Nanning, Guangxi, China – September 15, 2026

If you have ever experienced a Grade 10.9 hot-dip galvanized bolt snapping unexpectedly — sometimes hours or even days after installation — you know the frustration and safety concerns that come with it. These failures are not just inconvenient; they can be catastrophic in structural applications. If you are sourcing or specifying hot-dip galvanized bolts, you probably have two pressing questions. First, why do these high-strength bolts break when galvanized, and is it a quality issue with the manufacturer? Second, how can I prevent this from happening in my projects — are there specific specifications or processes I should be looking for? These are exactly the right questions to ask.

In this guide, we will explain the science behind why Grade 10.9 hot-dip galvanized bolts are susceptible to brittle fracture, clarify whether this is a quality issue or an inherent material challenge, and provide practical guidance on how to specify and source these fasteners safely. We will draw on industry standards including ASTM F2329, ISO 10684, and ISO 4042, as well as technical guidance from fastener industry experts. By the time you finish reading, you will understand the root causes of these failures and know exactly what to look for when purchasing hot-dip galvanized bolts from hot-dip galvanized bolts manufacturers.

Understanding the problem: Why Grade 10.9 bolts breakHydrogen embrittlement — The hidden culprit

The primary reason Grade 10.9 hot-dip galvanized bolts are prone to breaking is a phenomenon called hydrogen embrittlement (HE) . This is a process where atomic hydrogen penetrates the steel’s grain boundaries, reducing its ductility and making it susceptible to sudden, brittle fracture under tensile stress.

Here is how it happens: Before hot-dip galvanizing, fasteners must be cleaned to remove rust and scale. This cleaning is typically done through acid pickling — immersing the bolts in hydrochloric acid (HCl) or sulfuric acid (H₂SO₄). The acid reaction releases latent hydrogen ions (H⁺) that can penetrate the grain boundaries of the steel. Once inside, these ions combine to form stable hydrogen molecules (H₂) on the grain boundaries, creating weaknesses that can lead to failure when the fastener is subjected to tensile loads.

The risk increases with the material’s strength properties. Grade 10.9 bolts have a tensile strength of approximately 1,000 MPa, and when the applied stress exceeds this level, the possibility of hydrogen embrittlement becomes significant. Fasteners with a strength class of 10.9 or higher are particularly at risk.

Delayed fracture — The ticking time bomb

What makes hydrogen embrittlement particularly dangerous is a phenomenon called delayed fracture . This occurs when a fastener under tensile load suddenly breaks without warning — not at the time of installation, but after several hours or even days. As one industry expert explains, “there will be no problem at the time when fastener is fastened, but after several hours or days, the fastener head will pop out like a bullet and break”.

This delayed failure explains why a bolt can appear perfectly fine during installation and inspection, yet fail unexpectedly later. In structural applications, this can have devastating consequences.

Is it a quality issue?

The short answer is: not necessarily. While poor quality control can certainly contribute to failures, the risk of hydrogen embrittlement in Grade 10.9 hot-dip galvanized bolts is an inherent challenge of combining high-strength steel with the hot-dip galvanizing process. The brittleness issue is a function of the material strength, the pickling process, and the galvanizing process itself — not simply a matter of “good” versus “bad” manufacturing.

That said, quality control does matter enormously . As noted by the Southern African Institute of Steel Construction (SAISC), failures have been traced back to either hydrogen embrittlement or causes associated with the quality of the raw material and/or the heat treatment requirements needed to achieve mechanical properties. Poor heat treatment control, inadequate post-galvanizing baking, and excessive pickling times can all turn a manageable risk into a catastrophic failure.

Industry standards and best practicesWhat the standards say

Several standards govern the hot-dip galvanizing of high-strength fasteners:

ASTM F2329 / F2329M is the primary U.S. specification for hot-dip zinc coating applied to carbon and alloy steel bolts, screws, washers, nuts, and special threaded fasteners. Importantly, this standard does not recommend hot-dip galvanizing for grades with a hardness of 33 HRC and above (approximately 1,100 MPa tensile strength) due to hydrogen embrittlement risk. Grades that are prohibited or not recommended include ASTM A490, ASTM A354 Grade BD, and SAE J429 Grade 8.

ISO 10684 specifies requirements for hot-dip galvanized coatings applied to coarse threaded steel fasteners from M8 up to M64, for property classes up to and including 10.9 for bolts, screws, and studs. This standard provides information for pre-treatment and galvanizing processes that minimize the risk of brittle fractures.

ISO 4042:2022 specifies requirements for electroplated coatings on steel fasteners and gives recommendations to minimize the risk of hydrogen embrittlement.

Prevention through process control

The good news is that hydrogen embrittlement can be prevented or significantly reduced through proper process controls. Industry experts and standards bodies have identified several key measures:

1. Eliminate or minimize acid pickling. The most effective solution is to completely eliminate acid pickling for Grade 10.9 fasteners. Pre-treatment cleaning should be achieved through mechanical cleaning methods such as shot blasting or sand blasting. This eliminates the generation of hydrogen ions entirely.

If acid pickling must be used, strict controls are essential. The SANS 10094 specification recommends restricting pickling times to less than 15 minutes. For high-strength fasteners (≥33 HRC), mechanical descaling with a flash pickle is recommended over prolonged acid immersion.

2. Stress relief before galvanizing. Fasteners that have undergone severe work hardening must be stress-relieved by the manufacturer prior to galvanizing to prevent hydrogen embrittlement or distortion.

3. Baking (hydrogen embrittlement relief). Baking at temperatures of 190-220°C for periods of 4 to 24 hours (depending on strength level and size) should be used to eliminate the deleterious effects of hydrogen. If the fastener hardness is ≥33 HRC, baking must be performed after pickling but before galvanizing. For electroplated fasteners, a “heat soak process” at 200°C is required immediately after coating to drive off hydrogen ions from grain boundaries.

4. Zinc flake coatings as an alternative. Zinc-flake coatings (such as Dacromet) are increasingly used as an alternative to hot-dip galvanizing for high-strength fasteners. These coatings do not introduce hydrogen into the steel and provide excellent corrosion resistance.

Specifications comparison: Grade 10.9 hot-dip galvanized bolts

Specification	Key Requirements	Grade 10.9 Applicability
ASTM F2329	Hot-dip zinc coating; hardness ≥33 HRC not recommended	Not recommended — risk of HE
ISO 10684	Hot-dip galvanized coatings for fasteners M8–M64	Permitted — up to Class 10.9
ISO 4042:2022	Electroplated coatings; HE prevention requirements	Permitted — with baking
Coating Thickness (ASTM F2329)	≥50μm (>M10); ≥43μm (≤M10)	Must meet thickness requirements
Baking Requirement	190-220°C, 4-24 hours	Required for ≥33 HRC
Pickling Restriction	≤15 minutes or mechanical cleaning	Required to minimize HE risk

Practical recommendations for procurement

When sourcing hot-dip galvanized bolts from hot-dip galvanized bolts manufacturers, here is what you should look for:

1. Specify process controls in your purchase order. Require that the manufacturer use mechanical cleaning (shot blasting) instead of acid pickling for Grade 10.9 bolts. If acid pickling is unavoidable, specify a maximum pickling time of 15 minutes.

2. Require baking documentation. Ensure the manufacturer provides documentation confirming that baking was performed at 190-220°C for the appropriate duration.

3. Verify heat treatment quality. Poor heat treatment control is a known contributor to embrittlement failures. Request heat treatment process documentation and mechanical test reports.

4. Consider alternative coatings. For critical applications, consider zinc-flake coatings (Dacromet) which provide excellent corrosion resistance without hydrogen embrittlement risk.

5. Work with reputable manufacturers. Choose hot-dip galvanized bolts manufacturers who have documented quality systems, experience with high-strength fasteners, and a track record of compliance with relevant standards.

Contact Us

Not sure whether hot-dip galvanized bolts are the right choice for your project, or need help sourcing Grade 10.9 hot-dip galvanized bolts from reliable hot-dip galvanized bolts manufacturers? Our team works with leading fastener manufacturers and bolt factories to source high-quality fasteners for construction, infrastructure, and industrial applications. Whether you need grade 10.9 bolts prices or assistance with technical specifications and hydrogen embrittlement prevention, we are here to help. Reach out today for a consultation or a quote tailored to your needs.

FAQQ1: Why do Grade 10.9 hot-dip galvanized bolts break?

Grade 10.9 hot-dip galvanized bolts can break due to hydrogen embrittlement — a process where hydrogen atoms from acid pickling penetrate the steel’s grain boundaries, making the metal brittle and susceptible to sudden fracture under tensile stress. This can result in delayed fracture, where the bolt fails hours or days after installation without warning.

Q2: Is this a quality issue with the manufacturer?

Not necessarily. Hydrogen embrittlement is an inherent risk when combining high-strength steel (≥1,000 MPa tensile strength) with hot-dip galvanizing. However, poor quality control — such as excessive pickling times, inadequate baking, or improper heat treatment — can significantly increase the risk. Working with reputable hot-dip galvanized bolts manufacturers who follow strict process controls is essential.

Q3: Can Grade 10.9 bolts be hot-dip galvanized at all?

Yes, but with strict precautions. ISO 10684 permits hot-dip galvanizing for property classes up to and including 10.9. However, ASTM F2329 does not recommend galvanizing for fasteners with hardness ≥33 HRC due to hydrogen embrittlement risk. Proper process controls — including mechanical cleaning instead of acid pickling, stress relief, and baking — are essential to minimize risk.

Q4: What are the alternatives to hot-dip galvanizing for Grade 10.9 bolts?

Zinc-flake coatings (such as Dacromet) are an excellent alternative that provide comparable corrosion resistance without introducing hydrogen into the steel. These coatings are non-electrolytic and do not require acid pickling, eliminating the primary source of hydrogen embrittlement. For many demanding applications, zinc-flake coatings are the preferred choice for high-strength fasteners.

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About Us

Established in 2012, Aozhan Hardware Fastener Co., Ltd. is a company specializing in the production and sale of quality hardware fasteners. We are committed to providing our customers with high quality and reliable products to solve the pain points in the industry.

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