¿How does hydrogen weaken materials by embrittlement?

Introduction

Hydrogen, although considered a clean source of energy, can have detrimental effects on the materials that come into contact with it, especially in the industrial context. Hydrogen embrittlement is a critical phenomenon that can compromise the structural integrity of components subjected to hydrogen-rich environments, such as in the petrochemical, automotive, and aeronautical industries.

This process involves the interaction of hydrogen with the microstructure of materials, weakening their mechanical properties and generating hydrogen-induced cracks that can lead to catastrophic failures. This article explores the mechanisms of hydrogen embrittlement, the effects of atomic hydrogen on materials and how hydrogen corrosion contributes to the degradation of mechanical properties.

Hydrogen in materials

Hydrogen is an element that can act as a weakening agent in many materials. It occurs in atomic, molecular or ionic form and, depending on its form and concentration, can interact differently with materials. In its atomic form, hydrogen is particularly reactive and can interact with some metals, altering their microstructure and reducing their mechanical strength.

When materials are exposed to hydrogen under certain conditions, it can penetrate metal surfaces and diffuse through the material’s microstructure. This process is known as hydrogen embrittlement, and is particularly problematic in high-strength metals such as steels and nickel alloys. The presence of atomic hydrogen within the structure of the material can cause alterations at the level of atomic bonds, which weakens its ability to resist mechanical stresses.

Hydrogen embrittlement mechanisms

These mechanisms can be divided into several interrelated phenomena. The most significant of these are detailed below:

Hydrogen corrosion

It is one of the main mechanisms contributing to the embrittlement of materials. In acidic or high hydrogen pressure environments, hydrogen can be produced by corrosion reactions, where atomic hydrogen diffuses into the material. This phenomenon is associated with degradation of mechanical properties and, in particular, with the formation of hydrogen-induced cracks. Hydrogen corrosion not only affects the surface, but can also penetrate into the internal structure of the material, causing mechanical failure.

Hydrogen-induced cracking or crazing

This phenomenon occurs when hydrogen penetrates the grains of a material and causes them to fracture. Hydrogen-induced cracks can form at the intergranular or transgranular level, depending on material characteristics and environmental conditions. In high-strength steels, hydrogen can accumulate at grain boundaries, which significantly reduces the toughness of the material and can lead to premature failure under relatively low loads.

Hydrogen blistering

Hydrogen blistering is a defect that occurs in steels when atomic hydrogen penetrates the steel structure and becomes trapped in certain internal areas. This process begins when hydrogen ions are reduced to atoms and adsorbed on the surface of the steel. Some of these atoms diffuse through the material and are retained in specific areas, such as microscopic voids around non-metallic inclusions.

Within these traps, hydrogen atoms combine to form gas molecules (H₂). Because molecular hydrogen cannot diffuse as easily as atomic hydrogen, the gas becomes confined in small spaces. Over time, the accumulation of gas generates increasing internal pressure which, when it overcomes the strength of the material, causes blistering on the surface of the steel.

Hydrogen embrittlement.

Reduction of mechanical strength

The presence of hydrogen in materials can significantly decrease their ability to withstand mechanical loads. Hydrogen embrittlement particularly affects ductility and tensile strength properties. Materials subjected to mechanical stresses and exposure to hydrogen can suffer considerable loss of strength, which can result in unanticipated structural failures.

Hydrogen corrosion: A complex phenomenon

It is an electrochemical process that occurs when metallic materials come into contact with environments containing hydrogen in high concentrations. This phenomenon not only causes the weakening of the material in terms of mechanical strength, but can also lead to surface defects and a reduction in the service life of the exposed components.

In this type of corrosion, atomic hydrogen is generated from proton reduction reactions at the surface of the material. These hydrogen atoms diffuse through the microstructure of the metal and can accumulate at grain boundaries, phase interfaces or even dislocations. This accumulation of atomic hydrogen can lead to the formation of intergranular cracks and other defects at the microstructural level.

Microstructure of hydrogen-affected materials

The microstructure of materials plays a relevant role in hydrogen embrittlement. Materials with fine-grained structures are more susceptible to the effects of hydrogen penetration, since atomic hydrogen can accumulate more easily at grain boundaries. In coarse-grained materials, hydrogen has less mobility, which can make embrittlement less pronounced.

Materials with low toughness or those containing impurities in their structures may also be more susceptible to embrittlement. For example, in aluminum or titanium alloys, the presence of elements such as carbon or oxygen can facilitate hydrogen absorption and increase crack formation.

Which materials are most vulnerable to hydrogen embrittlement?

They vary according to the microstructure, chemical composition and exposure conditions of each material. The most vulnerable materials include:

Vanadium, nickel and titanium alloys: They have a high solubility in hydrogen and, therefore, can absorb significant amounts of hydrogen.

High strength steels: Martensitic and bainitic steels are particularly sensitive, as their high hardness and strength favor hydrogen accumulation in crystalline defects.

Carbon and low alloy steels: These materials used in the oil and nuclear industry are prone to hydrogen embrittlement in the presence of acidic media or welding processes.

Refractory metals (tantalum, niobium, zirconium): They are affected by hydrogen absorption, which causes the formation of hydrides and reduces their mechanical strength in high temperature applications.

Dehydrogenation processes

To counteract the negative effects of hydrogen on materials, dehydrogenation processes can be employed. These processes are designed to remove atomic hydrogen that has accumulated within the structure of the material. Heat treatment is one of the most common forms of dehydrogenation, where materials are heated to elevated temperatures to allow hydrogen to diffuse to the surface and be released.

Although the dehydrogenation process can be effective in reducing the hydrogen concentration in a material, it does not guarantee complete removal of hydrogen that has infiltrated deep into the microstructure. Therefore, prevention of hydrogen embrittlement must be part of a comprehensive approach that includes selection of suitable materials and control of environmental conditions.

Intergranular phenomena and hydrogen induced cracking

Atomic hydrogen accumulating at grain boundaries can lead to intergranular phenomena, where cracks propagate along these boundaries. This is especially problematic in high strength materials, such as high alloy steels and stainless steels, where hydrogen accumulation can compromise structural integrity. Hydrogen-induced intergranular cracks are often invisible to the naked eye, making failure difficult to predict and prevent.

These intergranular cracks can trigger rapid crack propagation under mechanical stresses, which accelerates component fracture. The phenomenon of intergranular cracking is more prevalent in materials subjected to tensile stresses and high hydrogen concentration environments, underscoring the importance of controlling hydrogen exposure and using materials with high resistance to hydrogen penetration.

Degraded mechanical properties

The effect of hydrogen on the mechanical properties of materials cannot be underestimated. Materials that have been weakened by hydrogen penetration exhibit a reduction in their mechanical properties, such as toughness, ductility and fatigue strength. This degradation can occur without significant observable deformation, making affected materials particularly hazardous in critical applications.

In addition, hydrogen embrittlement can lead to catastrophic failures even at relatively low loading conditions. Degraded mechanical properties can compromise the safety of critical structures such as pipelines, reactors and aircraft components, requiring constant monitoring and implementation of preventive maintenance practices.

Weldability

Weldability problems in hydrogen-affected materials are due to hydrogen embrittlement, which compromises the integrity of the welded joint and can lead to premature failure. These problems arise mainly due to the following factors:

• Cold cracking (Hydrogen induced cracking): Occurs when atomic hydrogen, generated during welding, diffuses into the heat affected zone (HAZ) and accumulates in microstructural defects, causing delayed cracking, which is more likely to occur in high strength steels and materials with hardened structures.
• Weld porosity: The presence of hydrogen in the molten pool generates gas bubbles that are trapped in the solidified metal, reducing the mechanical strength and quality of the weld.

Main causes of these problems

• Hydrogen contamination in the welding consumables, moisture in the electrodes or in the shielding atmosphere.
• High residual stresses, which accelerate hydrogen-induced crack propagation.

Solutions to mitigate these problems

• Use of electrodes and protective gases low in hydrogen.
• Application of post-weld dehydrogenation heat treatments.
• Control of welding parameters to avoid brittle microstructures.
• Reduce the moisture content in the shielding gas or by decreasing the humidity in the welding area.
• Caution in design to reduce stress concentration in the welded joint.

How to identify hydrogen embrittlement?

One of the first methods of identification is visual inspection, which allows the detection of cracks, fractures or surface alterations, especially in areas subjected to high stresses. However, for a more accurate assessment, it is necessary to resort to advanced tests, such as:

• Mechanical tests: Tensile and fatigue tests are tests that measure the strength and ductility of the material, allowing the detection of hydrogen embrittlement.
• Microscopic analysis: With this technique we can observe microcracks not visible to the naked eye.
• Non-destructive testing (NDT): NDT methods such as ultrasound and radiography are used to examine the internal integrity of the material and can detect hydrogen attack.
• Electrochemical methods: Hydrogen permeation tests help to evaluate the presence and impact of hydrogen on the structure.

Conclusion

Hydrogen embrittlement is a complex and damaging phenomenon that affects a variety of materials, especially in industries operating in high hydrogen concentration environments. Embrittlement mechanisms, such as hydrogen corrosion, hydrogen-induced cracking and intergranular phenomena, must be thoroughly understood to prevent structural failure and ensure component integrity. Proper management of hydrogen exposure, selection of resistant materials and the use of dehydrogenation processes are critical to mitigate the risks associated with this phenomenon.

References

1. https://www.corrosionclinic.com/types_of_corrosion/hydrogen-induced_cracking_HIC.htm
2. https://faculty.kfupm.edu.sa/me/hussaini/corrosion%20engineering/Images/Hydrogen-wallblister.jpg
3. https://www.twi-global.com/technical-knowledge/job-knowledge/defects-hydrogen-cracks-in-steels-identification-045