Hydrogen embrittlement presents a risk to fasteners in construction, but Hobson Engineering is tackling the challenge with zinc flake coatings.
For decades, the engineers and specialists at Hobson Engineering have been at the forefront of fastener technology, helping the industry navigate risks and best practices. As Australia increases its use of high-tensile steels, the company is raising awareness of hydrogen embrittlement – a hidden threat that can lead to sudden failures – and how zinc flake coatings offer a reliable solution.
Hydrogen embrittlement is a critical failure mechanism in high-tensile steels, occurring when specific conditions align. For embrittlement to take place, three factors must be present: high steel hardness, a high tensile load, and exposure to hydrogen. High-tensile fasteners, particularly those at Property Class (PC) 10.9 and above, are most vulnerable. The risk increases with higher tensile load on the fastener and when hydrogen is introduced into the metal.
Hydrogen exposure often happens during coating processes. Electroplated zinc carries the highest risk, while hot-dip galvanising can also introduce hydrogen, especially if acid cleaning is involved. Once a fastener under load becomes embrittled, failure can occur without warning.
Gavin McPherson, lead engineer at Hobson Engineering, explains that hydrogen embrittlement can occur in two ways: internally during manufacturing or externally from environmental exposure.
“Internal hydrogen embrittlement arises from processes such as electroplating, where hydrogen is introduced during manufacturing,” says McPherson.
“Environmental hydrogen embrittlement results from external sources, such as certain chemicals used onsite near the fastener. While both are uncommon, they are important considerations in failure analysis.”
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When a fastener fails unexpectedly, hydrogen embrittlement is often one of the first suspects. It can affect bolts, case-hardened screws, and other high-strength components, usually within 24 hours of being placed under load.
“As Australia transitions to greater use of high-tensile steels, the risks associated with hydrogen embrittlement become more pressing,” says McPherson.
“In the structural bolt market, PC 8.8 bolts – particularly our K0 range – are widely used. These bolts have a hardness below the critical threshold, removing the risk of hydrogen embrittlement. However, the use of PC 10.9 structural bolts is increasing, and these are at risk of susceptibility to hydrogen embrittlement.”
To mitigate risk, Hobson Engineering sources its PC 10.9 structural bolts from a trusted fastener manufacturer in limited factories. These bolts are hot-dip galvanised, providing a more controlled and reliable option.
While it is possible to source hot‑dip galvanised PC 10.9 structural bolts, the increased control over forming, heat treatment, coating and testing is cost‑prohibitive, and any oversight puts the entire manufacturing run at risk. For this reason, zinc flake carries a much lower risk of non-conforming products being put into the market.
Another way that engineers can design out the risk is by using high‑tensile stainless steel, such as Bumax, which has no coating.
Socket screws, traditionally zinc-plated or electroplated, require additional safeguards. Manufacturers manage embrittlement risk through controlled processing, including a baking procedure that allows trapped hydrogen to dissipate. This post-plating treatment reduces risks in high-strength products such as PC 12.9 fasteners.
“We want to raise awareness of this issue. Some engineers understand the risks and specify that they do not want zinc-plated or hot-dip galvanised high-strength fasteners, while others design out the risk by opting for lower tensile fasteners,” says McPherson.
A key development in mitigating hydrogen embrittlement is the use of zinc flake coatings, which Hobson Engineering has recently introduced to its range. Zinc flake is a cold‑applied, dry coating composed of zinc and aluminium in an inorganic binder. Unlike electroplating or acid-based treatments, this process eliminates the primary sources of hydrogen exposure, reducing embrittlement risk. As a result, it allows for the use of high‑tensile fasteners in demanding applications without compromising reliability.
“Zinc flake coatings have been in use for some time and are similar to ZAM coating, a blend of zinc, aluminium and magnesium used on cold-formed steel. This combination provides superior corrosion protection compared to zinc alone,” explains McPherson.
“The aluminium protects the steel while also providing cathodic protection, a key function of coatings. Zinc flake coatings perform comparably to hot-dip galvanising, but with added benefits. For high-tensile fasteners, I would recommend zinc flake coatings over hot-dip galvanising.”
Zinc flake coatings come at a higher cost than standard hot-dip galvanising, but McPherson believes the investment is worthwhile. The coatings provide a consistent thickness, typically between 12 to 18 microns, compared to over 50 microns for hot‑dip galvanising.
“This allows it to fit seamlessly with other threaded fasteners. With galvanising, threads must be over-tapped to accommodate the added thickness, whereas zinc flake, being thinner, fits a standard thread profile and has good lubricating properties,” he says.
“This ensures a more consistent finish when applying torque and improves precision when tightening fasteners.”
McPherson sees further innovations emerging that could enhance fastener durability in demanding environments, though gaining traction in the industry remains a challenge.
“We offer a range of coatings, including zinc flake,” he says. “We also use Ruspert, a ceramic coating, on some anchors and screws, as well as e-coating, an electronically applied epoxy coating that performs well without relying on cathodic protection.”
While hot-dip galvanising has long been the industry standard due to its durability, zinc flake coatings provide an alternative where hydrogen embrittlement is a concern.
As awareness of the risks grows, and the demand for high-performance fasteners increases, zinc flake coatings are set to play a greater role in securing the integrity of structures across the construction industry.
For more information or tailored advice on zinc flake coating solutions, contact the Hobson Engineering team.