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.

More on Hydrogen Embrittlement

The characteristics most often associated with hydrogen-embrittlement failures include:

  • Delayed failure often associated with stress rupture/static fatigue
  • Stress-state-dependent
  • Variable time to failure (seconds to many months)
  • Brittle fracture most often associated with loss of toughness
  • Microstructure/hardness dependent

How Does Hydrogen Get Out?

Hydrogen absorption need not be a permanent condition. If cracking does not occur and the environmental conditions are changed so that no hydrogen is generated on the surface of the metal, the hydrogen can re-diffuse out of the steel, and ductility is restored. Performing an embrittlement relief, or hydrogen bake-out cycle (the term "bake-out" involves both diffusion within the metal and outgassing) is a powerful method in eliminating hydrogen before damage can occur. Key variables are temperature, time at temperature and concentration gradient (atom movement).

For example, electroplating provides a source of hydrogen during the cleaning and pickling cycles, but the most significant source by far is cathodic inefficiency. A simple hydrogen bake-out cycle can be performed to reduce the risk of hydrogen damage (Table 1). 

Low Hydrogen Concentrations can be Problematic

Of concern today is embrittlement from very small quantities of hydrogen where traditional loss-of-ductility bend tests cannot detect the condition. This atomic-level embrittlement manifests itself at levels as low as 10 ppm of hydrogen (in certain plating applications, it has been reported that 1 ppm of hydrogen is problematic).

Although difficult to comprehend, numerous documented cases of embrittlement failures with hydrogen levels this low are known. This type of embrittlement occurs when hydrogen is concentrated or absorbed in certain areas of metallurgical instability. This concentrating action occurs either via residual or applied stress, which tends to "sweep" through the atomic structure, moving the infiltrated hydrogen atoms along with it. These concentrated areas of atomic hydrogen can coalesce into molecular-type hydrogen, resulting in the formation of high localized partial pressures of the actual gas. This underscores the importance of a proper bake-out procedure whenever hydrogen is suspected.
 

References

1. Herring, Daniel H., “Embrittlement Issues with Fasteners,” Fastener Technology International, December 2008.
2. Krause, George, Steels: Processing, Structure and Performance, ASM International, 2005.
3. Herring, D.H., “The Embrittlement Phenomena in Hardened and Tempered Steels,” Industrial Heating, October 2006.
4. Herring, D.H., “A Heat Treaters Guide to Hydrogen Embrittlement,” Industrial Heating, October 2004.