Question

I saw a demonstration of a corrosion phenomenon in which a piece of flexible copper tubing was immersed for about a minute in liquid mercury, removed and bent until it broke apart like glass. Can you explain what happened?

Answer

The phenomenon you observed is called Liquid Metal Embrittlement.

Liquid metal embrittlement (LME) occurs when a normally ductile metal experiences drastic loss of mechanical properties (e.g., tensile strength, ductility) and undergoes brittle fracture when it comes in contact with liquid metal and is exposed to some form of external stress (tensile stress). LME is normally associated with a molten liquid under particular conditions (e.g., temperature) penetrating into the metals surface, causing cracking.

LME is particularly problematic in that it can occur at loads below the yield stress and without significant deformation or (obvious) deterioration of the component part. Intergranular or transgranular cleavage fractures are common modes.

In general, the following is believed to occur with LME:
  • Instantaneous failure under applied or residual stress
  • Delayed failure at a static stress level below the material’s tensile strength
  • Microstructure is important but not critical in predicting attack
  • Plasticity is often present
  • Grain-boundary penetration is believed to be stress independent
  • A high-temperature corrosion effect
The presence of an external stress is a prerequisite as is a pre-existing crack (or plasticity) and the presence of an obstacle to dislocation movement (e.g., grain boundaries, twinning planes, precipitates).

Some of the processes in which LME has been known to occur include: soldering/brazing/welding, heat treatment, hot working and elevated temperature service.

Controlling Corrosion

Various protection methods can be used to prevent a metallic system from corrosion, including:
  • Thermodynamic – selecting materials with a high (positive) value for the free energy change for conversion of the free metal to a corrosive product
  • Kinetic – reducing the corrosion rate of the anodic or cathodic reaction by lowering current density
  • Barrier – isolation of the material from the corrosive medium by use of a coating, inhibitor, oxide layer, etc.
  • Structural design – minimizing exposure time to a corrosive environment
  • Environmental control – elimination of the principle constituent in a corrosive reaction from a (closed) environment
  • Metallurgical design – the proper alloy for the environment