Understanding Hydrogen Embrittlement: How It Impacts Metal Durability
Last updated on August 12, 2024 01:53:00 PM
What is Hydrogen Embrittlement
Hydrogen embrittlement is a phenomenon where, in the presence of hydrogen, metals (especially high-strength steels and alloys), become brittle and susceptible to cracking and failure.
Causes of Hydrogen Embrittlement
Absorption of Hydrogen
Hydrogen Embrittlement is the result of the introduction of hydrogen into a material. This may occur during the manufacturing process or it may be absorbed from moisture in the environment.
Factors Influencing Hydrogen Embrittlement
There are three main factors that influence hydrogen embrittlement:
- Material composition
- Hydrogen source
- Environmental factors
Material Composition
High-strength steels and alloys with high hardness and tensile strength are more susceptible due to their microstructure.
A materials’ microstructure simply describes the way in which the atoms are arranged on the microscopic level. For example, if you imagine a cube of material, the atoms may reside on every corner and every face of the cube (this is called a face-centered cubic), on every corner and in the body of the cube (referred to as a body-centered cubic), or even in a hexagonal pattern (Figure 1). Examples of face-centered cubic materials (FCC) are Aluminum,Copper, and Nickel, while examples of body-centered cubic (BCC) materials are Tungsten and Chromium. You can see how the space and bonding between the packing structures vary.
Figure 1. Face-centered cubic (FCC), left, and body-centered cubic (BCC), right.
Iron (the main ingredient in steel alloys) is unusual in that it can exist in both the FCC and BCC configurations, depending on temperature. At lower temperatures, iron exists as ferrite, a BCC atomic arrangement, while at higher temperatures it transforms into austenite, an FCC arrangement. In order to create a strong steel, carbon is added to the iron while it is hot. As it is cooled quickly (known as quenching), the atomic structure changes and the carbon atoms are trapped. Since hydrogen diffuses (travels through the material) much faster through BCC materials than through FCC materials, high-strength steels are more susceptible to hydrogen embrittlement.
There are other facets to a material’s microstructure that may increase the risk of hydrogen embrittlement. A repeating unit of the cubic structures described above, is called a crystal, or a grain. Materials are made of many crystals (polycrystalline) with many grain boundaries (Figure 2). These boundaries form an imperfect alignment in which hydrogen can accumulate.
Figure 2. Grain Boundaries.
Hydrogen Source
Metals may be exposed to either gaseous or chemically generated hydrogen. The gaseous hydrogen does not cause embrittlement while the atomic hydrogen from chemical attack dissolves into the metal at room temperature, increasing the susceptibility of the material to hydrogen embrittlement.
The introduction of chemical hydrogen may be due to acids, corrosion, or electroplating.
Metals may be exposed to hydrogen-containing acids during pickling, etching, or cleaning. Furthermore, during manufacturing, the presence of moisture during welding (damp welding rods) or while the metal is molten, introduces hydrogen. Therefore, it is important to bake the material after manufacturing in order to remove or immobilize the hydrogen.
High- Stress Conditions
Stress is a unit of force per unit area. Applying force to a large area produces less stress than applying force to a small area. As an example, if you place the same amount of force using a thumbtack or a coin, the thumbtack will produce more stress. Stress can be applied in a variety of ways- compression, tension, shear, torsion, and bending (Figure 3).
Figure 3. Types of stress. From left to right: tension, compression, shear, torsion, and bending.
The introduction of hydrogen alone does not cause embrittlement. It’s the combination of hydrogen and a high tensile stress condition that alters the material’s mechanical properties, making it more brittle, and leading to cracking.
Prevention and Mitigation:
Mitigating hydrogen embrittlement may be done through material selection (using low carbon steels or other materials designed to resist embrittlement), controlling the material’s exposure to hydrogen during the manufacturing process, storage, and service, or applying a coating or surface treatment to the material to help minimize hydrogen absorption. Lastly, a post-production heat treatment can help remove trapped hydrogen from the material.
