RCFA Hydrogen Embrittlement

‚ÄčIdentification of fracture mechanisms is a major stage to performing Root Cause Failure Analysis (RCFA).  ESR Technology carry out metallurgical failure analysis on a wide range of materials industry wide.  An example is the increasing use of high strength steels for bolting and fixtures.  High strength steels are generally obtained by quenching from the austenite phase to form martensite and subsequently tempering can obtain the required material properties.

Tempered martenite has relatively low ductility which increases the risk of hydrogen embrittlement (HE). In general, materials with increased hardness are more susceptible to HE.

The precise mechanism of HE is complex and several theories and mechanisms have been proposed such as an internal pressure theory, reduction in lattice cohesion, reduction of surface energy, brittle crack tip theory, hydride induced cracking and a localised slip model.  In reality it is likely that several mechanisms work in synergy, however for HE to occur the following are required:

  • HE must start with hydrogen atoms diffusing through the metal.
  • Increased temperature increases the mobility of the hydrogen atoms which can drive the hydrogen further through a metal.  Hydrogen however will still diffuse through a material at low temperature, as long as there is a concentration gradient

  • The hydrogen atoms will tend to collect at discontinuities within the steel matrix. The most obvious discontinuities are grain boundaries and inclusions; the grain boundaries become embrittled, significantly reducing the mechanical properties.

  • Atomic hydrogen can form molecular hydrogen which results in an increase in the internal pressure. This internal pressure can further weaken the steel and if sufficient hydrogen is present produce cracks (hydrogen induced cracking, or HIC).

HE can occur during various manufacturing operations, or, through operational use.  In fact, any situation where a metal may come into contact with atomic or molecular hydrogen. 

Manufacturing processes include electroplating, cathodic protection, phosphating, pickling, and arc welding.  HE is a well-documented problem in bolts and fasteners that are electroplated i.e. galvanising.  Bolts have been known to suddenly whilst in storage (delayed cracking).  This risk can be eliminated by baking the bolts 2000C for 24 hours after coating so that the hydrogen can diffuse out of the structure.

Another mechanism of hydrogen introduction is through corrosion, Figure 1.   Chemical reactions with acids or other chemicals (notably hydrogen sulphide in sulphide stress cracking, or SSC, a process of importance for the oil and gas industries).  However, corrosion of high strength steels is the most insidious. During the corrosion process the surface of the steel is attacked, this can occur in five stages:

  • General surface corrosion
  • Formation of U-shaped corrosion pits

  • U-shaped pits develop into V-shaped pits

  • Cracks form at the base of the pits

  • Cracks propagated through the material until failure occurs

  • HE mechanisms can be identified Scanning Electron Microscopy (SEM). Steels that fail under tensile loading will possess either a brittle mode (Figure 2) or in a ductile manner (Figure 3). A brittle fracture occurs when grains cleave apart resulting in relatively flat facets on the surface of the fracture. A ductile fracture has a cup and cone or dimpled appearance as a result of the material elongating before breaking. Often fracture surfaces can exhibit both types of fracture and are described as mixed mode.

  • HE however shows different fracture characteristics; the embrittled grain boundaries fail preferentially resulting in an intergranular fracture where the Individual grains can be seen with narrow gap around the grains (Figure 4).

 

 

If you require metallurgical support or failure analysis or have any questions please contact Eirwyn Davies, Principal Metallurgist on 01925 843417 or email This email address is being protected from spambots. You need JavaScript enabled to view it..

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