We continue our discussion of the different types of embrittlement mechanisms, namely: environmentally induced cracking, stress corrosion cracking, hydrogen-induced cracking (aka hydrogen embrittlement), corrosion fatigue and liquid- and solid-metal embrittlement.
Corrosion fatigue is the result of the combined action of an alternating stress and a corrosive environment. The fatigue process is thought to cause rupture of the protective passive film, upon which corrosion is accelerated. The introduction of a corrosive environment often eliminates the normal fatigue limit of a ferrous alloy, thereby creating a finite life regardless of stress level.
Liquid- and Solid-Metal Embrittlement
It is well known that certain metals when exposed to other metals in the liquid (or solid) state are susceptible to a phenomenon known as liquid-metal embrittlement (LME), aka liquid-metal-induced embrittlement or liquid-metal cracking (LMC). The embrittlement of aluminum in contact with liquid mercury is a classic example. This is why any mercury-filled device is prohibited on aircraft due to concerns over loss of structural integrity of the aircraft in flight.
Other examples include carbon steels and stainless steels, which are susceptible to LME by zinc and lithium. Copper and copper alloys are susceptible to LMC by mercury and lithium, and aluminum and aluminum alloys are susceptible to LME from mercury and zinc. While the exact mechanisms of embrittling are complicated, the penetration by the embrittling agent is normally intergranular, and the requirements for embrittlement tend to vary depending on the materials involved.
The minimal conditions required for liquid-metal embrittlement of steels are:
- The alloy must be in a state of tension (applied or internal).
- The surface must be clean and free of oxides (which act as a barrier).
- The embrittling species (liquid) must intimately wet the metal surface.
The embrittling effect is most severe when high hardness exists (above 40 HRC). How fast a material will fail due to LME depends on many factors. Under certain conditions, fracture can take place in seconds. Crack growth and propagation rates in the range of 0.82-3.3 feet/second (0.25-1.0 m/s) have been measured. An incubation period and a slow pre-critical crack-propagation stage generally precede final fracture.
It is not uncommon for certain steels to experience ductility losses and cracking during manufacturing processes such as hot-dip galvanizing or during subsequent fabrication.This effect can occur in the cutting or even metallographic sectioning of component parts if inadequate cooling is provided.
Liquid-metal embrittlement effects can be observed even in the solid state, when one of the metals is brought close to its melting point. This type of phenomenon is also known as solid-metal-induced embrittlement.
3. C. J. McMahon, Jr., Brittle Fracture of Grain Boundaries, Interface Science 12, 141- 146, 2004.
4. Bogner, B., G. Rorvik, and L. Marken, “Bolt Failures–Case Histories from the Norwegian Petroleum Industry,” Microscopy and Microanalysis, Volume 11 Supplement S02, August 2005.
5. Kolman, D. G., Environmentally Induced Cracking, Liquid Metal Embrittlement, ASM Handbook, Volume 13A, Corrosion: Fundamentals, Testing and Protection, ASM International, 2003, pp. 381 392.