Galvanized steel sheet is a composite material of steel and zinc. Galvanized products combine the advantages of the two materials. The composite material has the strength and plasticity of steel and a corrosion-resistant coating. It is widely used in industrial fields such as construction, home appliances and automobiles.
What Is Galvanized Steel Sheet?
Galvanized steel sheet is a steel sheet with a layer of metallic zinc coated on the surface to protect the steel substrate. Since the electrochemical activity of zinc is higher than that of iron, the presence of the zinc layer can delay the oxidation and corrosion of the steel. The thickness of common galvanized steel sheets is generally between 0.25mm and 5mm, and there are many specifications to choose from according to different application requirements.
Corrosion Resistance Principle Of Galvanized Steel Sheet:
The corrosion resistance of galvanized steel sheet comes from the protective effect of the zinc layer. Its corrosion resistance principle mainly includes the following three principles, namely the physical barrier effect of the zinc layer, the electrochemical protection principle, and the self-healing function of corrosion products.
(1) Physical Barrier Effect Of The Zinc Layer
The core anti-corrosion mechanism of galvanized steel sheet is that the surface is covered with a layer of zinc. This layer of zinc can effectively isolate air, moisture and corrosive substances (such as salt, acid rain, etc.), preventing the steel substrate from directly contacting these corrosive media. This physical barrier greatly delays the oxidation of the steel plate.
(2) Principle Of Electrochemical Protection
Zinc is a more active metal than iron. When the zinc coating is damaged, zinc undergoes oxidation reaction first to protect the steel. This principle is called "sacrificial anode protection". Even if the coating is partially damaged, the steel can still be protected.
(3) Self-Healing Function Of Corrosion Products
The corrosion products generated by zinc oxidation (such as basic zinc carbonate) are a dense film that can cover the surface of the steel, forming a second barrier and slowing down the corrosion rate. This characteristic makes the galvanized steel sheet have a certain "self-healing" ability during use.
Types Of Galvanizing Process:
Currently, the production processes of galvanized steel sheets mainly include hot-dip galvanizing and electrogalvanizing:
- Hot-dip galvanizing: The steel is immersed in molten zinc liquid to form a zinc coating through physical adhesion and chemical reaction. Under this process, the zinc layer is thicker (usually more than 50 microns), providing a stronger protective effect and high corrosion resistance. It is suitable for building materials exposed to the atmosphere and steel components in harsh environments.
- Electrogalvanizing: The zinc is evenly attached to the surface of the steel using the principle of electrolytic deposition. The coating is thinner (generally less than 20 microns) and has a smoother surface. It is usually used for products that require high precision or high aesthetic requirements. Although its corrosion resistance is not as good as hot-dip galvanized steel sheets, it has good effects in indoor environments or low-corrosive environments.
Factors Affecting The Corrosion Resistance Of Galvanized Steel Sheets:
Thickness Of Zinc Coating
The thicker the zinc coating, the stronger the corrosion resistance. The zinc layer can prevent corrosive media (such as water, oxygen and salt) from contacting the steel substrate, so a thicker zinc layer can provide longer protection. Hot-dip galvanizing can usually achieve a thicker coating, while the electroplated zinc layer is relatively thin, and the coating thickness may need to be increased when used in a humid environment.
Galvanizing Process Quality
Different galvanizing process quality also directly affects corrosion resistance. Hot-dip galvanizing is usually more corrosion-resistant than electroplating due to the higher thickness of the zinc layer.
Quality Of The Steel Itself
The cleanliness, flatness, carbon content, etc. of the steel surface will affect the adhesion of the galvanized layer. High-purity, low-carbon steel has better coating adhesion, which helps to improve corrosion resistance.
Environmental Conditions
- Normal atmospheric environment: The zinc layer of galvanized steel panel corrodes slowly and the service life can reach 20-30 years.
- Industrial or urban environment: The corrosion rate will be accelerated due to the presence of pollutants (such as sulfur dioxide and chlorides).
- Marine environment: Salt and high humidity significantly accelerate the corrosion rate of the zinc layer, and products with thick zinc layer or alloy coating are required.
- High temperature environment: High temperature will accelerate the oxidation reaction of the zinc layer and reduce the protective performance of the coating.
- Acid-base environment: Galvanised steel plates are more sensitive to acidic or alkaline solutions, and additional coating protection should be considered in these environments.
How To Improve The Corrosion Resistance Of Galvanized Steel Sheets?
Use Alloy Zinc Coating
Adding a small amount of elements such as aluminum or magnesium to the zinc coating can improve the corrosion resistance of the coating. For example, the aluminum in the galvanised steel sheet helps to form a more stable oxide film, and magnesium can further improve the corrosion resistance. Alloy zinc coatings are generally more durable than pure zinc coatings.
Apply Double Or Composite Coatings
Double coating is to cover the zinc coating with a protective coating, such as epoxy resin or polyurethane coating, which can increase corrosion resistance. Composite coatings can block corrosive media and are suitable for more severe environments.
Optimize The Thickness Of The Zinc Coating
Select the appropriate coating thickness for different environments. Generally speaking, galvanized steel plates in humid or acidic environments should choose thicker coatings to provide longer-term protection. In actual use, the reasonable coating thickness should be determined according to the corrosion level and the environmental conditions of the application site.
Surface Passivation Treatment
Passivation treatment is to chemically generate a dense protective film on the surface of the zinc coating to enhance corrosion resistance. Passivation treatment can be used to extend the service life of steel plates and reduce maintenance requirements.
Application Areas Of Galvanized Steel Sheets:
- Construction industry: galvanized steel sheets can be used for building components such as roofs, wall panels, and pipes.
- Automobile manufacturing: galvanized steel sheets can be used for body shells and chassis.
- Household appliances: galvanized steel sheets are mainly used for shells of refrigerators, washing machines, etc.
- Energy and power industry: galvanized steel sheets are used for the shells of power equipment and structures such as transmission towers.
- Traffic facilities: galvanized steel sheets are used in the manufacture of public facilities such as bridges, guardrails, and traffic signs.
With the advancement of technology, the addition of composite corrosion-resistant materials to galvanized steel sheets and the research and development of self-healing coatings will further enhance their application value in the future. In practical applications, choosing the appropriate galvanizing process and thickness according to different environmental characteristics and project requirements is the key to ensuring the long-term and stable use of galvanized sheets in various environments.
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