Hot-dip galvanized seamless pipe production process

Hot-dip Galvanized Seamless Steel Pipe (GI seamless pipe) is a steel pipe that has been hot-dip galvanized. It has the advantages of superior corrosion resistance, strong pressure resistance, strong impact resistance, beautiful and durable, safe and reliable, and economical and efficient. It is widely used in gas, heating, and water supply pipelines.

Principle of Hot-dip Galvanizing Process:

Hot-dip seamless pipe is to make molten metal react with iron matrix to produce alloy layer, so that the matrix and coating are combined. Hot-dip galvanizing is to pickle the steel pipe first. In order to remove the iron oxide on the surface of the steel pipe, after pickling, it is cleaned in an ammonium chloride or zinc chloride aqueous solution or a mixed aqueous solution of ammonium chloride and zinc chloride, and then sent to the hot-dip galvanizing tank. Hot-dip galvanizing has the advantages of uniform coating, strong adhesion, and long service life.

The production process flow of hot-dip galvanized seamless steel pipe is:
Black pipe—Alkali cleaning—Water washing—Pickling—Water rinsing—Immersion aid—Drying—Hot dip galvanizing—Outside blowing—Inside blowing—Air cooling—Water cooling—Passivation—Water rinsing— Inspection—Weighing—Storage. 

Specifically:

1. Pre-galvanizing process

There are 5 processes of alkali washing, pickling, rinsing, soaking aid, and drying.
1) The surface of some steel pipes has oil stains and needs to be washed with alkali.
2) Use hydrochloric acid for pickling to remove the scale on the surface of the steel pipe.
3) In order to remove the residual acid and iron salt attached to the surface of the steel pipe, the steel pipe needs to be rinsed twice.
4) The role of the flux is to remove all impurities from the surface of the steel pipe to ensure clean contact between the steel pipe and the zinc liquid to form a good coating.
5) In order to prevent the steel pipe from being immersed in the zinc pot and blasting, it is necessary to dry the steel pipe coated with flux. The drying temperature is 120-200°C and the drying time is not less than 150s.

2. Hot-dip galvanizing process

1) The temperature of the zinc liquid in the zinc pot is strictly controlled at 450±5°C.
2) The dried steel pipes are removed one by one through the chain of the drying furnace, immersed in the roller table of the furnace, then transported to the galvanizing furnace, and rolled into the three galvanizing spirals in the galvanizing machine.
3) The phases of the three helices are different, so that the steel pipe is in an inclined state on the helix. With the rotation of the helix, the steel pipe moves downward on one side and forms an inclination angle, then enters the zinc liquid, and continues to move downward.
4) The steel pipe automatically falls on the slide rail in the zinc pot, and is pushed to the outside by the material shifting mechanism, and then the zinc liquid is lifted out by the pipe lifter.
5) When the steel pipe is lifted to the surface of the magnetic roller, it is sucked and moved to the pull-out roller table.

3. Post-galvanizing treatment process

The post-plating treatment process includes: external blowing, pulling out, internal blowing, water cooling, inspection, passivation, and printing.
1) The steel pipe passes through the outer blowing ring, which has a row of small holes, and the excess zinc liquid of the steel pipe is blown away by compressed air to obtain a smooth appearance.
2) Under the condition of meeting the production rhythm, reducing the dragging speed can control the amount of zinc added and reduce zinc consumption.
3) Internal blowing can remove excess zinc liquid on the inner surface of the steel pipe to obtain a smooth inner surface. The removed zinc liquid forms zinc powder and is recycled.
4) The temperature of the water cooling tank is controlled at 80°C, and the cooling water is pumped to the cooling tower for cooling and then recycled.
5) After water-cooling, move out of the water-cooling tank and drop it on the inspection bench. After inspection, put the missed plating tubes into the waste basket, and the finished tubes are sent to the output roller table.
6) Spray and blow the finished tube with passivation solution during transportation to passivate the surface of the tube, and blow off excess passivation solution with compressed air after blowing the ring outside.
7) During the transportation, it passes through the drying bench laid with steam heating pipe rows, and after drying, it is printed and packaged.

Differences between Hot-dip Galvanizing and Cold-dip Galvanizing Processes:

a. Hot-dip galvanized seamless pipe

The steel pipe matrix undergoes complex physical and chemical reactions with the molten plating solution to form a corrosion-resistant and tightly structured zinc-iron alloy layer. The alloy layer is integrated with the pure zinc layer and the steel pipe matrix. Therefore, it has strong corrosion resistance.

b. Cold-dip galvanized seamless pipe

The zinc layer is an electroplated layer, and the zinc layer is layered independently from the steel pipe matrix. The zinc layer is thin, and the zinc layer is simply attached to the steel pipe matrix and is easy to fall off. Therefore, its corrosion resistance is poor. In newly built houses, it is forbidden to use cold-dip galvanized steel pipes as water pipes and gas pipes.

Formal galvanized pipe manufacturers, in order to ensure quality, mostly do not use electrogalvanizing (cold plating). Only small enterprises with small scale and outdated equipment use electrogalvanizing, and of course their prices are relatively cheaper.

Advantages of Hot-dip Galvanized Seamless Steel Pipes:

The advantages of hot-dip galvanized seamless steel pipes mainly include the following aspects:

1. Excellent anti-corrosion performance
The anti-corrosion performance of hot-dip galvanized seamless pipes is very good. The zinc layer can effectively isolate the steel pipe from the external environment and significantly slow down the corrosion rate. Zinc will form a dense zinc oxide protective film in the air, which further prevents the steel pipe substrate from contacting the corrosive medium, thereby extending the service life.

‌2. High strength and good mechanical properties‌
Since the seamless pipe (SMLS) itself adopts a seamless structure, its strength and mechanical properties are excellent and can withstand greater pressure and tension. The hot-dip galvanizing treatment does not reduce its strength, but enhances its surface hardness to a certain extent, making it more durable. Although the steel pipe needs to be immersed in high-temperature zinc liquid during the hot-dip galvanizing process, this has little effect on its mechanical properties, and the strength and hardness of the steel pipe can basically be kept within a reasonable range‌.

‌3. Uniform coating and strong adhesion‌
The hot-dip galvanizing process makes the zinc layer evenly attached to the surface of the steel pipe, with strong adhesion and not easy to peel off. The thickness of the zinc layer is generally around 65-100μm, providing better protection‌.

4. Beautiful appearance 

The appearance of hot-dip galvanized seamless steel pipe is more beautiful. The zinc layer has a silvery-white luster, which makes the seamless pipe look more neat and atmospheric. In some occasions with high requirements for appearance, such as architectural decoration, municipal engineering, etc., hot-dip galvanized seamless pipes can play a good decorative role.

5. Long service life‌
Compared with ordinary steel pipes, hot-dip galvanized seamless pipes have a longer service life. Even in highly corrosive environments, such as high humidity and high salinity environments, hot-dip galvanized seamless pipes can show better corrosion resistance and extend service life.

‌6. Wide range of applications ‌

Hot-dip galvanized seamless pipes are widely used in special pipelines for the transportation of various solid materials such as oil transportation, natural gas transportation, coal gas and water. It is light in weight, easy to construct and carry, and is an economical and applicable pipe material.

Read more: Seamless Steel Pipe Pickling Process or Black Steel Pipe VS Galvanized Steel Pipe