Bonding Zinc Painted/Passivated/Galvanized Substrates
Bonding Zinc Substrates: Surface Prep and Choosing a Zinc Adhesive
What is zinc and why is it used?
We come across bonding zinc substrates all the time, zinc is a very popular metal used for plating other metals (such as steel) to prevent corrosion, oxidation or rust – as it is commonly referred to. It crosses over many industries and applications including automotive, valves, plumbing and even down to basic items such as nuts, bolts and screws. Zinc is a sacrificial anode which prevents oxidation of the base metal. When zinc is oxidised it turns to a fine white dust (this is the zinc alternative to the common red rust, iron oxide).
In bonding zinc, is plating /passivation/galvanization treated the same?
No, there are differences and some are easier to bond than others. It is important to ascertain the type of zinc on the surface to be able to make the best adhesive selection.
Zinc plating is the more general term. Normally zinc plating would either be applied by electroplating, galvanization or will have a passivated layer to protect it from the “white rust”.
Bonding Zinc Plating that has been Painted
Often vulnerable metals will be zinc plated and then be painted. Paint adheres really well to the zinc plating and helps provide an effective barrier for the zinc (preventing that characteristic white dust effect). If the paint gets scratched, the zinc’s cathodic action prevents the steel underneath from corroding. This is ideal for applications such as roofing and metal fencing panels.
If you are bonding zinc with a painted surface, it is quite important to remove the paint (or better still, mask the bonding area before it is painted). Adhesion strength of paint is a lot lower than that of a structural adhesive. Removal can be done by paint stripper, abrasion or gritblasting. Ensure the surface below is clean, dry and free of any lose particles, use a solvent such as isopropanol to degrease the surface ready for bonding (if you use acetone it is important not to spill it or let it spread onto other paintwork as it can attack paint).
When bonding zinc that has been painted, you may want to use a structural acrylic-based adhesive or epoxies. These provide high structural bond strengths and excellent environmental protection. Other adhesives will also work well depending on the joint design and bonding requirements.
Bonding Galvanized Zinc
This is characterised by its “spangly” appearance, commonly seen in construction, air conditioning ducting etc. Hot dip galvanization – sheet metal e.g. steel, is dipped into hot zinc. Electrogalvanization gives a thinner layer (which is cheaper but weaker).
When bonding zinc that has been galvanized, you should first prep the surface. Degrease with isopropanol or acetone and make sure substrates are clean, dry and grease free. The zinc is designed to be a very robust layer so abrasion is highly unlikely to result in getting back to the parent metal underneath and may cause too much damage to thin substrates.
Adhesive selection: It is best to use a rubber toughened, flexible adhesive which will flex with the thin sheeting. Rigid adhesives are prone to snapping clean off the surface.
Bonding Passivated Zinc
After metal surfaces are electroplated with zinc, it is often passivated (to prevent the white dusty appearance) and therefore keeps the appearance looking shiny and new. Passivation involves using a type of chromate – the colour of the finished surface is dictated by this chemical. Most commonly seen are “gold” passivate (although no gold is involved) which is very frequently seen in automotive parts, “black zinc”, blue or clear zinc to name a few.
Surface preparation: degrease with isopropanol or acetone and ensure surfaces are clean, dry and grease free. Bonding zinc that has been passivated is easy with most adhesives so no abrasion necessary. Adhesive choice is more dependent on the joint design, gap fill required, service conditions, application process and cure time.
Bonding Zinc Alloy
Zinc alloy is widely used as a die casting material due to its low melting temperature, versatility, strength and durability properties (this is a solid zinc alloy item rather than a plated metal).
Again, a solvent degrease is probably sufficient for this surface and many adhesives bonding zinc alloy will work well so there is great freedom of choice of adhesive product.
For more information on bonding zinc substrates or product recommendations, please contact Permabond’s technical team.