Hydrogen Embrittlement Coefficient Determination Methods: Standards, Procedures, and Calculation

The hydrogen embrittlement coefficient is a quantitative index that measures the loss of ductility in metallic materials caused by absorbed hydrogen, and it is a critical quality gate for high-strength steels, fasteners, and electroplated components. This article explains the principle, calculation formula, sampling requirements, testing procedures, and applicable international standards for hydrogen embrittlement coefficient determination, helping manufacturers select the right method for process control and failure prevention.

Technical Overview

Hydrogen embrittlement (HE) is a degradation mechanism in which atomic hydrogen enters a metal during pickling, acid cleaning, electroplating, welding, cathodic protection, or service in hydrogen-containing environments. Once absorbed, hydrogen concentrates at stress risers, grain boundaries, and microstructural defects, dramatically reducing ductility and causing delayed brittle fracture. The risk is especially severe for high-strength steels with a tensile strength above approximately 1000 MPa or a hardness above 320 HV.

The hydrogen embrittlement coefficient expresses this ductility loss numerically. It is most commonly calculated from the reduction of area or elongation obtained in tensile testing of uncharged (reference) and hydrogen-charged specimens, using the following relationship:

• HE coefficient, F (%) = (Z0 − ZH) / Z0 × 100, where Z0 is the reduction of area of the uncharged specimen and ZH is the reduction of area of the hydrogen-charged specimen.
• An alternative expression uses elongation: F (%) = (A0 − AH) / A0 × 100.
• A complementary index is the notch tensile strength ratio, defined as the notch tensile strength of the charged specimen divided by that of the uncharged specimen; values approaching 1.0 indicate low susceptibility.

A larger coefficient indicates greater susceptibility to hydrogen embrittlement. Commonly used reference standards include ASTM F519, ASTM F1624, ASTM F1940, ASTM G129, ASTM G142, ISO 7539-7, ISO 11114-4, and GB/T 3098.17. Accredited third-party laboratories routinely apply these methods for incoming material qualification, plating process validation, and failure analysis.

Testing Samples

High-Strength Fasteners

Bolts, screws, studs, and anchor rods of property classes 10.9 and 12.9, as well as inch-series fasteners above grade 8, represent the largest sample category. These parts are typically electroplated or zinc-flake coated, which introduces hydrogen during pretreatment, and their high hardness makes them inherently susceptible. Both full-size parts and machined proportional specimens can be tested.

Electroplated and Coated Components

Automotive and hardware parts finished with zinc, zinc-nickel, cadmium, or nickel plating are tested to verify that plating lines and post-plating baking cycles do not leave damaging residual hydrogen. Witness coupons plated alongside production lots are widely accepted for process control under ASTM F1940.

Spring Steels and Wire Products

Cold-drawn spring wire, valve springs, prestressing steel strand, and music wire are evaluated because drawing and pickling operations charge hydrogen into the material and the finished parts operate under sustained high stress, a worst-case condition for delayed fracture.

Pipeline Steels and Pressure Components

Linepipe steels from X52 through X100, wellhead equipment, and pressure vessel steels exposed to wet hydrogen sulfide (sour service) or cathodic protection are tested in the as-received, welded, and precharged conditions to assess environmental cracking resistance.

Standard Specimen Geometries

• Smooth cylindrical tensile specimens per ASTM E8 or ISO 6892-1 for ductility-based coefficient calculation.
• Notched tensile specimens with a stress concentration factor of approximately 3 to 4, such as ASTM F519 Type 1a round notched bars, for sustained-load and incremental step loading tests.
• Flat sheet specimens, C-ring specimens, and small coupons for hydrogen content analysis by inert gas fusion.

Testing Items

• Hydrogen embrittlement coefficient based on reduction of area loss.
• Hydrogen embrittlement coefficient based on elongation loss.
• Notch tensile strength retention ratio and delayed failure time under sustained load.
• Threshold stress or threshold stress for hydrogen-induced cracking via incremental step loading.
• Total and diffusible hydrogen content in the material.
• Effectiveness verification of post-plating hydrogen relief baking.
• Comparative slow strain rate behavior in air versus hydrogen-charging environments.

Testing Methods

Slow Strain Rate Testing (SSRT)

Specimens are strained to failure at a constant low strain rate, typically between 10⁻⁶ and 10⁻⁵ s⁻¹, in an inert reference environment and in a hydrogen-producing or hydrogen-charged condition. SSRT per ASTM G129 and ISO 7539-7 accelerates hydrogen transport to growing cracks and produces a measurable ductility gap within one to three days. The coefficient is computed from the difference in reduction of area or elongation between the two environments. It is the preferred method for material ranking and research studies.

Sustained-Load Notched Specimen Testing

Following ASTM F519, notched round specimens are loaded in tension to a defined fraction, commonly 75 percent, of their notch tensile strength and held for a sustained period, typically 200 hours or more. Absorbed hydrogen diffuses to the notch root, and any delayed fracture constitutes failure. This pass/fail method is the aerospace industry standard for qualifying plating processes, baking cycles, and maintenance chemicals. ASTM F326 provides a similar notched-bar approach for plated high-strength steels.

Incremental Step Loading

ASTM F1624 describes a technique in which the load on a notched specimen is raised in programmed increments while the specimen absorbs hydrogen. The stress level at which cracking initiates defines the hydrogen embrittlement threshold. Because failure is forced within a controlled schedule, the method delivers quantitative threshold data in significantly less time than conventional sustained-load testing and is well suited to evaluating baking effectiveness.

Cathodic Hydrogen Precharging with Tensile Comparison

In this widely used laboratory approach, specimens are cathodically charged in an acidified or saline electrolyte, often containing a recombination poison such as thiourea or arsenic trioxide, at current densities of roughly 0.5 to 10 mA/cm² for several hours to overnight. Charged specimens are pulled to failure promptly, and their elongation and reduction of area are compared with those of uncharged reference specimens from the same heat. The relative loss values yield the hydrogen embrittlement coefficient directly.

Hydrogen Content Determination

Complementary chemical analysis quantifies the hydrogen that drives embrittlement. Inert gas fusion with thermal conductivity or infrared detection, per methods such as GB/T 223.82 and ASTM E1447, measures total hydrogen in a few milligrams to grams of material. Thermal desorption spectroscopy (TDS) separates diffusible from trapped hydrogen by ramping temperature and recording hydrogen evolution peaks, which is valuable for confirming that a baking treatment has removed mobile hydrogen.

Procedure and Calculation Summary

• Machine specimen sets in identical orientation from the same material lot; include at least three specimens per condition for statistical validity.
• Measure reference tensile properties in the uncharged state to obtain baseline elongation, reduction of area, and notch tensile strength.
• Charge hydrogen by cathodic electrolysis, corrosive exposure, or service-environment simulation, controlling current density, electrolyte chemistry, and duration.
• Test charged specimens under SSRT, sustained load, or step loading without unnecessary delay, since diffusible hydrogen escapes at room temperature.
• Calculate the coefficient from the ductility loss and report alongside hydrogen content, hardness, microstructure, and any baking treatment applied.

Applications

• Automotive engineering: validating electroplated body, chassis, and brake fasteners in property classes 10.9 and 12.9 before production release.
• Aerospace: qualifying cadmium, zinc-nickel, and aluminum coatings on landing gear and structural hardware, and approving maintenance chemicals per ASTM F519.
• Oil and gas: ranking linepipe and OCTG materials for sour service and verifying resistance under cathodic protection potentials.
• Construction and bridge engineering: assessing hydrogen exposure of prestressing strand and high-strength anchor systems during and after installation.
• Precision fastener and spring manufacturing: routine incoming inspection, plating line monitoring, and baking furnace validation.
• Hydrogen energy systems: screening cylinder, valve, and fuel cell component materials for hydrogen compatibility under ISO 11114-4.
• Failure analysis: quantifying embrittlement in fractured parts to distinguish hydrogen damage from overload, fatigue, or quench cracking.

FAQ

What hydrogen embrittlement coefficient value is acceptable?

Acceptance limits depend on the product specification, but as general guidance a coefficient below 10 percent based on reduction of area loss indicates low susceptibility, values between 10 and 25 percent indicate moderate susceptibility requiring process review, and values above 25 percent are typically rejected for high-strength applications. Contractual specifications, not the test method itself, define the pass/fail limit.

Which materials must be tested?

Testing is most critical for steels above roughly 1000 MPa tensile strength or 320 HV hardness, especially when they are pickled, electroplated, or exposed to hydrogen-generating service environments. Lower-strength materials rarely fail from hydrogen embrittlement but may still be tested for specific applications.

How can hydrogen embrittlement risk be reduced after plating?

The standard mitigation is a post-plating bake, generally performed within one to four hours of plating at 190 to 230 °C for 4 to 24 hours, with time and temperature scaled to the strength level of the part as guided by ASTM B850. The effectiveness of baking is verified by retesting the coefficient or by measuring residual diffusible hydrogen.

What is the difference between diffusible and total hydrogen?

Total hydrogen includes both mobile and trapped hydrogen within the metal lattice, while diffusible hydrogen is the fraction that can migrate at room temperature and concentrate at stress concentrations. Diffusible hydrogen correlates most closely with embrittlement risk and is the fraction removed by baking and measured by thermal desorption spectroscopy.

How long does a determination take?

Tensile comparison methods with precharging typically require two to five working days. SSRT programs usually need three to seven days depending on strain rate and the number of environments. Sustained-load notched specimen tests per ASTM F519 require at least the full 200-hour hold period plus specimen machining and coating time.

Can coated production parts be tested directly?

Yes. Full-size fasteners can be tested under sustained preload in accordance with GB/T 3098.17 and ASTM F1940, and witness coupons plated with production lots are commonly used for process control. Machined specimens are preferred when a numeric coefficient based on reduction of area is required.