How Is Galvanized Steel Made?

Galvanized steel is made by applying a layer of zinc to the surface of steel or iron to protect it from rust and corrosion. Meanwhile, the smooth and fine zinc coating also offers decoration purposes. This process is known as galvanization. There are two common methods, namely, hot-dip galvanizing and electro-galvanizing (cold galvanizing).

Hot-dipped Galvanizing (HDG) Process

Hot-dip galvanizing is the process of immersing the steel or iron in the molten zinc at about 480 ℃ to apply a zinc layer on the surface. Here is the process:

Uncoiling & welding — Degreasing & cleaning — Annealing & Pickling — Drying — Galvanizing — Cooling  — Inspection

At Wanzhi Steel, we often use cold-rolled or hot-rolled coils as the base metal. The base metal is uncoiled and welded first. The next step is the pre-treatment process, including degreasing, washing, and drying. Then the steel coil is heated to 700-800 degrees for recrystallization annealing. After that, cool down the metal to 480 ℃. Finally, it is immersed in a zinc pot under sealed conditions. After cooling and quality inspection, the galvanized steel will be rolled up for sale. The hot-dip galvanizing process offers a thick and dense pure zinc layer to protect the base metal from corrosion.

Electro-galvanizing Process (EG)

Electro galvanizing, also known as cold galvanizing, is the process of forming a zinc layer on the surface of steel or iron by electrolysis. It adopts an electrochemical method. Regarding zinc ingot as the anode, the zinc atoms lose electrons and become ions and dissolve into the electrolyte. And the steel metal is the cathode. The zinc ions get electrons on the steel and become zinc atoms and are deposited on the surface of the steel to form a zinc coating. Before electroplating, it is necessary to remove the grease and oxides on the base metal to ensure its adhesion. Here is the process:

Degreasing (at 5 -40°C) — Washing with water (at room temperature) —  rust cleaning — Washing with water (at room temperature) — Electro galvanizing  (5-60°C) — Washing with water (at room temperature) — Passivating — Washing with hot water (≤70°C) — Drying — Cooling — Quality Inspection

Conclusion

In general, electro-galvanizing is through an electrode reaction, while hot-dip galvanizing is galvanizing by immersing raw materials in a zinc bath, which is a physical reaction and intermetallic reaction. So there is a layer of intermetallic compound and a zinc layer on the surface of HDG steel.

Moreover, the zinc layer of electro-galvanized steel is thinner (≤30 g/㎡), and that of hot-dip galvanized steel is thicker, say 30-600 g/㎡. That’s why HDG has better corrosion resistance than EG. However, the surface EG is smoother than HDG. At Wanzhi Steel, the maximum zinc layer is 275 g/㎡ (Z275 or G90). In terms of their uses, electro-galvanized sheets are suitable for small parts. While hot-dip galvanized sheets are suitable for large parts and equipment. HDG products are widely used in construction, chemical facilities petroleum, municipal facilities, agricultural facilities, railways, and other fields. Welcome to contact us for more details!