In this article, we'll explore how corrosion silently undermines structural joints, why coating performance matters far beyond rust prevention, and the key questions engineers should ask when assessing corrosion resistance claims. If long-term reliability matters to your application, these are factors you can't afford to overlook.
Why corrosion is a mechanical problem
Corrosion impacts fastener performance in three ways:
- Interface Degradation: Early corrosion between the fastener and joined materials can impact load transfer characteristics, and therefore affect retained clamp force and slip resistance.
- Coating Failure and Galvanic Attack: As protective coatings continue to break down, electrochemical corrosion accelerates, further impacting the fastener's performance. Once the coating wears away, moisture combined with contact between dissimilar metals can trigger a reaction that corrodes the fastener faster than normal rust.
- Section Loss: Advanced material degradation reduces the fastener's effective cross-sectional area, reducing both tensile and shear strength. As preload declines, joints shift from friction-controlled load transfer to fastener-controlled load transfer. With this, cyclic stresses increase, fatigue life decreases, and therefore reliability declines.
Why do coatings matter?
Whilst many applications will not introduce conditions severe enough to affect the fastener performance during the expected application lifetime, joints installed in harsh environments experience severe premature performance drops or even failure within product lifetime unless they are specified with the correct coating.
Whilst fastener performance is primarily judged by values such as tensile strength, shear strength, clamp load, and fatigue resistance, coatings allow these performance values to be retained for significantly longer time periods.
How do coatings prevent corrosion?
Coatings act as a physical barrier between the base metal and corrosive elements. In most scenarios exposure to moisture, oxygen, salts and chemicals can act as a catalyst to the oxidation process. A coating essentially shields the base metal from the elements which cause and accelerate the oxidisation process. A fastener coating therefore extends the mechanical performance.
To ensure optimal results, the coating should be an engineering-oriented decision during the design stage, and not a best-fit choice later down the line. The different types of coatings typically used for industrial fasteners include:
|
Material |
Description |
Limitations |
|
Zinc Electroplating |
Thin, electrochemically applied zinc layer |
Limited lifespan in harsh or outdoor conditions; coating is relatively thin and wears quickly, and can also cause hydrogen embrittlement in the base material |
|
Hot-Dip Galvanizing |
Thicker zinc coating applied by immersing fasteners in molten zinc |
Coating thickness can affect thread fit/tolerance; less precise than plating methods, as well as reducing fatigue performance |
|
Zinc Flake (e.g., Geomet, Dacromet) |
Chromate-free coating applied in thin layers, cured at low temperatures |
Higher cost than standard plating; application process is more specialised |
|
PTFE / Fluoropolymer |
Low-friction coating applied over a base layer |
Primarily functional (friction control), not a standalone corrosion solution; added cost |
|
Phosphate Coating |
Chemical conversion coating, typically used as a base layer |
Offers minimal corrosion resistance on its own; usually requires a secondary coating or oil |
How are coatings tested?
There are many ways to test coatings for their corrosion resistance, but the most widely used is neutral salt spray (or "NSS") testing. Fasteners are placed in a controlled chamber and exposed to a continuous saline mist, simulating accelerated corrosive conditions. The key benchmark is hours to red rust. Across the fastener industry, results typically range anywhere from 96 hours for basic coatings up to 720+ hours for high-performance options. Testing for CX goes even further, introducing cyclic conditions, with NSS, UV, humidity and temperature changes.
How are coating test results categorised?
To make corrosion resistance easier to compare, the International Organisation for Standardisation (ISO) has written a category specifically addressing this: ISO 12944 corrosivity categories. Coatings are given one of the following classifications based on the type of conditions they have been tested and proven to work in:
- C1 - Very low: dry, indoor environments
- C2 - Low: low humidity, minimal pollution
- C3 - Medium: urban or industrial atmospheres
- C4 - High: industrial areas or coastal locations with moderate salinity
- C5 - Very high: industrial or marine settings with high humidity
- CX - Extreme: offshore environments with high salinity and aggressive conditions
How is CX testing different?
The tests for CX certification are by far the most rigorous. CX testing aims to replicate continuous cycles of salt exposure, humidity, UV and temperature over a 25-week period. Paired with scribe tests, seawater immersion, cathodic disbondment testing (which tests whether the coating actually sticks to the base material).
For high-demand, extreme conditions, sourcing industrial fasteners with CX certification is crucial, especially when installed in locations with access limitations, which make inspections difficult.
Looking beyond fastener cost
Corrosion can lead to unplanned repairs, maintenance, equipment downtime, and increased safety risks.
Validated coatings, controlled manufacturing processes, independently verified testing, and proven field performance provide risk-management mitigation.
When qualifying a fastening supplier, engineers should request third-party test certificates, coating specifications, and quality documentation relevant to the intended service environment.
Striking the right balance between fastener and coating is essential if you want your fastener to perform reliably, for the lifetime of the application. Clearly communicating the fastener requirements early on in the development process can mitigate risks further down the line.
Why corrosion claims should be independently verified
On fasteners from lower-cost overseas suppliers, coating quality is often one of the first areas where shortcuts are taken. For critical applications, third-party testing or coating by an approved supplier is essential. The need becomes more important in extreme environments. Continue reading about different Huck coating options.
Final thoughts
Corrosion resistance is not just about delaying visible rust. It is about helping the joint retain structural performance in service.
A coating may pass a laboratory test, but engineers need to know whether the fastening system will still perform after years of real exposure, vibration, moisture, and maintenance constraints.
For critical applications, coating choice should be verified, specified, and approved before installation.
Huck offers a range of coating options for demanding environments, with a new CX coating developed for more aggressive applications. We will share more detail on CX coating performance and application suitability in the next article.
To discuss corrosion-resistant coating options for Huck® lockbolts and blind fasteners, contact our engineering team.
