Introduction
High strength bolts of grade 10.9 and above are widely used in steel structure, crane, mining equipment and heavy machinery. In order to improve surface anti-corrosion, many overseas engineering projects directly adopt conventional acid pickling and electro-galvanizing process without additional hydrogen bake-out treatment. The assembled bolts look completely normal, but after being placed for several days to several weeks, they suddenly break without external overload.
Industry failure statistics show that more than 80% of sudden fractures of high strength bolts after electroplating are not caused by improper torque or overload, but by hydrogen embrittlement delayed fracture. The hydrogen absorbed during acid pickling and electroplating slowly diffuses into the material matrix, leading to internal crack initiation and sudden brittle fracture under preload. This kind of batch hidden danger can cause large-scale scrap in overseas heavy industry projects, with rework and compensation costs far exceeding the saved plating cost.
This article deeply analyzes the hydrogen embrittlement mechanism of high strength bolts, compares the risk level of different coating processes, sorts out high frequency pitfalls in foreign trade procurement, and provides standard hydrogen embrittlement prevention schemes. We supply hydrogen-safe high strength bolt solutions including bake-out treatment and chromium-free zinc flake coating for heavy-duty projects.
Why High Strength Bolts Crack Days After Assembly
Hydrogen embrittlement is a delayed brittle fracture phenomenon of high strength steel under the combined action of tensile stress and diffusible hydrogen. The core hazard lies in the fact that there is no obvious abnormality before fracture, and the failure time is delayed.
Hydrogen Source in Acid Pickling and Electroplating
During acid pickling and electro-galvanizing, hydrogen atoms are generated on the steel surface and partially penetrate into the material matrix. Ordinary strength bolts have low sensitivity and can tolerate a small amount of hydrogen; however, high strength bolts above 10.9 grade have high tensile strength and high hydrogen sensitivity, and diffusible hydrogen will gradually accumulate at grain boundaries under preload stress.
Delayed Fracture Process
After assembly, the bolt remains under tensile preload. The diffusible hydrogen slowly diffuses to the stress concentration area (thread root, under head fillet), where microcracks initiate and expand. When the crack reaches a critical size, the bolt suddenly fractures. The whole process has no obvious deformation or warning, which is why "assembly looks fine but breaks days later" occurs.
Key Risk Accelerating Factors
Strength level: the higher the grade (10.9 / 12.9), the higher the hydrogen sensitivity.
Plating thickness and process: thick electro-galvanizing without bake-out increases hydrogen intake.
Preload level: higher preload accelerates crack propagation.
Surface defects: decarburization, scratches and corrosion pits become crack initiation points.
Coating Process Risk Comparison
Coating Process | Hydrogen Risk | Typical Failure Mode | Hydrogen Bake-out | Recommendation |
Electro-galvanizing (without bake-out) | High | Delayed brittle fracture in days | Usually omitted | Not for 10.9+ |
Electro-galvanizing + bake-out | Medium | Reduced but not zero risk | Required 4-8h after plating | Conditional use |
Dacromet / Geomet zinc flake | Very low | Virtually no hydrogen ingress | Not required | Preferred for high strength |
Mechanical galvanizing | Low | Limited hydrogen intake | Light bake-out | Acceptable alternative |
Stainless steel A2-70 / A4-80 | None | No hydrogen embrittlement | Not required | Premium option |
High-Frequency Foreign Trade Pitfalls
Blindly Choosing Cheap Electro-Plated High Strength Bolts
Many overseas buyers only compare unit price and grade, ignoring the plating process and bake-out record. Cheap electro-galvanized 10.9 bolts often skip the bake-out step, and batch delayed fracture occurs after site assembly, resulting in large-scale component scrap and project rework.
Misjudging Delayed Fracture as Over-Torque Quality Complaint
When a batch of bolts breaks after installation, the customer usually judges it as insufficient material strength or improper manufacturing, and directly claims compensation. In fact, the root cause is hydrogen embrittlement caused by the electroplating process, which should have been controlled at the supplier stage.
No Bake-out Record in Quality Agreement
Foreign trade orders often only specify grade and plating, without requiring hydrogen embrittlement test or bake-out process record. The buyer cannot verify whether the supplier has actually completed bake-out, leaving the whole batch hidden danger.
Standard Hydrogen Embrittlement Prevention Rules
For bolts grade 10.9 and above, avoid conventional acid pickling electro-galvanizing without bake-out.
If electro-galvanizing is adopted, hydrogen bake-out must be completed within 4 hours after plating, with process record provided.
Prefer chromium-free zinc flake coating (Dacromet / Geomet) for high strength bolts, which introduces virtually no hydrogen.
Require batch hydrogen embrittlement test report for safety-critical structural connections.
Stainless steel A2-70 / A4-80 is the premium option when budget allows.
FAQ
Q1: Why do high strength bolts break days after normal assembly?
A: This is typical hydrogen embrittlement delayed fracture. Hydrogen absorbed during pickling and plating slowly diffuses under preload and causes sudden brittle cracking.
Q2: Can ordinary 8.8 grade bolts have hydrogen embrittlement?
A: The risk is much lower. Hydrogen embrittlement sensitivity rises sharply with strength grade; 10.9 and above are the high-risk range.
Q3: Is bake-out sufficient to eliminate hydrogen risk?
A: Bake-out significantly reduces diffusible hydrogen, but process control and record are essential. Zinc flake coating is the safer route for high strength bolts.
Q4: Can NINGBO TOPBOLT METALWORKS CO., LTD. provide hydrogen-safe high strength bolts?
A: Yes. We provide 10.9 / 12.9 high strength bolts with bake-out process or zinc flake coating, plus hydrogen embrittlement test reports for heavy-duty projects.
