Hydrogen Embrittlement

*Updated 2026

Hydrogen embrittlement occurs when steel cracks due to hydrogen that is trapped between the grains of the steel. Although steel commonly absorbs hydrogen during the hot-dip galvanizing process, it is usually expelled due to the temperature of the zinc in the galvanizing kettle. In some cases, however, the grain size of the steel is too small to allow release of atomic hydrogen. This can later cause cracking due to increased stress at the location of the hydrogen between the grains. Grains of steels with a tensile strength below 150 ksi (150,000 psi) are usually big enough to allow escape of hydrogen, but for steels having a tensile strength of 150 ksi or greater, there is a potential for hydrogen to remain trapped between grains leading to hydrogen embrittlement.

These measures are particularly important regarding hydrogen embrittlement:

1. Source of Hydrogen

The primary source of hydrogen associated with galvanizing is the acid cleaning solution (pickling) process used to remove mill scale prior to coating. During this step, atomic hydrogen is generated and can be absorbed into the steel surface.

In most structural steels, absorbed hydrogen is released during subsequent processing, particularly at elevated temperatures. As a result, hydrogen does not typically accumulate to levels that cause embrittlement in conventional steels.

2. Steels at Risk

Hydrogen embrittlement is generally only a concern for steels with specific characteristics:

• Ultimate tensile strength greater than approximately 150 ksi (1,100 MPa)
• Fine grain structure capable of trapping hydrogen
• Severe cold work or localized hardening prior to pickling

Steels below this strength range typically have a coarser grain structure that allows hydrogen to diffuse out during processing. As a result, hydrogen embrittlement is highly unlikely in conventional structural steels.

3. Mitigation Measures Before Galvanizing

ASTM A143 provides several practical methods to reduce the likelihood of hydrogen embrittlement when susceptible steels are involved:

• Thermal treatment after cleaning: Heating at approximately 300 °F (150 °C) to promote hydrogen diffusion
• Mechanical surface preparation: Abrasive blast cleaning rather than acid solution cleaning will reduce hydrogen exposure
• Controlled pickling: Use of rapid or “flash” pickling to limit hydrogen absorption

Hydrogen embrittlement is not observed until the part is under load and can be avoided in several ways. The most obvious way is to ensure the designer has chosen steel with a tensile strength less than 150 ksi. These types of steels have grains large enough to allow escape of any trapped hydrogen. When it is necessary to galvanize high strength steel, a modified galvanizing process can be used to minimize the chances of hydrogen embrittlement. Rather than chemical cleaning the steel for the normal length of time, this modified process includes mechanically cleaning the steel, such as by blast cleaning, and then flash chemical cleaning (less than 30 seconds in the chemical cleaning bath) the steel. Flash chemical cleaning is necessary to remove any residues from the blasting operation. This reduces the amount of hydrogen the steel is exposed to and thus reduces the chances of hydrogen becoming trapped in the small grains of high strength steel.