Various methods of applying zinc films to steel items offer unique and long-lasting anti-corrosion benefits, according to various zinc experts who spoke during a webinar hosted by Creamer Media Contract Publishing on behalf of the International Zinc Association (IZA) on November 23.
IZA technology and market development director Martin van Leeuwen said that close to 60% of zinc is used for galvanising or applying zinc coatings on steel for corrosion protection.
“The other 40% is then split over semi-manufacturing [applications]. It is also used for brass . . . die-cast products. The last 9% goes into oxides and chemicals.”
As a result of this composition, he said, it was “no surprise” that about half of zinc was used in applications in the construction industry to protect outdoor steel from corrosion. Twenty-one per cent was also used in the transportation sector, while the infrastructure sector accounted for 16%, consumer products 6% and industrial machinery 7%.
Zinc coatings – providing a barrier coating or galvanic protection to steel – can be used in various different applications, including metallising, hot-dip galvanising, zinc-rich paints, continuous galvanising and electro-plating.
“Zinc is impermeable to all kinds of aggressive materials like water, chlorides or other kinds of aggressive elements or compounds,” said Van Leeuwen.
Another form of zinc coating – zinc-rich paints – provide protective paintable coatings, and are made up of four main ingredients, including a binder, a pigment, a solvent and additives, with zinc-rich paints often using zinc dust or zinc phosphate as a pigment, said paints and stains company PPG Europe, Middle East and Africa protective and marine coatings technical support manager Michael Taylor.
“Zinc is used in its powder or dust form, is grey in colour [and] is used as an anti-corrosive pigment designed to protect steel structures in construction or marine applications,” he said.
The main ingredient in paint is the binder, while solvents enable processes like application, drying and curing. The third main ingredient is pigments.
Besides using zinc dust or phosphates as an anti-corrosion pigment, other anti-corrosion pigments can include titanium dioxide and red iron oxides, as well as aluminium flake or micaceous iron oxide – the latter two of which are flatter in shape and serve as a barrier to water permeability, said Taylor.
He noted that alternative pigments, otherwise known as fillers, which usually come from mining, include china, clay and talc. Taylor said these “tend” to provide volume and are fairly inert in composition.
Further, the main types of zinc-rich paint coatings using zinc are primers based on silicates (inorganic coatings) and primers based on epoxy.
Zinc silicates form an abrasion-resistant film, have “excellent” corrosion and high-temperature resistance, as well as “good” chemical resistance and can be complicated in terms of overcoating, he said.
Zinc silicates have a typical durability of between 6 to 12 months.
Alternatively, zinc epoxy coatings are organic and have tough film properties that provide “very good” corrosion resistance when used as part of a coating system, said Taylor.
He added that zinc epoxies are also easier to apply and repair than zinc silicates, are easier to overcoat, but are not as chemically-resistant as zinc silicates.
Zinc coatings on steel initially act as a barrier and if the coating system is damaged down to the bare steel substrate, then the zinc provides sacrificial protection, explained Taylor.
ZINC THERMAL SPRAY
Metal spray equipment manufacturer Metallisation regional sales manager Iain Blacklock explained that zinc can also be applied by line-of-sight to steel substrates using a thermal spray technique, similar to spray painting.
This coating is 100% metallic and is applied by spraying a liquified form of zinc, which instantly adheres to steel.
The process involves a feedstock, which is either a zinc powder or a wire; and a heat source which can be electric or flame – this is used to melt the wire, creating molten particles which are propelled onto a prepared surface.
“The molten particle will then splat, cool and shrink instantly and will build up a coating.”
You can [further] build up a coating with [increased] passes done. “It is instant curing. There is no drying or curing time. It is not like paint. As soon as it is sprayed, it is cured,” he said.
The process is metallurgically cold, resulting in “virtually no” heat input to the substrate, which can typically reach temperatures of about 80 oC, said Blacklock.
Thermal spray zinc is also typically a mechanical bonding process, and can be used to spray a range of materials, including zinc, aluminium, copper, steels, bronzes and ceramics at various thicknesses of typically between 150 μ and 350 μ.
This zinc coating provides long-life corrosion protection of about 20 years to first maintenance in splash zone.