Hydrogen Embrittlement (HE) of metallic hardware and connectors in the mass timber industry has become a discussion topic and a major point of consideration for specifiers and structural designers in recent years. A wide variety of factors can contribute to stress failure caused by HE, which can be generally categorized into two main types:
In this article, we focus on IHE: what it is, how it affects mass timber screws, how it may occur, and what measures can be taken to prevent it.
Approximately 5-minute reading time.
Environmental vs. Internal Hydrogen Embrittlement
Hydrogen embrittlement (HE) occurs when hydrogen infiltrates a metal’s microstructure, making it brittle under sustained tension and significantly raising the risk of sudden, catastrophic failures. This issue is particularly concerning for mass timber screws, making preventive measures crucial.
Hydrogen can originate from various sources, typically categorized based on when the screw is exposed to tension:
Note that the term “internal” typically relates to the hydrogen absorbed during the manufacturing process rather than the screw’s inherent properties. If delayed failure occurs that suggests HE, a combination of fracture analysis and chemical-based investigation is required to confirm the phenomenon.
While EHE and IHE are distinguished by the timing of tensioning, the responsibility for minimizing hydrogen absorption depends on when the products are transferred from the manufacturer or seller to the installer. The manufacturer can only mitigate IHE. On the other hand, the installer bears responsibility for the risk of both EHE and IHE, the latter of which could arise during storage and handling before installation.
Main Internal Sources of Hydrogen
Acid Cleaning
Acidic solutions are used to clean screws at a microscopic level after production, preparing their surface for electroplating, a process that deposits a protective coating (e.g., zinc or nickel). While effective, these solutions are a significant source of hydrogen, making them a leading cause of IHE.
Electroplating
During electroplating, the screw is placed in a solution bath where a current is applied between a cathode (screw) and anode (coating metal). The aqueous electrolytic solution used in this process is often acidic and can facilitate hydrogen absorption into the screw. Additionally, plating parameters such as time and current are factors that can result in hydrogen production at the screw surface.
Steel Quality
The quality of the steel used in manufacturing screws is crucial. If hydrogen levels aren’t carefully monitored during steel production, contamination can occur, increasing the risk of IHE later.
Storage and Transit
Hydrogen may infiltrate screws during storage or transit if ambient conditions (e.g., humidity) are not properly controlled. This is particularly concerning when screws are outsourced for electroplating, although this represents a much smaller exposure risk compared to the other scenarios.
Preventative Measures
To prevent IHE and ensure your screws remain reliable, consider the following strategies:
Acid Cleaning Enhancements
Work with your supplier to understand their quality-control processes and ensure they are minimizing hydrogen exposure during acid washing. This can be accomplished by lowering the acid concentration, reducing the time in the solution, and using an inhibitor additive.
Hardness Control
CSA O86:24 guidelines, which reference ISO 2702, mandate that self-tapping screws with a core hardness above 390 HV should not be used in mass timber structures. Select steel with adequate strength without exceeding this hardness threshold, which is shown to mitigate the risk of IHE.
IHE Testing
For manufacturers, CSA O86:24 guidelines require IHE testing on screws during manufacturing as a part of a quality-control program to catch any potential issues and correct them before screws are sold for installation. For customers, make sure to request IHE testing information from your supplier.
Baking
If screws are at risk of IHE, baking them at an elevated temperature can help remove hydrogen. Unfortunately, this process could take up to 24 hours, and the additional energy needed makes this solution industrially prohibitive for manufacturers.
Material Susceptibility
Some materials are inherently less prone to HE, which is said to be attributed to mechanisms like hydrogen traps or dislocation mobility. As HE is a complicated and detrimental process, enhancing material resistance is a current area of research.
Delivering reliable solutions that comply with the latest design codes and standards in North America is at the core of MTC Solutions’ business model. This is why we have recently established an internal Quality Control service, which plays a key role in managing the risk of HE in our structural fasteners. If you’re interested in how we ensure top-quality hardware, don’t miss our next blog post release about it!
As always, our Technical Support Team is here to answer any questions you might have. 😉
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