How is Liquid Metal Embrittlement defined?

It is a deterioration mechanism that affects industrial equipment when it works in the presence of fluids that may contain molten metals and is visualized in the form of a crack that attacks alloys specific.

Quote (APA)

Candela Rocío Barbisan | Aug. 2022
Chemical engineer

Certain alloys are affected by certain low melting point metals such as zinc, mercury, cadmium, lead, copper and tin. It should be noted that, at very low concentrations of these metals coming into contact with susceptible material, cracking is generated, promoting the fissure. These metals can come either from the working fluid (outside the material) or from the material itself, as in the case of lead in a steel lead-free machining. Here a fundamental role is played by temperature, since for embrittlement to occur due to metal In liquid, not only the concentration of the metal is important, but also the temperature.

Materials and conditions

The most affected materials, defined by API 571, are carbon steels, stainless steels and aluminum alloys. However, NACE studies have detected higher susceptibility in aluminum alloys. And it can be mentioned, as a general rule (although there are exceptions) that the following combinations can be critical: series 300 stainless steels with Zinc, copper alloys with Mercury, 400 alloys with Mercury and aluminum alloys with Mercury.

If we look at the history of oil and gas operation in the world, we will find some disasters caused by this threat. Historically, it is a deterioration mechanism that attacks cryogenic gas plants, when liquid mercury condenses from the process gas. In 2004, in Algeria, an explosion caused the death of 27 people and 74 people were injured due to the failure of a heat exchanger due to the presence of liquid mercury in its gas.

Typically, heat exchangers of this type (plate) are designed with 5083 and 3003 series aluminum alloys, the plates inside (of 3003 alloys) are not very susceptible to this damage mechanism, however, in the external structure of the exchanger, the embrittlement

Well, where does mercury come from? In gas and oil producing wells we can find mercury, we can also find it in the form of you go out or as part of different organic compounds. The triple point of mercury is known to be -39°C, since the temperature of extraction of gas is above the triple point, it will be in the liquid or gaseous state.

In these cases, what happens is due to the removal of the protective layer that protects the surface, aluminum oxide. This layer is removed by thermal and mechanical stress or abrasion. Aluminum and its alloys lose ductility when "wet" by certain liquid metals and, being stressed, are susceptible to embrittlement.

The characteristic of this mechanism is that amalgamation can occur, that is, the formation of amalgams. When the metal comes into contact with the surface of the alloy (once the protective layer has been removed), amalgams are formed preferably in welds, generating the loss of endurance mechanics in them. On the other hand, corrosion can occur in these amalgams. When amalgam forms in the presence of moisture, amalgam corrosion is said to exist, since the The main difference with amalgamation is that, as it requires water, it propagates with lower concentrations of mercury.

When amalgamation occurs at the grain boundaries, followed by a fracture due to applied or residual stresses, we are dealing with liquid metal cracking. In these cases, the presence of water is not necessary to give rise to the mechanism.

Unlike other mechanisms, this is accelerated in terms of crack propagation and the low stresses necessary to generate it. and it is known that concentrations as low as 0.1 µg/Nm3 can be sufficient to cause damage to aluminum alloys such as mentioned.