Hydrogen embrittlement is a phenomenon that affects high-strength steels, causing unexpected brittle failure when certain conditions are met. In bolts, this type of failure represents a critical risk, as it compromises the safety of structures and fasteners.
What is hydrogen embrittlement?
Hydrogen embrittlement, or hydrogenation, occurs when hydrogen is introduced into the crystalline structure of steel, causing a loss of ductility and increasing its tendency to brittle fracture.
This condition, combined with tensile stresses, can lead to delayed bolt failure, i.e. a failure that occurs hours or days after installation, with no prior visible signs.
Why does hydrogen embrittlement failure occur?
For a bolt to suffer a hydrogen embrittlement failure, three factors must coincide simultaneously:
- A material susceptible to hydrogen absorption.
- The presence of hydrogen.
- High tensile stresses on the part.
Material susceptibility
The degree of hydrogen embrittlement of a bolt depends on the hardness of the steel. It has been shown that hydrogen embrittlement is more likely when the hardness of the part exceeds 390 HV, as the ductility of the material decreases significantly and embrittlement increases.
To avoid this problem, it is recommended that concrete bolts have a hardness of less than 350 HV.
Presence of hydrogen
Hydrogen can be introduced into the fastener during the fabrication of the steel or bolt, or through environmental exposure.
Stress on the bolt
The degree of stress caused by the external load applied to the fastener is one of the triggers.
Types of hydrogen embrittlement
There are two main categories depending on the source of hydrogen:
Internal hydrogen embrittlement (IHE)
This is the residual hydrogen that originates during the production phases of the fastener, specifically the chemical pickling phase and its coating.
In fact, this protective layer prevents the hydrogen from leaving the part by natural diffusion tendency, causing it to be trapped inside the part.
If hydrogen is trapped inside the steel, brittle fracture can occur in the first 48 hours after installation, even at loads well below the strength limit of the bolt. Rupture occurs in areas of stress concentration, e.g. under the head or in the thread start area.
External hydrogen embrittlement (EHE)
Hydrogen can also penetrate the inside of the part due to galvanic corrosion in service.
This process occurs when bolts are exposed to humid or aggressive environments, generating hydrogen on the surface of the steel.
Measures to minimise the risk of internal hydrogen embrittlement
To mitigate the likelihood of hydrogen embrittlement, the industry adopts practices such as:
- Dehydrogenation heat treatments (baking), where bolts with hardnesses above 390 HV are subjected to temperatures of 190-220 °C for a period of 8 to 10 hours to remove retained hydrogen.
- Substitution of chemical pickling by mechanical cleaning, avoiding the introduction of hydrogen into the structure of the material.
Measures to minimise the risk of external hydrogen embrittlement
To mitigate the likelihood of external hydrogen embrittlement, we recommend:
- Use materials with similar electrolytic potentials to reduce galvanic corrosion.
- Opt for non-electrolytic coatings, which offer corrosion protection without the risk of introducing hydrogen.
How concrete bolts are tested for resistance to hydrogen embrittlement
To approve concrete bolts and ensure their resistance to hydrogen embrittlement, product-specific tests are carried out to assess their behaviour in the presence of hydrogen, both internally and externally.
In addition, during manufacturing, strict quality controls are implemented to avoid excessive hardness levels that could compromise the safety of the product due to hydrogen embrittlement failure.
