Hot Dip Galvanizing Process

Hot dip galvanizing (HDG) process is a process of coating steel with zinc by immersing the base metal in the molten zinc bath at about 450 °C. It is a form of galvanizing, and the other one is cold galvanizing, also known as electro-galvanizing. The base metal can be hot-rolled steel or cold-rolled steel. The aim of galvanization is to protect the base metal from further corrosion and prolong its service life. Here let’s review the hot-dip galvanizing process steps.

1. Uncoiling & welding

Uncoil the steel coil and weld the coils together for continuous operation. Generally, two uncoilers are used.

2. Degreasing & cleaning

This process, including degreasing, pickling, and water cleaning, aims to remove the oil, grease, dirt, and paint on the metal surface. First, use chemical degreasing or water-based metal degreasing cleaners to remove the oil. Poor degreasing and pickling treatment will result in poor coating adhesion so that the zinc cannot be plated or the zinc layer falls off.

3. Annealing

Recrystallization annealing aims to improve mechanical properties. At the same time, controlling the atmosphere in the furnace can also improve the coating performance. The annealing process consists of preheating, heating, soaking, and cooling.

4. Galvanizing

Galvanizing is applying a layer of zinc coating on the surface of the base metal by immersing the base metal in the molten zinc bath at about 450 °C. It is also suitable for galvalume steel. During hot-dip galvanizing, it is important to control the temperature of the zinc solution, the dipping time, and the speed at which the workpiece is drawn out of the zinc solution. If the temperature is too low, the fluidity of the zinc liquid is poor, the coating is thick and uneven, and easy to sag. However, if the temperature is too high, a large amount of zinc slag will be produced, which affects the quality of the zinc layer and easily causes color differences, and the zinc consumption is high.

5. Cooling

Water cooling is generally used, and the temperature should not be lower than 30 °C and not higher than 70 °C.

6. Flattening and straightening

Roll the strip with a small rolling force to obtain a good flatness, etc. The strip is stretched and straightened to further improve the shape of the plate.

7. Post-processing

The steel coil is passivated or other chemical surface treatment to improve its corrosion resistance, lubrication, or other requirements.

8. Quality inspection

Check the appearance of the coating whether it is bright and free from sagging and wrinkling. Also, check the zinc coating thickness with a thickness gauge. Moreover, the properties can be inspected by bending tests, impact tests, salt spray tests, sulfate corrosion tests, etc.

9. Oiling

After quality inspection, apply oil to the strip to achieve better anti-rust and lubricating properties.

10. Coiling

Roll up the coils for sale.

Here are the full steps of the hot dip galvanizing process. Hot-dip galvanized steel combines the strength of steel with the resistance to corrosion of zinc. It is more corrosion-resistant than unpainted steel, but it is more affordable than stainless steel. At Wanzhi Steel, there are two continuous hot-dipped galvanizing lines and an electro-galvanizing line. Welcome to contact us for an online or on-site visit!