What is zinc, and why is it popular?
We come across bonding zinc substrates all the time. Zinc is a very popular metal for plating other metals. It prevents corrosion, oxidation, or ‘rust’. It crosses over many industries and applications, including automotive, valves, plumbing, and even basic items such as nuts, bolts, and screws. Zinc is a sacrificial anode that prevents oxidation of the base metal. When zinc is oxidized 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 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”.
Often vulnerable metals will be zinc plated and then 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 you scratch the paint, the zinc’s cathodic action prevents the steel underneath from corroding. This is ideal for applications such as roofing and metal fencing panels.
When bonding painted zinc remove the paint. Or, better still, mask the bonding area before painting. Adhesion strength of paint is a lot lower than that of a structural adhesive. Remove the paint with paint stripper, abrasion or grit blasting. Ensure the surface below is clean, dry, and free of any loose 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 painted zinc, 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.
This is characterized 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 galvanized zinc, you should first prep the surface. Degrease with isopropanol or acetone and make sure substrates are clean, dry, and grease free. The zinc is 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.
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 color of the finished surface is dictated by this chemical. “Gold” passivate (although there is no gold in it) is the most common. It is popular in automotive parts. There are also, “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 passivated zinc is easy with most adhesives, so no abrasion is necessary. Adhesive choice is more dependent on the joint design, gap fill required, service conditions, application process, and cure time.
Zinc alloy is ideal for die casting. 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.