Hot-dip galvanizing is one of the most effective and widely used anti-corrosion treatments for steel props. Different from ordinary painting or electroplating, hot-dip galvanizing forms a dense and firm metallurgical bond between the zinc layer and the steel substrate, which not only can effectively isolate the steel from the external corrosive environment (such as moisture, rainwater, salt fog), but also has the function of cathodic protection—when the zinc layer is damaged, the zinc will sacrifice itself to corrode first, so as to protect the underlying steel from rusting. The hot-dip galvanizing process of steel props is a systematic project with strict operation specifications, and each link directly affects the quality of the zinc layer and the anti-corrosion effect of the props.
The first core stage of the hot-dip galvanizing process of steel props is surface preparation, which is the key to ensuring the bonding quality between the zinc layer and the steel substrate, accounting for 90% of the overall process quality. The purpose of surface preparation is to completely remove oil, mill scale, rust, oxides and other impurities on the surface of steel props, so as to expose a clean and activated pure iron surface, which lays a foundation for the subsequent metallurgical reaction between zinc and steel. This stage mainly includes four consecutive steps: degreasing, rinsing, pickling and fluxing.
Degreasing is the first step of surface preparation. Steel props will be contaminated with drawing oil, workshop grease, fingerprints and dust during the production, processing and transportation process. These organic contaminants will prevent the zinc liquid from contacting the steel substrate, resulting in poor bonding of the zinc layer. The degreasing operation is usually carried out by immersing the steel props in an alkaline solution (such as sodium hydroxide and sodium carbonate mixed solution) at a temperature of 60-80℃. Through saponification and emulsification, the oil stains on the surface are dissolved and removed. For steel props with heavy paint or thick oil stains, mechanical cleaning (such as sandblasting) should be carried out first to ensure the degreasing effect.
After degreasing, rinsing is required immediately to wash away the residual alkaline degreasing solution on the surface of steel props. This step is crucial to avoid cross-contamination—if the degreasing solution is carried into the subsequent pickling stage, it will reduce the service life of the acid solution and cause defects such as uneven zinc layer. The rinsing is usually carried out with clean tap water, and the surface of the props should be free of obvious water stains and residual chemicals after rinsing.
Pickling is the step to remove mill scale and rust on the surface of steel props. Mill scale is a dense oxide layer formed on the surface of steel during hot rolling, and rust is the product of steel oxidation in the air. Both will prevent the metallurgical reaction between zinc and steel. The pickling operation is to immerse the steel props in a diluted hydrochloric acid solution (or sulfuric acid solution, but hydrochloric acid is preferred for its faster reaction speed and easier control) at room temperature. The acid solution will chemically react with mill scale and rust to dissolve them, exposing the pure metallic iron surface. It is necessary to strictly control the pickling time and acid concentration to avoid "over-pickling" which will corrode the steel substrate or cause hydrogen embrittlement. After pickling, the steel props should be rinsed again with clean water to stop the acid reaction and keep the surface neutral.
Fluxing is the last step of surface preparation, which aims to prevent the cleaned steel surface from re-oxidizing before entering the zinc bath and improve the wettability between the steel surface and the molten zinc. The steel props are immersed in a zinc ammonium chloride solution (ZnCl₂·2NH₄Cl) for a certain period of time, and a thin protective film will be formed on the surface after fluxing. This film can isolate the air and prevent oxidation, and can also reduce the surface tension of the molten zinc, making the zinc liquid better adhere to the steel surface. After fluxing, the steel props need to be dried to remove surface moisture—any moisture left on the surface will cause zinc liquid splashing or even explosion when entering the high-temperature zinc bath, which is not only dangerous but also affects the zinc layer quality.
The second core stage is hot-dip galvanizing, which is the key link to form the zinc layer. After surface preparation, the steel props are hung on a special hanger (ensuring that all surfaces can be fully contacted with the zinc liquid) and slowly immersed into the molten zinc bath. The temperature of the molten zinc is strictly controlled between 445-465℃, and the zinc purity is not less than 90% to ensure the quality of the zinc layer. When the steel props are immersed in the zinc bath, the iron on the steel surface will have a metallurgical diffusion reaction with the molten zinc, forming a gradient zinc-iron alloy layer (including Γ phase, δ phase and ζ phase) between the steel substrate and the pure zinc layer. This alloy layer has high bonding strength (about 3600 psi) and good wear resistance, which is the key to ensuring the firmness of the zinc layer.
The immersion time of steel props in the zinc bath depends on the thickness of the props and the required zinc layer thickness, usually 2-5 minutes. Too short immersion time will lead to insufficient reaction between zinc and iron, resulting in thin zinc layer and poor bonding; too long immersion time will cause excessive growth of the alloy layer, making the zinc layer brittle and easy to peel off. After the reaction is sufficient, the steel props are lifted out of the zinc bath at a uniform speed. During the lifting process, the excess zinc liquid on the surface will flow back into the zinc bath under the action of gravity, forming a uniform pure zinc layer on the surface of the alloy layer.
The third core stage is post-treatment, which is used to improve the quality and corrosion resistance of the zinc layer, and ensure the appearance and performance of the steel props. This stage mainly includes cooling, passivation (optional), finishing and inspection.
Cooling is the first step of post-treatment. The steel props just lifted out of the zinc bath have a high temperature, and need to be cooled quickly to solidify the zinc layer and prevent the zinc-iron alloy reaction from continuing. The cooling method can be natural air cooling or water cooling—natural air cooling is suitable for small-size steel props, while water cooling is more efficient and suitable for large-scale production, but it is necessary to avoid rapid cooling which may cause the zinc layer to crack. The steel props should be cooled to room temperature before entering the next step.
Passivation is an optional but recommended step. The purpose of passivation is to form a thin passivation film on the surface of the zinc layer, which can effectively prevent the zinc layer from producing "white rust" (zinc corrosion product) during storage and transportation. The passivation treatment is usually carried out by immersing the cooled steel props in a chromate or non-chromate passivation solution (non-chromate passivation is more environmentally friendly and widely used now). After passivation, the steel props are dried again to ensure the stability of the passivation film.
Finishing is to trim the surface of the steel props, mainly to remove excess zinc nodules, zinc slag and burrs on the surface, so as to make the appearance of the props flat and beautiful, and avoid affecting the subsequent installation and use. The finishing operation can be carried out by manual polishing or mechanical grinding, and the surface of the props should be smooth and free of obvious defects after finishing.
Inspection is the final step of the hot-dip galvanizing process, which is used to verify whether the zinc layer meets the relevant standards and requirements. The inspection items mainly include three aspects: first, appearance inspection, which requires the zinc layer to be uniform, dense and free of missing plating, blistering, peeling, cracks and other defects; second, thickness inspection, which is measured by a magnetic thickness gauge, and the average thickness of the zinc layer of steel props is usually not less than 65 microns (complying with ISO 1461 standard); third, adhesion inspection, which can be carried out by hammering or bending—after hammering or bending, the zinc layer should not peel off or crack. Only the steel props that pass all inspections can be put into use.
It should be noted that the hot-dip galvanizing process of steel props must comply with relevant international and domestic standards, such as EN ISO 1461:2022 (European standard), ASTM A123 (American standard) and GB/T 2518-2019 (Chinese national standard). These standards clearly specify the process parameters, zinc layer quality requirements and inspection methods, ensuring the standardization and reliability of the galvanizing process. In addition, the design of steel props should also consider the galvanizing process—for example, reasonable venting and draining holes should be set to ensure that the zinc liquid can fully fill all parts of the props and avoid missing plating.
In practical application, the hot-dip galvanizing process can significantly improve the corrosion resistance of steel props, making them suitable for outdoor, coastal, humid and other harsh construction environments, and extending their service life to 15-25 years (with minimal maintenance). This is why hot-dip galvanizing has become the preferred anti-corrosion treatment method for steel props in construction projects.
References
1. GB/T 7714 Format
Yuantai Derun. Hot Dip Galvanized Process: A Practical Guide to the Steel Protection That Actually Lasts[EB/OL]. [2026-02-05].
2. MLA Format
American Galvanizers Association. "What is Hot-Dip Galvanizing?" American Galvanizers Association, 2026. Accessed 17 Apr. 2026.
3. APA Format
European General Galvanizers Association. (2026). Hot Dip Galvanizing Standards: EN ISO 1461:2022.