When fasteners such as bolts, screws and nuts are zinc plated, the coating provides corrosion protection and improves the appearance of the fastener. Common finishes include zinc blue, zinc black, zinc yellow and zinc-nickel.
But when high-strength steel fasteners are plated, there is another important factor to consider: hydrogen embrittlement.
In simple terms, hydrogen can enter the steel during the plating process. In high-strength fasteners, this hydrogen can contribute to cracking and unexpected failure. This is why hydrogen embrittlement control is an important part of plating high-strength fasteners.
What Is Hydrogen Embrittlement?
Hydrogen embrittlement occurs when hydrogen enters steel and reduces its resistance to cracking, particularly when the steel is under high stress.
During processes such as acid cleaning, pickling and electroplating, hydrogen can be generated at the surface of the fastener. Some hydrogen escapes, but some can enter the steel.
Once inside, hydrogen can move towards highly stressed areas such as thread roots, notches and other stress-concentrated regions. This can become a problem when the fastener is highly loaded.
For example: a Grade 10.9 bolt may look perfectly normal after zinc plating. However, if hydrogen has entered the steel and the bolt is subsequently tightened and placed under load, hydrogen-assisted cracking can potentially occur.
Why Does Zinc Plating Need Hydrogen Embrittlement Control?
The issue is not the colour of the zinc coating.
Zinc blue, zinc black and zinc-nickel plating can all require hydrogen embrittlement control when applied through an electroplating process.
The concern comes mainly from the processes used to prepare and plate the steel. A simplified process looks like this:
- Step 1Steel fastener
- Step 2Cleaning / Pickling
- Step 3Electroplating
- RiskHydrogen enters steel
- ControlHydrogen-relief treatment
ISO 4042:2022 covers electroplated coating systems for fasteners, including zinc, zinc-nickel and zinc-iron, and provides requirements and recommendations for minimizing hydrogen-embrittlement risk.
Why Are 10.9 and 12.9 Bolts More Sensitive?
The higher the strength and hardness of a steel fastener, the greater its general susceptibility to hydrogen embrittlement. A simple comparison is:
| Fastener Grade | Nominal Tensile Strength | HE Susceptibility |
|---|---|---|
| 8.8 | 800 MPa | Lower |
| 10.9 | 1,000 MPa | Higher |
| 12.9 | 1,200 MPa | Very high |
This does not mean that every 8.8 bolt is automatically safe or that every 10.9/12.9 bolt will fail. The actual risk depends on the steel, hardness, plating process, stresses and applicable specification.
However, as fastener strength increases, hydrogen embrittlement becomes a much more important consideration.
What Is Hydrogen De-Embrittlement?
After electroplating, a controlled hydrogen-relief baking process may be used to encourage hydrogen to diffuse out of the steel. The basic idea is:
- Step 1Plating
- RiskHydrogen enters steel
- Step 2Controlled heating
- Step 3Hydrogen diffuses out
- ResultReduced risk
This process is commonly referred to as hydrogen de-embrittlement or hydrogen relief baking.
Important: baking does not mean that all hydrogen is guaranteed to be removed. It is a risk-reduction process, and the plating process itself must also be properly controlled.
The timing is important as well. Hydrogen-relief treatment is generally performed as soon as practicable after plating, according to the applicable requirements.
What About 12.9 Grade Bolts?
There is a common misconception that:
"12.9 bolts should not be baked after plating."
This is too broad.
The real issue is that 12.9 fasteners are highly susceptible to hydrogen embrittlement while also being highly heat-treated materials. Therefore, any post-plating baking must be carefully controlled.
The temperature must be suitable for the fastener's heat-treatment condition. Excessive or inappropriate heating can affect the mechanical properties that the bolt was originally heat-treated to achieve.
12.9 does not mean "never bake." It means hydrogen control and any hydrogen-relief treatment must be carefully specified and controlled.
A Simple Example
Suppose you are purchasing:
- Product
- M12 × 60 Hex Bolt
- Standard
- ISO 4017
- Grade
- 10.9
- Finish
- Zinc Blue
Instead of telling the plating supplier only:
"Zinc blue plating required."
it is better to specify the fastener grade and applicable coating requirements and ask how hydrogen embrittlement is being controlled.
For a 12.9 fastener, this becomes even more important.
What Should Fastener Buyers Remember?
When specifying a plated fastener, don't look only at the basics. Consider the plating as well:
Don't look only at
- Size
- Thread
- Grade
- Zinc colour
Also consider
- Plating process
- Fastener strength / hardness
- Hydrogen embrittlement risk
- Hydrogen-relief requirements
- Applicable coating standard
The Key Takeaway
The colour of a plated fastener tells you what the coating looks like. It does not tell you whether hydrogen embrittlement has been properly controlled.
For high-strength fasteners, particularly 10.9 and 12.9 grades, the fastener grade, plating process and hydrogen-relief process should all be considered together.
For reliable performance, always ensure that the plating process follows the applicable fastener and coating standards, as well as any specific OEM or customer requirements.
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Frequently Asked Questions
What is hydrogen embrittlement in plated fasteners?
Hydrogen embrittlement occurs when hydrogen enters steel and reduces its resistance to cracking, particularly when the steel is under high stress. In plated fasteners, hydrogen can be generated at the surface during acid cleaning, pickling and electroplating, and some of it can enter the steel. It then moves towards highly stressed areas such as thread roots and notches, where it can contribute to cracking and unexpected failure once the bolt is tightened and placed under load. High-strength grades such as 10.9 and 12.9 are the most susceptible, which is why a controlled hydrogen-relief baking process may be used after plating to reduce the risk.
Do Grade 10.9 and 12.9 bolts need hydrogen de-embrittlement after zinc plating?
Hydrogen embrittlement control should be considered whenever Grade 10.9 (1,000 MPa) or Grade 12.9 (1,200 MPa) fasteners are electroplated, whether the finish is zinc blue, zinc black, zinc yellow or zinc-nickel, because susceptibility rises with strength and hardness. A controlled hydrogen-relief bake, generally performed as soon as practicable after plating, encourages hydrogen to diffuse out of the steel; it reduces risk but does not guarantee that all hydrogen is removed. For Grade 12.9, the idea that the bolts should never be baked is too broad: any baking must be carefully specified, at a temperature suitable for the fastener's heat-treatment condition. Follow the applicable coating standard, such as ISO 4042:2022, and any OEM or customer requirements.
Does the zinc plating colour show that hydrogen embrittlement has been controlled?
No. The colour of a plated fastener only tells you what the coating looks like; zinc blue, zinc black and zinc-nickel can all require hydrogen embrittlement control when applied by electroplating. To cover the risk when ordering, state the fastener grade and the applicable coating requirements, and ask the plating supplier how hydrogen embrittlement is being controlled, instead of specifying only the colour. Alongside size, thread, grade and colour, also consider the plating process, the fastener's strength and hardness, the hydrogen-relief requirements and the applicable coating standard.
Excel Trading Corporation supplies fasteners for industrial applications across India. For product availability, technical specifications and bulk pricing, contact us at +91 7204435253 or info@exceltrading.in.