Metal coating is a very common method to protect steel against corrosion. The most used metal is zinc. Bonding zinc to steel – usually referred to as galvanized steel – can make it resistant to corrosion in wet and humid environments for decades. It’s a cost-efficient method of protecting steel, particularly compared to using stainless steel. It also has environmental benefits since it doesn’t produce emissions and ensures practically maintenance-free use. It’s mostly used unpainted, but if it’s painted the paint coating has good adhesion to the zinc surface.
Because of the sacrificial nature of the zinc, the metal coating provides corrosion protection also for areas of exposed steel surfaces, such as cutting edges and areas where coating has been damaged by scratches or impacts.Full corrosion protection is even achieved in areas that have been heavily formed because there is a low peeling tendency of this low-friction and tightly bonded coating.
Galvanized steel is found everywhere, from nuts and bolts to power line pylons and other large-scale installations. Car bodies are nowadays fully galvanized, making them last longer without rusting. The agriculture industry is an extensive user of galvanized sheet metal and galvanized pipe for cattle gates, fences, wires, and much more.
The building industry uses a lot of galvanized steel for infrastructure such as ventilation, heating, and electrical installations. It’s also used for balconies, staircases, ladders, gutters, rails, tubing, and walkways. The aesthetic appearance is another factor which makes it a favorite material for many architects.
Hot dip galvanizing creates a thick, resilient layer of zinc iron alloys on the surface of a steel product. The hot dip process does not generally reduce the strength of the base metal, with the exception of high-strength steels where hydrogen embrittlement can become an issue.
In the process of hot dip galvanizing the base metal is immersed in a bath of molten zinc at a temperature of around 450 °C (842 °F). The zinc alloys with the surface of the steel, forming a matt grey surface that protects the steel from further corrosion.
Hot dip galvanized steel offers a wide protection range from Z100 to Z600 and a tight bond between the hot dip coating and the steel. That’s why zinc coatings are well suited to forming.
The composition of zinc coating consists almost entirely of zinc (>99%) and is lead free, resulting in finely crystallized zinc spangle that meets high requirements for visual appearance. The corrosion resistance provided by the zinc coating is in direct proportion to the coating's thickness. The Z600 coating (42 µm on both sides) can achieve a service life of up to 80 years.
The process of electrogalvanizing or electroplating involves running a current of electricity through a saline/zinc solution with a zinc anode and steel conductor. Such zinc plating or cold galvanizing was developed at the beginning of the 20th century. At that time, the electrolyte was cyanide based. A significant innovation occurred in the 1960s, with the introduction of the first acid chloride-based electrolyte. The 1980s saw a return to alkaline electrolytes, only this time, without the use of cyanide.
Compared to hot dip galvanizing, electroplated zinc offers these advantages:
Production of galvannealed steel sheet begins with hot dip galvanization of sheet steel. After passing through the galvanizing zinc bath the sheet steel is heated in an annealing furnace for several seconds. This makes iron and zinc layers diffuse into one another, creating zinc-iron alloy layers at the surface. The galvanizing bath contains slightly over 0.1% aluminum, added to form a layer bonding between the iron and coated zinc. Annealing temperatures are around 500 to 565 °C (932-1049 °F).
Galvannealed coating (ZF) feature excellent spot-welding properties, resulting in longer electrode life and low galling on electrodes. The surface of the galvannealed steel sheet is finely structured, giving an attractive appearance to the painted surface and strong paint adhesion.
Galfan (ZA) is a zinc-aluminum alloy coating that provides excellent corrosion protection and forming properties compared to traditional zinc coating.
Galfan steel coating features excellent corrosion resistance compared with traditional zinc coating. This is why it can be used to achieve a longer service life for a steel part, or to replace traditional zinc coating with a thinner galfan coating, thereby simultaneously offering improved weldability and formability properties.
A bright metallic and mildly cellular-patterned surface is characteristic for galfan coating. The eutectic composition of 95% Zn and 5% Al improves the coating adhesion and enables a lamellar coating structure in micro-scale. This makes it ductile and well suited for deep drawing.
Metal coated Docol® AHSS steels are advanced high-strength steels that are hot-dip metal coated for corrosion protection. Using metal coated Docol® AHSS in structural and safety parts provides lightweight vehicles without compromising passenger safety and corrosion resistance.
Metal coated SSAB Domex® HSLA steels are designed for applications that require galvanic corrosion protection and good formability in relation to high guaranteed yield strength. Metal coated SSAB Domex® structural steels are made for thin-gauge structural applications that call for load-bearing capacity and good galvanic corrosion protection.
SSAB is a metal powder supplier focusing on gas atomized steel powder. With this material, the properties of high-strength steel can be transferred into any complex shape by additive manufacturing – 3D printing. This will give new opportunities to design and produce steel components with high strength, impact toughness and hardness.