The image above shows a bolt manufactured by a certain factory that has fractured. The customer has been unable to determine the cause of the fracture and is seeking to understand the underlying reasons.

There are many possible causes of bolt failure; after gathering some information from the customer, it can generally be determined that the fracture was due to hydrogen embrittlement.

On this occasion, let’s discuss the issue of hydrogen embrittlement in bolts and how to prevent it.

I. What is hydrogen embrittlement?

Simply put, “hydrogen” refers to hydrogen atoms. Because hydrogen atoms are extremely small, during electroplating and acid pickling, they can penetrate from the surface into the steel matrix, becoming trapped in the bolt head or within the thread gaps.

Due to the ingress of hydrogen atoms, minute internal cracks have already formed in the bolt. When the bolt is installed in a product, the tensile stress generated during tightening, combined with these internal cracks, means that each vibration experienced during operation could cause it to fracture suddenly and without warning.

This is hydrogen embrittlement, also known as delayed fracture.

II. Which types of bolts are prone to hydrogen‑induced brittle fracture?

Hydrogen embrittlement is predominantly observed in high-strength bolts, with higher hardness generally correlating with a greater risk of hydrogen embrittlement.

For example: 12.9 high-strength bolts, 10.9 high-strength bolts, 40Cr quenched and tempered bolts, etc.

Among high-strength bolts, those with a surface electroplated zinc coating are most prone to hydrogen embrittlement. By contrast, blackening, Dacromet, and powder‑zinc coatings virtually eliminate the risk of hydrogen embrittlement; therefore, electroplated zinc is generally not recommended for high-strength bolts.

III. How does hydrogen permeate into the interior of a bolt?

The main reasons are as follows:

1. During electroplating, the electrolyte decomposes to release a large number of hydrogen atoms. Because hydrogen atoms are extremely small, the longer the plating time and the higher the current, the more hydrogen atoms penetrate into the steel substrate.

2. During the pickling process, the reaction between the acid and iron generates hydrogen gas; hydrogen atoms also diffuse into the steel matrix. Similar to electroplating, the longer the pickling time and the higher the acid concentration, the greater the amount of hydrogen that penetrates the steel.

IV. How can it be prevented?

Domeda’s main approaches to preventing hydrogen embrittlement are as follows:

1. Control the electroplating or pickling process

As mentioned above, electroplating and acid pickling are the primary causes of hydrogen embrittlement in bolts. Therefore, the most effective approach is to avoid electroplating or acid pickling for high-strength bolts and instead adopt alternative treatment methods.

For example, for high-strength bolts, we recommend against using electroplating; instead, shot blasting or sandblasting can be employed as alternatives to acid pickling. We will explain the causes of hydrogen embrittlement and leave the decision on whether to adopt these alternatives to the customer.

If the customer insists on using electroplating or acid pickling, selecting a low‑hydrogen galvanizing bath and implementing post‑plating forced hydrogen‑removal baking (dehydrogenation) can effectively minimize the ingress of hydrogen atoms into the steel and facilitate their removal.

2. Hydrogen atoms are released in advance during the processing stage.

After pickling, high-strength bolts must undergo a heat‑treatment tempering process. During tempering, the tempering temperature should be carefully controlled to prevent excessive hardness and minimize internal cracks in the steel.

3. Other Stages

Upon receipt of high-strength bolt steel, fracture testing must be conducted to proactively identify and mitigate the risk of hydrogen embrittlement.

During the optional‑equipment selection process, customers are advised to use compatible components. For high‑strength bolts used in safety‑critical structures and load‑bearing applications, electro‑galvanized coatings are prohibited.

During the assembly process, use tools calibrated to the specified torque; the applied torque must not exceed the bolt’s maximum allowable torque.

The above are some of the more common causes of hydrogen embrittlement, along with several preventive measures. Given the wide range of applications for bolts and the complex nature of hydrogen‑embrittlement mechanisms, please feel free to discuss this further with us if needed.