Liquid metal attack

Immediately before the experiment, the solid specimen was rinsed with ethanol and dried. Then, it was pressed into a high-purity graphite tube, 16 mm diameter, to protect its lateral surface from the liquid-metal attack. Therefore, only the disc surface, 1 cm2 area, dissolved in liquid aluminium during the run. If a larger surface area was desirable (for example, when determining the solubility values), while its instantaneous value was unimportant, the specimens with unprotected lateral surfaces were used. In such cases, graphite tubes served only as holders for the transition-metal specimens. 

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 addition to hydrogen embrittlement, brittle failure can occur as a result of stress corrosion, liquid metal attack, or strain-age hardening. The last mentioned cause is well known and can occur on strained steel of any strength but seldom actually occurs in modem steels it is adequately documented in Appendix E of BS 729 (British Standards Institution, 1971 reaffirmed in 1986) users often erroneously refer to this effect as hydrogen embrittlement. This clearly states that strain-age embrittlement is the only type of embrittlement that can be aggravated by the hot dip galvanizing process. 

Liquid metal attack of steel is again an entirely separate phenomenon. The matter is adequately summarized in the latest draft revision of ISO 1461 (1994) document as follows ... 

Chemical Properties. Molybdenum has good resistance to chemical attack by mineral acids, provided that oxidizing agents ate not present. The metal also offers excellent resistance to attack by several liquid metals. The approximate temperature limits for molybdenum to be considered for long-time service while in contact with various metals in the hquid state ate as follows ... 

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). 

A special form of crevice attack can occur at a waterline or at the edges of water droplets. At the water surface, a meniscus region is present where surface tension causes water to climb up the metal surface it contacts. In effect, a crevice is formed between the air-liquid and liquid-metal interface at the meniscus. Oxygen concentration is high at the meniscus due to the greater accessibility of this region to the air. The meniscus region becomes cathodic to the adjacent metal surface. Corrosion occurs just below the meniscus, and chloride, if present, is... 

Non-metallic impurities in liquid alkali metals play a major role in the corrosion of materials either by affecting metal solubilities, f orming spalli-ble corrosion products on the metal surface, promoting liquid metal embrittlement or bulk embrittlement of the surface or by sensitising the structure for further attack by other impurities e.g. O2. As in other corrosive environments the direction and magnitude of these impurity reactions... 

There are two cases in which a metal can be attacked by a salt melt if it is soluble in the melt, or if it is oxidised to metal ions. In the first case, attack occurs by direct dissolution without oxidation of the metal and the mechanism is likely to be closely similar to attack by liquid metals (Section 2.9). If the solubility is appreciable, excessive corrosion can be expected, but with few exceptions metals appear to be appreciably soluble only in their own salts. Most of the metals of the first and second groups of the periodic... 

In its resistance to liquid metals, titanium shows variable behaviour, the rate of attack often depending upon temperature and increasing with rise in temperature. By thickening the surface film of oxide, resistance to attack is enhanced, and, for example, repeated repair of the surface film renders titanium resistant, on a limited-time basis, to molten zinc in galvanising baths. A surface-oxide thickening technique also enables titanium to be employed in contact with molten aluminium. Titanium equipment is also used in applications involving lead-tin solders, and it is resistant to mercury, at least up to 150 C. 

Nickel-manganese-palladium brazes are resistant to attack by molten alkali metals and And applications in sodium-cooled turbine constructions. Their freedom from silver and other elements of high thermal neutron-capture cross-section allows them to be used in liquid-metal-cooled nuclear reactors. 

Nickel and nickel-rich alloys must be considered as having generally poor resistance to molten metals. Eldred has made a systematic investigation of the attack of liquid metals on solid metals and alloys, and his results for nickel, and nickel-chromium and nickel-copper alloys, are summarised in Table 7.35. These are for tests at up to 500 C and apart from potassium and sodium all the low-melting-point metals investigated produced moderate to severe attack on the nickel-rich materials. Furthermore, the values for many of the combinations given in the table indicate a marked tendency to preferential intergranular attack. 

Table 7.35 Attack of liquid metals on nickel and nickel-rich alloys ...

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... 

Snell et al, "Colorimetric Methods of Analysis," vol IIA, Van Nostrand(1959), 156-87 Aluminum Alkyls were prepd in 1865 hy the action of aluminum on mercury alkyls(Refs 1 6)(see also Note below). Later they were made by the action of "electron metal (alloy of Al and Mg) on a sola of the alkyl halide in ether (Refs 2 6). The Al trialkyU are volatile liquids, violently attacked by air or water. Following are examples trimethylaluminum A1(CHS), d 0.752 at 20°/4°, mp 15.0°, bp 126. la, tri-ethyl-, Al(CaH5) d 0.837 at 20°/4°, mp -52.5°, bp 185.6a, tri-n-propyl-, Al(n-CjH,),. d 0.823 at 20°/4°, mp -107°, bp ca 250° (Refs 3,4,5 6). These three compds are inflammable in air and for this reason may be of interest as components of liquid propellants for rockets Note The prepn of a compd called "Aethyl-aluminium" was claimed by W.Hallwachs A.Schaffarik, Ann 109, 207(1859) hut it was not properly identified and its props (except that it is violently decomp by water) were not detd... 

Snell et al, "Colorimetric Methods of Analysis," vol II A, Van Nostrand(1959), 156-87 Aluminum Alkyls were prepd in 1865 by tbe action of aluminum on mercury alkyls(Refs 1 6)(see also Note below). Later they were made by the action of "electron metal (alloy of Al and Mg) on a soln of the alkyl halide in ether (Refs 2 6). The Al trialkyls are volatile liquids, violently attacked by air or water. 

Vanadium is resistant to attack by hydrochloric or dilute sulfuric acid and to alkali solutions. It is also quite resistant to corrosion by seawater but is reactive toward nitric, hydrofluoric, or concentrated sulfuric acids. Galvanic corrosion tests mn in simulated seawater indicate that vanadium is anodic with respect to stainless steel and copper but cathodic to aluminum and magnesium. Vanadium exhibits corrosion resistance to liquid metals, eg, bismuth and low oxygen sodium. 

It is available in steel cylinders, with purity approximately 99.5% it can be purified further by distillation. Although liquid HF attacks glass rapidly, it can be handled conveniently in apparatus constructed either of copper or Monel metal or of materials such as polytetrafluoroethylene (Teflon or PTFE) and Kel-F (a chlorofluoro polymer). 

Thallous Salts. — The oxide, TljO, is a heavy black powder, which is formed when the metal is oxidized at low temperatures, or when Tl(OH) is heated at 100°. It absorbs water readily, producing the hydroxide, and at 300° it melts to a liquid which attacks glass rapidly. It is easily reduced or oxidized. 

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