Author: Site Editor Publish Time: 2026-09-19
In 2026, why the hydrogen embrittlement alarm bell rings frequently?
In 2026, cases of hydrogen embrittlement failure of high-strength fasteners will emerge in many key areas. Wind power, aviation, automobiles, nuclear power - these industries with extremely high safety requirements have all focused on the same question: why does a high-strength bolt break without warning after assembly?
Judging from public academic literature and industry reports, the failure cases disclosed in 2026 show highly consistent characteristics: delayed fracture, flat fractures without plastic deformation, no abnormalities during assembly, but sudden failure after hours, days or even weeks of service. All evidence points to the same source - hydrogen atoms introduced during the electroplating process.
Typical cases in 2026:
1. Wind power bolts: break after pre-tightening
A high-strength bolt of a wind turbine component broke after being pre-tightened. Failure analysis adopts methods such as macroscopic observation, chemical composition analysis, mechanical property testing, scanning electron microscopy and energy spectrum analysis. The results show that the fracture property of the bolt is hydrogen-induced brittle fracture. During the electroplating process, a large number of hydrogen atoms are introduced into the bolt and gather at the grain boundary, causing pressure there. At the same time, the bolt is subjected to static stress, and cracks initiate at the grain boundary under the combined action of the two forces. The cracks continue to expand and eventually lead to fracture.
The measured tensile strength of this bolt is 1310 MPa and the core hardness is 43 HRC, which is a typical high-strength fastener above grade 10.9.
2. 30CrMnSi steel screws: the price of insufficient hydrogen removal
Another study analyzed the causes of fracture of 30CrMnSi steel screws used in a certain product. The fracture surface showed an intergranular fracture morphology, with "chicken claw" characteristics and secondary cracks visible, which are typical hydrogen embrittlement delayed fracture characteristics. The structure and hardness analysis shows that there is 3.0-level band segregation, the overall hardness is high, and the microhardness distribution is uneven. Combined with the production process, it is speculated that the main reason for the fracture is insufficient hydrogen removal after hydrogenation in the galvanizing process. At the same time, the high hardness of the material and the uneven structure together lead to hydrogen embrittlement delayed fracture.
3. Aviation-specific bolts: double risks of pickling and galvanizing
A failure analysis published in "Aviation Maintenance and Engineering" in 2026 pointed out that a certain high-strength special bolt broke during assembly. The analysis results show that hydrogen embrittlement is mainly related to the pickling and galvanizing processes. It needs to be prevented and controlled by optimizing the process flow and adding a hydrogen removal process after the pickling process.
4. Inner barrel of ejection barrel: sensitivity of hydrogen removal parameters
The abnormal fluorescence on the outer surface of the seat ejection tube inner tube was determined to be a hydrogen-induced brittle crack. Research clearly points out that hydrogen embrittlement cracks on the outer surface of the inner cylinder are mainly related to the hydrogen removal temperature and hydrogen removal time after the electroplating process. The hydrogen removal temperature and hydrogen removal time must be strictly controlled to reduce hydrogen embrittlement susceptibility.
The common characteristic of these cases is sobering: the "culprit" of the failure is not the material itself, but the hydrogen introduced during the manufacturing process.
Nature and Characteristics of Hydrogen Embrittlement Failure
Hydrogen embrittlement is a phenomenon in which hydrogen causes the toughness of materials to decrease and delayed brittle fracture occurs under tensile stress. The essence is environmental embrittlement caused by the interaction of three factors: "material-environment-stress". Three conditions must be met for it to occur:
① The material is sensitive to hydrogen damage;
② There is tensile stress (external load or residual stress;
③Hydrogen atoms exist.
The more the three conditions overlap, the faster the damage will develop until it breaks.
Regarding the mechanism of hydrogen embrittlement, there are five main theories in the academic community:
Hydrogen pressure theory (hydrogen atoms combine into hydrogen molecules in traps such as grain boundaries, dislocations, and inclusions to generate internal pressure, which induces cracking when the yield strength is reached), weak bond theory (hydrogen atoms enter the lattice gap to reduce the bonding strength of metal atoms), hydrogen reduces surface energy theory, hydrogen promotes local plastic deformation theory (hydrogen segregation dislocations form Cottrell air clusters), and hydrogen promotes strain-induced vacancy theory. The most common one in engineering is the synthesis of hydrogen pressure theory and weak bond theory - hydrogen enrichment reduces the binding energy of grain boundaries, superimposes the internal pressure of hydrogen molecules, and cracks slowly expand under sustained tensile stress.
Hydrogen embrittlement failure of high-strength fasteners has three distinctive characteristics:
Brittle fracture without plastic deformation: fractures mostly occur at stress concentration locations such as the thread bottom and the transition fillet between the head and the rod. The fracture is flush, perpendicular to the axial direction, and has no necking. The fracture source area has an intergranular morphology ("chicken claw-shaped" secondary crack), which is obviously different from overload ductile fracture;
Hysteresis and suddenness: If the fastener passes the factory inspection, it will suddenly break within hours to months after assembly and pre-tightening, without obvious signs;
Coupling trigger: Simple hydrogen-rich or simple stress does not necessarily cause fracture, it must occur under the combined action of hydrogen source and tensile stress.
For high-strength fasteners, material factors are the "root cause": as the strength of steel increases, the hydrogen embrittlement susceptibility index increases and the lasting strength decreases. Due to its high dislocation density and many grain boundary defects, quenched martensitic steel is significantly more susceptible to hydrogen embrittlement than bainite, pearlite, and ferrite structures. The stronger the material, the lower the critical hydrogen concentration allowed, and even trace amounts of hydrogen introduced during the process can be fatal.
Inherent Limitations of the Electroplating Process
The entire electro-galvanizing process involves multiple sources of hydrogen: "introducing potential failure triggers while simultaneously providing protection."
Core Conflict:
The formation mechanism of anti-corrosion coatings inherently involves side reactions such as hydrogen evolution and hydrogen ingress. Baking for hydrogen removal is merely a remedial measure applied after the fact; it cannot fundamentally eliminate the risk of hydrogen embrittlement introduced during the process.
Pre-treatment Stage: Acid pickling is a significant source of hydrogen ingress.
After heat treatment, high-strength fasteners often develop surface oxide scale, which is typically removed in industrial settings through pickling with hydrochloric or sulfuric acid. During this process, the steel substrate undergoes a chemical reaction that releases hydrogen, leading to the infiltration of a significant number of hydrogen atoms into the material's grain boundaries and structural defects. Engineering data indicate that the amount of hydrogen absorbed by the steel during pickling increases linearly with the square root of time until saturation is reached; furthermore, lower pH levels result in higher saturation values.
Real-world failure cases underscore the critical risks associated with pickling. For instance, the fracture of a main-shaft connecting bolt in a wind turbine was attributed to the following conclusion: "Improper control of the pickling process prior to surface treatment led to significant hydrogen infiltration, which was the primary cause of the hydrogen-induced brittle fracture." Additionally, cathodic electrolytic degreasing also triggers hydrogen evolution as a side reaction, further increasing the hydrogen content within the substrate. While the use of corrosion inhibitors or shorter pickling durations can mitigate hydrogen infiltration, they cannot eliminate it entirely.
Electroplating Process: Cathodic Hydrogen Evolution and the Generation of Active Hydrogen
During electroplating, the fastener acts as the cathode. As zinc ions are reduced and deposited, hydrogen ions on the workpiece surface are also reduced to form atomic hydrogen (H⁺ + e⁻ → H). While some of this hydrogen recombines and escapes, a portion infiltrates the high-strength steel substrate. The rate of hydrogen evolution is directly determined by the electroplating current density, pH level, and temperature.
More critically, the resulting coating acts to "trap" the hydrogen: the deposited zinc layer seals the surface of the substrate, preventing any hydrogen that has already infiltrated from escaping. Subsequent passivation and sealing treatments further obstruct these diffusion pathways. Engineering research confirms that zinc acts as a barrier to hydrogen permeation; if hydrogen has already infiltrated the workpiece prior to galvanizing, it becomes even more difficult for that hydrogen to escape afterward [1]. This creates a fundamental paradox in the electroplating process: it provides surface protection while simultaneously introducing a catalyst for failure.
Hydrogen-Removal Baking: The Inherent Limitations of Remedial Measures
In the industry, it is standard practice to perform a hydrogen-removal bake—heating the components to between 190°C and 230°C for several hours (typically 2 to 24 hours, depending on the strength grade)—within four hours of the electroplating process. However, the de-embrittlement baking process has four inherent limitations:
1. **Narrow time window:** As hydrogen diffuses deeper into the steel over time, it becomes increasingly difficult to remove from deep-seated traps. Consequently, baking must be performed within a 4-hour window following the electroplating process.
2. **Restricted temperature range:** Excessive temperatures can reduce the tempering hardness of high-strength steel and compromise its mechanical properties, while insufficient temperatures fail to effectively drive out the hydrogen.
3. **High process variability and poor batch-to-batch consistency:** Fluctuations in plating bath composition, unstable electrical current, or inadequate control over pickling duration can lead to inconsistent hydrogen absorption, making it difficult to ensure uniform resistance to hydrogen embrittlement.
4. **Inadequacy for ultra-high-strength steel (e.g., 12.9-grade fasteners):** In safety-critical applications—such as those in the wind power industry—electroplating is generally prohibited or strictly restricted for 12.9-grade fasteners, as baking alone cannot sufficiently mitigate the risk of hydrogen embrittlement. Real-World Cases of Hydrogen Embrittlement Failure in Electroplating Processes
Fracture of Grade 12.9 Bolts in Wind Turbine Units: Three Grade 12.9 bolts (made of 35CrMo steel with a hot-dip galvanized coating) used in wind turbine components fractured within one week of being pre-tensioned. Physical and chemical analysis confirmed the failures were caused by hydrogen-induced brittle fracture—a result of a large number of hydrogen atoms being introduced during the electroplating process and accumulating at grain boundaries. This, combined with static pre-tension stress, triggered the initiation and propagation of cracks.
Fracture of Main Shaft Connection Bolts in Wind Turbines: Improper control of the pickling (acid cleaning) process led to significant hydrogen ingress, resulting in hydrogen-induced brittle fracture.
Fracture of Ultra-High-Strength Steel Screws Under Low Stress: A 30CrMnSiNi2A steel screw used in a solid-propellant rocket engine fractured at 11.8 MPa—well below the design requirement of 24.1 MPa. The fracture occurred at the first thread, with the origin zone exhibiting an intergranular morphology. A primary cause of the failure was that the material's actual strength (tensile strength σb ≈ 1690 MPa) exceeded the design specification (1500 ± 98 MPa), which increased its susceptibility to hydrogen embrittlement.
Hydrogen Threshold Cases: In one instance, a hydrogen mass fraction of just 0.0004% within the matrix of a 30CrMnSiNi2A bolt was sufficient to cause hydrogen-induced brittle fracture. In another case, an engine intake duct bolt failed following a secondary galvanizing process; the microstructure of the fractured bolt consisted of tempered martensite (hardness 452 HV, tensile strength ~1493 MPa)—significantly higher than the tempered sorbite microstructure (375 HV, ~1222 MPa) found in intact bolts from the same batch. The combination of excessive strength and high hydrogen content led to the failure.
Summary: The fundamental conflict in electroplating processes is that the mechanism used to form anti-corrosion coatings inherently involves side reactions that generate and introduce hydrogen—essentially "introducing a failure trigger while attempting to provide protection." Post-plating baking is merely a remedial measure and cannot completely eliminate the risk of hydrogen embrittlement.
Electro-galvanizing vs. Zinc-Aluminum Coatings: A Comparison of Hydrogen Introduction Pathways
Principles and Process Systems for Hydrogen Embrittlement Prevention in Zinc-Aluminum Coatings
Zinc-aluminum coatings (commonly known as Dacromet, Geomet, etc.) consist of flake-like zinc and aluminum particles bound together by an inorganic matrix—traditionally containing chromates, though modern formulations are predominantly chromium-free. These coatings form a protective film through physical application followed by low-temperature curing. As they do not rely on electrochemical reactions, they effectively eliminate the root cause of hydrogen evolution—a common issue in electroplating. Consequently, this technology serves as an environmentally friendly and highly efficient method for the anti-corrosion protection of metals.
Core Mechanisms for Hydrogen Embrittlement Prevention and Control
Replacement of strong-acid pickling with mechanical pretreatment: Alkaline degreasing combined with mechanical descaling (via sandblasting or shot blasting) removes oxide scale through high-speed abrasive impact. Since no acid reacts with the steel substrate, hydrogen ingress—a common byproduct of acid pickling—is eliminated at the source.
Physical coating and curing without electrolytic hydrogen evolution: As there is no external power source for electrodeposition, no cathodic hydrogen evolution side reactions occur, and no new active hydrogen atoms are generated.
Hydrogen removal during the curing process: The coating is cured at approximately 300°C, a temperature that facilitates the diffusion and escape of any residual hydrogen trapped within the substrate. Furthermore, the coating features a porous, layered, flake-like structure that is permeable to hydrogen, preventing the "trapping" of hydrogen often seen with the passivation films used in electroplating.
Anti-corrosion mechanism (Layered shielding + Sacrificial cathodic protection): Zinc and aluminum flakes are stacked in parallel within the coating, creating a "labyrinth" barrier that significantly slows the penetration of corrosive media. If the coating is damaged, the zinc and aluminum—having a lower electrochemical potential than the steel substrate—undergo preferential anodic dissolution, thereby protecting the steel. These coatings can withstand 500–1,200 hours in neutral salt spray tests—roughly 5 to 10 times the performance of standard electro-galvanized coatings (100–200 hours).
Operational Constraints and Considerations
While zinc-aluminum coatings can eliminate the risk of hydrogen embrittlement stemming from the coating process itself, they do not guarantee absolute zero risk. Key considerations include:
Inability to remove pre-existing hydrogen: If hydrogen is already present in the workpiece due to heat treatment, raw material composition, or prior processing steps, the coating cannot remove this "trapped" hydrogen. Strict quality control of incoming materials is therefore essential.
Temperature limitations: Prolonged exposure to high temperatures can cause the coating to age and fail. (Some systems can withstand temperatures up to approximately 300°C—exceeding the threshold at which micro-cracks form in electro-galvanized layers.)
Lower mechanical strength: The surface of the coating is relatively soft; care must be taken during handling and assembly to avoid impacts or scratches.
Requirement for a sealant: In highly corrosive environments, an additional sealing layer may be necessary. When selecting a sealant, its hydrogen permeability characteristics must be evaluated. International Standards and Applicable Strength Levels
ISO 10683:2018, "Fasteners — Non-electrolytically applied zinc flake coatings," explicitly states that such coatings are particularly suitable for high-strength fasteners with a tensile strength of ≥ 1000 MPa (i.e., property class 10.9 and above) to mitigate the risk of internal hydrogen embrittlement (IHE). This standard confirms, from a regulatory perspective, the suitability of zinc flake coatings as a solution for preventing hydrogen embrittlement in ultra-high-strength fasteners.
Selection of Hydrogen Embrittlement Prevention Solutions for High-Strength Fasteners and Quality Inspection Systems
Principles for Selecting Surface Treatments
Quality Verification Methods for Hydrogen Embrittlement
Constant-Load Delayed Fracture Test (ISO 15330 / GB/T 3098.17): Involves subjecting a fastener to a constant tensile load over an extended period to evaluate its susceptibility to delayed fracture. This is the primary test for determining hydrogen embrittlement in high-strength fasteners.
Process Control Verification (ASTM F1940): Utilizes Incremental Step Loading (ISL) to monitor residual hydrogen resulting from electroplating or coating processes in real time; used for process qualification and periodic audits.
Fractographic Analysis: Employs scanning electron microscopy (SEM) to determine the fracture mode—distinguishing between intergranular fractures (indicative of hydrogen embrittlement) and fractures caused by overload or fatigue—by identifying characteristic "chicken-claw" (quasi-cleavage/intergranular) features.
Supporting Physical and Chemical Testing: Includes metallographic examination, hardness/strength conversion (as hydrogen embrittlement is strongly correlated with high-hardness martensitic microstructures), and thermal desorption analysis for hydrogen content (ISO 16573-1).
Batch Sampling: Increases the sampling frequency for electroplated products to monitor batch-to-batch consistency.
Conclusion
The root cause of hydrogen embrittlement in electroplated high-strength fasteners lies in the electroplating process itself, which introduces multiple sources of hydrogen: hydrogen ingress during acid pickling (pretreatment), hydrogen evolution at the cathode during electroplating, and the entrapment of hydrogen during passivation or sealing. Baking for hydrogen removal is merely a remedial measure; it is constrained by a narrow time window and limited temperature ranges, making it incapable of fully eliminating hydrogen trapped deep within the material. This poses a significant safety risk for ultra-high-strength fasteners (grade 10.9 and above)—a fact repeatedly confirmed by numerous fracture incidents in the wind power and aerospace sectors.
Zinc-aluminum coatings eliminate the need for acid pickling through mechanical pretreatment and replace electrolytic deposition with physical coating and curing. This approach cuts off the path for hydrogen ingress at the source. Furthermore, these coatings provide both a layered barrier and sacrificial cathodic protection against corrosion. Consequently, ISO 10683 explicitly identifies zinc-aluminum coatings as the preferred solution for preventing internal hydrogen embrittlement in high-strength fasteners with a tensile strength of ≥ 1000 MPa. In engineering practice, the selection of surface treatments should not be based solely on anti-corrosion criteria; rather, it must be determined by holistically considering material strength grades, loading conditions, and environmental corrosivity levels. Furthermore, a comprehensive quality control system—encompassing raw materials, surface preparation, coating/electroplating processes, and final-product verification for hydrogen embrittlement—must be established to effectively prevent delayed fracture accidents caused by hydrogen embrittlement.
