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.

Hydrogen Embrittlement

Hydrogen embrittlement, also known as hydrogen-induced cracking, involves the ingress of hydrogen into the metal, causing reduced ductility and load-bearing capacity, subsequent cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials (Fig. 1). The materials that are most vulnerable include high-strength steels, titanium and aluminum alloys.

How Hydrogen Gets In

Parts that are undergoing surface treatments such as etching, pickling, phosphating, corrosion removal, paint stripping and electroplating are susceptible to hydrogen ingress. The severity of the hydrogen damage depends on the:

  • Source of hydrogen (external or internal)
  • Exposure time
  • Temperature and pressure
  • Presence of solutions/solvents that could react with metals
  • Alloy type and production method
  • Number and type of discontinuities in the metal
  • Treatment of exposed surfaces to form barrier layers (e.g., oxide layers as hydrogen permeation barriers on metals)
  • Final surface treatment
  • Heat-treatment method
  • Level of residual and applied stresses

Hydrogen-Embrittlement Mechanism

Hydrogen enters the steel as atomic hydrogen, diffuses along grain boundaries (areas of low energy) as a gaseous phase and collects at areas of high triaxial stress where it recombines in molecular form (Fig. 2). This gas is not mobile and collects in small voids along the grain boundaries, resulting in localized loss of ductility and a reduction of the cohesive strength of the material. These gases build up enormous internal pressure, which often relieves itself by initiating cracks.

Two common sources of hydrogen exposure are plating operations and, to a lesser degree, the heat-treat atmosphere. Plating is generally considered the most common source of hydrogen. Therefore, an area of emphasis must be on controlling the plating operation in combination with a robust post-plating bake-out cycle. A deep case depth observed in many applications may also contribute, allowing more hydrogen to enter the part than would normally occur if the targeted case depth 0.076-0.127 mm (0.003-0.005 inches) had been produced.

References

1. Herring, Daniel H., “A Heat Treaters Guide to Hydrogen Embrittlement,” Industrial Heating, October 2004.
2. Walton, Harry, Ubiquitous Hydrogen, Heat Treat Conference Proceedings, ASM International, 1999.
3. Professor Rick Sisson, Worchester Polytechnic Institute, private correspondence.
4. VanAiken, Dave, “Engineering Concepts: Hydrogen Embrittlement of Copper,” Industrial Heating, October 2001.