A Class 12.9 bolt passes all quality controls, is installed correctly, and fails after 72 hours. There is no visible corrosion. There was no overloading. The problem occurred before assembly, during the coating process. It's called hydrogen embrittlement, and in the metalworking industry, it's one of the most underestimated failure modes.
What is Hydrogen Embrittlement?
Hydrogen embrittlement is a phenomenon in which atomic hydrogen penetrates the microstructure of steel during certain industrial processes, especially acid pickling and electrodeposition. Once absorbed, the hydrogen becomes trapped in the metal's crystal lattice and generates internal stresses that drastically reduce ductility and fracture toughness.
The most dangerous aspect of this phenomenon is its delayed nature: the part can pass a visual inspection, a torque test, and even dimensional tests without showing any apparent defects. Microcracks propagate internally and silently, and failure occurs hours, days, or even weeks after installation, under normal service loads.
Hydrogen embrittlement is not always visible at the time of inspection. A part may appear perfect and fail days after being put into service.
In which high-strength components does hydrogen embrittlement occur?
The risk of hydrogen embrittlement is directly proportional to the hardness and strength of the steel. The greater the mechanical strength, the greater the material's susceptibility to absorbing hydrogen and developing internal fractures. The most vulnerable components are those that combine high strength with critical geometries—such as threads, recesses, or changes in cross-section—where stresses are concentrated.
How is hydrogen introduced during the coating process?
The source of hydrogen is not in service: it is in the coating application process. Surface treatments involving acidic media or electric current are the main causes of hydrogen absorption in the metallic substrate.
Process stages that generate risk
- Acid Pickling (Pretreatment): During cleaning and surface activation with acids, atomic hydrogen is produced as a reaction byproduct. Some of this hydrogen penetrates the steel before the coating process begins.
- Electroplating (Electrolytic Zinc Plating): During the passage of electric current through the zinc plating bath, a fraction of the hydrogen generated at the cathode is absorbed into the metal structure instead of being released as a gas.
The dehydrogenation process can reduce the risk, but it does not eliminate it because it is a palliative treatment. For high-hardness parts, ASTM B-850 and other international specifications require dehydrogenation heat treatments within 4 hours of coating, with the temperature and dwell time depending on the steel class.
Conclusion
Hydrogen embrittlement is a real, silent, and preventable risk. The good news is that the problem has a solution: non-electrolytic coatings like Laurentcoat® eliminate the risk at the source, without compromising corrosion resistance or the dimensional accuracy of the parts.
Do you work with high-strength parts and want to assess the risk in your current process?
You can contact us at info@chousa.com.ar and get in touch with our technical team.