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.
Galvanized sheet is a steel sheet with a layer of metallic zinc coated on the surface of the steel 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.
The corrosion resistance of galvanised steel sheet comes from the protective effect of the zinc layer. Its working principle is mainly achieved through three mechanisms, namely physical barrier effect, sacrificial anode effect and passivation phenomenon during corrosion.
Physical barrier effect
As the outermost layer of galvanized sheet, the zinc layer isolates the direct contact between air and moisture and steel through physical barrier effect, thereby preventing oxidation and rust. Zinc itself has good oxidation resistance. The thickness of the coating determines the length of the protective effect. The thicker the zinc layer, the more significant the physical protection effect provided.
Sacrificial anode effect
When the galvanized steel plate is partially exposed to the surface of the steel due to mechanical wear or chemical corrosion, the "sacrificial anode effect" of zinc begins to play a role. Zinc has a lower potential than steel and a higher electrochemical activity. It will preferentially undergo oxidation reactions to protect the steel from corrosion. This feature allows the galvanized steel plate to extend its life even when the coating is damaged, showing high corrosion resistance.
Passivation phenomenon during corrosion
In a humid or oxygen-containing environment, a thin oxide film or basic zinc carbonate will form on the surface of the zinc layer. This passivation film can further slow down the corrosion rate, making the galvanized steel panel have good corrosion resistance in a variety of harsh environments.
Currently, the production processes of galvanized steel sheets mainly include hot-dip galvanizing and electro-galvanizing:
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), provides a stronger protection effect, and has high corrosion resistance. It is suitable for building materials exposed to the atmosphere and steel components in harsh environments.
Electro-galvanizing: 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 that of hot-dip galvanized steel sheets, it has a good effect in indoor environments or low-corrosion environments.
The zinc layer provides an effective physical barrier for the steel plate, preventing external moisture and oxygen from directly contacting the steel, thereby reducing the corrosion rate of the steel plate. At the same time, as a sacrificial anode material, zinc's electrochemical protection mechanism can further enhance the anti-corrosion effect.
Thickness of zinc coating
The thicker the zinc coating, the stronger the corrosion resistance. Hot-dip galvanizing can usually achieve a thicker coating, while the electroplated zinc coating is relatively thin, and the coating thickness may need to be increased when used in a humid environment.
Quality of galvanizing process
Different galvanizing process quality also directly affects corrosion resistance. Hot-dip galvanizing usually has better corrosion resistance than electroplating due to its higher zinc coating thickness.
Quality of the steel itself
The cleanliness, flatness, carbon content, etc. of the steel surface will affect the adhesion of the zinc coating. High-purity, low-carbon steel coatings have better adhesion and help improve corrosion resistance.
Environmental conditions
Galvanized steel sheets corrode faster when exposed in coastal areas, hot and humid areas, or acidic environments, so thicker coatings or regular maintenance are required to extend service life.
The durability of galvanized plates in different corrosive environments varies. The following is its specific performance in several typical corrosive environments.
Coastal areas
The air in coastal areas contains high salt content, which is highly corrosive to metal materials. Hot-dip galvanized steel sheets can effectively resist salt corrosion in this environment and have a long service life, usually more than 15 to 20 years.
Industrial pollution areas
The air in industrial areas contains a large amount of acidic or alkaline substances, such as sulfur dioxide and chlorine, which can cause chemical reactions in the galvanized layer and accelerate the corrosion process. Hot-dip galvanized steel sheets can better resist acidic gases, but they need regular cleaning and maintenance.
High humidity and high temperature environment
Humid and high temperature environments can easily cause the zinc layer on galvanized steel sheets to oxidize, especially in environments with poor air circulation. Therefore, when using galvanized steel sheets under high humidity or high temperature conditions, a thicker zinc layer should be selected and good ventilation should be ensured to delay corrosion.
In order to evaluate the corrosion resistance of galvanized steel sheets, the following standard test methods are usually used:
Salt spray test
The salt spray test evaluates the corrosion resistance of steel sheets in coastal or high-salt environments by exposing them to salt spray environments. After a certain period of time, the degree of corrosion on the surface of the zinc layer is observed. The test results can reflect the corrosion resistance strength of coatings of different thicknesses.
Wet heat test
The wet heat test simulates the corrosion resistance of galvanized steel sheets under extreme climatic conditions (especially tropical and marine climates) by placing them in a high temperature and high humidity environment. This experiment is suitable for testing the overall durability and stability of steel plate coatings.
Outdoor exposure test
By exposing galvanized steel sheets in the natural environment, their performance is monitored for a long time. This test can obtain more realistic durability data, but the cycle is long, and it usually takes several years to obtain complete test data.
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 the outer shells of refrigerators, washing machines, etc.
Energy and power industry: galvanized steel sheets are used for the outer shells of power equipment and structures such as transmission towers.
Transportation 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 steel sheets in various environments.
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