0 > 60° it should occur as discrete inclusions at four-grain junctions. 

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). 

Mercuiy is the primary liquid metal that degrades aluminum. Liquid mercuiy does not wet an aluminum oxide surface, but if the natural oxide film in penetrated aluminum dissolves in the mercury to form an amalgam starting a very rapid reaction. The dissolved aluminum oxidizes immediately in the presence of moisture and more aluminum dissolves. This reaction is assisted by the presence of halides. The mercury penetration tends to proceed along grain boundaries, and if tensile stresses are present in the metal, drastic splitting and the exposure of further film free metal occurs. Mercury can plate out of aqueous solutions to produce this effect. A mercuiy content of greater than 0.01 ppm is cause for concern. Detection of even lesser amounts of mercury may indicate a problem, since mercury tends to evaporate and low levels are difficult to analyze. Common sources of mercuiy are broken thermometers and mercury vapor bulbs, or mercury manometers that have been over-pressurized. 

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