A grade 12.9 bolt passes every quality control, is correctly installed, and fails within 72 hours. No visible corrosion. No overload. 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, particularly during 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 resistance.
The most dangerous aspect of this phenomenon is its delayed nature: the part can pass a visual inspection, a torque test, and even dimensional checks without showing any apparent defect. 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 can look perfect and fail days after being put into service.
In which high-strength parts does hydrogen embrittlement occur?
The risk of hydrogen embrittlement is directly proportional to the hardness and strength of the steel. The higher the mechanical resistance, the greater the material’s susceptibility to absorbing hydrogen and developing internal fractures. The most vulnerable parts are those that combine high strength with critical geometries — such as threads, undercuts, or cross-section changes — where stresses concentrate.
How does hydrogen enter the part during the coating process?
The source of the problem is not in service: it’s in the coating application process itself. Surface treatments involving acidic media or electric current are the primary responsible for hydrogen absorption into the metal substrate.
Process stages that generate risk:
- Acid pickling (pretreatment): During surface cleaning and activation with acids, atomic hydrogen is produced as a byproduct of the reaction. Part of that hydrogen penetrates the steel before the coating process even begins.
- Electrodeposition (electrolytic zinc plating): During the passage of electric current through the zinc bath, a fraction of the hydrogen generated at the cathode is absorbed into the metal’s crystal structure instead of being released as gas.
The de-embrittlement (baking) process can reduce the risk, but does not eliminate it — it is a palliative treatment. For high-hardness parts, ASTM B-850 and other international specifications require thermal de-embrittlement treatment within 4 hours after coating, with temperature and holding time defined according to the steel grade.
Conclusion
Hydrogen embrittlement is a real, silent, and preventable risk. The good news is that the problem has a solution: non-electrolytic coatings such as Laurentcoat® eliminate the risk at the source, without compromising corrosion resistance or the dimensional precision 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 connect with our technical team.
