Choosing metal finishes for products is one of the major decisions in the project. Galvanized metal has become one of the popular multifunctional metal surface treatments due to its higher strength, durability, and formability.
Definition of galvanizing
Galvanizing, as the name suggests, is the process of coating the surface of metals, alloys, and other materials with a layer of zinc. It is a surface treatment technique that has both aesthetic and rust resistant properties. Due to zinc being a sacrificial coating, it can protect the underlying substrate.
Therefore, galvanizing is commonly used to extend the service life of metal components. This metal coating has a wide range of applications and can be used for components in different industries such as automobiles, solar energy, electronics, and construction.
What types of metals can be galvanized?
The most common metals suitable for galvanizing are steel and iron. However, other types of metals can also be galvanized. Generally speaking, ferrous metals such as cast steel, cast iron, malleable cast iron, hot-rolled steel, and cold-rolled steel can be galvanized.
On the other hand, there are other materials that cannot form a galvanized layer. Due to the importance of iron in the galvanizing reaction, it is difficult for metals such as copper or aluminum to form a galvanized layer. In addition, although materials such as chromium molybdenum steel may form coatings, there will be many exposed areas and incomplete galvanized layers.
Galvanizing process: How does the galvanized layer protect the base metal?
Galvanizing is the process of adding a layer of zinc to the surface of metals such as steel and iron. There are various methods for galvanizing. Therefore, the process varies depending on the chosen technology. However, they all share a common principle of coating zinc around steel or iron, which can be in liquid or powder form.
When zinc is introduced into the component, the iron in the metal substrate reacts with zinc to form a tightly bonded alloy coating. This is a relatively simple coating that forms a fairly thick layer. The entire process can be divided into three stages:
- Preparation of metal surface
- Galvanizing process
- Surface treatment after galvanization
Generally speaking, galvanizing protects metals based on the following principles:
- The zinc layer can protect the base metal from corrosive substances such as acids, caustic alkalis, and gases.
- Zinc is a sacrificial metal. When scratching the coating, zinc easily sacrifices its anode to protect the base metal from rusting.
- The corrosion rate of zinc is faster than that of base metals, making it an excellent method for protecting metals.
However, the preparatory work and post-treatment methods in the early stage will determine the effectiveness of the galvanizing process. Insufficient preparation work can affect the reaction between the base metal and the molten steel. In addition, insufficient post-processing can also affect the final appearance of the galvanized layer. This will affect the overall effect of the product.
Different galvanizing methods
There are multiple methods for galvanizing, each with its own characteristics and advantages. Let's take a look below.
hot-dip galvanizing
As the name suggests, this method involves immersing the base metal into a molten zinc pool. Before actual galvanizing, the mechanic must perform chemical or mechanical cleaning on the base metal. Cleaning is a preparatory process to ensure the bonding quality between metal and zinc coatings. After cleaning, flux the metal to remove any residual oxides.
Then immerse in a heated zinc bath, typically maintained at a temperature of around 460 ° C. When metallurgical bonding begins to form between zinc and the metal substrate, the metal substrate will react with oxygen to form zinc oxide after being removed from the melt. Zinc oxide also reacts with carbon dioxide to form zinc carbonate, which is the final protective layer.
Hot dip galvanizing is a fast and economical method that can be applied to both simple and complex metal sheet components. However, if operated improperly, you may find that the coating on the metal surface is somewhat inconsistent.
Pre galvanized
The pre galvanizing process is similar to hot-dip galvanizing. However, it is carried out in the first stage of production (steel mill). This process involves rolling metal plates using mechanical or chemical cleaning agents to prepare for galvanizing. After cleaning, the mechanic passes the base metal through a molten zinc pool and immediately rolls it up again.
This process ensures rapid galvanizing of large coil steel plates, forming a more uniform coating. However, when starting to manufacture pre galvanized metal, there may be some uncoated and exposed areas. For example, when cutting a longer steel plate coil into small pieces, the cutting edge is exposed due to the lack of galvanized layer.
Electrogalvanizing
Unlike the two processes discussed above, electroplating does not require the use of a molten zinc bath. On the contrary, electroplating involves introducing current into an electrolyte solution and then applying it to the substrate. The function of the current is to transfer zinc ions to the metal substrate.
This method of galvanizing metal involves reducing positively charged zinc ions to metallic zinc, which is then deposited onto a positively charged metal (steel or iron). Manufacturers will also add grain refiners to ensure a smooth zinc layer on the metal. Like pre galvanizing, electroplating galvanizing is also applied to long coil metal sheets in the initial stage of product development process.
This process can form precise and uniform coating thickness on metals. Moreover, the coating of this method is usually thinner than that of hot-dip galvanizing.
Annealing galvanizing
Galvanizing annealing is a combination of hot-dip galvanizing and annealing processes, aimed at coating galvanized steel with a special layer. Annealing and hot-dip processes are carried out instantaneously to produce a matte gray surface.
The hot-dip process is usually performed before the coated metal passes through an air knife, which helps to remove excess zinc from the metal. Then, the metal is briefly heated in an annealing furnace at a temperature of 500 to 565 ° C, and the zinc layer and iron layer diffuse into each other, forming a zinc iron alloy on the surface.
The galvanized steel sheet produced by this method can withstand welding well, and its surface can also ensure excellent paint adhesion.
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