Liquid Metal Attack - Environmental Degradation of Metals - page 434

addition of small amounts of zirconium 1 ( 1%) to the niobium. Zirconium

reacts with the oxygen dissolved in the metal to form ZrO 2 , thereby lowering the
chemical activity of oxygen in solid solution. The attack on tantalum is likewise
reduced by the addition of hafnium.
The corrosion or reaction products sometimes form protective layers on the
containment metal surface, thus reducing further attack. For example, in molten
lead of high oxygen activity steel develops such a protective layer. An addition
of aluminum or silicon to steel helps in forming surface-protecting corrosion
products in the melts of low oxygen activity. The addition of zirconium to liquid
bismuth or mercury has an inhibiting effect or the corrosion of steel in these
liquid metals. The nitrogen present in steel forms a surface layer of ZrN that is
thermodynamically a very stable compound and is an effective diffusion barrier.
Elemental transfer refers to the net transfer of interstitials or impurities to or
from a liquid metal. In such a case the liquid metal atoms do not react with the
atoms of the containment metal atoms. Carburization of refractory metals and of
austenitic stainless steels has been observed in liquid sodium contaminated with
carbon. Decarburization of iron-chromium-molybdenum steels, particularly
lower chromium steels, in liquid sodium or lithium is another example of element
transfer. When two solid metals are in contact with a liquid metal, elemental
transfer can lead to the intermetallic compound formation. For example, for alu-
minum and molybdenum exposed to molten bismuth, intermetallic compounds
Al 3 Mo, Al 5 Mo, and Al 12 Mo have been found on the molybdenum surface.
An alloying action can be observed between the atoms of the liquid metals
and the constituents of the material. It is advisable to avoid systems that form
alloys or stable intermetallic compounds, e.g., nickel in molten aluminum.

REFERENCES

1. W. Rostoker, J. M. McCaughey, and H. Markus, Embrittlement by Liquid Metals ,

Reinhold, New York, 1960.

2. D. W. Fager and H. F. Spurr, Corrosion , Vol. 24, (209, 1969); Corrosion , Vol. 27,

p. 72, 1971.

3. M. H. Kamdar, Liquid-Metal Embrittlement in Metals Handbook , 9th ed., Vol 11,

American Society for Metals, Metals Park 1986, pp. 225-238.

4. A. R. C. Westwood, C. M. Preece, and M. H. Kamdar, Am. Soc. Metals Trans.

Quart ., Vol. 60, p. 723, 1967.

5. N. S. Stoloff, R. G. Davies, and T. L. Johnston, Environment Sensitive Mechanical

Behavior , A. R. C. Westwood and N. S. Stoloff (eds.), Gordon and Beach, New
York, 1966.

6. V. I. Likhtman, P. A. Rebinder, and G. V. Karpenko, Physico-Chemical Mechanics

of Metals , Acad. Sci. USSR, Moscow, 1962.

7. W. M. Robertson, Trans. AIME , Vol. 236, p. 1478, 1966.