Hydrogen Embrittlement

Hydrogen embrittlement is the unexpected embrittlement of metallic materials caused by hydrogen penetrating into their interior. In other words, when metals absorb hydrogen atoms, they lose their ductility (capacity for elastic deformation) and become prone to cracking. This condition can lead to fracture at loads far below what the material would normally withstand. The severity of hydrogen embrittlement depends on the amount of hydrogen absorbed by the metal and its microstructure. Particularly high-strength (hard) metals are more susceptible to hydrogen. Additionally, a tensile stress must typically be present on the material for this effect to manifest.

Hydrogen Entry into Metal

When hydrogen atoms enter the metal lattice, they rapidly diffuse and accumulate at defects such as voids and dislocations. Metals can be exposed to hydrogen in various ways during production and use. Internal hydrogen is usually acquired during manufacturing. For example, atomic hydrogen generated during contact of molten metal with water, use of moist electrodes during welding, or acid cleaning (pickling) and electroplating processes can penetrate the metal. External hydrogen originates from the service environment. Metals can absorb hydrogen atoms produced by the reduction of water during corrosion. Similarly, in systems such as tanks and pipelines exposed to high-pressure hydrogen gas, H₂ molecules dissociate into atoms at the surface and diffuse into the metal. In practice, it is known that high-strength steel components can crack within days due to hydrogen embrittlement following poorly controlled acidic plating operations or humid welding applications.

Embrittlement Mechanisms

The presence of hydrogen within the metal facilitates crack initiation and propagation by interacting with microstructural defects. Various theories have been proposed in the literature to explain this phenomenon. For instance, hydrogen may weaken interatomic bonds and reduce fracture resistance (Hydrogen-Induced Decohesion Theory, HEDE), or it may facilitate dislocation motion, leading to localized plastic deformation (Hydrogen-Enhanced Localized Plasticity Theory, HELP). Furthermore, at high hydrogen concentrations, atoms may combine to form H₂ molecules within voids, generating internal pressure that leads to microcracks and surface blisters. In some metals such as titanium, it has been shown that hydrogen forms brittle metal hydrides whose fracture causes failure. In reality, several of these mechanisms may operate simultaneously, and no single explanation can account for all cases of hydrogen embrittlement.

Materials Affected

Hydrogen embrittlement occurs most severely in high-strength steels. Steels with tensile strength below approximately 1000 MPa (softer than about Rockwell C 32) are generally not susceptible to this effect. As strength increases, so does the risk of ductility loss and unexpected fracture due to hydrogen. In addition, metals with HCP (hexagonal close-packed) or BCC (body-centered cubic) structures, such as titanium and nickel alloys, can also be adversely affected by hydrogen. For example, in titanium alloys, hydrogen forms titanium hydride (TiH₂) phases that embrittle the material. On the other hand, austenitic stainless steels and metals with FCC (face-centered cubic) structures such as aluminum and copper are more resistant to hydrogen embrittlement at room temperature. In aluminum alloys, hydrogen primarily reduces strength by forming porosity during casting, but classical hydrogen embrittlement does not occur at ambient temperatures.

Fracture Indicators

A metal component affected by hydrogen embrittlement often fractures suddenly and brittlely without any prior warning. Moreover, this fracture typically occurs after a delay following the application of load. For this reason, hydrogen embrittlement is also known as delayed fracture. A high-strength material exposed to hydrogen can fail abruptly hours or even days after entering service. Upon examination of the fractured surface, necking or tensile marks characteristic of ductile fractures are absent. Instead, the fracture surface exhibits a bright, granular structure that propagates along grain boundaries.

Prevention Methods

Material selection and process control are critical in preventing hydrogen embrittlement. First, where possible, more resistant alloys should be chosen instead of high-strength materials sensitive to this effect. In particular, electroplating processes in acidic environments should be avoided for high-hardness steel components. If plating is necessary, hydrogen that has penetrated the material should be removed by baking at approximately 190 °C for several hours after the process.

^【1】

In addition, measures should be taken during manufacturing to minimize hydrogen ingress. For example, using dry electrodes during welding and adding inhibitors to acidic baths to reduce hydrogen generation from corrosion are effective methods. Furthermore, surface treatments such as shot peening, which induce compressive stresses, make the material more resistant to hydrogen-induced crack initiation. Current research aims to add hydrogen-trapping elements to alloys or optimize the microstructure to accumulate hydrogen atoms in harmless traps. This approach seeks to prevent hydrogen from accumulating in critical regions of the material, thereby reducing the risk of embrittlement.

Hydrogen embrittlement is a critical issue from both engineering and safety perspectives. High-strength metal components can fail unexpectedly due to hydrogen ingress, leading to malfunctions and accidents. Therefore, precautions must be taken during material selection and throughout production and operational processes to mitigate the harmful effects of hydrogen. In particular, for applications involving the storage and transport of hydrogen gas, it is essential that the materials used are resistant to this effect.