Galvanizing (such as
galvanized seamless pipe) is the process of adding a zinc coating to the surface of steel or iron. Because zinc acts as a sacrificial coating, it protects the steel or iron underneath, thereby extending the service life of the metal component.
But galvanizing is a complex process and can be done in a variety of ways. Galvanizing comes in a variety of forms, including hot-dip galvanizing and thermal diffusion galvanizing, and these forms have their own advantages and disadvantages. Therefore, it is helpful to understand the different forms of galvanizing before incorporating galvanizing into your prototyping or manufacturing project.
Definition of galvanizing
Galvanizing in chemistry refers to a simulated method of stimulating action or effort. In engineering, it means that we apply a protective layer of zinc to metals suspected of corrosion. This is a relatively inexpensive method of corrosion prevention. Corrosion prevention is the main purpose of galvanizing. Zinc forms a barrier layer between the surface of the material and the environment to protect the material.
What is galvanized steel pipe (GI pipe)?
Galvanized steel pipe is a steel pipe that is coated with a layer of hot-dip galvanizing or electroplated galvanizing on the surface of the welded steel pipe. Galvanizing can increase the corrosion resistance of the steel pipe and extend its service life. Galvanized pipes have a wide range of uses. In addition to being used as pipeline pipes for conveying water, gas, oil and other general low-pressure fluids, they are also used as oil well pipes and oil pipelines in the petroleum industry, especially offshore oil fields, oil heaters, condensing coolers, coal distillation and washing oil exchangers for chemical coking equipment, and trestle pipe piles and support frames for mine tunnels.
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. Since the iron element is important 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 chrome-molybdenum steel may form a coating, there will be many exposed areas and the galvanized layer will be incomplete.
Advantages of galvanizing
The purpose of galvanizing is to form a protective coating of zinc on steel or iron parts. But why is this useful? What are the main advantages of galvanizing?
Stops Corrosives: The zinc coating on the galvanized metal blocks corrosives from reaching the weaker underlying base metal. This can extend the life of the base metal, especially in demanding applications.
Prevents Rust: Galvanizing significantly delays rusting.
Creates Sacrificial Anode: The zinc coating is consumed by corrosives, so it provides protection to the metal underneath until it is completely used up. Even if a scratch penetrates the coating, the zinc will be used up before the iron.
Durable: The zinc coating formed by galvanizing is tough and long-lasting, often providing decades of protection to the underlying metal.
Affordable: Galvanizing is a fairly simple and affordable procedure that provides significant benefits for minimal expenditure.
Creates Thick Coatings: Although not suitable for all applications, galvanizing produces thicker coatings than, for example, electroplating.
Different Galvanizing Methods
There are a variety of methods for galvanizing, each with its own characteristics and benefits. Let's take a look at them below.
Hot-Dip Galvanizing
As the name implies, this method involves dipping the base metal into a pool of molten zinc. Prior to the actual galvanizing, a machinist must chemically or mechanically clean the base metal. Cleaning is a preparatory process to ensure a quality bond between the metal and the zinc coating. After cleaning, the metal is fluxed to remove any residual oxides.
This is followed by a dip in a heated zinc bath, which is usually maintained at around 460°C. Once the metallurgical bond between the zinc and the base metal has begun to form, it reacts with oxygen to form zinc oxide when the base metal is removed from the bath. The zinc oxide also reacts with carbon dioxide to form zinc carbonate, which is the final protective layer.
Hot-dip galvanizing is a quick, economical process that can be performed on both simple and complex sheet metal parts. However, if it is not done correctly, you may find that the coating on the metal surface is somewhat inconsistent.
Pre-galvanizing
The pre-galvanizing process is similar to hot-dip galvanizing. However, it is performed at the first stage of production (the steel mill). The process prepares the metal sheet for galvanizing by passing it through a mechanical or chemical cleaning agent before rolling it. After cleaning, the machinist passes the base metal through a bath of molten zinc and then immediately re-rolls it.
This process ensures that large coils of steel are galvanized quickly, resulting in a more uniform coating. However, when pre-galvanized metal is initially manufactured, some uncoated and bare areas may be present. For example, when a long coil of steel sheet is cut into smaller pieces, the cut edges are exposed without the zinc coating.
Electrogalvanizing
Unlike the two processes discussed above, electrogalvanizing does not require the use of a molten zinc bath. Instead, electroplating involves the introduction of an electric current into an electrolyte solution before it is applied to the substrate, which acts to transfer zinc ions to the metal substrate.
This method of zincizing metals involves reducing positively charged zinc ions to metallic zinc, which is then deposited onto the positively charged metal (steel or iron). Manufacturers also add grain refiners to ensure a smooth zinc layer on the metal. Like pre-galvanizing, electrogalvanizing is applied to long coils of metal sheet at the very beginning of the product development process.
This process produces a precise, uniform coating thickness on the metal. Also, the coating is typically thinner than that of hot-dip galvanizing.
Annealed galvanizing
Galvanizing annealing is a combination of hot-dip galvanizing and annealing processes, the purpose of which is to give the galvanized steel a special coating. The annealing and hot-dip processes are carried out in a split second to produce a matte grey surface.
The hot-dip process is usually carried out before the coated metal passes through an air knife, which helps to remove excess zinc from the metal. The metal is then briefly heated in an annealing furnace at a temperature of 500 to 565 °C, and the zinc and iron layers diffuse into each other, forming a zinc-iron alloy on the surface.
Galvanized steel sheets produced in this way withstand welding very well, and their surface also ensures excellent paint adhesion.
Galvanizing process: How does the zinc coating protect the base metal?
Galvanizing is the addition of 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 technology chosen. However, they all have a common principle, which is to coat the steel or iron with zinc, which can be in liquid or powder form.
When zinc is introduced to a part, the iron in the metal substrate reacts with the zinc to form a tightly bonded alloy coating. This is a relatively simple coating that forms a fairly thick coating. The entire process can be divided into three stages:
Preparation of the metal surface
Galvanizing process
Surface treatment after galvanizing
Generally speaking, galvanizing protects metals based on the following principles:
The zinc layer protects the base metal from corrosive substances such as acids, caustics, gases, etc.
Zinc is a sacrificial metal. When the coating is scratched, zinc easily sacrifices its anode, thus protecting the base metal from rust.
Zinc corrodes faster than the base metal and is an excellent way to protect metals.
However, the preliminary preparation work and post-treatment methods will determine the effectiveness of the galvanizing process. Inadequate preparation will affect the reaction between the base metal and the molten steel. In addition, inadequate post-treatment will also affect the final appearance of the galvanized layer. Thus affecting the overall effect of the product.
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