#EpicFail – Liquid Metal Embrittlement

Liquid metal embrittlement (LME) can lead to catastrophic failure of various products across a wide range of industries. The phenomenon occurs when molten metals come into contact with susceptible materials. Crack growth rates can be incredibly rapid, and failure can occur within seconds of exposure. The fast crack propagation associated with LME can have serious consequences. LME played a role in the tragic explosion at the Flixborough plant in the UK in 1974 whereby the flame produced by the ignition of cyclohexane led to the zinc on galvanised wire to contact stainless steel pipework and induce liquid metal embrittlement (1). The mechanism formed part of a series of events which ultimately led to the catastrophic incident which tragically claimed 28 lives and caused extensive damage to 2,000 properties. In fact, products which contain mercury are prohibited by all airlines as they pose a serious risk to the structural integrity of the aircraft which is produced predominantly from aluminium alloys.

Figure 1 – Flixborough UK plant, 1975 – explosion due to LME

LME can occur during fabrication or in service. Galvanised steel products have been found to exhibit LME upon welding. In addition, brazed stainless steel products are also commonly associated with this phenomenon. In service, a susceptible material in contact with a low melting point metal at low temperatures may crack when the temperature rises above the melting temperature of the low melting alloy. In refineries, mercury is found in some crude oils and can condense in the atmospheric tower overhead system thereby embrittling Monel 400, titanium and aluminium components.

The mechanism occurs initially with the molten metal weakening the surface layer, disrupting the stress equilibrium. Areas of high stress between the grains of the base material are relieved by the liquid filler metal, which quickly causes visible cracks along the grain (see Figure 2) and/or failure in the base material. There are three conditions necessary for LME to occur:

1. A susceptible alloy
2. The presence of a tensile stress (applied or residual)
3. The presence of a liquid metal

(1) https://www.icheme.org/media/17752/the-flixborough-disaster-report-of-the-court-of-inquiry.pdf.

Figure 2 – Copper LME of a stainless steel product

Numerous materials can be affected by this phenomenon such as austenitic stainless steel, carbon steel, alloy steel, nickel alloys, aluminium alloys and copper alloys. LME tends to occur in specific combinations of metals in contact with low melting point metals such as mercury, zinc, copper, cadmium, tin and lead. Typical combinations are given in the table below.

Prevention for this phenomenon can be summarised as follows:

Materials Selection: Select materials that are less susceptible to LME. For instance, some high-strength steels and alloys have better resistance to LME. Nickel alloys and austenitic stainless steels are often more resistant than carbon steels.

Avoid Exposure to Liquid Metals: Keep materials away from liquid metals that are known to cause LME.

Surface Coatings: Application of coatings to the material to prevent direct contact with liquid metals.

Welding Techniques: Be cautious when welding materials that may be susceptible to LME. Adjust welding parameters, such as temperature, to minimize the risk of liquid metal coming into contact with the base metal during the welding process.

Avoid High-Temperature Exposure: Control the operating temperatures to minimise exposure to temperatures where liquid metal embrittlement is likely to occur.

For more information or expert advice on LME and its prevention, feel free to contact us at info@r-techmaterials.com. Our team of specialists can assist with material integrity challenges and provide support to ensure the safety and longevity of your engineering systems.