High-Corrosion-Resistant Galvanized Steel Sheet
Galvanized steel sheet, as the name suggests, is a material created by applying a zinc coating to a steel sheet. Let’s start by introducing the manufacturing methods and fundamental concepts.
Manufacturing Method
Galvanized steel is made by applying a zinc coating to SPCC (Cold-Rolled Steel Sheet). SPCC is a readily available and relatively inexpensive steel sheet with excellent formability, making it widely used for forming processes like bending and deep drawing. However, SPCC is highly susceptible to rust, so it almost always requires a surface treatment. Various surface treatments for SPCC exist, such as painting and plating, with galvanization being one of the most common.
Features
Plating is a surface treatment that forms a thin metal film on the surface of a metal or resin. Among these, plating with a zinc film is called galvanization. Zinc reacts with oxygen in the air to form a passive oxide film. This gives it the characteristic of having an extremely high anti-rust effect.
Furthermore, zinc has a higher ionization tendency than iron, meaning it oxidizes faster. Therefore, when iron (steel) is galvanized, even if a small scratch temporarily exposes the underlying iron, the surrounding zinc will corrode first, protecting the iron from rust. This phenomenon is known as sacrificial protection and is the primary reason why galvanization is applied to iron and steel.
Types of Galvanization
Galvanization is classified by the method of plating and the type of metal used.
There are two main methods: hot-dip galvanizing and electro-galvanizing.
Hot-dip galvanizing involves immersing a steel sheet or component in molten zinc. Imagine a fondue pot or a chocolate fountain. It is also known as hot-dip galvanization or hot-dip zinc plating. A key feature is the ability to achieve a thick plating layer. It also forms an alloy layer of zinc and the base metal between the plating and the substrate, making it very durable and providing long-lasting corrosion resistance. On the other hand, because the product is immersed in hot, molten zinc, it can sometimes cause thermal deformation.
Electro-galvanizing is a method where zinc and the material to be plated are placed in a solution and an electric current is applied, causing the zinc dissolved in the solution to deposit on the surface of the material. There are various electro-galvanization methods, such as zinc ammonium baths and potassium chloride baths (using acidic solutions) and cyanide baths and zincate baths (using alkaline solutions). Its key features are uniform thickness and excellent decorative properties. Unlike hot-dip galvanizing, electro-galvanized coatings cannot be used directly. They typically undergo chromate treatment to improve corrosion resistance and prevent the formation of white rust (white powder from corrosion). Chromate treatment uses a solution similar to that used for chrome plating to make the galvanized coating more durable. It can also be used for decorative purposes, adding a yellowish (gold-like) color, black, or gloss.
In the past, hexavalent chromium was the main component used in chromate treatment. However, hexavalent chromium is toxic, so trivalent chromium is now used. Although trivalent chromium is non-toxic, a global trend towards “chrome-free” treatments is increasing the use of nickel-molybdenum alloys as an alternative. In recent years, due to environmental concerns, there have been increasing restrictions on the metals used in manufacturing. Therefore, plating methods and treatments may change over time. It’s important to always check for the latest information.
A comparison of the features of hot-dip galvanizing and electro-galvanizing is provided in the table below.
| Feature | Hot-Dip Galvanizing | Electro-Galvanizing |
| Coating Thickness | Thick, non-uniform | Thin, uniform |
| Durability | High | Low |
| Processing | May deform due to heat | Excellent, retains shape |
| Application | Outdoor, harsh environments | Indoor, decorative applications |
| Appearance | Spangle patterns may appear | Smooth, uniform finish |
Metals can be classified into pure zinc plating and zinc alloy plating. The types of metals added in zinc alloy plating differ between hot-dip and electro-galvanizing. Hot-dip galvanizing often uses lead, and recently, the addition of aluminum and magnesium is also increasing. In electro-galvanizing, zinc alloy plating is done using nickel or iron.
Electro-Galvanized Steel Sheet: SECC
What kind of material is SECC? Here, we introduce its features and applications.
Features of SECC
SECC is an electro-galvanized steel sheet. The English acronym stands for Steel Electrolytic Cold Commercial. It is also known as “Bond Steel Sheet,” which was the original product name when it was first sold by Nippon Steel.
The “CC” at the end of SECC is the same as the “CC” in SPCC (Cold-Rolled Steel Sheet), the base material before electro-galvanization, indicating that it is a cold-rolled material for general use.
Because its base material is SPCC, SECC also has excellent formability. Additionally, since it is electro-plated, it has a beautiful, glossy appearance, yet it has excellent paintability and can be coated in various colors.
It is one of the most widely used treated steel sheets on the market.
Applications of SECC
As a general-purpose steel, SECC cannot be expected to have extremely high strength. Furthermore, the galvanized layer is thinner than that of hot-dip galvanized steel, making it unsuitable for use in harsh environments. It is often used for the casings of household appliances and indoor electrical equipment.
Pros and Cons of SECC
Pros:
Relatively low price and easy to obtain
Beautiful surface finish
Excellent formability, easy to shape
Excellent paintability
As the most common type of treated steel sheet, SECC is readily available at a low cost. Using SPCC as its base material, combined with a thin and uniform electro-plated layer, makes it easy to perform processes like stamping.
Cons:
Requires chromate treatment
Lower corrosion resistance than SGCC
Because SECC is electro-galvanized, it requires chromate treatment. Its thinner plating layer compared to hot-dip galvanized steel also results in lower corrosion resistance. Therefore, it is not suitable for outdoor use or in places where it is likely to come into contact with water.
Common Processing and Surface Treatments for SECC
Here are some typical processes performed on SECC.
Bending and Deep Drawing: SECC’s properties are fundamentally similar to SPCC, so it undergoes the same processes. It is a material well-suited for bending and deep drawing.
Painting: Electro-galvanized coatings have good paint adhesion, giving SECC excellent paintability. It is often painted for parts of household appliances and other items that require a high-quality or decorative appearance.
Hot-Dip Galvanized Steel Sheet: SGCC
Next, we introduce the features and applications of SGCC.
Features of SGCC
SGCC is a hot-dip galvanized steel sheet. The English acronym stands for Steel Galvanized Cold Commercial.
Since it is made by hot-dip galvanizing an SPCC base, its basic properties are very similar to SPCC. It is also commonly called galvanized iron sheet. The plating layer is thicker than SECC, giving it high corrosion resistance. It has “partners” like galvannealed steel and Galvalume steel sheet.
Applications of SGCC
SGCC is not a particularly high-strength material, but its excellent corrosion resistance makes it widely used. In addition to being used for materials like transmission towers and guardrails, it is also used for vehicle parts. It has many applications in construction, serving as a material for roller shutters and rain shutters, and in the form of corrugated iron sheets for building exteriors and roofs.
Pros and Cons of SGCC
Pros:
Provides long-lasting, high corrosion resistance
Relatively inexpensive and easy to obtain
Excellent formability
SGCC, like SECC, uses SPCC as its base material, so its ease of processing is similar. Its superior corrosion resistance compared to SECC is a major advantage.
Cons:
Can sometimes develop a dendritic pattern (spangle)
Zinc can transfer to molds during stamping
Zinc can crystallize with impurities in the air, sometimes forming a branch-like geometric pattern on the surface, known as a spangle. The spangle does not affect the material’s properties, but since it’s often used for external decorative purposes, it may be used selectively based on aesthetic design requirements.
During stamping, the surface zinc can be slightly scraped off and adhere to the mold. This increases the friction of the mold, leading to various forming defects.
Common Processing and Surface Treatments for SGCC
Here are some typical processes performed on SGCC.
Bending: SGCC is often bent and is used for corrugated galvanized sheets, pipes, and more. It’s basically the same as SPCC—a material suitable for bending and stamping.
Painting: SGCC is also widely used in applications like guardrails and roller shutters, which require a good appearance and design. Therefore, it is often painted.
SGCC Derivatives
SGCC has derivative materials that offer improved corrosion resistance and formability. Here we introduce some of the most well-known “partners” of SGCC.
Galvannealed Steel Sheet: Standard SGCC has the disadvantage of zinc adhering to the mold during stamping. Galvannealed steel was created to solve this problem. While standard SGCC has an alloy layer between the base metal and the plating, galvannealed steel eliminates this distinction, alloying the entire plating layer to improve adhesion. Galvannealed steel is typically produced by heat-treating SGCC. This improves not only formability but also paintability and weldability.
Galvalume Steel Sheet: Recently, Galvalume steel sheet has gained attention as a construction material and is another partner of SGCC. Galvalume is an alloy of zinc, aluminum, and silicon. Galvalume steel sheet is made by hot-dip galvanizing with this alloy and was commercialized by Bethlehem Steel. Standard SGCC does not have strong resistance to acids and bases. As a result, galvanized sheets can corrode long before their expected lifespan due to acid rain. Galvalume steel sheet was developed to address this. It is one of the most durable materials among plated steel sheets.
Mechanical and Physical Properties of SECC and SGCC
SECC and SGCC do not have officially specified values for mechanical or physical properties. The values provided here are for reference only. For detailed information, please consult the catalogs of individual material manufacturers.
Mechanical Properties
Apart from what’s mentioned above, SECC and SGCC do not have standardized mechanical properties. This means that, for example, the values for SECC materials of the same thickness can sometimes vary by manufacturer. When calculating strength, it’s better to refer to the product catalog rather than the JIS (Japanese Industrial Standards) values.
Physical and Chemical Properties
SECC and SGCC also do not have standardized physical or chemical properties. These can be confirmed by consulting the manufacturer’s product catalogs and manufacturing process specifications.
Standard Sizes for SECC and SGCC
While the standard thickness of the steel sheet before plating exists for SECC, the final thickness varies with the plating thickness, so there is no standard final size for SECC. The standard thickness of the pre-plated steel sheet is the same as SPCC, ranging from 0.4mm to 3.2mm in various thicknesses.
SGCC has a specified standard thickness after plating, including the coating. The standard thickness for SGCC is from 0.4mm to 6.0mm in various thicknesses. Although there are specified standard thicknesses, the thicknesses available can vary by manufacturer, so it is better to check each manufacturer’s product catalog rather than relying on a single standard.
For sheet sizes, it is also recommended to check the operating sizes listed by each manufacturer rather than a standard size.
Summary
Plating is a process of forming a thin metal film on the surface of a metal or resin. Among these, zinc plating is characterized by its high corrosion resistance, making it commonly used as a surface treatment for iron products.
SECC is a material made by applying electro-galvanization to SPCC (Cold-Rolled Steel Sheet).
SGCC is a material made by applying hot-dip galvanization to SPCC.
Both are easily workable steel sheets with corrosion resistance imparted by galvanization, making them widely used for casings of electrical products, building materials, and more.
