What is Hydrogen Embrittlement?

If you’re working with steel tanks and pressure vessels, you’ve probably heard of hydrogen embrittlement. It’s a type of corrosion that affects many metals when exposed to atomic hydrogen. This article will help you understand why it’s important, how it acts, and what strategies people use to mitigate it.

Why is hydrogen embrittlement important?

Materials exposed to atomic hydrogen can experience changes in their chemical properties, including a reduction in tensile strength and ductility. In less technical terms, they become less flexible and therefore less able to withstand stress without cracking or breaking.

In other words, hydrogen embrittlement can cause your object to break before its expected lifetime has passed. In particular, this form of corrosion typically affects large surface areas of the material, and when exposed to too much stress, forms tiny cracks which combine with each other to cause a complete failure of the material.

What materials are at risk?

Hydrogen embrittlement affects metals, usually alloys. The most common type of alloys affected by it are steels, although some types of steel are more resistant than others. In general, the stronger the steel, the more vulnerable it is to hydrogen embrittlement, while steels with high ductility are more resistant. The arrangement of atoms in the steel’s lattice structure can also change how much of an impact hydrogen embrittlement is likely to have – FCC alloys are more resistant.

Some examples of steels that are more resistant are:

• Austenitic stainless steels
• Alloys containing nickel, aluminum, and nickel
• Ferritic steels

How does hydrogen embrittlement cause failure?

Hydrogen in a stable form is made of two hydrogen atoms bonded together, denoted H2. When H2 dissolves in some liquids it splits into two individual hydrogen atoms, or atomic hydrogen.

The material properties of a metal depend on the size, type, and arrangement of atoms making up the metal. When hydrogen embrittlement occurs, atomic hydrogen sneaks its way into the material by slipping through gaps between atoms. This is possible because atomic hydrogen has the smallest diameter of any atom, and the balance of forces holding the metal atoms together creates gaps just big enough for it to pass through.

Once inside the material, the atomic hydrogen pushes on the forces holding the metal atoms in their place, changing how they react to stresses. When a stress is applied, it’s harder for the metal atoms to slide against each other like they would normally in a neat lattice because now there’s something in the way. So instead of stretching or sliding, the lattice cracks. This further concentrates stresses in the material, which leads to more cracks, which leads to more stress concentrations until the entire process snowballs into a total failure of the material.

How is failure prevented?

Mitigating failure from hydrogen embrittlement requires separate strategies from most other measures preventing other types of corrosion. The first step is to understand the environment the material will be used in (including during the manufacturing process), and understand the risk factors for hydrogen embrittlement to occur.

Some risk factors include:

• Cathodic protective measures meant to alleviate other forms of corrosion
• Steels exposed to sulfuric acid
• A combination of high temperatures and exposure to an environment with hydrogen in it (e.g. welding)
• Exposure to sulfur and arsenic compounds

Once you know if hydrogen embrittlement is likely to occur, there are several strategies to employ to reduce its effects. Some of these are:

• Strain hardening the metal alloy
• Heat treating the metal to reduce the tensile strength
• Using a more resistant alloy
• Heating the metal to force the atomic hydrogen out

It’s important to regularly inspect components for signs of corrosive failure even if steps are taken to prevent it. Corrosion is something that is slowed, not prevented.

Learn more

Here at Rexarc, we use multiple strategies to mitigate corrosion in our steel tanks and pressure vessels, depending on your project needs and the environments your product will be exposed to. We’re happy to work with you to answer questions you have about managing corrosion, including hydrogen embrittlement. Reach out to us today!