Galvanized steel sheet with high rust resistance

Galvanized steel sheet with high rust resistance

Galvanized Steel Sheet, as the name implies, is a material made by applying “galvanizing” to “steel sheet”. First, let’s introduce the manufacturing method and basic knowledge.

Manufacturing method

Galvanized steel sheet is a material made by applying zinc to SPCC (cold rolled steel sheet). SPCC is a relatively cheap and easily available steel sheet with excellent workability, so it is often used for forming processes such as bending and drawing. However, SPCC is very easy to rust, so surface treatment is basically required. There are various methods for surface treatment of SPCC, such as painting and plating, among which galvanized steel sheet is galvanized.

Features

Plating is a surface treatment that forms a metal film on the surface of metal or resin. Among them, forming a zinc film is called galvanizing. Zinc combines with oxygen in the air to form a passive oxide film. Therefore, it has a very high rust prevention effect.
In addition, zinc has a higher ionization tendency than iron, so it also has the characteristic of oxidizing faster than iron. Therefore, if iron (steel) is galvanized, even if a small scratch is temporarily generated on the electroplated surface, the internal iron is exposed, and the surrounding zinc will be corroded preferentially, thereby having the effect of preventing iron corrosion. This phenomenon is called sacrificial corrosion protection. Therefore, galvanizing is mainly performed on iron (steel).

Galvanizing is classified according to the plating method and metal.
There are two methods: hot-dip galvanizing and electro-galvanizing.

Hot-dip galvanizing is a method of dipping steel plates and parts into heated and molten zinc. Think of fondue, chocolate fondue, etc. It is also called hot dip galvanizing or hot dip galvanizing. In addition to being able to obtain a thicker electroplated layer, an alloy layer of zinc and base metal can be obtained between the electroplated layer and the base metal layer, so it is very strong and can obtain a long-term corrosion resistance effect. This is its characteristic. On the other hand, since the product and parts are immersed in high-temperature molten zinc, deformation due to heat may occur.

Electro-galvanizing is a method of placing zinc and the material to be plated in a solution and passing electricity to cause the zinc dissolved in the solution to precipitate on the surface of the material to be plated. There are many methods of electroplating, such as zinc ammonium bath using acidic solution, potassium zinc chloride bath, and cyanide bath and zincate bath using alkaline solution. Its characteristics are uniform thickness and excellent decorative properties. On the other hand, unlike hot-dip galvanizing, electrogalvanizing cannot be used directly. Chromate treatment is usually applied to improve corrosion resistance and prevent white powder from being generated due to corrosion. Chromate treatment is a treatment that makes zinc plating more durable by using a solution with a composition similar to that of the treatment solution used in chrome plating. Chromate treatment can be used for decoration, such as adding a color close to yellow (gold), black, or gloss.
In the past, hexavalent chromium was the mainstream for chromate treatment. However, hexavalent chromium is toxic, so trivalent chromium is now used. Although trivalent chromium is non-toxic, the use of nickel alloys and molybdenum instead of trivalent chromium is increasing due to the global trend towards chromium-free. In recent years, from the perspective of environmental issues, restrictions are sometimes imposed on metals used in the manufacturing stage, and electroplating methods and treatments may also change. Be sure to check the latest information frequently.

The following table shows a comparison of the characteristics of hot-dip galvanizing and electrogalvanizing.