Hydrogen Embrittlement in Fastener Applications

Hydrogen embrittlement has been a problem for decades and should not be taken lightly since it mechanically degrades a range of different structural materials. Even though it has been a problem for more than 100 years, many uncertainties are still present surrounding this issue. We’ve compiled the most recent information on what hydrogen embrittlement is to help you understand how it can affect your fasteners.

What Is Hydrogen Embrittlement?

Hydrogen embrittlement is when metals become brittle and fracture after being exposed to hydrogen. Most cases can typically be divided into two categories. The first is due to hydrogen coming from the environment, normally present in corrosion. The second and most common type of hydrogen embrittlement, called internal hydrogen embrittlement, occurs due to the supply of hydrogen from processes during manufacturing.

Metals Affected By Hydrogen Embrittlement

These types of fractures tend to be more complex than the normal fracture processes even though not all metals and alloys are affected. Materials that are most susceptible include high-strength steels, titanium and aluminum alloys. The process starts with the entry of hydrogen into these metals and alloys. This alone is a complicated process and the rate at which hydrogen enters the metals and alloys is dependent on many different factors. Besides corrosion, there are other sources of hydrogen during the steelmaking process, during heat treatment and the vast majority of hydrogen embrittlement occurring during electrochemical surface treatments such as acid cleaning and electroplating.

Usually associated with carbon and alloy steels, hydrogen embrittlement in fasteners can still affect other metals and alloys as well. The hardness of the fastener is often a contributing factor when dealing with dealing with this type of fracture. In most cases, it occurs when dealing with a hardness of above 320HV. Some experts have even discovered that when dealing with a fastener with a hardness exceeding 360HV extra steps need to be taken to manage the risk.

Factors Leading to Hydrogen Embrittlement in Fasteners

The failure of a fastener as a result of hydrogen embrittlement is a delayed, brittle failure and only occurs when fasteners are installed and exposed to ensile stresses. It usually progresses in five stages with stages two through five continuing until the fastener cannon resist the load any longer, eventually breaking.

Progression of Hydrogen Embrittlement
1.   Entry of hydrogen into the fastener
2.   Diffusion of hydrogen to high tensile stress regions
3.   Hydrogen segregation to grain boundaries, inclusions, dislocations and other traps
4.   Critical value of hydrogen concentration
5.   Crack development

It is obvious that hydrogen embrittlement can pose a serious threat to the structural soundness of your product. To protect your quality products, monitor and take the necessary steps to prevent hydrogen embrittlement when conditions allow for it to happen.