Zinc Coatings - How They Work
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 which, left unchecked, is estimated to cost an industrialized country’s economy at least 4% of GDP each year.
Zinc-coated or galvanized steel offers a unique combination of properties unmatched by any other material. These include:
- high strength
- light weight
- corrosion resistance
- low cost
For this reason, galvanized steel sheet is an ideal material for a multitude of building and manufacturing applications - from automobiles to household appliances to residential, commercial and industrial construction.
Technical Performance of Zinc Coatings
There are many factors to consider when selecting the most appropriate zinc coating. In addition to corrosion protection, the coating’s formability, adherence, appearance and cost should also be considered. These factors each impact on selecting the correct zinc coating for a given application. All zinc coatings, like the steel they protect, are recyclable.
When left unprotected, steel will corrode in almost any environment. Zinc coatings protect steel by providing a physical barrier as well as cathodic protection for the underlying steel. It is important that the correct zinc coating is specified to provide optimal performance under the exposure conditions to which the coating will be subjected.
When painted zinc-coated steel is scratched, zinc protects both
the underlying steel from corrosion and the overlying paint coat from lifting.
Zinc coatings provide a continuous, impervious metallic barrier that does not allow moisture to contact the steel. Without moisture, there is no corrosion, except in certain chemical atmospheres. The effectiveness of zinc coatings in any given environment is directly proportional to coating thickness. Coating life is determined by the coating corrosion rate, itself a function of many factors such as time, composition of the atmosphere and the type of coating.
In situations of outdoor exposure, the acidity level of rain will influence the zinc corrosion rate. With indoor exposure - ventilation ducts, floor decks and steel framing, for example - moisture may also be present. In industrial indoor situations, the atmosphere may be corrosive. Thus the type and weight of coating required depends both on the service life needed and the exposure conditions.
Corrosion resistance of coatings can also be improved by using a zinc alloy coating, such as Galfan® or Galvalume®, or by applying paint top coats. These two methods, individually or together, are recommended for exposed sheet applications where enhanced corrosion protection is required.
Another outstanding protection mechanism is zinc’s remarkable ability to galvanically protect steel. When base steel is exposed, such as at a cut edge or scratch, the steel is cathodically protected by the sacrificial corrosion of the zinc coating adjacent to the steel. In practice, this means that a zinc coating is not undercut because the steel cannot corrode adjacent to a zinc coating. This contrasts with paint and aluminum coatings where the corroding steel progressively undercuts the surrounding barrier film. The extent of this cathodic protection is determined by the type of coating, its thickness and that of the underlying steel, as well as by the area of damage.
Painted Zinc Coatings
Zinc coatings are easily painted. The term "duplex coating" is used for galvanized and painted steel parts, whereas the term "coil coating" or "pre-painting" is used for continuous galvanized and painted steel sheet. Paint acts as a barrier protecting the underlying zinc coating. Zinc is an excellent substrate for paint coatings because if the paint film is broken, zinc’s high corrosion resistance prevents undercutting of the paint film. Even if the coating damage does reach the steel base, zinc’s cathodic action will prevent the steel from corroding. Zinc’s ability to extend the life of paint coatings is what makes pre-painted galvanized steel sheet such a durable product that continues to extend its market share in commercial and residential roofing and cladding applications.
Combigram of duplex systems.
The zinc and steel industries have for many years conducted both research and field trials on the performance of various combinations of painted and coated sheet steel. The result of this extensive base of information is that existing performance can be predicted for a wide range of atmospheric conditions. In the example shown for a highly corrosive industrial atmosphere, a 70 µm galvanized coating plus a 100 µm layer of paint will provide a coating life of over 50 years.
Source: Stichting Doelmatig Verzinken / Progalva
Formability and Adhesion
The formability and adhesion of continuous galvanized zinc coatings are excellent and in most cases match the formability of the underlying steel. The formability of galvanized steel - which is defined as the resistance to cracking and loss of adhesion of the zinc coating during forming - is inversely proportional to coating and steel substrate thickness. There are, however, some coatings that are more ductile than others, an important consideration for deep draw stamping applications. It is therefore necessary to balance the requirements for corrosion resistance and formability .
Assembly refers to the technique of joining galvanized sheet steel products, mainly to themselves. In any application, the joining method should suit the metallic coating and will be determined either on the basis of its performance or the properties and characteristics of the galvanized product.
There are several effective joining methods:
Welding is the most common method of joining steel products. Resistance welding is a technique that uses resistance to the flow of electrical current to generate heat and thereby join two materials.
Pre-finished galvanized steel can be spot welded if the zinc coating is not too thick to enable the welding current to pass from one electrode to the other. This technique is often used in the downstream sectors.
Mechanical fixing methods such as screws, rivets, self-piercing rivets and lock forming can be used for a wide range of steel substrate, zinc coatings and applications.
Adhesive bonding has become more popular and can also be allied to mechanical fixing. The range of adhesive systems is wide and the selection depends on numerous variables such as surface condition, adhesion, strength and cure speed.
Zinc and zinc alloy coatings can differ in appearance depending on customer needs and consumer preferences. Galvanized coating finishes can vary from extra smooth and featureless to a flowery "spangle" pattern. Galvanneal (Zn-Fe) coatings have a matt grey appearance. Electrogalvanized coatings have the smoothest finish among zinc coatings and provide the best substrate for a high quality paint finish.
There are standards covering all aspects of surface appearance - coating finishes, surface qualities (from regular to best quality) and surface treatments (chemical passivation, oiling) - all targeted to enhance further processing.