Hydrogen wear vs hydrogen embrittlement: what’s the difference?

Author: NEOL

Hydrogen is the most abundant element there is, estimated to contribute to around 75% of the mass of the universe. Many people believe it to be the key to achieving net zero, offering an alternative energy source that could greatly reduce carbon emissions.

It is, however, also responsible for a considerable amount of mechanical damage. This is due to two separate occurrences: the well-known hydrogen embrittlement and the lesser-known hydrogen wear.

Both can cause significant damage to metals and greatly reduce the useful life of machinery. But, while there are similarities between the two, there are also some major differences – and understanding these can help us combat the negative effects of both, improving the performance and longevity of our machines as a result.

Hydrogen embrittlement

The term hydrogen embrittlement is used to describe the process whereby metals become brittle and fracture as a result of absorbing hydrogen. It is a common issue for steel makers, lowering the level of stress required for cracks to appear in the metal and therefore reducing its load bearing capabilities.

Hydrogen embrittlement typically occurs during the manufacturing process, or in other situations where hydrogen is present or produced, such as electroplating. When hydrogen atoms come into contact with a susceptible metal – at room temperature or higher – they can enter into the metal lattice and diffuse through the grains. These atoms then come together, forming small bubbles at the metal grain boundaries (the interface between two grains of the same material).

The bubbles then increase pressure between the metal grains, forming tiny cracks inside the material and lowering its ductility.

Fortunately, hydrogen embrittlement is nothing new, and there has been plenty of research conducted into methods for preventing it. There are ways in which we can limit metal’s exposure to environments that introduce hydrogen during manufacture, and post-fabrication heat treatments that can be used to diffuse absorbed hydrogen before embrittlement happens.

There are also several industry tests that can be performed to determine if a certain process will lead to hydrogen embrittlement, including the ASTM F519 test used to evaluate plating/coating processes and service environments.

Hydrogen wear

Hydrogen wear isn’t as well documented as embrittlement, but it is no less important. It works in a similar way, with hydrogen invading the metal surface, but instead of happening during production it occurs during the process of friction.

As metals surfaces connect with each other they generate heat and pressure, causing the particles to separate. This provides gaps which allow hydrogen atoms to infiltrate the metal, pushing out its own particles and causing the deterioration of that machine part. The hydrogen itself can emerge from moisture that has found its way to the friction point and, in many cases, is a byproduct of the lubricant used to protect the machine.

An example of this in action is what has become known as white etching cracking (WEC). WEC is a common cause of bearing failure in wind turbine gearboxes, responsible for around 60% of bearing failures in the industry and illustrates the damage hydrogen can cause in areas of high friction – leading to expensive maintenance and replacement costs.

Though research into hydrogen wear isn’t as commonplace as it is for embrittlement, there has still been a considerable amount conducted. Not only that, but this research has also led to a major discovery; that hydrogen wear can be used to polish a friction surface, and copper can be introduced to control it and prevent the hydrogen atoms from causing damage.

Understand the cause to determine the response

While both causes of mechanical degradation stem from the same element, they elicit entirely different responses. Hydrogen embrittlement poses a significant risk to metals, making them brittle and prone to fracture, and should be eliminated where possible. Hydrogen wear on the other hand, if controlled, can be harnessed and used to improve machine performance.

Understanding the differences between the two is more than an academic exercise, it is a practical necessity for enhancing the longevity and functionality of machinery. As we move towards a future where hydrogen is set to play a central role in achieving net-zero carbon emissions, the importance of recognizing and addressing the element’s various effects on metal continues to grow.

By deepening our understanding of these phenomena, we can continue to develop the knowledge and solutions needed to overcome the challenges they present – helping to reduce the waste caused by mechanical failure and further advance our pursuit of green energy.