Hydrogen Embrittlement

Hydrogen Embrittlement in Fasteners: Comprehensive Technical Guide

Hydrogen embrittlement represents one of the most critical failure mechanisms affecting high-strength fasteners in modern engineering applications. This comprehensive guide addresses the fundamental science, prevention strategies, testing methodologies and industry standards governing hydrogen embrittlement in threaded fasteners.

Understanding Hydrogen Embrittlement

What is hydrogen embrittlement?


Hydrogen embrittlement (HE) is a metallurgical phenomenon whereby atomic hydrogen penetrates the crystalline structure of steel, causing severe loss of ductility and load-bearing capacity that can result in catastrophic delayed brittle fracture. Unlike conventional mechanical failure, hydrogen embrittlement occurs at stress levels significantly below the material's yield strength, often manifesting hours, days or even weeks after initial exposure to hydrogen sources. The phenomenon is particularly insidious because affected fasteners may appear structurally sound during inspection yet fail unexpectedly under normal service loads. Hydrogen embrittlement specifically affects the mechanical properties of ferrous metals and certain other alloys, transforming normally ductile materials into brittle components susceptible to sudden fracture under tensile stress.

What causes hydrogen embrittlement in fasteners?

Hydrogen embrittlement arises from multiple sources throughout the fastener lifecycle. Primary causes include electroplating processes (zinc, cadmium, nickel, chromium), where electrolytic action generates atomic hydrogen that readily absorbs into the steel substrate. Acid pickling during surface preparation introduces hydrogen through chemical dissolution of mill scale and oxides. Phosphating treatments, commonly employed for corrosion protection, can contribute hydrogen through chemical reactions. Hot-dip galvanising, whilst not directly introducing hydrogen during the molten zinc immersion, can release trapped residual hydrogen from previous manufacturing steps through thermal activation. Welding operations generate hydrogen from moisture in electrode coatings, shielding gases or base metal contaminants. Cathodic protection systems in service environments continuously generate hydrogen at the protected metal surface through electrochemical reduction reactions. Corrosion processes, particularly in acidic or marine environments containing hydrogen sulphide (H₂S), produce atomic hydrogen as a reaction product. Manufacturing processes including heat treatment (particularly from carburising atmospheres), cold forming, machining with unsuitable lubricants, and grinding operations can all introduce or trap hydrogen within the fastener material structure.

How does hydrogen cause embrittlement at the atomic level?

The embrittlement mechanism begins when atomic hydrogen (H) penetrates the steel surface, diffusing through the crystalline lattice structure until reaching regions of high triaxial tensile stress or specific metallurgical features. These regions include grain boundaries, inclusions, dislocations, precipitates and other microstructural discontinuities that act as "trap sites". Hydrogen atoms accumulate at these locations, where several competing mechanisms contribute to embrittlement. The hydrogen-enhanced decohesion mechanism proposes that hydrogen reduces the cohesive strength of atomic bonds, particularly at grain boundaries, facilitating crack initiation and propagation at applied stresses well below normal fracture levels. The hydrogen-enhanced localised plasticity mechanism suggests hydrogen increases the mobility of dislocations, creating localised plastic deformation zones that promote crack formation. Hydrogen may also recombine into molecular form (H₂) at internal discontinuities, generating enormous internal pressures that create voids and blisters, providing crack initiation sites. Additionally, hydrogen can stabilise specific phases within the microstructure that exhibit reduced ductility. The threshold stress for hydrogen embrittlement failure typically ranges from 60-90% of the material's yield strength, varying with hydrogen concentration, material hardness, and microstructural characteristics.

Which fastener materials and grades are susceptible?

Material susceptibility to hydrogen embrittlement increases exponentially with hardness and strength. Fasteners with hardness above 39 HRC (approximately 390 HV, corresponding to 1200 MPa tensile strength) are classified as susceptible according to ISO 4042, ASTM F1941 and industry consensus. This encompasses metric property classes 10.9 and 12.9 per ISO 898-1, SAE J429 Grade 8 bolts, ASTM A574 socket head cap screws, and all case-hardened fasteners regardless of core hardness. Fasteners between 34-39 HRC represent a transitional susceptibility range where manufacturing process control becomes critical. Below 34 HRC (320 HV), including property classes 8.8, 5.8, 4.8 and SAE Grade 5, materials demonstrate very low susceptibility and generally do not require hydrogen embrittlement relief measures. However, critical applications may specify precautionary measures even for lower-strength materials. Property class 12.9 fasteners merit special consideration, as ISO 898-1 includes specific warnings regarding stress corrosion cracking risks due to their specified hardness range of 39-44 HRC (385-435 HV). Material composition significantly influences susceptibility, with boron-treated steels, high-carbon steels and those with elevated phosphorus or sulphur content demonstrating increased sensitivity. Fastener geometry also affects risk, with smaller diameter fasteners, threads under 6mm diameter, components with stress concentrations, and parts subjected to cold working after heat treatment showing elevated vulnerability. Martensitic and precipitation-hardened stainless steels require particular attention, whilst austenitic stainless steels (300 series) generally demonstrate good resistance except when sensitised or cold-worked to high strength levels.

Dangers and Effects

What are the consequences of hydrogen embrittlement?


Hydrogen embrittlement failure presents catastrophic safety and economic consequences across multiple domains. The delayed failure characteristic means fasteners may pass installation torque verification yet fail unexpectedly during service, potentially occurring minutes, hours, days or even weeks after assembly. Structural consequences include sudden brittle fracture in critical load-bearing applications, complete loss of joint integrity without warning, and potential progressive failure of surrounding components. Safety implications are severe, particularly in automotive, aerospace, construction, pressure vessel, lifting equipment and marine applications where human life depends on fastener reliability. The unpredictable nature of hydrogen embrittlement complicates maintenance scheduling and inspection protocols. Economic impacts encompass warranty claims, product recalls, litigation costs, equipment downtime, emergency repairs, and potential reputational damage to manufacturers and end users. Particularly concerning are applications involving residual stresses from installation torque, cyclic loading conditions, corrosive service environments, and elevated operating temperatures that can accelerate hydrogen mobility. Historical failures attributed to hydrogen embrittlement have resulted in numerous industrial accidents, structural collapses, vehicle recalls and fatalities, establishing hydrogen embrittlement prevention as a fundamental requirement in modern fastener specification and quality control.

How quickly can hydrogen embrittlement failure occur?

The timeline for hydrogen embrittlement failure varies considerably based on multiple interacting factors. Immediate failures may occur within minutes to hours of hydrogen exposure when hydrogen concentration exceeds critical thresholds in highly stressed, very hard materials. Short-term failures typically manifest within 24 hours to one week, representing the most common timeframe for manufacturing-related hydrogen embrittlement (internal hydrogen embrittlement). Medium-term failures can emerge weeks to months after exposure, particularly in fasteners subject to sustained service loads or moderate environmental hydrogen sources. Long-term failures may develop over months to years through environmental hydrogen embrittlement mechanisms including stress corrosion cracking, where continuous low-level hydrogen generation from corrosion or cathodic protection gradually builds sufficient hydrogen concentration for delayed failure. The incubation period depends critically on applied stress level (failures occur faster at higher stress), hydrogen concentration in the material, material hardness and susceptibility, temperature (elevated temperatures accelerate hydrogen diffusion), and the presence of stress concentrators or metallurgical defects. This time-dependent nature makes hydrogen embrittlement particularly dangerous, as fasteners may pass initial quality checks, survive proof testing, and perform satisfactorily during early service before failing catastrophically without warning when critical hydrogen concentration and stress conditions align.

What are the characteristic failure modes?

Hydrogen embrittlement failures exhibit distinctive characteristics that differentiate them from normal ductile failures or fatigue. Fracture surfaces display brittle, intergranular or quasi-cleavage morphology with minimal macroscopic plastic deformation, contrasting sharply with the necking and cup-cone appearance of normal tensile failure. Scanning electron microscopy reveals characteristic faceted, crystalline fracture surfaces with grain boundary separation patterns. Failures typically initiate at or near the surface, often at thread roots where stress concentration and hydrogen content are highest, propagating rapidly through the cross-section perpendicular to the tensile stress axis. Multiple crack initiation sites may be observed. Fractures frequently occur in the threaded section rather than the unthreaded shank, though case-hardened fasteners may fail at the case-core interface due to hydrogen accumulation at the microstructural transition. The absence of visible corrosion or mechanical damage before failure often puzzles investigators. Hydrogen-embrittled fasteners may exhibit reduced breaking torque during disassembly compared to installation torque. Failed components show no loss of hardness or apparent metallurgical degradation beyond the embrittlement effect itself. Fractographic examination often reveals "fish-eye" patterns indicative of internal hydrogen pressure-induced crack initiation. Unlike fatigue failures that show progressive beach marks or striations, hydrogen embrittlement fractures appear sudden and complete, with limited or no indication of incremental crack growth prior to catastrophic failure.

Prevention Strategies

How can hydrogen embrittlement be prevented during manufacturing?


Manufacturing process control provides the primary defence against hydrogen embrittlement. Material selection begins with choosing appropriate steel grades and hardness levels for the application, potentially avoiding susceptible hardness ranges where alternative materials can meet performance requirements. Heat treatment processes must be meticulously controlled, ensuring proper quenching and tempering procedures to achieve intended microstructures, avoiding excessive hardness from over-quenching or inadequate tempering, preventing surface carburisation, and ensuring complete martensitic transformation without retained austenite. Surface preparation methods should minimise hydrogen introduction by preferring mechanical cleaning (shot blasting, tumbling, grinding) over acid pickling where possible. When pickling is unavoidable, inhibitors should be employed to suppress hydrogen evolution, with pickling time, acid concentration and temperature strictly controlled, followed by thorough rinsing and immediate processing to subsequent steps. Phosphate coating processes require careful control of bath chemistry, temperature, and immersion time, with complete removal of phosphate coatings before heat treatment of high-strength parts to prevent phosphorus diffusion forming embrittlement-susceptible surface layers. Manufacturing sequence is critical, with mechanical operations (threading, forming, machining) completed before final heat treatment wherever possible to avoid introducing hydrogen into hardened material. Thread rolling should be performed before hardening for optimal fatigue strength without hydrogen risk. Any unavoidable machining of hardened parts must use appropriate cutting fluids that do not decompose into hydrogen sources, with sulphur-containing lubricants avoided on susceptible materials.

What baking procedures prevent hydrogen embrittlement after plating?

Post-coating hydrogen embrittlement relief baking represents the most effective method for removing absorbed hydrogen from electroplated fasteners. The process must commence within 4 hours of electroplating completion, with 1 hour preferred, before hydrogen has time to diffuse into stressed regions and become trapped. Baking should always occur before application of chromate conversion coatings or any topcoat, as temperatures above 150°C damage these finishes. Standard baking parameters per ISO 4042 and ASTM F1941 specify part temperatures of 190-230°C (375-450°F) for a minimum duration of 8 hours, though 3-24 hours may be required depending on fastener size, strength, coating thickness and process specifics. Larger diameter fasteners require longer baking times due to greater hydrogen diffusion distances. Thicker electroplated coatings retain more hydrogen and require extended baking. Zinc and zinc-alloy coatings, the most common electroplates, are typically baked at 200-220°C (390-430°F) to avoid approaching zinc's melting point of 419°C. The maximum baking temperature must never exceed the original tempering temperature of the fastener to avoid inadvertent softening and loss of mechanical properties. Lower baking temperatures may be employed for coatings sensitive to temperature, but require proportionally longer durations to achieve equivalent hydrogen removal. Intermediate baking after partial coating thickness (2-5 μm) can improve hydrogen removal efficiency when final coating thickness exceeds 8-10 μm. Baking effectiveness should be verified through hydrogen embrittlement testing per ASTM F519, F1624, F1940, ISO 15330 or equivalent methods. The process removes hydrogen through two mechanisms: effusion (outward diffusion through the coating) and irreversible trapping at metallurgical sites where hydrogen is immobilised. Critical factors include achieving proper part temperature throughout (not just furnace temperature), maintaining temperature for the full duration, using adequate circulation in batch furnaces, and documenting time-temperature profiles for quality records. Fasteners must be cleaned and dry before baking to prevent oxidation or contamination.

Which coating systems minimise hydrogen embrittlement risk?

Coating selection significantly influences hydrogen embrittlement susceptibility. Mechanical coatings applied through non-electrolytic processes eliminate hydrogen introduction during coating application. Hot-dip galvanising introduces no hydrogen during the zinc immersion process itself, though residual hydrogen from previous manufacturing steps can be released by the thermal cycle, potentially causing embrittlement if adequate pre-treatment wasn't performed. Sherardising, a thermal diffusion process, similarly avoids electrochemical hydrogen generation. Electroless nickel plating introduces less hydrogen than electrolytic processes but is not hydrogen-free and still requires baking for susceptible materials. Organic coatings including paint, powder coating, PTFE, and anaerobic threadlockers introduce no hydrogen and are recommended for high-strength fasteners in applications where sacrificial corrosion protection is not essential. When electroplating is specified, coating system selection impacts hydrogen permeability and retention. Zinc and zinc-alloy electroplate (zinc-nickel, zinc-iron, zinc-cobalt) are most common and relatively hydrogen-permeable, facilitating baking effectiveness. Cadmium plating is highly effective for corrosion protection and relatively permeable to hydrogen, but environmental regulations increasingly restrict cadmium use. Tin and tin-alloy coatings may be adversely affected by standard baking temperatures, requiring lower-temperature, longer-duration treatments. Chrome plating creates a relatively impermeable hydrogen barrier, making hydrogen removal difficult and limiting use on susceptible fasteners. Nickel plating similarly exhibits low hydrogen permeability. Copper underplates may be specified beneath zinc or cadmium to serve as a hydrogen barrier, though this adds process complexity. Alkaline zinc processes typically introduce less hydrogen than acid zinc processes. Modern trivalent chromate conversion coatings replacing hexavalent chromates must be applied after baking. The ideal approach for susceptible fasteners combines mechanical coating methods with appropriate lubrication systems, or specifies organic coatings that entirely eliminate electrochemical hydrogen introduction.

What design and specification measures prevent hydrogen embrittlement?

Comprehensive hydrogen embrittlement prevention begins at the design and specification stage. Specify appropriate property classes and hardness ranges for the application, avoiding unnecessary over-specification that unnecessarily elevates embrittlement risk. Consider using larger diameter fasteners at lower strength rather than smaller diameter at higher strength to remain below susceptibility thresholds whilst maintaining joint capacity. Incorporate design features that reduce stress concentrations in threads, fillets and head-to-shank transitions. Specify thread rolling before heat treatment where possible for superior fatigue strength without post-hardening machining operations. Avoid specification of electroplated coatings on fasteners above 39 HRC (390 HV) unless absolutely unavoidable, preferring mechanical or organic coating alternatives. When electroplating is essential, mandate specific hydrogen embrittlement relief requirements including maximum time from plating to baking, specific temperature-time profiles, and verification testing protocols per recognised standards. Require material certifications confirming chemical composition and mechanical properties, particularly for high-strength grades. Specify prohibited manufacturing sequences such as threading or forming after heat treatment for susceptible materials. Include testing requirements in procurement specifications, mandating sustained load testing, incremental step load testing, or equivalent methods as appropriate. For critical applications, require statistical process control data from the fastener manufacturer demonstrating consistent hydrogen embrittlement test results over production runs. Develop and communicate clear acceptance criteria based on relevant standards (ISO 4042, ASTM F1941, BS 7371-1). Consider alternative materials entirely, such as non-ferrous options (aluminium alloys, titanium, nickel alloys) for extreme corrosion resistance without hydrogen embrittlement susceptibility, or lower-strength stainless steels where hydrogen sensitivity is minimised. Specify appropriate installation procedures including controlled torque sequences and adequate joint relaxation periods before final torque verification to allow detection of any early-stage embrittlement failures. Document all requirements clearly in engineering drawings, material specifications and quality control procedures to ensure consistent implementation throughout the supply chain.

Testing and Standards

What standards govern hydrogen embrittlement testing?


Comprehensive international standards provide detailed test methodologies and acceptance criteria for hydrogen embrittlement evaluation. British and European standards include BS ISO 16573-1:2020 ("Steel. Measurement method for the evaluation of hydrogen embrittlement resistance of high strength steels. Constant load test") providing sustained load testing methodology, BS ISO 16573-2:2022 ("Steel. Measurement method for the evaluation of hydrogen embrittlement resistance of high-strength steels. Slow strain rate test") specifying alternative dynamic testing approaches, BS ISO 9588:2007 ("Metallic and other inorganic coatings. Post-coating treatments of iron or steel to reduce the risk of hydrogen embrittlement") establishing baking process requirements, BS ISO 9587:2007 ("Metallic and other inorganic coatings. Pretreatment of iron or steel to reduce the risk of hydrogen embrittlement") addressing surface preparation protocols, BS EN ISO 15330:1999 ("Fasteners. Preloading test for the detection of hydrogen embrittlement. Parallel bearing surface method") describing fastener-specific testing, BS ISO 10587:2000 ("Metallic and other inorganic coatings. Test for residual embrittlement in both metallic-coated and uncoated externally-threaded articles and rods. Inclined wedge method") providing an alternative fastener test method, BS EN ISO 4042 editions including :2000, :2018 and :2022 ("Fasteners. Electroplated coatings" or "Electroplated coating systems") specifying coating requirements and hydrogen embrittlement relief procedures, BS 7371-1:2009 ("Coatings on metal fasteners. Specification for general requirements and selection guidelines") providing comprehensive coating selection guidance. International standards include ISO 2080 ("Metallic and other inorganic coatings. Surface treatment, metallic and other inorganic coatings. Vocabulary"), ISO 898-1 ("Mechanical properties of fasteners made of carbon steel and alloy steel. Part 1: Bolts, screws and studs") containing hardness limits and cautionary notes for susceptible property classes, ISO 898-2 addressing nuts and including hydrogen embrittlement considerations, ISO 898-5 covering set screws and case-hardened fasteners, ISO 7539 series including Part 7 ("Corrosion of metals and alloys. Stress corrosion testing. Slow strain rate testing") and Part 11 ("Guidelines for testing the resistance of metals and alloys to hydrogen embrittlement and hydrogen-assisted cracking"), ISO/TR 20491:2019 ("Fasteners. Fundamentals of hydrogen embrittlement in steel fasteners") providing comprehensive technical guidance consolidating current knowledge. American standards encompass ASTM F519 ("Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments") describing sustained load testing with multiple specimen types, ASTM F1624 ("Standard Test Method for Measurement of Hydrogen Embrittlement Threshold in Steel by the Incremental Step Loading Technique") providing accelerated testing methodology, ASTM F1940 ("Standard Test Method for Process Control Verification to Prevent Hydrogen Embrittlement in Plated or Coated Fasteners") enabling process monitoring, ASTM F1941/F1941M ("Standard Specification for Electrodeposited Coatings on Mechanical Fasteners") specifying comprehensive coating and baking requirements, ASTM B633 ("Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel"), ASTM B850 ("Standard Guide for Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement"), ASTM F606 ("Standard Test Methods for Determining the Mechanical Properties of Externally and Internally Threaded Fasteners, Washers, Direct Tension Indicators, and Rivets") Section 7 addressing hydrogen embrittlement testing, ASTM G129 and G142 covering slow strain rate testing. SAE J429 ("Mechanical and Material Requirements for Externally Threaded Fasteners") includes hydrogen embrittlement considerations for Grade 8 and higher. NACE standards address environmental applications including NACE TM0177 ("Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H₂S Environments"), NACE TM0284 ("Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking"), NACE TM0103 ("Laboratory Test Procedures for Evaluation of SOHIC Resistance of Plate Steels Used in Wet H₂S Service"), NACE MR0175/ISO 15156 multi-part standard for petroleum and natural gas industries covering materials for H₂S-containing environments.

What testing methods detect hydrogen embrittlement?

Multiple standardised test methods evaluate hydrogen embrittlement susceptibility and verify process effectiveness. Sustained load testing per ASTM F519, ISO 15330 and related standards subjects specimens or actual fasteners to constant tensile stress (typically 75% of specified yield strength) for extended periods (commonly 200 hours) in air or controlled environments. Specimens are pre-charged with hydrogen through processing or cathodic charging, installed in loading fixtures, and monitored for fracture. This pass/fail method effectively simulates service conditions but requires extended test duration. Incremental step loading (ISL) or rising step load (RSL) testing per ASTM F1624 provides accelerated quantitative evaluation. Applied load increases incrementally with holding periods at each step until specimen fracture. The threshold stress at which hydrogen-induced crack growth initiates is determined, with values typically expressed as a percentage of baseline strength. Testing typically completes within 24-72 hours compared to weeks for sustained load methods, providing rapid feedback for process optimisation. The preloading test per BS EN ISO 15330:1999 specifically addresses threaded fasteners, applying controlled assembly torque to simulate installation stresses and monitoring for delayed failure at room temperature. This test is suitable for in-process control but must commence within 24 hours of the final manufacturing step for maximum effectiveness. The inclined wedge method per BS ISO 10587:2000 provides a simple go/no-go test where fasteners are installed into a wedge-shaped fixture applying increasing tensile stress along the length. Residual hydrogen causes fracture in susceptible regions. Slow strain rate testing (SSRT) per ASTM G129, ISO 7539-7 and related methods subjects specimens to very slow tensile strain rates (typically 10⁻⁶ to 10⁻⁴ s⁻¹), measuring reduction in ductility (notch tensile strength, elongation, reduction of area) compared to specimens tested in inert environments. C-ring and bent-beam tests per ASTM F519 Type 1 specimens apply constant deflection stress through mechanical fixtures, offering simplicity for comparative evaluations. Process control verification per ASTM F1940 uses witness specimens processed alongside production fasteners to continuously monitor plating bath hydrogen embrittlement characteristics. Fracture mechanics approaches including ASTM E1681 determine threshold stress intensity factors (K_IH) for hydrogen-assisted cracking. Each method serves specific purposes: sustained load for qualification and acceptance testing, incremental step load for development and process optimisation, preloading for routine production monitoring, and slow strain rate for fundamental research and material ranking.

How do you interpret hydrogen embrittlement test results?

Test result interpretation depends on the specific method employed and application requirements. For sustained load testing, specimens must survive the specified stress level and duration without fracture to pass. Typical acceptance criteria require no failures in a minimum sample size (commonly 3-10 specimens per lot) when tested at 75% specified yield strength for 200 hours in air. Any fracture within the test period constitutes failure, indicating unacceptable hydrogen embrittlement risk. Time-to-failure data from multiple specimens at varying stress levels can construct threshold curves defining safe operating stress ranges. For incremental step load testing per ASTM F1624, the threshold stress (σth) is determined as the stress level at which subcritical crack growth begins. Results are typically expressed as an embrittlement ratio, comparing hydrogen-charged threshold stress to baseline (uncharged) strength. Acceptance criteria vary by application but commonly require embrittlement ratios above 0.60 (60%) for general applications, above 0.75 (75%) for critical applications per ASTM F1940 process qualification. Values below these thresholds indicate excessive susceptibility requiring process corrective action. Preloading test results per ISO 15330 are binary: specimens either survive or fail within the test period. Acceptance requires all test specimens survive without fracture. Any failure necessitates investigation and process modification. Slow strain rate testing compares mechanical properties between specimens tested in hydrogen-generating environments versus inert controls. The notch tensile strength ratio (NTS%), elongation ratio, and reduction of area ratio are calculated. Values approaching 100% indicate minimal embrittlement; values below 60-80% suggest significant susceptibility depending on material and application. Fracture surface examination through optical and scanning electron microscopy confirms embrittlement mechanisms, revealing characteristic intergranular or quasi-cleavage fracture morphology rather than ductile cup-cone appearance. Test result significance depends on production versus qualification contexts. Process qualification establishes that manufacturing procedures reliably produce non-embrittled parts under normal production variation. Lot acceptance testing verifies that specific production batches meet requirements. Trend analysis of ongoing process control data (e.g., ASTM F1940) identifies gradual degradation requiring maintenance before unacceptable parts are produced. All testing must be documented with complete traceability including specimen source, processing history, test conditions, failure modes, fractographic observations, and acceptance criteria applied.

What quality control measures prevent hydrogen embrittlement in production?

Effective quality control combines process control, verification testing, and comprehensive documentation. Process qualification must be completed before production begins, establishing that all manufacturing sequences, material specifications, and hydrogen embrittlement relief procedures reliably produce conforming product under normal production variation. This includes material certification verifying steel chemistry and mechanical properties comply with specifications, and process capability studies demonstrating consistent results across multiple production batches. In-process control monitors critical parameters at each manufacturing stage: heat treatment temperature-time profiles recorded through thermocouples or data logging systems, surface preparation chemistry and duration controls for pickling and phosphating, electroplating current density, voltage, temperature, bath chemistry and plating thickness measured at defined intervals, post-plating baking temperature-time profiles with furnace instrumentation and part temperature verification, and final dimensional and visual inspection. Statistical process control (SPC) methods track key variables over time, identifying trends before out-of-specification conditions develop. Verification testing periodically confirms process effectiveness through hydrogen embrittlement testing per appropriate standards. The frequency depends on production volume and part criticality, ranging from continuous witness specimen testing per ASTM F1940 for high-volume critical applications, to batch testing of representative samples for lower-volume production. Test results must be documented with full traceability to production lots, enabling rapid isolation and containment if failures occur. Supplier qualification and oversight ensures external coating applicators and heat treaters maintain appropriate controls, with periodic audits verifying continued compliance. Material traceability systems track steel lot numbers through all manufacturing operations to finished product, enabling investigation of field failures back to material source if necessary. Non-conformance procedures define actions when test failures occur, including immediate production hold, root cause investigation, corrective action implementation, re-qualification testing, and disposition of affected inventory. Preventive maintenance schedules for all process equipment, particularly plating baths and heat treatment furnaces, maintain consistent operating conditions. Personnel training ensures operators understand hydrogen embrittlement mechanisms, critical process parameters, and proper implementation of controls. Comprehensive documentation including material certifications, process parameter records, test results, non-conformance reports, and corrective actions must be retained for the specified period per industry quality standards (ISO 9001, AS9100, IATF 16949 as applicable).

Minimising Effects

What to do if hydrogen embrittlement is suspected after installation?


When hydrogen embrittlement is suspected in service, immediate action is required to prevent catastrophic failure. First, implement precautionary measures by reducing or eliminating applied loads where possible, redistributing loads to backup systems, evacuating personnel from areas of potential failure, and installing temporary supports or barriers. Conduct preliminary inspection documenting installation dates, torque values, visual condition, evidence of corrosion or other damage, and service environment history. If feasible, disassemble suspect joints carefully, recording disassembly torque values (reduced values suggest embrittlement), preserving failed or suspect fasteners in clean, dry conditions, and photographing fracture surfaces before handling. Perform diagnostic evaluation including hardness testing to verify material specifications, fracture surface examination through optical or electron microscopy to identify characteristic brittle intergranular fracture, dimensional verification confirming parts meet specifications, and coating analysis to determine plating type and thickness. Conduct hydrogen embrittlement testing on representative fasteners from the same lot if available, using accelerated methods (ASTM F1624) to rapidly assess susceptibility. Material analysis determines whether steel chemistry and heat treatment produced intended properties. Investigation scope depends on failure criticality and quantity of potentially affected fasteners. For isolated incidents, focused investigation of failed components and immediate surroundings may suffice. For systemic issues involving multiple installations, comprehensive field survey and testing of all potentially affected assemblies becomes necessary. Remediation strategies include immediate replacement of all suspect fasteners with properly processed alternatives, de-embrittlement treatment (low-temperature baking at 150-200°C for 24 hours) for accessible assemblies if hydrogen concentration is recent and not yet deeply diffused, or redesign using lower-strength fasteners below embrittlement susceptibility thresholds, alternative materials immune to hydrogen effects, or modified joint designs reducing fastener stress levels. Prevention of recurrence requires identifying root causes in manufacturing, coating application, or installation procedures; revising specifications to eliminate problematic processes or materials; implementing enhanced quality controls and verification testing; and training personnel on proper fastener selection, handling and installation procedures. All findings, actions and corrective measures must be comprehensively documented for liability protection, regulatory compliance, and continuous improvement purposes.

Can hydrogen embrittlement be reversed?

Hydrogen embrittlement is potentially reversible if action is taken before permanent damage occurs, but success depends critically on timing and degree of embrittlement. The reversibility window exists because hydrogen in steel is mobile at room temperature and above, capable of diffusing outward from the material if concentration gradients favour efflux. De-embrittlement treatment employs low-temperature baking similar to post-plating hydrogen relief but adapted for assembled components or finished fasteners. Parts are heated to 150-200°C (300-390°F) for extended periods (24 hours minimum, potentially 48-72 hours for heavily embrittled or large-diameter fasteners). This temperature range maximises hydrogen diffusion rate whilst remaining below the tempering temperature to avoid softening. The process removes mobile interstitial hydrogen that has not yet caused microstructural damage. However, several critical limitations apply. Treatment must occur before hydrogen has reached sufficient concentration and residence time to cause irreversible cracking or void formation. Once microcracks initiate, they remain as permanent defects even after hydrogen removal, severely limiting load-bearing capacity. Parts exhibiting visible fracture, reduced load capacity, or failed verification testing typically cannot be salvaged. The effectiveness of de-embrittlement diminishes with increasing material hardness, as higher-strength steels may suffer damage at lower hydrogen concentrations and shorter exposure times. Surface coatings may impede hydrogen effusion, particularly dense, impermeable coatings like chromium plate that effectively trap hydrogen. Such coatings must be removed before de-embrittlement can succeed, then re-applied if corrosion protection is required. Stress state during treatment matters; parts should be unloaded during baking to avoid propagating incipient cracks. Practical application of de-embrittlement treatment is limited to situations where fasteners can be removed, treated, and reinstalled, or where entire assemblies can tolerate elevated temperatures without damaging adjacent components, sealants, or finishes. Cost considerations often favour replacement with properly processed new fasteners rather than attempting rehabilitation of questionable parts. The most reliable approach remains prevention through proper manufacturing processes, appropriate material selection, and effective quality control rather than depending on post-embrittlement remediation.

What environmental service conditions require special consideration?

Certain service environments continuously generate hydrogen, creating ongoing embrittlement risk throughout the fastener service life termed environmental hydrogen embrittlement (EHE). Sour gas environments containing hydrogen sulphide (H₂S) in oil and gas production, refining, and petrochemical industries represent the highest-risk applications. H₂S promotes hydrogen entry into steel by poisoning the normal recombination of atomic hydrogen into molecular form, allowing absorption at much lower corrosion rates than ordinary aqueous corrosion. Even low H₂S partial pressures (parts per million) combined with water significantly elevate risk. Specifications for these applications invoke NACE MR0175/ISO 15156 material requirements and NACE TM0177/TM0284 qualification testing to ensure resistance to sulphide stress cracking (SSC), stress-oriented hydrogen-induced cracking (SOHIC), and hydrogen-induced cracking (HIC). Marine environments with saltwater exposure generate hydrogen through general corrosion and particularly from cathodic protection systems designed to prevent corrosion on ships, offshore platforms, and coastal structures. The protective cathodic potential deliberately generates hydrogen at protected metal surfaces, requiring careful material selection to balance corrosion protection against embrittlement risk. Acidic industrial environments including chemical processing, acid storage tanks, battery manufacturing, and pickling operations produce hydrogen through direct acid attack on steel surfaces, with pH below 4 presenting elevated concern. Elevated temperature and pressure conditions accelerate both hydrogen generation and diffusion, increasing embrittlement rates. Nuclear reactors and certain chemical processes can expose fasteners to atomic hydrogen at elevated temperatures and pressures requiring special nickel-based alloys or other hydrogen-resistant materials. Alternating wet-dry cycles or thermal cycling can pump hydrogen into materials through repeated adsorption-desorption processes. Applications involving galvanic couples where fasteners are cathodic to adjacent materials experience continuous hydrogen generation at the fastener surface. High-strength fasteners in any of these environments warrant conservative material selection, possibly avoiding susceptible hardness ranges entirely in favour of lower-strength alternatives, non-ferrous materials, or special alloys. Coating selection must consider both corrosion protection and hydrogen permeability, potentially favouring barrier coatings that prevent environmental access rather than sacrificial coatings that rely on electrochemical protection. Periodic inspection, non-destructive testing, and planned replacement schedules may be implemented for critical applications where environmental hydrogen exposure cannot be eliminated. Design considerations include avoiding crevices where stagnant electrolytes concentrate, providing drainage to minimise moisture accumulation, and incorporating electrochemical isolation between dissimilar metals to prevent galvanic coupling. Material testing should replicate actual service environment conditions including temperature, pressure, pH, H₂S partial pressure, chloride concentration, and applied cathodic potential to accurately predict in-service performance.

Industry Applications

Which industries are most affected by hydrogen embrittlement concerns?


Hydrogen embrittlement considerations impact numerous critical industries where high-strength fasteners operate under demanding conditions. Automotive manufacturers extensively utilise high-strength fasteners in chassis, suspension, engine, and safety-critical systems to achieve lightweighting and performance objectives. The combination of high assembly stresses, corrosion-prone environments, and liability exposure drives rigorous hydrogen embrittlement prevention. Aerospace applications demand maximum strength-to-weight ratios using very high-strength materials operating under extreme conditions with zero tolerance for failure. Aircraft structural joints, engine components, and landing gear employ fasteners at the upper limits of hardness ranges where embrittlement risk is maximal. Construction and structural engineering in buildings, bridges, towers, and infrastructure depend on Grade 8.8, 10.9 and higher bolts preloaded to substantial percentages of yield strength. Delayed failures could precipitate catastrophic structural collapse, making hydrogen embrittlement prevention mandatory in modern building codes. Oil and gas production, refining, and petrochemical processing encounter the particularly aggressive combination of H₂S, high pressure, elevated temperature, and high-strength materials in wellhead equipment, valves, pressure vessels, and piping systems. NACE specifications governing these applications impose stringent material and testing requirements. Marine and offshore structures including ships, submarines, oil platforms, and port facilities face continuous corrosion attack coupled with cathodic protection systems generating persistent hydrogen. Stainless steel and high-strength alloy fasteners must resist both corrosion and embrittlement simultaneously. Power generation in nuclear, fossil fuel, and renewable energy facilities employs high-strength fasteners in turbines, pressure vessels, and structural applications operating at elevated temperatures and pressures with extremely high reliability requirements. Rail transport utilises high-strength fasteners throughout locomotives and rolling stock, particularly in bogies, couplings, and structural members experiencing dynamic loading and vibration. Mining, quarrying and earthmoving equipment operate in aggressive environments with large, highly-stressed fasteners subject to shock loading and corrosion. Manufacturing machinery, presses, and production equipment increasingly employ high-strength fasteners for compact, efficient designs. Pressure vessels and boilers in industrial and commercial applications must comply with codes (ASME, PED) addressing hydrogen embrittlement for high-pressure rated equipment. Defence and military applications mirror aerospace requirements with additional environmental exposure scenarios. Medical devices and surgical implants represent another critical domain where titanium and stainless alloys must avoid hydrogen-assisted cracking despite physiological fluid exposure and cyclic loading. Each industry has developed application-specific standards, specifications, and quality control practices reflecting their unique risk profiles and regulatory frameworks.

What special considerations apply to heritage and restoration applications?

Heritage engineering and vintage vehicle restoration present unique hydrogen embrittlement challenges when reproducing obsolete fasteners or maintaining historic machinery. Pre-war and early post-war fasteners often utilised materials, grades, and manufacturing methods no longer common in modern production. British Standard fasteners including BSW, BSF, and BA threads specified in original equipment must be faithfully reproduced using historically appropriate materials whilst incorporating modern understanding of hydrogen embrittlement risks not recognised during original manufacture. Period-correct finishes including cadmium plating, black oxide, and phosphate coatings must balance authenticity with safety, potentially requiring modern hydrogen embrittlement relief procedures not originally employed. Steam locomotive restoration exemplifies these challenges, where Victorian and Edwardian engineering specified materials and processes predating hydrogen embrittlement awareness. Reproduction studs, bolts, and nuts for cylinder heads, motion assemblies, and pressure vessels must meet original specifications for thread form and material properties whilst incorporating post-plating baking or alternative coating methods to ensure reliability. Orginal specifications such as BS 190 for steam locomotive nuts require interpretation into modern equivalents with appropriate property classes and embrittlement prevention. Heritage railways and industrial preservation rely on maintaining operational vintage equipment for both historical education and commercial tourist operation, necessitating stringent safety standards for components that may have operated satisfactorily for decades but require replacement using modern manufacturing understanding. Classic automobile restoration similarly demands reproduction of obsolete fasteners with period-appropriate head styles, thread forms, and finishes whilst ensuring components safely withstand modern use on public roads. Concours judging may penalise visible deviation from original specifications, creating tension between authenticity and safety that must be resolved through inconspicuous modern improvements like hydrogen embrittlement relief baking that leaves no visible evidence. Marine heritage including historic ships, canal boats, and naval vessels encounters additional complexity from the combination of traditional mild steel or wrought iron structures, brass and bronze components, saltwater exposure, and the use of modern cathodic protection systems on originally unprotected structures. The galvanic interactions and hydrogen generation from cathodic protection may create embrittlement risks absent in original service. Museum curators and restoration professionals must balance preservation of original components where possible against replacement with reproductions incorporating modern safety standards where structural integrity is compromised or critical safety applications demand higher reliability than period manufacturing could achieve. Documentation of all deviations from originality, reasoning for modern interventions, and evidence of appropriate hydrogen embrittlement prevention forms an essential component of professional heritage engineering practice, ensuring current safety whilst preserving historic authenticity and facilitating future conservation work by clearly identifying which components reflect period practice versus modern intervention.

Conclusion

What is the current state of hydrogen embrittlement knowledge?


Contemporary understanding of hydrogen embrittlement represents over 140 years of research since the phenomenon was first documented in 1875, yet significant aspects remain incompletely understood. Multiple competing theories including hydrogen-enhanced decohesion, hydrogen-enhanced localised plasticity, and hydrogen-induced phase transformation each explain certain aspects but no single comprehensive model fully captures all observed behaviours across different materials, conditions, and stress states. Modern analytical techniques including scanning electron microscopy, transmission electron microscopy, secondary ion mass spectrometry, and atom probe tomography have revealed nanoscale hydrogen distribution and microstructural interactions previously invisible, whilst in-situ mechanical testing under controlled hydrogen environments provides real-time observation of crack initiation and propagation mechanisms. Computational modelling using finite element analysis, molecular dynamics simulations, and multi-scale approaches increasingly predicts hydrogen distribution, diffusion kinetics, and embrittlement susceptibility from first principles, though validation against real-world complexity remains challenging. The fastener industry has progressively refined manufacturing practices, coating technologies, and testing methodologies, codifying best practices into international standards that continue evolution through ongoing research and field experience. ISO/TR 20491:2019 represents recent efforts to consolidate current knowledge into practical guidance specifically for fasteners. Industry consensus recognises susceptibility thresholds, baking requirements, and testing protocols that effectively prevent hydrogen embrittlement in routine manufacturing when properly implemented. However, gaps remain in predicting long-term environmental hydrogen embrittlement in service, understanding synergistic effects of combined stresses and environmental factors, developing accelerated testing that accurately predicts field performance, and establishing hydrogen embrittlement resistance in emerging ultra-high-strength materials exceeding 1500 MPa where traditional approaches may be inadequate. Future developments will likely emphasise materials designed from first principles for hydrogen resistance rather than retrospective testing of existing alloys, advanced coating systems that both prevent corrosion and block hydrogen entry, real-time hydrogen monitoring sensors integrated into critical structures, and predictive modelling enabling lifetime assessment under complex variable loading and environmental exposure. The fundamental challenge—balancing the engineering demand for ever-higher strength materials against the exponentially increasing hydrogen embrittlement susceptibility—will continue driving innovation in alloy development, surface engineering, and manufacturing process control for decades to come.

Summary

Hydrogen embrittlement represents a critical yet manageable failure mechanism in high-strength fastener applications. Understanding the fundamental causes, implementing proven prevention strategies, adhering to established testing standards, and maintaining rigorous quality control enables safe, reliable use of high-strength fasteners across demanding industrial, structural, and engineering applications. Material selection below susceptibility thresholds, elimination or control of hydrogen sources during manufacturing and service, effective hydrogen embrittlement relief through post-plating baking, appropriate coating system selection, and comprehensive verification testing form the foundation of hydrogen embrittlement prevention. Ongoing vigilance through process control, periodic testing, and awareness of environmental service conditions ensures continued component integrity throughout the fastener service life. The comprehensive standards framework established by ISO, BS, ASTM, NACE, SAE and industry organisations provides detailed guidance enabling manufacturers, engineers, and end users to systematically address hydrogen embrittlement risks appropriate to their specific applications and risk tolerance levels.

Standards Reference Summary

British and European Standards

  • BS ISO 16573-1:2020 – Steel. Measurement method for the evaluation of hydrogen embrittlement resistance of high strength steels. Constant load test
  • BS ISO 16573-2:2022 – Steel. Measurement method for the evaluation of hydrogen embrittlement resistance of high-strength steels. Slow strain rate test
  • BS ISO 9588:2007 – Metallic and other inorganic coatings. Post-coating treatments of iron or steel to reduce the risk of hydrogen embrittlement
  • BS ISO 9587:2007 – Metallic and other inorganic coatings. Pretreatment of iron or steel to reduce the risk of hydrogen embrittlement
  • BS EN ISO 15330:1999 – Fasteners. Preloading test for the detection of hydrogen embrittlement. Parallel bearing surface method
  • BS ISO 10587:2000 – Metallic and other inorganic coatings. Test for residual embrittlement in both metallic-coated and uncoated externally-threaded articles and rods. Inclined wedge method
  • BS EN ISO 4042:2000 – Fasteners. Electroplated coatings
  • BS EN ISO 4042:2018 – Fasteners. Electroplated coating systems
  • BS EN ISO 4042:2022 – Fasteners. Electroplated coating systems (current edition)
  • BS 7371-1:2009 – Coatings on metal fasteners. Specification for general requirements and selection guidelines
  • BS 7371-3:1993 – Coatings on metal fasteners. Specifications for electroplated zinc and cadmium coatings (Withdrawn)
  • BS EN ISO 898-1 – Mechanical properties of fasteners made of carbon steel and alloy steel. Part 1: Bolts, screws and studs with specified property classes
  • BS EN ISO 898-2 – Mechanical properties of fasteners made of carbon steel and alloy steel. Part 2: Nuts with specified property classes
  • BS EN ISO 898-5 – Mechanical properties of fasteners made of carbon steel and alloy steel. Part 5: Set screws and similar threaded fasteners with specified hardness classes

International Standards

  • ISO 2080 – Metallic and other inorganic coatings. Surface treatment, metallic and other inorganic coatings. Vocabulary
  • ISO 2081:2018 – Metallic and other inorganic coatings. Electroplated coatings of zinc with supplementary treatments on iron or steel
  • ISO 7539-7 – Corrosion of metals and alloys. Stress corrosion testing. Part 7: Slow strain rate testing
  • ISO 7539-9 – Corrosion of metals and alloys. Stress corrosion cracking. Part 9: Preparation and use of pre-cracked specimens for tests under rising load or rising displacement
  • ISO 7539-11 – Corrosion of metals and alloys. Stress corrosion cracking. Part 11: Guidelines for testing the resistance of metals and alloys to hydrogen embrittlement and hydrogen-assisted cracking
  • ISO/TR 20491:2019 – Fasteners. Fundamentals of hydrogen embrittlement in steel fasteners
  • ISO 19598:2016 – Metallic coatings. Electroplated coatings of zinc and zinc alloys on iron or steel with supplementary Cr(VI)-free treatment
  • ISO 16540 – Corrosion of metals and alloys. Test method for determining susceptibility to stress corrosion cracking of low-alloy steels by exposure to boiling acidified sodium chloride solution
  • ISO 11114-4 – Gas cylinders. Compatibility of cylinder and valve materials with gas contents. Test methods for selecting metallic materials resistant to hydrogen embrittlement
  • ISO 12135 – Metallic materials. Unified method of test for the determination of quasistatic fracture toughness
  • ISO 17081 – Method of measurement of hydrogen permeation and determination of hydrogen uptake and transport in metals by an electrochemical technique

American Standards (ASTM)

  • ASTM F519 – Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments
  • ASTM F1624 – Standard Test Method for Measurement of Hydrogen Embrittlement Threshold in Steel by the Incremental Step Loading Technique
  • ASTM F1940 – Standard Test Method for Process Control Verification to Prevent Hydrogen Embrittlement in Plated or Coated Fasteners
  • ASTM F1941/F1941M – Standard Specification for Electrodeposited Coatings on Mechanical Fasteners (Inch and Metric)
  • ASTM F606/F606M – Standard Test Methods for Determining the Mechanical Properties of Externally and Internally Threaded Fasteners, Washers, Direct Tension Indicators, and Rivets (Section 7)
  • ASTM B633 – Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel
  • ASTM B850 – Standard Guide for Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement
  • ASTM F2660 – Standard Test Method for Qualifying Coatings for Use on A490 Structural Bolts Relative to Environmental Hydrogen Embrittlement
  • ASTM A490 – Standard Specification for Structural Bolts, Alloy Steel, Heat Treated, 150 ksi Minimum Tensile Strength
  • ASTM A574 – Standard Specification for Alloy Steel Socket-Head Cap Screws
  • ASTM G129 – Standard Practice for Slow Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to Environmentally Assisted Cracking
  • ASTM G142 – Standard Test Method for Determination of Susceptibility of Metals to Embrittlement in Hydrogen Containing Environments at High Pressure, High Temperature, or Both
  • ASTM G38 – Standard Practice for Making and Using C-Ring Stress-Corrosion Test Specimens
  • ASTM G39 – Standard Practice for Preparation and Use of Bent-Beam Stress-Corrosion Test Specimens
  • ASTM E1681 – Standard Test Method for Determining Threshold Stress Intensity Factor for Environment-Assisted Cracking of Metallic Materials
  • ASTM E1820 – Standard Test Method for Measurement of Fracture Toughness
  • ASTM A1032 – Standard Test Method for Hydrogen Embrittlement Resistance for Steel Wire Hard-Drawn Used for Prestressed Concrete Pipe
  • ASTM B577 – Standard Test Methods for Detection of Cuprous Oxide (Hydrogen Embrittlement Susceptibility) in Copper
  • ASTM B858 – Standard Test Method for Ammonia Vapor Test for Determining Susceptibility to Stress Corrosion Cracking in Copper Alloys

American Standards (SAE)

  • SAE J429 – Mechanical and Material Requirements for Externally Threaded Fasteners
  • SAE J78 – Steel Self-Drilling Tapping Screws
  • SAE J81 – Thread Rolling Screws
  • SAE J1237 – Metric Thread Rolling Screws

NACE Standards

  • NACE TM0177 – Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H₂S Environments (Methods A, B, C, D)
  • NACE TM0284 – Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking
  • NACE TM0103 – Laboratory Test Procedures for Evaluation of Stress-Oriented Hydrogen Induced Cracking (SOHIC) Resistance of Plate Steels Used in Wet H₂S Service
  • NACE TM0198 – Slow Strain Rate Test Method for Screening Corrosion-Resistant Alloys for Stress Corrosion Cracking in Sour Oilfield Service
  • NACE TM0316 – Four-Point Bend Testing of Materials for Oil and Gas Applications
  • NACE MR0175/ISO 15156 (multi-part) – Petroleum and natural gas industries. Materials for use in H₂S-containing environments in oil and gas production
  • NACE MR0176 – Metallic Materials for Sucker-Rod Pumps for Corrosive Oilfield Environments

German and European Federation of Corrosion

  • DIN 267 (multiple parts) – Mechanical fasteners. Technical specifications
  • DIN EN ISO 4042 – See ISO 4042 above (German adoption)
  • EFC Publication 16 – Guidelines on materials requirements for carbon and low alloy steels for H₂S-containing environments in oil and gas production
  • EFC Publication 17 – Corrosion resistant alloys for oil and gas production: guidelines on general requirements and test methods

Withdrawn/Obsolete Standards (Referenced for Historical Context)

  • DIN 986 – Hexagon domed cap nuts (Withdrawn, superseded by ISO 4161)
  • BS 190 – Specification for black hexagon nuts for steam locomotives and locomotive-type boilers (Withdrawn)

Note: Standards are subject to periodic revision. Users should verify the current edition of any standard before application. Some standards listed may have been superseded or revised since the publication of this guide.