 Information on this topic can be viewed in PDF format (40kb) from our manual:
How They Work and How They Are Applied
For over a century, zinc has enhanced the longevity and performance of steel. Zinc coatings provide the most effective and economical way of protecting steel against corrosion. Zinc-coated (galvanized) steel offers a unique combination of positive features, of which the ones of key interest to residential construction are as follows:
- high strength, determined by the steel substrate
- formability, a key feature for roll forming coated steel sheet
- light weight of steel framing and roofing, as compared to competitive materials
- corrosion resistance, for both long life and the maintenance of esthetic appearance
- recyclabilty, both for the scrap materials of construction and end-of-life demolition
- low cost, competitive with all construction materials of matching quality.
For these reasons, galvanized steel is an ideal material for a multitude of building
applications. In the residential construction market, galvanized steel has particular and
cost-effective applications in framing, roofing, rainware (gutters and downspouts),
ductwork (heating/cooling and venting) and household appliances.
Galvanized steel resists corrosion. Since, for the industrialized nations, at least 4%
of GDP is lost to corrosion each year, the trend of at least the past fifteen years has been
toward customer and manufacturer demands for increased protection through both
higher contents of zinc and additional applications of zinc-coated steel.
Zinc Coatings
How They Work and How They Are Applied
Roger Wildt
- How Zinc Protects
When left unprotected, steel will corrode in almost any environment. Zinc
coatings stop corrosion by providing two protections - a physical barrier and
cathodic action.
Barrier Protection
Zinc coatings provide a continuous, impervious metallic barrier that does
not allow moisture to contact steel. Without direct moisture contact, there is
no corrosion. However, since zinc gradually erodes due to its much slower
degradation in the presence of water and atmospheric pollutants in open air
applications, barrier life is proportional to coating thickness. This subject has been
researched for many years and the literature is well supplied with reports on zinc’s
performance in different climates, with different alloy additions to the coating and
at different coating thicknesses.
Within the interior of a structure (wall framing and roof trusses) corrosion is not
a consideration provided the exterior membrane maintains its integrity.
Barrier coating longevity can be improved a number of ways other than by just
increasing coating thickness. The addition of aluminum for alloy coatings called
Galfan® and Galvalume®, or the application of paint, individually or in combination,
will significantly and economically extend the life of coated steel sheet.
Cathodic Protection
Another outstanding protection mechanism is zinc’s remarkable ability to
galvanically protect steel. When bare steel is exposed to moisture, such as at a
cut edge or surface scratch, steel is protected by the sacrificial loss of zinc in the
vicinity of the exposed steel. In the immediate presence of zinc, steel will not
corrode until all the zinc has been sacrificed. This is particularly important for
coated steel sheet since corrosion will continually undercut both aluminum or paint
barrier coatings.
The presence of zinc is the key to cathodic protection. All zinc-containing
metallic coatings, including Galfan® and Galvalume®, share this beneficial
characteristic.
Roger Wildt is a consultant to International Zinc Association.
- Formability and Adhesion
For residential framing and roofing applications, all zinc coatings are
continuously applied by dipping pre-treated, pre-heated sheet steel in a bath of
molten zinc or zinc alloy, a process called "Continuous Galvanizing".
The bond between the zinc and steel is metallurgical so that a coil of zinccoated
steel can be cut, punched and formed without damaging the zinc coating.
(There are steel thickness, bending radius and coating weight limitations, but they
are well known and respected by the producers of coated steel sheet, framing and
roofing products.)
- The Coating Products and their Definitions
Galvanized A zinc coating, usually hot-dipped, in which the zinc and steel form a metallurgical
bond. The thickness of a hot-dipped coating can be varied from a thin zinc/iron
alloy layer to heavy applications suitable for extended outdoor exposure.
Electrogalvanized Also a zinc coating, but applied in a cold, electrolytic bath rather than a molten zinc
bath. Traditionally the coatings are thinner than hot-dipped and not suitable for
extended outdoor exposure.
Galvanneal A zinc-iron coating produced by post-heating a hot-dipped coating. It is often used
where paint is to be applied to the coated sheet.
Galfan®
A proprietary zinc alloy coating (5% aluminum) with improved corrosion resistance
and formability compared to zinc alone.
Galvalume®
A proprietary zinc alloy coating (55% aluminum) with superior corrosion resistance.
- The Continuous Galvanizing Process
Globally, some 550 continuous galvanizing lines produce about 70 million metric tons of zinc-coated steel each year. For many years the capacity for
applying zinc coatings has been in a growth mode.
In the continuous hot-dip galvanizing process, coils of rolled steel are
continuously unwound and fed through cleaning and annealing sections before
entering a molten zinc bath at speeds of up to 200 metres/minute (650 feet/minute).
As the steel exits the molten steel bath, coating thickness is controlled by gas "knives" which wipe excess zinc from the steel sheet. The steel sheet then
undergoes a number of mechanical and chemical treatments specified by the
customer.
This process is a very capital-intensive, high-speed, precise, factory-controlled
operation that controls not only the coating, but the strength and formability of the
steel substrate.
- Sustainable Development
Zinc is natural and fully recyclable. It is an element essential for life and the 17th
most common element in the earth’s crust. Today, over 80% of the zinc available for
recycling is recycled. The presence of zinc coating on steel does not restrict steel’s
recyclability and all types of zinc-coated products are recyclable, both from the
construction phase (job-site scrap) and the demolition phase (end-of-life scrap).
Zinc coated steel is recycled along with other steel scrap during the steel production
process - the zinc volatilises and is then recovered.
Zinc coating is an energy-efficient process. Zinc residues from the galvanizing
process are recycled. Zinc coating extends the life of all steel products and thus
improves steel’s life-cycle performance and removes the need for regular
maintenance normally associated with paint and other coatings. Today’s
technology enables thinner zinc coatings to provide higher performance. Moreover,
as observed in many industrialised countries, zinc coatings now last even longer
thanks to decreasing levels of atmospheric sulphur dioxide.
Specifying a zinc coating is in step with today’s need for sustainable materials.
Need More Information?
The information contained in this leaflet has been drawn from an IZA
publication titled "Zinc Coatings - Protecting Steel". Single copies of the reference
document may be obtained free of charge from International Zinc Association.
Members of the
International Zinc Association have
adopted a
Sustainability Charter – view their commitmen
to sustainable development at www.zincworld.org
|
