Embrittlement

Embrittlement
 
Embrittlement Failures

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:

  1. Susceptible Material
  2. Hydrogen Source
  3. 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.

 
Mitigating Hydrogen Embrittlement
 
Embrittlement Failures

Three conditions are required to induce a hydrogen embrittlement failure

Avoid electroplating fasteners having a hardness above 39 HRC. Electroplating processes will generally utilize an acid cleaning step in preparation for the plating. Hydrogen can diffuse into the base material and subsequently become trapped by the plating due to the plating’s reduced hydrogen permeability.

If electroplating is still desired, ensure that proper procedures and baking practices are utilized. ASTM F1941/F1941M outlines hydrogen embrittlement relief requirements for coated steel fasteners that have been quenched and tempered to a hardness above Rockwell C39 through C44, which includes baking plated fasteners such as ASTM A574 and ISO 898-1 Property Class 12.9 fasteners for a minimum of 14 hours. Baking temperatures should be high enough to cause hydrogen effusion, but well below the tempering temperature of quenched and tempered fasteners to avoid altering mechanical properties. The temperature should also be chosen to minimize the risk of solid or liquid metal embrittlement. For zinc plated through-hardened fasteners, the baking temperature should be 375 to 425°F (190 to 220°C). Although effective, post-electroplating baking it does not completely eliminate the possibility of a hydrogen embrittlement failure.

Using a coating process that does not introduce hydrogen into the material (particularly those that do not utilize acids for cleaning) will help avoid HE. Additionally, coatings that can entrap hydrogen such as zinc electroplating should be avoided. A number of zinc-flake dip-spin coatings are considered hydrogen embrittlement “free” because they use mechanical cleaning processes (abrasive blasting) for descaling and the coatings do not entrap hydrogen. Most of these coatings offer higher corrosion resistance than general electroplatings.

Material hardness is the major contributing factor to the susceptibility of hydrogen embrittlement (HE). Harder, stronger, materials will be more susceptible than lower strength alternatives. Through-hardened fasteners having a hardness greater than 39 HRC will have a distinct susceptibility to HE failure. In some applications, fastener hardness below 35 HRC or lower will be required to reduce HE potential. Considerations should be made at the time of joint design to avoid using ultra high-strength fasteners with hardness greater than 39 HRC.

The service environment is a critical consideration in bolted joint design. Proper selection of the fastener material/strength for the service environment can reduce the risk of embrittlement failures. The potential for hydrogen embrittlement cracking (even for fasteners below HRC 35) is accelerated if the fastener is acting as the cathode in a galvanic couple. Caustic or “sour” environments may require much lower hardness levels, or specialty materials to lower the susceptibility to hydrogen embrittlement.

Liquid Metal Embrittlement
 

Liquid Metal Embrittlement(Metal-Induced Embrittlement) is an embrittlement phenomenon that forms under tensile stress and involves contact of certain alloys with another metal in either liquid or close-to-liquid state (solid metal embrittlement). This plays a factor particularly with coated fasteners where the use of the fastener may be limited to approximately ½ the melting point of the plating material. For example, a zinc plated A193/A193M B7 would be limited to applications no more than 390°F since the melting point of zinc is 780°F. This is one of the main reasons baking temperature for zinc plated fasteners is approximately 400°F.

 
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