According to recent studies, the global galvanised steel market was valued at over USD 222 billion in 2023 and continues to grow steadily with infrastructure expansion worldwide.
Galvanisation plays a crucial role in protecting steel and iron from corrosion, extending their lifespan across countless construction and industrial applications. This zinc-coating process ensures long-term durability, making it an essential step in modern metalwork.
Key Takeaways:
- Galvanisation is the process of applying a protective zinc coating to iron or steel to prevent rusting and corrosion.
- The zinc coating acts as both a physical barrier (blocking moisture and oxygen) and a sacrificial layer (corroding before the base metal).
- For businesses or contractors in Malaysia seeking galvanised metal products or related solutions, Unitrade is a trusted supplier offering a full range of construction and building materials, including galvanised steel systems.
What is Galvanisation or Galvanised Metals?
Galvanisation, or galvanising, is the essential process of applying a protective zinc coating to steel or iron to prevent rusting and deterioration – also known as galvanised metals. This coating safeguards the base metal by acting as a physical barrier and providing sacrificial, or cathodic, protection. The most common technique involves submerging the clean metal into a bath of molten zinc (hot-dip galvanising).
Historical Context
The practice of coating iron to protect it has roots extending back to the 17th century, with one of the earliest examples being found on Indian armour. The term “galvanisation” itself is derived from the Italian scientist, Luigi Galvani (1737–1798). Galvani’s research involved the study of electrical currents generated by chemical actions, notably demonstrating the stimulation of a dead frog’s leg using electrical means. This work inspired the writer Mary Shelley’s novel, Frankenstein. The term was formally applied to the coating process in 1837 when the Frenchman Stanislas Sorel patented his “galvanic paint,” coinciding with the development of the industrial hot-dipping methods in England and France.
Principle of Corrosion Protection
Galvanised coatings protect the underlying metal through two crucial and complementary mechanisms: acting as a physical barrier against environmental threats and providing sacrificial protection. The intact zinc coating forms a tight, impervious barrier that does not allow moisture, the critical electrolyte needed for corrosion, to contact the steel beneath. This characteristic adherence and impervious nature ensure the coating possesses excellent abrasion resistance and will not crack or peel over time in the manner that some other coatings, such as paint, might.
The second, and perhaps most unique, function of galvanisation is cathodic protection, commonly known as sacrificial corrosion. This mechanism exploits the fact that zinc is a more reactive (electropositive) metal compared to steel. When a galvanised coating is scratched or damaged, exposing the base steel to the atmosphere, the adjacent zinc preferentially oxidises. This sacrificial corrosion of the zinc coating prevents the iron from rusting, even across small exposed areas, because the remaining zinc acts as a sacrificial anode. The corrosion products generated by the zinc are deposited on the exposed steel surface, effectively resealing it from the environment and halting further degradation. The overall corrosion rate of zinc is significantly slower than that of iron, making the zinc coating a resilient shield, and the service life of the coating is directly proportional to its thickness.
Methods of Applying Zinc Coatings
While several methods exist for applying zinc coatings, hot-dip galvanising (HDG) remains the most common and widely utilised technique.
Hot-Dip Galvanising
The standard hot-dip method involves fully immersing fabricated steel or iron articles in a bath of molten zinc, a process often referred to as batch galvanising. The molten zinc bath is typically maintained at high temperatures, often ranging between 450°C and 470°C. During immersion, the zinc reacts with the iron to form a series of intermetallic zinc-iron alloy layers that develop a strong metallurgical bond with the surface of the steel.
To ensure successful hot-dip galvanising, the steel must undergo extensive surface preparation to be entirely free of contaminants, such as oil, grease, scale, rust, and dirt. This preparation sequence usually involves several steps: first, degreasing the metal with a suitable solvent; second, pickling with acid to dissolve any oxide film; third, thorough rinsing and drying; and finally, treating the material with a flux solution, usually zinc ammonium chloride, which prevents further oxidation prior to coating. When the component is removed from the molten bath, it carries an outer layer of pure zinc that solidifies upon cooling, resulting in the characteristic crystalline surface patterns known as “spangles.” Because the component is fully immersed, protection extends to all areas, including recesses and inaccessible internal sections.
Continuous Hot-Dip Galvanising
For sheet steel and strip products, the continuous process is employed. Coils of cold-rolled steel are continuously fed through a cleaning apparatus and an annealing furnace before being immersed in the molten zinc bath. As the strip exits the zinc, gas knives are used to precisely control the thickness of the coating.
A specific variation of the continuous process is the Sendzimir process, named after Tadeusz Sendzimir, which involves adding a small amount of aluminium to the zinc bath. This results in a durable, highly corrosion-resistant coating that contains very little iron-zinc alloy.
Galvannealing
Following continuous galvanising, the strip may undergo an additional step called galvannealing. This involves immediately reheating the coated strip to a temperature of around 590°C. This thermal treatment restarts the iron-zinc diffusion reaction, converting the coating into a dull grey matte finish composed entirely of zinc-iron alloys. Galvanneal coatings are frequently chosen by the automotive industry because they exhibit exceptionally good paintability.
Other Galvanisation Methods
Beyond hot-dip techniques, alternative methods are used for specific applications. Electrogalvanizing employs electrolytic action to deposit a thin, uniform coating of pure zinc onto the steel surface, often used for sheet steel in automotive applications where thinner coatings are required prior to painting. Another method, Sherardising, is a form of thermal diffusion galvanising typically used for small articles like bolts and nuts. This involves heating the components in a sealed, rotating container packed with metallic zinc dust below the zinc’s melting point. Lastly, Metallic Spraying, or metallising, projects atomised particles of molten zinc onto a prepared surface, commonly used on-site to cover areas like welds.
Applications and Uses
Galvanised steel is vital across numerous sectors due to its resistance, strength and longevity.
In the construction industry, hundreds of thousands of tons of galvanised steel products are used every year worldwide. This material forms the basic construction components for steel frame buildings, road safety barriers (guard rails), bridges, walkways, staircases, and street furniture. Galvanised materials are also crucial for sheet metal applications, such as flashing, gutters, and downspouts.
The automotive sector relies heavily on galvanised steel, particularly the thinner coatings produced by electrogalvanizing, for exterior body panels to enhance corrosion performance before painting. Hot-dip galvanised and galvannealed products are common for structural and unexposed parts.
Furthermore, galvanised iron and steel are commonly found in the manufacturing of household appliances, including washing machines and refrigerators, and in historical applications, such as cold-water plumbing pipes used in the early 20th century.
Durability and Limitations
Galvanised steel is noted for its good durability and minimal maintenance requirements. In benign rural environments, galvanised coatings often boast a life expectancy exceeding 50 years, while in more aggressive urban or coastal environments, the expected service life is typically 20 to 25 years. Zinc’s self-protecting nature is largely attributed to the formation of a stable, tenacious layer of zinc carbonate on its surface when exposed to the atmosphere.
However, the zinc coating will eventually corrode. The rate of deterioration is governed by factors like the coating thickness, the time of wetness, and the concentration of atmospheric pollutants, particularly sulfur dioxide, which can lead to corrosion in non-salty, industrial environments. In highly acidic environments, such as where severe acid rain or high salt concentrations are present, the protective coating may be insufficient, and stainless steel might be a preferable choice.
Certain technical issues can also arise: for instance, some steel components may be either too small or too large for standard hot-dip processes. Additionally, if galvanised components are exposed to continuous temperatures exceeding 200°C, the zinc intermetallic layer may peel. When designing components for hot-dip galvanising, it is critical to ensure proper venting and draining holes are included, as completely sealed fabrications cannot be galvanised effectively.
Work with Malaysia’s Trusted Galvanised Metal Supplier
Galvanised steel is a cornerstone of modern construction, infrastructure, and manufacturing thanks to its durability and corrosion resistance. But the long-term performance of any project depends not just on the process — it also relies on sourcing materials from a reliable and experienced supplier.
Unitrade is a leading Malaysian supplier of construction materials, specialising in pipes, fittings, valves, structural steel, cement, and solar PV systems. With more than 40 years of experience and a proven track record in landmark projects such as KLIA and KLCC, Unitrade serves as a trusted one-stop provider for major developments.
Unitrade has also expanded into supplying complete solar system solutions, including solar panels, mounting structures, battery storage, and inverters for both residential and commercial projects. Learn more at https://solar.unitrade.com.my/.
Contact us today for a free quotation and expert advice tailored to your project’s specific needs.