Can galvanized steel rust?

1.What is galvanized steel

Galvanized steel is an ordinary carbon steel treated by galvanizing process. By covering the surface with a layer of zinc metal, it forms a physical and electrochemical double protection barrier.

-Physical protection: the zinc layer can isolate air, moisture and other corrosive media, and the steel matrix to form a protective barrier.

-Electrochemical protection: Zinc is more active than iron, even if the coating is partially broken, zinc will be “sacrificial anode” way to give priority to the oxidation reaction, thus protecting the steel from corrosion.

Zinc is chemically active and can react with oxygen and water vapor to produce a dense oxide film (ZnO), thus preventing further corrosion of the steel. There are two main categories depending on the process:

-Hot-dip galvanized steel: steel is immersed in a molten zinc solution (approx. 450°C) to form a 50-150 micron thick zinc layer with strong adhesion, suitable for outdoor environments.

-Electro-galvanized steel: A 5-30 micron zinc layer is deposited on the surface of the steel through electrolysis, which provides a smoother surface and is commonly used for home appliances and automotive panels.

2. Six characteristics of galvanized steel

(1) Excellent corrosion resistance

The zinc layer can effectively block the contact of air and moisture, even if partially damaged, can still rely on electrochemical action to protect the substrate. In ordinary atmospheric conditions, hot-dip galvanized steel can have a service life of more than 50 years, much higher than that of unprotected steel.

(2) Cost-effective

Although the initial cost of galvanized steel is 10-20% higher than that of ordinary steel, its life-cycle cost is extremely low due to the reduction of frequent painting, repair and replacement costs. For example, galvanized steel for bridges reduces painting and maintenance costs by 90% (source: International Zinc Association), and galvanized steel greatly reduces the burden of late maintenance.

(3) Good processing adaptability

Galvanized steel can be stamped, welded and bent, but note that the melting point of the zinc layer is low (about 420°C), and the temperature needs to be controlled when welding. Electro-galvanized steel is more suitable for precision stamping due to its uniform surface.

(4) Aesthetics and functionality

Hot-dip galvanized steel often has a silver-gray “zinc bloom” on the surface, while electro-galvanized steel is more subtle and flat. Through the passivation or color coating process, it can also show different effects such as blue-white, green-yellow, etc., taking into account both decorative and anti-corrosion properties.

(5) Environmentally friendly

Zinc is 100% recyclable, and the slag from the galvanizing process can be recycled through the Waelz process. EU research shows that the carbon emissions of galvanized steel are 60% lower than stainless steel (Source: European Steel Technology Platform).

(6) Wide range of application scenarios

-Architecture:Roofing, curtain walls, steel frames (galvanization ≥275g/㎡, in accordance with GB/T13912 standard).

-Automobile:Body panels, electro-galvanized steel plate thickness about 7-15 microns.

-Electric power: transmission towers, communication towers, hot-dip galvanized life of up to 30 years or more.

-Appliances: refrigerator, washing machine shell, taking into account the rust and aesthetic.

3. Will galvanized steel rust?

The answer is: yes, but the probability and speed are much lower than ordinary steel. Galvanized steel is not absolutely rust-free material, but has long-term corrosion resistance in most environments. If the use of harsh environments, substandard plating or maintenance is not in place, galvanized steel may also appear rust.

4. Galvanized steel rust causes

(1) Environmental factors

-Humid environment:Humid environment is one of the main causes of rusting of galvanized parts. In a high humidity environment, the water vapor in the air will react with the zinc on the surface of the galvanized parts to generate zinc oxide and water, thus forming an iron oxide layer.

-Acid and alkali environment:The presence of strong acid or alkali will destroy the structure of the galvanized layer and make it lose its protective properties, thus accelerating the rusting process.

(2) Material factors

-Bad quality of galvanized layer:Bad quality of galvanized layer will lead to rusting of galvanized parts. If the thickness of the galvanized layer is not uniform, the galvanized layer coverage is incomplete or there are defects such as air holes, it will make its anti-corrosion properties decline, easy to rust.

-Galvanized layer and the substrate combination is not firm: If the galvanized layer and the substrate combination is not firm, cracks and voids will be formed, so that the galvanized layer is subject to the erosion of the external environment, which leads to rusting of galvanized parts.

(3) High humidity or sufficient oxygen: 

galvanized sheet is easy to rust in a humid environment, because high humidity and sufficient oxygen will make the galvanized layer lose its original anticorrosive properties, leading to rusting.

5. Treatment of galvanized steel rust

After galvanization of steel products, if local rust is found, the treatment method should take different measures according to the situation of rust.

(1) Slight rusting (point or small area)

Rust point is shallow, accounting for a small amount of area, can be used to dip a wooden stick in dilute vinegar coated on the rust point, leave it for a while, and then wash it with water. Then dry it with a cloth and apply transparent antirust paint on the rusted parts.

(2) Medium rust (localized chunks)

can be toothbrush or copper wire brush to brush off the surface rust spots, sprayed on the rusty parts of the lubricating oil or rust preventive agent, and then wipe dry with a cloth after a period of time.

(3) Serious rust (cracks, spalling, deformation)

This type of situation usually can not be repaired, can only replace the damaged parts to ensure structural safety.

6. Methods of preventing rusting of galvanized steel

In order to prevent the rusting of galvanized steel, the following measures can be taken:

(1) Timely cleaning

Regularly remove oil, dust and salt deposits on the surface of galvanized steel to avoid the formation of corrosion cells, which can make the galvanized layer on the steel surface more stable, thus preventing the rusting of galvanized steel.

(2) Coating protective layer

In harsh environments, epoxy paint and fluorocarbon coating can be sprayed on top of the galvanized layer to form a protective film to enhance the corrosion resistance life and prevent the steel from rusting and corrosion.

(3) Reasonable material selection

Choose hot-dip galvanized or electro-galvanized products according to the usage environment. For example, hot-dip galvanized steel should be preferred for outdoor projects, while electro-galvanized steel is more suitable for indoor or scenes with high surface requirements.

(4) Avoid damaging the plating

During transportation, installation and use, bumps, scratches and welding burns should be prevented. Once the localized plating is damaged, the anti-corrosion material should be reapplied in time.

(5) Regular inspection and maintenance

establish an inspection system for large facilities such as transmission towers, bridges and other regular testing, once the first signs of corrosion are found in a timely manner to prevent the expansion of corrosion.

In summary, galvanized steel is not “never rust”, but in the vast majority of applications, its corrosion life can reach decades. The physical barrier and electrochemical protection provided by the zinc layer makes it ideal for use in construction, transportation, energy and other industries. Rusting of galvanized steel is only possible in extreme environments or with improper maintenance. Through scientific selection, proper use and regular maintenance, galvanized steel is fully capable of maintaining stable performance throughout its entire life cycle, making full use of its value in modern industry.