Galvanizing: The Galvanizing Process

Galvanizing is a coating process in which semi-finished products, components, or assemblies made of iron or steel are coated with a thin layer of pure zinc. This can be accomplished using various methods. Compared to other types of coatings, galvanizing provides active corrosion protection in addition to passive protection. This means that the zinc layer has a cathodic effect relative to the iron and acts as a sacrificial anode. This significantly increases the service life of the coated parts.

There are a number of different methods for applying a zinc coating to a component. The most commonly used methods in practice are:

• Hot-dip galvanizing (melt-dip galvanizing)
• Electrolytic galvanizing
• mechanical galvanizing
• Spray zinc coating
• Sherardizing (diffusion galvanizing)

Due to the sometimes considerable differences between these processes, the term “galvanizing” is only meaningful with certain limitations. According to technical terminology, the common element of all zinc plating processes is the creation of a corrosion-resistant coating made of pure zinc. Coating methods that use zinc dust paints or zinc flake coatings, therefore, are not considered zinc plating.

In hot-dip galvanizing, the zinc coating is applied by immersing the pretreated workpiece or component in a molten zinc bath. The temperature of the zinc bath is approximately 450 °C. In addition, technicians distinguish between batch galvanizing and continuous hot-dip galvanizing, often referred to as Sendzimir galvanizing, continuous galvanizing, or strip galvanizing.

A typical application for batch galvanizing is the coating of a custom-made stair railing to provide lasting protection against rust. Semi-finished products such as sheet metal, strips, or profiles are usually galvanized continuously. Different zinc coating thicknesses are achieved in both processes. While the zinc coating in batch galvanizing ranges from 50 to 150 micrometers, continuous hot-dip galvanizing typically yields coating thicknesses of up to 40 micrometers.

Piece-by-piece galvanizing

• Ideal for individual, custom-made components and structures
• Complete immersion of the pretreated workpiece into the molten zinc bath
• High coating thicknesses can be achieved
• Robust, long-lasting corrosion protection
• Components are reliably protected against corrosion all around—inside and out, including at the cut edges

Continuous galvanizing (strip galvanizing)

• Intermediate products such as steel strips or profiles pass continuously through a zinc bath prior to further processing.
• Thinner zinc coating than with piece galvanizing
• Subsequent punching, cutting, etc., destroys the zinc coating at the cut edges

The most important set of standards for piece-by-piece hot-dip galvanizing is DIN EN ISO 1461. This standard is valid throughout Europe. As an ISO standard, it is also recognized worldwide. The standard specifies all requirements for the hot-dip galvanizing of individual parts as well as the corresponding testing procedures. However, DIN EN ISO 1461 defines only the services that the galvanizing contractor must provide. The hot-dip galvanizing plant is not responsible for defects that arise during subsequent transport or during the assembly of the galvanized parts. For continuous strip galvanizing, DIN EN 10346 and DIN EN 10143 apply. These standards specify all requirements and test procedures regarding the quality and thickness of the zinc coating. The proper repair of defects is also explained in detail. The galvanizing of barbed wire is governed by its own standard, DIN EN ISO 10244-1 and -2.

In electroplating, zinc plating is a process used both to protect against corrosion and to finish the surfaces of a wide variety of steel parts. Because it is relatively cost-effective, electrogalvanizing is used very frequently in the automotive industry, for example. In the galvanizing process, the zinc coating is formed using an electric current. To do this, the components to be galvanized are immersed in an acidic or alkaline electrolyte bath, where they act as the cathode. The electrode acting as the anode consists of pure zinc. During the galvanizing process, the steel does not form an alloy with the zinc. A relatively thin layer of zinc forms, which provides excellent protection against corrosion for the galvanized parts.

In mechanical zinc plating—often referred to by experts as “mechanical plating”—the zinc coating that protects against corrosion is applied without the use of heat or electrolytes. This makes this type of zinc plating ideal for parts prone to warping as well as for hardened parts such as screws, nuts, or small parts made of spring steel. The zinc plating process takes place fully automatically in a rotating drum filled with a chemical substance, glass powder, and zinc powder. While the glass beads essentially “hammer” the zinc powder onto the component, a controlled chemical reaction ensures that a strong bond forms between the steel and the zinc. Mechanical zinc coating produces very uniform zinc surfaces. However, the decorative appearance and adhesion are significantly inferior to those achieved with electrogalvanizing, so mechanical zinc coating is used relatively rarely in practice, and its application is always carefully evaluated.

In spray galvanizing, a pure zinc wire is melted using a flame or an electric arc, atomized into tiny particles by compressed air, and sprayed precisely onto the workpiece. It is important that the components, workpieces, or structures be sandblasted prior to galvanizing. Sandblasting creates a porous surface to which the zinc particles adhere very well, resulting in a strong, durable corrosion-resistant coating. As with mechanical galvanizing, the thermal stress involved in spray galvanizing is negligible. This makes the process particularly well-suited for structures prone to warping and for large surfaces. However, spray galvanizing reaches its limits when it comes to hard-to-reach areas, undercuts, and cavities, making it unsuitable for containers, inner tubes, and similar components.