When designing sheet metal parts, galvanized steel sheets are a commonly used material. This article introduces the applications, characteristics, SECC, and SGCC of galvanized steel sheets.
Galvanized steel sheets with high anti-rust effect
As the name suggests, galvanized steel sheets are made by adding a layer of zinc to steel sheets. First, let’s introduce the manufacturing method and basic knowledge.
Manufacturing method
Galvanized steel sheets are made by coating SPCC (cold-rolled steel sheets) with zinc. SPCC is an affordable and readily available steel sheet with excellent workability, making it suitable for forming processes like bending and deep drawing.
However, SPCC is prone to rusting, so it requires surface treatment. Surface treatments for SPCC include coating, plating, and other methods, but galvanization is often used.
Characteristics
Galvanization is a surface treatment that forms a thin metal film on the surface of metals or resins.
Galvanization specifically refers to the formation of a zinc film. Zinc reacts with oxygen in the air to form a passivation oxide film, making it highly resistant to rust.
Additionally, zinc ions have a higher tendency to ionize than iron, so zinc also has a faster oxidation rate than iron.
Therefore, even if there are small scratches on the galvanized surface, the zinc around it will corrode first, effectively preventing corrosion of the iron underneath. This phenomenon is called sacrificial corrosion protection. Therefore, galvanization is mainly carried out on iron (steel).
Types:
Galvanized steel can be classified according to the method and metal used for the plating.
There are two methods: hot-dip galvanizing and electroplating.
Hot-dip galvanizing involves immersing steel sheets or parts into molten zinc. One can imagine it’s similar to dipping food into melted cheese or chocolate.
It is also known as hot-dip immersion or hot-dip galvanizing. In addition to being able to obtain a thicker plating layer, a zinc-metal alloy layer can be obtained between the plating layer and the base metal layer, making it very strong and able to achieve long-lasting corrosion resistance, which is its characteristic feature.
On the other hand, since the product or component is submerged in molten zinc at high temperatures, deformation due to heat can sometimes occur.
Electroplating zinc involves placing zinc and the material to be plated into a solution and using electricity to allow the zinc to dissolve in the solution and deposit onto the material’s surface.
There are several methods for electroplating, such as using acidic solutions like zinc ammonium baths or zinc chloride baths, or using alkaline solutions like cyanide baths or zinc acid salt baths. Its feature is a uniform thickness and excellent decoration.
On the other hand, unlike hot-dip galvanizing, electroplated zinc cannot be used directly. It is usually treated with chromate to improve its corrosion resistance and prevent the formation of white powder due to corrosion.
Chromate treatment is a process that uses a solution similar in composition to the treatment liquid used for chromium plating to make the galvanized coating more durable. Chromate treatment can be used for decoration purposes, such as adding colors close to yellow-gold, black, or gloss.
In the past, hexavalent chromium was used in chromate treatment as the mainstream. However, hexavalent chromium is toxic, so now trivalent chromium is used.
Although trivalent chromium is non-toxic, there is a growing trend to use nickel alloys and molybdenum for treatment instead of using trivalent chromium due to the global trend of eliminating chromium altogether.
In recent years, from an environmental standpoint, there have been restrictions placed on the metals used in the manufacturing process, and electroplating methods and treatment may change. Be sure to check for the latest information regularly.
The characteristics of hot-dip galvanizing and electroplating are compared in the following table.
| Type | Electroplating thickness | Advantage | Disadvantage |
| Electrogalvanizing | About 2-25 μ m | Uniform coating thickness and excellent decorative performance | Chromate treatment is required |
| Hot dip galvanizing | About 50-100 μ m | Strong corrosion resistance | Produce deformation due to heat |
According to the metal used for plating, there are two categories: pure zinc plating and zinc alloy plating.
In zinc alloy plating, the type of metal added to hot-dip galvanizing and electroplating is different. Lead is commonly used in hot-dip galvanizing, and recently, the addition of aluminum and magnesium has increased.
On the other hand, in electroplated zinc, an alloy of zinc with nickel or iron is used.
SECC: Steel sheet with electroplated zinc coating
What are the characteristics of SECC? Let’s introduce the features and applications of SECC.
Characteristics of SECC:
SECC is a steel sheet that has been electroplated with zinc. CC at the end of SECC is the same as the base material for electroplating, which is SPCC (cold-rolled steel sheet), which is commonly used for general purposes. Since the base material for SECC is SPCC, it has excellent workability.
In addition, because it has been electroplated, it has a beautiful and glossy appearance, and it can withstand various types of coatings.
It is the most common type of steel in the processed steel sheets.
Applications of SECC:
Since it is a general-purpose steel, it cannot be expected to have high strength.
Also, the galvanized layer is thinner than that of hot-dip galvanized steel sheet, making it unsuitable for use in harsh environments. It is often used for household appliances, indoor electrical equipment enclosures, and so on.
Advantages and disadvantages of SECC:
Advantages:
- Low cost and easy to obtain
- Attractive surface appearance
- Excellent workability and easy to form
- Excellent coating properties
- It is the most common type of processed steel sheet, making it easy to obtain at a low price. It uses SPCC, which has excellent workability, as the base material, and the electroplated layer is thin and uniform, making it easy to process through pressing and other methods.
Disadvantages:
- Requires chromate treatment
- Inferior corrosion resistance compared to SGCC
SECC has been electroplated with zinc, so it requires chromate treatment. Additionally, the thickness of the coating is thinner than that of hot-dip galvanized steel, so the corrosion resistance is worse. Therefore, it is not suitable for outdoor use or in places that are exposed to water.
Common processing and surface treatments for SECC:
The following are some of the representative processing methods used for SECC:
- Bending and deep drawing
SECC shares many properties with SPCC, so it is widely used in the same kinds of processing. It is a suitable material for bending and deep drawing processes.
- Coating
The zinc coating on SECC has good permeability for paint, and thus SECC has good coating properties. It is often used in household appliances and other products that require an attractive appearance and decorative surfaces, and is therefore usually coated.
- Chromate treatment
Since SECC requires chromate treatment, it is often implemented before painting. Chromate treatment is a process that uses a solution similar in composition to the treatment liquid used for chromium plating to make the galvanized coating more durable.
- Plating with other metals
SECC can also be plated with other metals to enhance its properties for specific applications. For example, nickel plating can be used to improve its resistance to corrosion, and tin plating can be used to enhance its solderability.
SGCC: Implementation of hot-dip galvanized steel.
Here are the characteristics and applications of SGCC.
Characteristics of SGCC:
SGCC is a type of hot-dip galvanized steel. Its English name is Steel Galvanized Cold Commercial. Because it is a hot-dip galvanized version of SPCC, its basic properties are almost the same as those of SPCC.
It is also known as a galvanized sheet. Its coating is thicker than that of SECC, and it has a high level of corrosion resistance. Other types of galvanized steel sheets include alloy hot-dip galvanized steel sheets and coated aluminum-zinc steel sheets.
Applications of SGCC:
Though not a particularly strong material, SGCC has excellent corrosion resistance, and therefore is used in many places. It is used not only for materials for power transmission iron towers and rails, but also for vehicle components.
Its usage in construction materials is also widespread, as it is used not only as a material for rolling shutter doors and window guards, but also in the form of galvanized sheets for building exteriors and roofs.
Advantages and Disadvantages of SGCC
Advantages:
- High corrosion resistance for long periods of time.
- Relatively low cost and easy to obtain.
- Excellent workability.
- As SGCC has the same origin as SECC, it can be easily processed, as with SPCC. Its superior corrosion resistance over SECC is also a significant advantage.
Disadvantages:
- Sometimes, a pattern similar to tree branches (spangle) may occur.
- During pressing and processing, zinc coating adheres to the mold.
- Zinc and impurities in the air can form crystals and sometimes float out as geometrical spangle patterns. This is called spangle pattern. Regardless of whether spangle occurs, the properties do not change. However, as it is often used as an external decorative material, it is sometimes differentiated according to appearance design requirements. During pressing and processing, the surface zinc is slightly shaved off and adheres to the mold. This increases the frictional resistance of the mold and leads to various molding problems.
Common Processing and Surface Treatment for SGCC
The following are representative processes and surface treatments used for SGCC.
Bending:
SGCC is often used for bending processes, such as for corrugated galvanized sheets and pipes. Basically, it is similar to SPCC and is suitable for bending and pressing.
Coating:
SGCC is also commonly used for places that require appearance and design, such as rails and rolling shutter doors. Therefore, it often undergoes coating.
Derivative Materials of SGCC
SGCC has derivative materials that enhance corrosion resistance and workability. Here are some of the most well-known materials in the SGCC family.
Alloy Hot-Dip Galvanized Steel Sheet:
The general SGCC has the disadvantage that the electroplated zinc adheres to the mold during pressing and processing.
Therefore, alloy hot-dip galvanized steel sheets were developed to solve this problem. In the original SGCC, there was an alloy layer between the base metal layer and the electroplated layer.
However, in the alloy hot-dip galvanized steel sheet, the difference is eliminated, and the electroplated layer is completely alloyed, improving the adhesion.
Usually, alloy hot-dip galvanized steel sheets are made by heat-treating SGCC. Not only workability, but also coating and welding properties are improved.Coated
Aluminum-Zinc Steel Sheet:
In recent years, coated aluminum-zinc steel sheets have gained attention as construction materials and are also partners of SGCC.
Aluminum-zinc plating is an alloy of zinc with added aluminum and silicon. Steel sheets that have undergone hot-dip galvanizing using this alloy become coated aluminum-zinc steel sheets, which were commercialized by Bethlehem Steel Corporation.
The acid and alkali resistance of general SGCC is not strong. Therefore, so-called galvanized sheets are often corroded by acid rain before their expected lifetime.
To solve this problem, coated aluminum-zinc steel sheets were developed. Coated aluminum-zinc steel sheets are materials with extremely high durability in electroplated steel sheets.
Mechanical and Physical Properties of SECC and SGCC
SECC and SGCC do not have specified numerical values for mechanical and physical properties. Here are some reference values, but please check the manufacturer’s catalogs for details.
Mechanical Properties
The mechanical properties of SECCT with T at the end of SECC are as follows:
| Yield point or endurance (N/mm²) | Tensile strength (N/mm²) | Elongation (%) | Coating sintering hardening amount (N/mm²) | |||||
| Display thickness (mm) | ||||||||
| Above 0.40 and less than 0.60 | Above 0.60 and less than 1.0 | Above 1.0 and below 1.6 | Above 1.6 and below 2.3 | 2.3 above and less than 2.5 | Above 2.5 and below 3.2 | |||
| Above 270 | Above 34 | Above 36 | Above 37 | Above 38 | Above 38 | Above 38 | ||
For SECC and SGCC without specified mechanical properties, the values can vary among different manufacturers even for materials with the same thickness. When calculating strength and other properties, it is recommended to refer to the manufacturer’s catalogs rather than JIS values.
Physical and Chemical Properties
SECC and SGCC do not have specified physical or chemical properties. These can be confirmed by checking the manufacturer’s catalogs and manufacturing process specifications.
Standard Sizes of SECC and SGCC
SECC does not have a standard thickness due to changes caused by plating thickness before electroplating. However, the standard size of SECC steel plates before electroplating is the same as that of SPCC, which is above 0.4 mm and below 3.2 mm in multiple thicknesses.
| Type | Material symbol | Shape | Unit | Standard dimensions |
| Cold rolled steel plate | SPCC | steel plate | t | 0.4, 0.5, 0.6, 0.7, 0.8, 1, 1.2, 1.6, 2, 2.3, 3.2 |
SGCC has a standard thickness including the coating after electroplating. The standard thickness of SGCC is above 0.4 mm and below 6.0 mm in multiple thicknesses. Although there are standard thicknesses, the thicknesses may vary among different manufacturers, so it is recommended to check each manufacturer’s catalog instead of relying on JIS standard sizes.
In addition, it is also recommended to confirm the operating size of each manufacturer instead of relying on standard sizes.
Summary
Plating is a processing method for forming a metal film on the surface of metals or resins, and galvanizing is a type of plating that is used for surface treatment of iron products due to its high corrosion resistance.
SECC is a material that has been galvanized with zinc onto SPCC (cold-rolled steel), while SGCC is a material that has been hot-dip galvanized onto SPCC.
Because these easy-to-process steel plates have corrosion resistance due to galvanization, they are widely used as enclosures for electrical products, building materials, and more.