What is Hot Dip Galvanizing?

Hot-dip galvanizing is a process where steel is coated with a layer of zinc by immersing it in molten zinc. This provides long-term corrosion protection through a durable, metallurgically bonded zinc-iron alloy layer.

This guide covers the process steps, key principles, and critical factors that affect the quality of the galvanized coating.

Hot Dip Galvanizing Process

The typical hot-dip galvanizing process follows these main steps:

Preparation → Pickling → Washing → Fluxing → Hot-Dip Galvanizing → Cooling → Passivation → Cleaning → Inspection → Packaging → Storage

Principle of Hot-Dip Galvanizing

The hot-dip galvanized pipe is formed by a reaction between the molten metal and the iron matrix, producing a zinc-iron alloy layer. This ensures that the substrate and coating are combined.

The process is as follows:
1. Pickling: The steel surface is pickled to remove iron oxide.
2. Fluxing: After pickling, it is cleaned in an aqueous solution of ammonium chloride or zinc chloride.
3. Hot-Dip: The cleaned steel is then immersed into a bath of molten zinc.

Key advantages of hot-dip galvanizing include: uniform coating, strong adhesion, and long service life.

Formation of the Galvanized Layer

● The steel substrate undergoes complex physical and chemical reactions with the molten bath, forming a corrosion-resistant, tightly structured zinc-iron alloy layer. This alloy layer is integrated with the pure zinc layer and the steel base, providing excellent corrosion resistance.

● The process can be simply described as: when the workpiece is immersed in molten zinc, a zinc-iron (body-centered) solid solution first forms at the interface. This is the base metal's iron dissolving in the solid state to form crystalline zinc atoms. A fusion occurs between the two metal atoms.

● When the zinc becomes saturated, zinc and iron atoms inter-diffuse. The zinc atoms migrate into the iron lattice, gradually forming an alloy with iron. Meanwhile, the diffused iron in the molten zinc forms intermetallic compounds (FeZn13), which sink to the bottom of the galvanizing pot as dross.

● When the workpiece is removed from the molten zinc, a pure zinc layer leaches onto the surface, forming hexagonal crystals. The iron content in this layer is typically not more than 0.003%.

Hot-Dip vs. Cold-Dip Galvanizing

Feature	Hot-Dip Galvanizing	Cold-Dip (Electro) Galvanizing
Coating Thickness	50-150μm	5-30μm
Corrosion Resistance	Excellent, long-term (20-50 years)	Limited, shorter lifespan
Adhesion	Strong, metallurgical bond	Weaker, mechanical bond
Application	Outdoor, industrial, harsh environments	Indoor, mild environments, precision parts
Typical Standards	ASTM A123, ISO 1461	ASTM B633

Key Factors Affecting Coating Quality

Two critical factors during the hot-dip galvanizing process are temperature and immersion time.

● Temperature Control:
Too low: Zinc has poor mobility, resulting in a thick, uneven coating that is prone to sagging and has poor visual quality.
Optimal: Good fluidity of liquid zinc, producing a thin, strong, and uniform coating with good appearance.
Too high: Excessive temperature leads to serious iron loss from the workpiece and the zinc pot, producing excessive zinc slag. This affects coating quality and increases zinc consumption, potentially leading to failure to coat.

● Immersion Time: At the same temperature, a longer dip time generally results in a thicker coating. Higher temperatures require shorter times to achieve the same thickness.

Frequently Asked Questions

Q1: What is the difference between hot-dip galvanized and electro-galvanized pipes?
A: Hot-dip galvanizing provides a much thicker, more durable coating that is metallurgically bonded to the steel, offering superior long-term corrosion protection for outdoor and industrial use. Electro-galvanizing provides a thinner, smoother coating suitable for indoor or mild environments.

Q2: How thick is the hot-dip galvanized coating?
A: The coating thickness typically ranges from 50 to 150 micrometers (μm), depending on the steel composition and immersion time. This is about 5-10 times thicker than electro-galvanized coatings. For more details on galvanized layer thickness, please read our [Guide to Selecting Galvanized Layer Thickness for Carbon Steel Pipes].

Q3: Does hot-dip galvanizing affect the mechanical properties of the steel?
A: The hot-dip galvanizing process itself (immersion in 445-465°C molten zinc) has minimal effect on the strength of standard structural steels. However, high-strength steels may require special consideration to avoid hydrogen embrittlement.

Read more: 

● BS 1387 Hot-Dip Galvanized Steel Pipe

● Galvanized ERW Pipes vs. Galvanized Seamless Pipe

● Hot-Dip Galvanizing Process: Seamless Pipe Production Steps