Galvanized Steel

Galvanized Steel Product Descriptions

Galvanized pipes & tubes

The galvanized coating applied to steel makes it possible to protect the steel from oxidation and rust, which ultimately prevents the steel pipe from deteriorating. Galvanized construction pipes are typically used for indoor projects to transport liquids such as water, but they can also be used for certain outdoor applications such as protective ballards. The protective quality of galvanized pipes is achieved by a hot-dip process in which the steel is completely coated with zinc.

The zinc then forms a sacrificial layer that will rust away before the steel is attacked. Overall, a galvanized steel pipe can last well over 100 years, depending on the level of corrosion.

Hot-Dipped Galvanized Angles

Angles are one of the most commonly used steel structures in the construction industry. The basic shape of steel gives it many practical uses.

They are often used in heating, ventilation and air conditioning systems as well as in process plants, shipping and offshore industry.

The advantages of galvanized steel

Many different industries use galvanised steel mainly because it has such a wide range of benefits for the industry to use.

Low initial cost

Low initial cost compared to most processed steels. Furthermore, galvanized steel is ready for immediate use upon delivery. It does not require any additional surface preparation, inspections, painting/coating, etc., which saves companies additional costs.

Longer service life

Longer service life. In galvanizing, it is expected that a piece of industrial steel will last more than 50 years in average environments and can last more than 20 years in high water exposure. No maintenance is required. The increased durability of the end product of the steel also increases the reliability of the product.

Steel is protected by zinc

The protective anode ensures that damaged steel is protected by the surrounding zinc layer. It does not matter if the steel profile is completely exposed; the zinc will still corrode first. The coating preferentially corrodes the steel, creating a sacrificial protection for the damaged areas.

Rust protection through zinc

Rust resistance through the zinc coating. The iron elements in steel are incredibly susceptible to rust, but the addition of zinc acts as a protective buffer between the steel and any moisture or oxygen. Galvanized steel is very protective, including sharp corners and depressions that could not be protected with other coatings, making it resistant to damage.

Types of galvanizing and other zinc coatings

There are a variety of zinc coatings used for corrosion protection, each with its own unique properties and performance.

Zinc coatings are applied to steel surfaces by hot-dip galvanizing, electroplating, sherardizing, mechanical coating, zinc-rich painting and zinc spraying (metallization). Of these, the HDG process is by far the most widely used process.

A brief explanation of each type of zinc coating follows. The figure below shows the typical relative thickness of each coating. In most cases the thickness of the coating is proportional to the durability of the article.

Batch hot dip galvanizing

Prepared articles are galvanized by immersion in molten zinc. The surface of the work is completely covered, creating a uniform coating of zinc and zinc-iron alloy layers, the thickness of which is mainly determined by the mass of the steel to be galvanized. This is an important advantage of the galvanizing process – a standard minimum layer thickness is applied automatically, independent of the operator.

The molten zinc in the galvanizing bath covers corners, seals edges, seams and rivets and penetrates into recesses to fully protect areas that are potential corrosion points in other coating systems. The galvanized coating is slightly thicker at corners and narrow edges, providing greatly increased protection compared to organic coatings that thin out in these critical areas. Complex shapes and open containers can be galvanized inside and outside in one operation.

Items ranging from small fasteners to structures hundreds of meters high can be protected using modular construction techniques. Large galvanizing baths combined with modular construction techniques and double-end immersion processes allow the galvanizing of almost any structure, with greatly reduced maintenance costs and extended service life.

Small objects can be immersed in the molten zinc in a tank which is centrifuged or spun after the molten zinc is removed. This removes excess zinc from filaments and edges and produces a smooth, albeit thinner, coating than items dipped in batches.

Continuous galvanizing processes (In-line galvanizing)

Steel sheet, pipe and wire can be continuously galvanized in specially developed galvanizing processes. These processes are widely used and generally allow precise control of the thickness, ductility and other properties of the zinc coating, allowing a wide range of products to be manufactured to meet the varying requirements of subsequent manufacturing processes and end uses.

These products should not be confused with batch hot dip galvanized items. In-line galvanized articles always produce thinner coatings than batch hot-dip galvanized articles for the same steel thickness and therefore offer less protection against corrosion when exposed to the same environment.

Continuously galvanized products can usually be further processed by bending or roll forming, for example as purlins and belts, without damaging the coating. In addition, welded hollow sections made from pre-galvanized strip are widely used. Note that welds, cut ends and drilled or punched holes may need to be repaired to restore corrosion protection, depending on the application and environment.

Thermal spraying (or metallization)

Thermal spraying or metallization is the process of spraying semi-molten zinc, other metals or their alloys onto manufactured objects using wire or powder heated by a flame, arc spray or plasma heat source.

Spraying of zinc has the advantage that zinc coatings up to a thickness of 250μm, corresponding to 1500 g/m2, can be applied either manually or mechanically and the process can be carried out in the factory or on site. The steel surface must be prepared by sandblasting and the coating cannot normally extend to the inner surfaces. In addition, it may be damaged or limited in application where there are sharp edges, tight corners, holes and poor surface preparation. The resulting zinc coating provides both barrier and cathodic protection to the underlying steel in the same way as a galvanized coating.

In most cases, thermal spraying is more expensive than discontinuous hot dip galvanizing for the relevant section, but these processes are complementary and are used together for large structures.

Electroplating

Electroplating is an economical, versatile and effective method of applying a protective coating to small steel parts. It is the most widely used method for applying metallic zinc coatings to small fasteners, especially those with fine threads. However, fasteners used with batch hot dip galvanized articles should have a comparable corrosion protection and composition to prevent bimetallic corrosion.

In general, there is an economic limit to the amount of zinc coating compound that can be applied by electroplating and therefore is not normally used outdoors without additional coatings.

Sherardisation and thermal diffusion

Sherardising involves heating steel objects to about 500°C in a closed rotating drum which also contains metallic zinc dust and normally an inert filler such as sand. At temperatures above 300°C, zinc evaporates and diffuses into the steel substrate, forming diffusion-bound Zn-Fe phases. The similar thermal diffusion process normally works with less filler and a lower temperature (400°C) and is therefore more efficient.

Sherardizing and thermal diffusion are most effective for small articles – typically those with fine filaments, although the article size is limited only by the size of the rotating drum. The process also eliminates hydrogen embrittlement and can therefore safely be used for very high strength steels above 1000 MPa. The coating thickness varies from 20 to 120 µm, although coating is usually in the range of 20 to 50 µm. The coating thickness typically depends on the time in the rotating drum, not on the steel thickness.

Mechanical coating

Mechanical plating or peen plating is an “electroless” plating process used to deposit coatings of ductile metals on metal substrates using mechanical energy and heat. It is used for plating zinc on steel parts, especially threaded parts and items with tight tolerances. The thickness of the coating is often similar to that of electroplated articles.

Zinc rich colours

Zinc-rich paint coatings consist of metallic zinc dust in organic or inorganic vehicles/binders.

Surface preparation by blasting or by the use of power tools to expose bare steel with a profile (e.g. SSPC SP-11) is required and coatings can be applied by brush or spray. Zinc-rich coatings are barrier coatings that provide cathodic protection even to small exposed areas of the steel, provided that the steel surface is properly prepared and the paint conforms to the relevant standards (e.g. AS/NZS 3750.9 and AS/NZS 3750.15). Suitable zinc-rich paint coatings also provide a useful repair coating for damaged or worn galvanic coatings.

Compared to hot dip galvanizing, these products have the advantage that they can be applied on site and on articles of any size. Some products can be applied with thicker or top coats to provide additional protection. The disadvantages are susceptibility to transport and field damage, curing times and the cost of equivalent corrosion protection (usually as part of a system).