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Liquid metal embrittlement and common embrittling couples


Liquid Metal Embrittlement (LME) is the loss of ductility in normally ductile metals when stressed under contact with liquid metal.

The general rules for the possible occurrence of liquid metal embrittlement are:

  1. Low mutual solubility between the liquid and solid metals.
  2. Absence of intermetallic compound formation between the solid-liquid couple.

Some significant examples of embrittling couples include: steel-Cu, stainless steel - Zn, aluminium-Hg.

Steels are embrittled by zinc at temperatures above 400°C. When stresses are present, embrittlement is manifested by extremely fast rates of crack propagation (at temperatures above 750°C). The disastrous explosion at Flixborough UK plant in 1975 killed 28 people and it was attributed to liquid metal embrittlement of a stainless steel pipe in contact with molten zinc.

Mercury embrittles mild steels in stressed or unstressed conditions at low temperatures in the presence of local stress raisers. Additions of solutes such as Ni, Si, Al, Co increase the susceptibility to mercury embrittlement. Note that laboratory studies have been reported to demonstrate such issues, but, this has not been a practical concern in industrial application.

Nickel alloys can also suffer from liquid metal embrittlement by zinc or mercury, but to a lesser degree than carbon and stainless steels (18Cr-8Ni type). The severity of liquid metal embrittlement depends on the magnitude of local stresses and the prevailing temperature. A relative ranking order from the most to least susceptible nickel alloys to zinc or mercury LME is: Monel 400, Monel K500, Alloy 625, Monel R405, Alloy X750, Alloy 718, Alloy 600, pure nickel, Alloy 825, Alloy 800.

The presence of zinc or mercury on mating surfaces in the above materials results in welding defects which would not have formed in a clean weld are more susceptible to cracking and therefore the mating surfaces should be free from any zinc contamination prior to welding. Carpenter 20Cb-3 has similar susceptibility to zinc LME as 18Cr-8Ni type stainless steels.

Liquid metal embrittlement by copper (also known as 'copper contamination cracking') has also been observed adjacent to welds in both cobalt and iron alloys. Austenitic stainless steels are particularly susceptible to this type of cracking but ferritic stainless steels and nickel alloys are relatively immune.

Embrittlement of aluminium by mercury is particularly important in LNG plants since many natural gas feeds contain some mercury.

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