Questions You Should Know about Difference Between Hot And Cold Dip Galvanizing

Corrosion has been a significant concern for metals, particularly in construction. For centuries, humans have sought effective methods to extend the lifespan of metals in corrosive environments. One prominent technique for safeguarding metals, especially steel, is galvanizing.

For more information, visit YiTong.

In 1837, a chemist named Melouin discovered that zinc could be applied to iron by immersing it in molten zinc. This finding launched extensive research in the scientific community, paving the way for the galvanization process we use today. Notably, Luigi Galvani, an Italian physicist, observed in his studies that the contact between dissimilar metals could lead to electrical currents, which helped in understanding corrosion (For additional insights, see Why Do Two Dissimilar Metals Cause Corrosion?). Over the years, the galvanizing method improved significantly, and currently, approximately 10,000 tons of zinc are utilized annually in Britain alone for protecting iron structures—demonstrating the continuous growth of this vital industry.

Galvanized steel is critical across various sectors, including construction, agriculture, transportation, and energy production, among others. The two primary galvanization techniques are hot-dip galvanizing and cold galvanizing. In this article, we will delve into these two methods and clarify their differences.

What is Galvanizing?

Galvanizing is fundamentally the process of applying a zinc layer onto the surface of a metal, usually steel or iron. This technique functions as a protective measure against corrosion and enhances the longevity of the metal component. When effectively applied, zinc coatings provide two primary protection types: barrier protection and galvanic protection.

Barrier protection

The zinc coating, once applied and dried, forms a protective layer of zinc oxide that is impervious to air and moisture, missing one of the key elements necessary for corrosion—the electrolyte. This barrier prevents further corrosion to the underlying steel substrate.

Figure 1. Zinc oxide layer provides barrier protection to the underlying substrate.

Galvanic protection

Independent of barrier protection, galvanizing is distinguished by its galvanic protection capability. This method (also known as cathodic protection) ensures that the zinc corrodes preferentially, thus protecting the underlying steel even when exposed due to surface damage. Due to zinc's high reactivity, it sacrifices itself, continuously offering protection until fully consumed.

Hot-dip Galvanizing

Hot-dip galvanizing stands as one of the most conventional forms of galvanization. This method involves immersing iron or steel objects into a molten zinc bath at approximately 840°F (449°C). Once removed, the zinc coating reacts with atmospheric oxygen, forming a stable zinc oxide that transforms into a protective zinc carbonate layer with carbon dioxide interaction.

This dull grayish layer closely bonds to the steel surface. Despite its straightforward appearance, hot-dip galvanizing involves several essential steps (Figure 2). The steel is subjected to thorough cleaning processes, including degreasing, pickling, and fluxing, to prepare it for galvanizing. These stages eliminate contaminants and ensure optimal bonding of the zinc coating.

Figure 2. Hot-dip galvanizing consists of a sequence of steps.

Once cleaned and dried, the steel is ready for immersion in the molten zinc bath, where the liquid envelops the object, providing comprehensive protective coverage before air-drying and inspection.

Cold Galvanizing

Cold galvanizing refers to applying zinc-rich paint to steel surfaces to shield them against rust. Some coating professionals consider this term a misnomer.

For detailed insights on the Difference Between Hot And Cold Dip Galvanizing, don’t hesitate to reach out to us.

The application techniques for cold galvanizing vary; zinc paint can be applied using brushes, rollers, or spray guns. Importantly, this technique utilizes a binder that allows the zinc to bond mechanically with the steel for adequate protection.

Similar to hot-dip galvanizing, cold galvanizing provides barrier and limited cathodic protection. However, for effective performance, the zinc concentration in the paint must remain sufficiently high to enable electrical conductivity between the zinc and steel.

Figure 3. Video describing cold galvanizing.

Interestingly, the surface preparation for applying zinc-rich coatings is less stringent than that of hot-dip techniques. The surface must be clean and dry, typically prepped with a wire brush to eliminate any rust or oxidation before applying multiple coats of zinc paint.

Comparing Hot-dip and Cold Galvanizing

While both methods of galvanizing provide similar protective purposes, they diverge considerably in application and performance. Cold galvanizing often falls short, as it lacks the chemical bonding and durability that hot-dip galvanizing inherently possesses.

Cold galvanizing's simplicity and cost-effectiveness make it ideal for smaller structures, while hot-dip galvanizing is better suited for extensive industrial applications. (For additional reading, refer to: Galvanization and its Efficacy in Corrosion Prevention.)

Ultimately, the choice between hot-dip and cold galvanizing will depend on balancing cost and performance needs specific to each project.

If you’re interested in understanding more about this topic, check out our resources about Metal Culverts.