What Material Is Hot-Dip Galvanizing Applied To?

What Is Hot-Dip Galvanizing?

Hot-dip galvanizing is a widely used anti-corrosion treatment in which a steel article is immersed in a bath of molten zinc at about 450 °C. A metallurgical reaction takes place between the iron in the steel and the zinc, forming a series of zinc-iron alloy layers covered by a pure zinc layer. The coating protects the steel in two ways: it acts as a barrier against moisture and oxygen, and it provides cathodic protection, because zinc is electrochemically more active than steel and corrodes preferentially at scratches and cut edges.

What Material Is Galvanized?

The base material of hot-dip galvanizing is steel. Most structural carbon steel grades, including low-carbon steel plates, sections, tubes and fasteners, are suitable for galvanizing. The steel surface must be clean and free of oil, rust and scale before immersion; otherwise the zinc will not wet the surface and the coating will be defective. For this reason the process always starts with degreasing, pickling and fluxing, followed by drying and immersion in the zinc bath.

Characteristics of the Zinc Coating

Corrosion resistance: the zinc layer protects the steel and extends its service life for decades in many environments.

Self-healing at edges: galvanized coatings sacrificially protect exposed steel at cut edges and scratches.

Mechanical durability: the hard zinc-iron intermetallic layers resist abrasion and impact.

Environmental performance: no toxic or harmful substances are released during the treatment, and the coating needs no further painting in many applications.

Application Fields

Hot-dip galvanizing is used wherever long-term corrosion protection of steel is required. Typical applications include steel buildings, bridges, transmission towers, communication towers, pipelines, guardrails, automotive parts, motorcycle frames and home appliances. In the power transmission sector, galvanized lattice towers and fittings are specified to survive coastal and industrial atmospheres with minimal maintenance.

Standards and Coating Thickness

The coating mass and thickness of hot-dip galvanized articles are specified in ISO 1461, GB/T 13912 and ASTM A123/A123M. The required average coating thickness depends on the steel section thickness: thicker sections generally carry a thicker zinc coating because the reaction time and heat input differ. Purchasers should specify the standard and the minimum average coating thickness in the order, and verify the coating by the magnetic gauge method or by weighing.

FAQ

1. Is the galvanized article made of zinc or steel?

The article itself is steel. Zinc is applied as a protective coating on the steel surface; the two materials bond metallurgically during immersion in the molten zinc bath.

2. Why does zinc protect steel?

Zinc corrodes preferentially because it is electrochemically more active than iron. It also forms a dense barrier layer. The combination of barrier protection and sacrificial cathodic protection is what makes galvanizing so durable.

3. What preparation is needed before galvanizing?

The steel must be degreased, pickled to remove rust and scale, rinsed, fluxed and dried. A clean surface is essential for the zinc to react with the steel and form a sound coating.

4. How long does a galvanized coating last?

Service life depends on the coating thickness and the environment. In many rural and industrial atmospheres, a standard galvanized coating protects steel for 30 to 70 years before maintenance is required.

5. Which standards govern hot-dip galvanizing?

Coating requirements are defined in ISO 1461, GB/T 13912 and ASTM A123/A123M, including coating thickness, appearance and sampling rules.