No matter the industry, creating a long-lasting and durable structure will always present challenges. Materials can degrade or corrode. Foundations can fail. Bonds can break.
Designing out these potential risks during the early stage of the process is essential.
Galvanised steel remains a popular material in all industries to increase the durability and longevity of structures. However, despite technological advancements, it can still present bonding challenges.
Here at Forgeway, we help companies find the right adhesive solution. Bonding galvanised steel is challenging but not impossible.
In this article, we will analyse why galvanised steel is difficult to bond. We will also describe which adhesives are best for this application. By the end of the article, you will understand how to create a strong bond with galvanised steel.
It's important to note that not all types of galvanised steel are difficult to bond. 'Galvanised steel' is not all the same. It will vary from grade to grade and from supplier to supplier.
So it wouldn't be correct to label all galvanised steel types as 'difficult to bond'. Nonetheless, some grades of this metal will present bonding challenges. Let's dissect why.
Firstly, we should look at the 3 main processes for galvanising a steel material:
It's important to analyse these methods as you can generally get a good idea of which galvanising method will present a more challenging surface.
Take, for example, galvannealed steel. The harder and more brittle surface by itself isn't any more challenging than the other types of galvanised steel.
The increased brittle nature of the galvannealed coating is more likely to fail when under strain. Rigid adhesives will exacerbate this issue. Rigid adhesives present an increased risk of bond failure with all galvanised steels.
Hot-dip galvanised coatings will also have this issue. Whilst the coating isn't as hard and brittle as galvannealing, it still presents a brittle layer. This can increase the likelihood of the bond failing.
It's also helpful to analyse why these galvanised materials can cause bonding challenges on a chemical level. As we explain in a separate article, strong adhesion will usually require a strong chemical bond to form between the material and the adhesive.
Low surface energy materials and materials with contamination (like grease, dirt, etc) will not provide a very receptive surface for most adhesives. It will be very difficult for the adhesive to form a strong bond.
Whilst the zinc coating used in the galvanising process isn't low surface energy, some of the other ingredients can create a low surface energy layer on the steel. This is where the grade and supplier of the product can determine its bondability.
You may also encounter scenarios where the adhesive isn't compatible with the zinc coating on the steel's surface. We've witnessed examples where some acrylic-based adhesives have struggled to form a strong chemical bond with the galvanised steel.
It's not just acrylic adhesives. Some epoxy-based and polyurethane-based adhesives will also not interact and form a strong chemical bond with the zinc coating. Some raw materials used in these adhesives are incompatible with the zinc surface.
As you can see, each application has its nuances. There are lots of different variables that can determine whether the galvanised steel will present bonding challenges. There is no easy way of distinguishing the bondability without specific testing.
You can use the guidelines listed above to give you a good idea. However, specific testing is the best way to ensure you are using the right adhesive/galvanised steel combination.
As we attempted to explain above, there are plenty of variables that can affect each bond. Your specific grade of galvanised steel might be compatible with most adhesives. Then again, it might not.
However, just as you can get an early indication of bond strength by the type of galvanised steel, you can also get an early indication of which adhesives are more likely to be compatible.
Due to the hard and often brittle nature of most galvanised coatings, it's important to choose an adhesive that has enough flexibility to absorb any stress or impact. Typically, an adhesive with more than 50% elongation will be flexible enough.
It's also important to be aware of the adhesive chemistry's compatibility with galvanised coatings. As we alluded to, some versions of epoxy, acrylic, and polyurethane adhesives will have poor chemical adhesion to the zinc coating.
Hybrid (MS) polymer adhesives will typically have good chemical adhesion to galvanised steel. They also have good flexibility. Both of these characteristics make Hybrid polymers the best adhesive solution for most galvanised steel bonding applications. Although, they will require the surface to be clean and free from contamination, as always.
This is because the chemical interaction between the Hybrid polymer and the zinc coating is strong.
They come with drawbacks though. Hybrid polymer adhesives will rarely be able to form strengths above 5 MPa. Companies will often deem this to not be structurally sufficient for their application.
The trade-off you will often see for increased flexibility is reduced strength.
At Forgeway, we have tested all of our adhesives on electro-galvanised and hot-dip galvanised steel. We've listed a recommendation for adhesives based on the results.
Now you understand why galvanised steel can be difficult to bond. You also understand which adhesives will generally perform better with this material. It's time to analyse which adhesive is best for you.
Here at Forgeway, we have been helping companies solve their bonding challenges for over 25 years. We want to help you solve yours.
If you are using galvanised steel and you can't seem to find the right adhesive, you have several options.
If your main concern is strength and the bond won't experience much movement or impact, we suggest Purok VX60.
Or, if your bond is likely to experience movement and impact, we would suggest Formoa 017FE.
Alternatively, you might want to explore an adhesive that has high strength and high flexibility. In that case, you will need to explore the vast range of adhesives available from suppliers all around the world.
The easier option is to speak to one of our adhesive experts. They will help you find the best adhesive solution for your application.