Galvanized steel is steel coated with layers of zinc oxide to prevent corrosion and provide a durable finish. The most common method is hot-dip galvanizing, but other methods exist. Zinc protects steel by resisting rust and acting as a sacrificial layer. Galvanized steel is used in many applications, but it has some disadvantages, including a shorter lifespan when exposed to rain or seawater. Alternatives include stainless steel.
Galvanized steel is steel that has undergone a chemical process to prevent it from corroding. The steel is coated with layers of zinc oxide because this protective metal does not rust easily. The coating also gives the steel a more durable, hard-to-scratch finish that many people find appealing. For countless outdoor, marine or industrial applications, galvanized steel is an essential fabrication component.
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One of the most common ways to make steel resistant to rust is by combining (alloying) it with a metal that is less likely to corrode: zinc. When steel is dipped in molten zinc, a chemical reaction permanently bonds the zinc to the steel. Therefore, zinc isn’t exactly a sealer, like paint, because it doesn’t just coat the other metal; it actually becomes permanently part of it. The outermost layer is all zinc, but the subsequent layers are a mixture of zinc and iron, with a pure steel interior.
This process, known as hot-dip galvanizing, is one of the most common methods of making galvanized steel, but it’s not the only one. Continuous sheet zinc plating passes a steel sheet or wire through molten zinc; it is also a form of hot dipping, but leaves a thinner layer of the protective metal. Steel can also be painted or sprayed with zinc, although neither method creates the strong bond that hot dipping does. Electrolytic or galvanic zinc plating uses electricity to create the bond between the two metals, which results in a relatively thin layer of zinc.
The degree of galvanization is usually described as the weight of zinc per surface area rather than its thickness, because this gives a better representation of how much metal has been applied. Thick steel is galvanized after forming individual parts, such as braces, nails, screws, beams or studs. However, continuous galvanizing of sheet metal is often done before the metal is used to create products and can resist bending and warping without flaking.
Advantages of galvanizing
Zinc protects steel in two ways. First, it is highly resistant to rust; iron, one of the main components of steel, reacts very easily with oxygen and moisture and eventually disintegrates. The zinc layer on the surface prevents these elements from reaching the steel so quickly. It also develops a patina – a layer of zinc oxides, salts and other compounds – which offers further protection. Zinc is also extremely durable and scratch resistant, and has a satin finish that many find attractive.
The outer layer also protects the steel by acting as a “sacrificial layer”. If, for some reason, rust takes hold on the surface of the galvanized steel, the zinc corrodes first. Even in areas where the surface is scratched or damaged, the surrounding zinc will still corrode sooner than the steel. The life of galvanized steel varies, but industrial steel with a 3.9 mil (0.0039 inch or 0.09906 mm) thick coating can last more than 70 years without maintenance under average conditions.
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Galvanized steel is found almost everywhere. Many people live in steel frame homes, and the buildings are often made with galvanized steel roofs. There are multiple steel parts in cars that help protect them from the elements and it is used in many marine applications due to its ability to withstand salt and the elements. In addition to being cheap, durable and effective, this metal is also popular because it can be recycled and reused over and over again.
Disadvantages
There are relatively few disadvantages to galvanizing, but it’s not a good method of protecting all steel items. Some items cannot be hot dipped, often because they are too large or too small, and other forms of galvanizing are often not as protective and durable. Eventually, the zinc will corrode, although how long this takes depends on the thickness of the coating and the environment. Galvanized steel has a much shorter life span when regularly exposed to rain or seawater.
Galvanized steel that is cooled too slowly or stacked together before it has completely cooled can also cause the outer zinc layer to peel off. In some cases, small gaps can form between the zinc and the underlying alloy, which can also cause flaking. When a very thick layer of zinc is applied, it can also become brittle and flake.
Alternatives to galvanizing
Some metal objects cannot be electroplated effectively; the zinc coating would be too thick on a small screw, for example. In many cases, stainless steel is used instead, made with chromium. Unlike galvanized metals, stainless steel is not coated in the metal; chromium is part of the steel alloy composition.
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