Understanding and Mitigating Hydrogen Embrittlement Risks in Mass Timber Buildings

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As mass timber construction gains momentum as a sustainable and versatile option, designers have sought to create larger structures with longer spans. These designs demand high-capacity structural connections, often relying on case-hardened fasteners manufactured from high-strength steel. Achieving desired strength typically involves increasing their core hardness. Figure 1 shows the key composition of a typical carbon steel fastener for mass timber applications.

While increasing core hardness correlates with higher ultimate strength, it also significantly elevates the risk of hydrogen embrittlement (HE)—a well-documented phenomenon in which infiltrated hydrogen reduces ductility, leading to failure under sustained tensile stress. This vulnerability—once an overlooked concern in mass timber, especially in highly seismic regions—now represents a critical challenge for the integrity and durability of today’s increasingly expansive mass timber buildings.

In response to industry failures attributed to HE, the Canadian wood design standard, CSA O86, has implemented updates in its 2024 revision on the core hardness of carbon steel fasteners. The new CSA O86 guidelines for fasteners produced under ISO 2702 introduce maximum core hardness limits of 360 and 390 HV for screws intended for wet and dry service conditions, respectively (36 and 38 HRC, respectively, if produced under ASME B18.6.3). These thresholds, derived from extensive empirical data, aim to limit HE risks. As detailed in ISO/TR 20491:2019 Fasteners—Fundamentals of hydrogen embrittlement in steel fasteners, surpassing 390 HV markedly heightens susceptibility to HE, with the effect following a sigmoidal relationship (Figure 2).

Crucially, two common misconceptions about HE warrant clarification.

First, ductility determined through bend tests does not guarantee immunity to HE. This bulk property reflects a material’s ability to deform plastically, but typical mechanical testing does not capture the prolonged tension required for HE development. Screws that appear ductile during testing can still suffer from HE and fail unexpectedly in service. As such, bend tests are not reliable indicators of HE resistance.

Second, it is important to distinguish between corrosion- and HE-induced failures in mass timber screws. While a zinc-plated wood screw may exhibit corrosion resistance and avoid corrosion-related failure, even minor damage to the zinc coating—a common occurrence in steel-to-wood connections—can trigger galvanic corrosion when water is present. This process generates hydrogen at the steel surface, which can readily infiltrate the tensioned screw and result in HE-induced failure, long before corrosion would compromise its structural integrity. Therefore, corrosion resistance must not be interpreted as protection against HE.

The CSA O86 standard emphasizes hardness and HE testing, in accordance with ISO 2702 and 15330, respectively, as some of the most reliable methods to evaluate HE susceptibility. Beyond compliance, mitigating HE risks calls for a collaborative effort among manufacturers, suppliers, engineers, and designers.

Manufacturers and suppliers play a vital role in minimizing risks associated with internal HE (IHE) during production. They can adopt techniques to limit hydrogen exposure, such as using inhibitors during acid cleaning and implementing electroplating methods designed to minimize hydrogen generation. Proper storage and transportation practices are equally critical to protect screws from moisture exposure.

Engineers and designers are responsible for addressing environmental HE (EHE) risks during service. This includes specifying screws with hardness levels appropriate for the intended service conditions (dry or wet). Robust environmental controls, such as measures to reduce moisture ingress, are vital to mitigating the conditions that promote HE.

By addressing both IHE and EHE through these strategies, stakeholders can significantly reduce the likelihood of HE-related failures, ensuring the durability and longevity of mass timber buildings.

The evolution of mass timber construction relies on the integration of high-performance structural screws in connections. While these screws enable innovative, high-capacity designs, they also present challenges like HE. Although HE-related standards for screws have yet to be established in every jurisdiction, designers need to be aware of the potential hazards of HE and the necessary measures to prevent it. Screws should be sourced from reputable suppliers that rigorously test their products and apply best practices to mitigate HE risks before installation.
By understanding HE mechanisms and adopting proactive strategies, the construction industry can confidently advance toward a future where mass timber stands as a reliable and sustainable alternative to traditional materials. ■

About the Authors

Dong Han, PhD, is a Senior Scientific Writer with MTC Solutions. He studied advanced materials for environmental, energy, and biomedical applications and has been involved in scientific publishing for over a decade (support@mtcsolutions.com).

Lori Koch, MS, PE, is a Senior Product Engineer with MTC Solutions. She is a board member for SEAVa, serves on the NCSEA Continuing Education Committee, and is a licensed Professional Engineer in Virginia (support@mtcsolutions.com).

Tyler A Davis, PhD, is a member of the R&D team at MTC Solutions. As a peer-reviewed author, he has a background in mechanical engineering and material science. His expertise also extends to many complementary fields such as sustainability and building development (support@mtcsolutions.com).