Although the most common hydrogen embrittlement failures are directly underneath the head of the fastener (A), they can also occur elsewhere given proper circumstances (B). Lab analysis is always necessary to determine the cause of a failure.
Hydrogen embrittlement (HE) is defined by ASTM F2078 as “a permanent loss of ductility in a metal or alloy caused by hydrogen in combination with stress, either externally applied or internal residual stress.” Hydrogen embrittlement is commonly associated with high-strength steels including fasteners made of carbon and alloy steels. However, it is worth noting that even precipitation hardened stainless steels, titanium, and aluminum alloys can also be vulnerable. Embrittled fasteners under stress can relax or fracture suddenly and without warning. Figure 1 shows some examples of hydrogen embrittlement failure.
Conditions for Hydrogen Embrittlement Failure
There are three conditions necessary to cause hydrogen embrittlement failure:
- Susceptible Material
- Hydrogen Source
- Sustained Mechanical Stress
If all three conditions are present in sufficient amount, and given time, hydrogen embrittlement failures can occur. The time to failure can vary depending on the severity of the conditions. Stress and hydrogen are contributing factors while material susceptibility is fundamentally the root cause of failure (Figure 2).
1. Susceptible Material -– Material strength or “hardness” of steel is a function of the material’s metallurgical and mechanical condition, which are the basis for material susceptibility to hydrogen embrittlement. As hardness increases, a given steel increases in strength, but loses ductility and toughness, resulting in an increase to susceptibility of hydrogen embrittlement. In fasteners, a hardness greater than 39 HRC is generally identified as being susceptible and should be avoided if possible.
2. Hydrogen Source - There are two main source categories of hydrogen: internal and environmental.
Internal hydrogen embrittlement (IHE) is the result of hydrogen pickup during the manufacturing process. One primary hydrogen source for fasteners is cleaning via acid-containing solutions before corrosion protective surface treatments are applied. Subsequent coating processes, specifically electroplating, can trap any hydrogen that was absorbed during these processes. Because the amount of hydrogen is finite and already present within the material, IHE failures will most commonly occur within 24-72 hours after installation.
Environmental hydrogen embrittlement (EH) is caused by the service environment via a corrosion reaction or interaction with hydrogen generating conditions. Coatings that utilize a dissimilar metal than that of the base material are sacrificial in nature. That is, they are anodic compared to the base material. An electroplating or other type of metallic protection coating does not eliminate the possibility of corrosion, but sacrifices itself to protect the base material during the service period. As corrosion progresses, a galvanic couple can develop between the base material and its coating. This can result in hydrogen generation that can diffuse into the base material. Because these conditions can vary, EHE failures are subject to the rate and severity of the corrosion and/or environment, and can occur anywhere from a week to years after installation. ISO/TR 20491:2019 Section 9.4.3 indicates the following in regards to EHE:
“From a failure analysis perspective, any amount of corrosion prior to failure of an in-service fastener can lead to EHE as the dominant failure mechanism, independently of the presence of internal hydrogen. With the passage of time, the localized contribution of corrosion generated hydrogen is cumulative, and the relative contribution of internal hydrogen becomes negligible."
3. Mechanical (Tensile) Stress - Mechanical fasteners are unique in that they are commonly assembled under a high static tensile stress. This will exploit stress concentrations, and as time progresses atomic hydrogen will diffuse to these concentrations. A materials critical threshold stress is dependent on the material susceptibility and the amount of hydrogen present in the material.