Facts About LME

LME – No, we’re not talking about the London Metal Exchange! 

LME in this case stands for Liquid Metal Embrittlement.

Molten metal can cause a variety of problems in a range of metallic systems. One of the more notable is the stress corrosion cracking of aluminum alloys by mercury. Molten aluminum, zinc and copper will corrode steels (including stainless) and nickel alloys. There are several different mechanisms by which corrosion can occur. General corrosion and intergranular embrittlement or cracking are two of the more common associated with stainless steel.

The solubility of iron in molten zinc (419° C melting point) is high and will result in significant general corrosion. Welding galvanized pipe without removing the coating can result in significant cracking or porosity in the weld in addition to generating noxious fumes. Molten zinc on austenitic (non-magnetic) stainless steels is likely to cause severe general corrosion through intergranular attack. However, 316L has provided useful service life in some galvanizing line components. The 6Mo superaustenitic alloy AL-6XN© has shown better resistance than 316L. Haynes alloy 556 has also exhibited useful resistance in this environment. The use of galvanized pipe is restricted in many chemical process plants where the risk of fire could cause molten zinc to contact low alloy steel and stainless equipment.

Figure 1

The solubility of iron and nickel in molten aluminum (660° C) is very high and will result in general corrosion and intergranular embrittlement in austenitic stainless and nickel alloys. Molten copper (1085° C) is also corrosive to steel and will selectively attack the austenitic grain boundaries and penetrate quickly into the base metal. Figure 1 illustrates this mechanism. Small amounts of copper or aluminum contamination can lead to significant cracking when stainless or nickel alloys are subjected to high temperature service. We are occasionally asked by aluminum and copper foundries for suggestions on materials for ladles, structurals, instrumentation, etc. The reality is that low alloy steels are usually the most economical choice. Their general corrosion rate is only slightly worse than a stainless steel and they do not suffer from the intergranular embrittlement that attacks a non-magnetic stainless steel. Ferritic stainless grades (430, 446) can offer better general corrosion resistance, but low alloy steels also have the advantage of lower cost. Duplex (austenitic/ferritic) alloy 2205 did outperform austenitic alloy 309 as a skimmer (Figure 2) for a bath of molten brass.

Figure 2

Bismuth (271° C), lead (327° C), cadmium (321° C), and magnesium (651° C) are other low melting point materials that can cause general or intergranular corrosion in stainless and nickel alloys. These elements are generally less aggressive than either copper or aluminum. Although, our Rolled Alloys Metallurgical Services (RAMS) group was called in to review a failure of Alloy 600 in a rechargeable battery metal reclamation process. The empirical evidence suggests that the Alloy 600 water cooled head suffered from stress corrosion cracking when vaporized cadmium condensed on the surface.

Stainless and nickel alloy producing mills and suppliers have practices in place to prevent low melting point metal contamination of products. The use of mercury containing instruments has all but vanished. Fabricators of high temperature equipment also need to be aware of the potential for contamination from aluminum or copper tooling. Equipment used to fabricate aluminum or copper alloys should never be used to fabricate stainless or nickel alloys that will see high temperature service.