Galvanization is a process for adding a coating of zinc to the surface of steel or iron. Since the zinc acts as a sacrificial coating, it protects the steel or iron underneath and can therefore extend the lifespan of metal parts.
But galvanization is a complex process that can be carried out in several ways. Different forms of galvanizing exist, including hot-dip galvanizing and thermal diffusion galvanizing, and these variations offer their own particular advantages and disadvantages. It is therefore helpful to understand the different forms of galvanization before incorporating it into a prototyping or manufacturing project.
Galvanized metal is more common than you might think. Structures like building frameworks, metal benches, balconies, ladders, and fire escapes can all be made from galvanized metal to ensure a longer lifespan and adequate protection from corrosion and damage. Other examples can be found in automotive parts and electrical appliances.
Furthermore, galvanized steel and galvanized iron can be used with different manufacturing processes, from sheet metal fabrication to CNC machining, making it a staple process in the manufacturing world.
This article goes over the basics of galvanization, answering questions like what is galvanization, which metals can be galvanized, what is galvanization used for, what types of galvanization are there, and how does galvanization improve metal parts.
What is galvanization?
Galvanization (or galvanizing) is the process of adding a layer of zinc to the outer surface of a metal, namely steel or iron. The purpose of this is to add a protective coating to the underlying metal, reducing the likelihood of corrosion or rusting.
The galvanization process varies depending on the exact technique. However, the most important part of the process is that it generally involves surrounding steel or iron in zinc, which may be in a liquid or dust-like state. When the zinc is introduced, the iron in the surrounded metal reacts with the zinc, creating a tightly bonded alloy coating.
The end-to-end galvanizing process can be divided into three stages:
- Preparing the surface of the steel or iron
- The actual galvanization
- Post-galvanization surface treatment
Galvanization is a relatively simple coating process, and it produces a fairly think surface coating, typically around 80 µm — around three times thicker than a coating created by electroplating.
The principle substrate materials that can be galvanized are steel and iron. However, other ferrous metals can also be galvanized.
The coating material in galvanization is always zinc. However, a similar process to galvanization is tinning, which substitutes zinc for tin. Tinning is fine for simple applications like food storage, but it is less reliable than galvanization: if the coating is damaged, the tin will eat away at the underlying iron.
Types of galvanization
The two most popular forms of galvanization are hot-dip galvanization and electrogalvanization. These two methods and others are discussed below. Knowing which type of galvanization to select may require the help of an expert.
During hot-dip galvanization, metal is immersed in a bath of hot zinc at around 450 °C. When it is removed, the coated surface reacts with oxygen to form zinc oxide, and then reacts with carbon dioxide to form zinc carbonate. Prior to galvanization, the metal must be cleaned using a caustic solution then pickled in an acidic solution; zinc ammonium chloride can also be applied to prevent premature oxidation of the metal before the galvanization takes place.
Electrogalvanizing combines galvanization with electroplating: a current of electricity is fed through a zinc bath with a zinc anode and a steel conductor. The process produces a thinner coating than hot-dip galvanization and also creates a brighter surface finish suitable for cosmetic applications.
This form of galvanization goes by several names: Sherardizing, thermal diffusion galvanizing, vapor galvanizing, and dry galvanizing. It involves heating steel parts up to 500 °C in a closed rotating drum that contains zinc dust. The evaporated zinc diffuses into the surface of the steel part to form a strongly bonded coating. It is ideal for small parts and parts that require coating of internal sections.
Galvannealing is a combination of hot-drip galvanizing and annealing. The hot-dip galvanization process occurs as normal, before the coated metal is passed through air knives to remove any excess zinc. The metal is then very briefly heated in an annealing furnace at 500–565 °C, causing the iron and zinc layers to diffuse into each other and create layers of zinc-iron alloy.
Advantages of galvanization
The purpose of galvanization is to create a protective coating of zinc on top of a steel or iron part. But why is this useful and what are the main galvanization advantages?
- Blocks corrosive substances: A zinc coating on galvanized metal stops corrosive substances reaching the weaker underlying substrate metal. This can prolong the lifespan of the substrate, especially if it is used in a demanding application.
- Prevents rust: Galvanization significantly delays the onset of rust.
- Creates sacrificial anode: The zinc coating is consumed by corrosive substances, so it offers protection to the underlying metal until is fully used up. Even if a scratch penetrates the coating, the zinc will be used up before the iron.
- Lasts a long time: Zinc coatings created by galvanization are tough and long-lasting, often providing decades of protection for the underlying metal.
- Is affordable: Galvanization is a fairly simple and affordable procedure that provides a notable benefit for minimal expenditure.
- Creates a thick coating: Although not suitable for all applications, galvanization produces a thicker coating than, for example, electroplating.
Applications of galvanization
The ability to prevent corrosion in steel and iron makes galvanizing widely used across several industries.
The most common application is galvanized steel for construction: hundreds of thousands of tons of galvanized construction steel is produced yearly to fabricate structures like building frameworks, street furniture, fire escapes, ladders, and balconies.
Galvanized steel is also common in the automotive industry, where it can be turned into rust-resistant parts, including the majority of a car’s “body in white”: its skeletal foundation. Bumpers, beams, frames, and chassis can also be made from galvanized steel.
Rust prevention is a desirable feature in telecommunications, where phone lines and electrical boxes are subject to the elements on a daily basis. Galvanization serves to increase the lifespan of such items.
Manufacturing with galvanized metal
Galvanized materials are widely available as galvanized sheet metal, suitable for sheet metal fabrication processes like bending and stamping, or as galvanized metal bars, tubes, and rods suitable for processes like CNC machining.
Galvanized materials tend to be easily machinable, since the zinc coating is softer than the underlying steel.
Galvanized metal can also be welded, but zinc can give off dangerous fumes, so proper ventilation is essential. Painting is not particularly effective on zinc-coated steel or iron unless the material has been galvannealed.
3ERP is a prototyping and low-volume manufacturing specialist with expertise in metal material selection. Read about how we improve corrosion resistance in aluminum parts with anodization, or contact us for a free quote.