Galvannealed steel’s zinc coating has a good absorption quality that enables paints to cling to the surface compared to galvanized steel. Read to learn more about galvannealed steel:
What is Galvannealed Steel?
Galvanneal, also known as galvannealed steel, is a product of galvanizing process before placing the steel under the annealing process. It is easy to identify a galvannealed steel by looking at its color. It has a uniform matte grey color, which absorbs paints quickly compared to galvanized steel. The paint does not adhere and becomes brittle in galvanized steel.
There are many stages involved in the processing of galvannealed sheets. It starts with hot dip galvanization, where the material passes through a galvanizing zinc bath. From there, the steel sheet undergoes air knives to expel excess zinc before heating it for a few seconds in an annealing furnace. This method diffuses the zinc and iron layers to form a zinc-iron alloy at the material’s surface.
Annealing takes place after galvanizing using a hot strip and zinc still liquid. The galvanizing zinc bath contains more than 0.1 percent aluminum, contributing to iron bonding, and coats the zinc layer. This process requires an annealing temperature range from 500 to 565 degrees Celsius.
Galvanneal Steel Specification
Galvanneal steel consists of two coating types: zinc and zinc-iron alloy. The zinc coating comprises fourteen coating designations, while the zinc-iron alloy has four coating designations. Z001 zinc coating and ZF001 zinc-iron alloy coatings have no minimum coating thickness and weight.
Other zinc coating descriptions include Z90, Z120, Z180, Z275, Z305, Z350, Z450, Z550, Z600, Z700, Z900, and Z1100. They have a corresponding both sides coating weight mass of 90, 120, 180, 275, 305, 350, 450, 500, 550, 600, 700, 900, and 1100 g/m2.
Each side has a coating weight mass of 30, 36, 60, 94, 110, 120, 154, 170, 190, 204, 238, 316, and 390 g/m2. The total coating weight for both sides ranges from 75 to 975 g/m2.
Other zinc-iron alloy coating descriptions are ZF75, ZF120, and ZF180, with corresponding both sides coating weights of 75, 120, and 180 g/m2. Each has a corresponding one-side coating weight of 24, 36, and 69 g/m2, with both sides coating weights of 60, 90, and 150 g/m2.
Galvannealed Steel vs. Galvanized Steel
Both galvanized and galvannealed steel have zinc coating at the mill, which gives them resistance to corrosion and rust. Both sheets of steel undergo a hot dip coating process.
Their difference is that galvanized steel does not require an additional annealing process. The purpose of annealing is to induce diffusion of the alloys between the steel and molten zinc coating.
Annealing reinforces the paint adhesion and formability of the steel. The dull matte finish makes the surface adhere to the paint because it has a high absorption ability compared to galvanized steel, which has a spangled finish.
Both sheets of steel follow the standard of ASTM A 653/A 653M grades. It is easy to spot their difference when it comes to their grades.
Galvanized steel has a designation of G, such as G40, G60, G30, G90, G100, and G115. Galvannealed’ designated grades start with A, like A40 or A60. The number after the letter signifies the coating weight. For example, G40 means 0.40 per square foot.
The galvanizing process of a bare steel metal is prone to rusting and corrosion as the iron content is high. The steel undergoes a hot bath at 850 degrees Fahrenheit of liquid zinc solution. After the hot bath, the air knives dispel excess zinc to achieve the coating thickness. The molecular bond between the steel and zinc prevents rust formation on the exterior.
In galvannealing, the sheet metal undergoes heating at 1050 degrees F annealing oven. This method draws excess iron to the steel’s outermost layers to form an alloy of zinc and iron. The material’s surface becomes more robust due to the high amount of iron on the surface, which makes it resilient and improves paint adherence.
Use of Galvannealed Steel
The durability of galvannealed steel makes it an excellent material for building bridges, highway signs, fencing, and guardrails. Areas that require less maintenance against rust, heavy use, and corrosion use galvannealed steel. The biggest market for galvanneal is the automobile industry. In the mid-1980s, Chrysler Corporation was the first car manufacturer that used galvannealed sheet steels.
Ten years later, Ford, Toyota, and Honda began using galvannealed coatings in their vehicles using electro-galvanized and hot dip galvanized coatings. The price differences and car frames influence the variations of galvanneal applications in manufacturing vehicles. Another use of galvannealed steel is building permanent line chute and debris systems.
Galvanneal steel last for years due to its corrosive-resistant properties. They require less maintenance because the zinc coating is an inhibitor against rusting and has excellent paint adhesion characteristics.