What are the differences between hot-dip galvanizing and electrogalvanizing?
Hot-dip galvanizing (HDG) and electrogalvanizing (also known as cold galvanizing) are the two most popular processes for galvanizing round pipes. Their core differences lie in the formation principle, thickness, performance, and application scenarios of the galvanized layer. These differences in process flow directly determine the characteristics of the final product. The following details the process flow from two perspectives: a "process breakdown" and a "core difference comparison":
1. Hot-dip galvanizing (HDG) process: High-temperature galvanizing forms a composite layer of "alloy + pure zinc"
The core principle of hot-dip galvanizing is to immerse the derusted round pipe in molten zinc (approximately 440-460°C). Through chemical reactions and physical wetting between the metals, a tightly bonded "zinc-iron alloy layer + pure zinc layer" is formed on the pipe surface. This is a "high-temperature chemical deposition" process. The complete process can be divided into three stages: pre-treatment, hot-dip galvanizing, and post-treatment. The specific steps are as follows:
1. Pre-treatment: Remove impurities and ensure zinc coating adhesion (a critical step).
The core goal of pre-treatment is to thoroughly remove oil, scale, and rust from the surface of the round tube, exposing the pure iron substrate. Otherwise, the zinc layer will easily fall off.
Degreasing (degreasing): Place the round tube in an alkaline degreaser (such as sodium hydroxide solution) or a solvent-based degreaser. Immerse, spray, or ultrasonically clean the tube to remove contaminants such as engine oil and rust preventative oil that may have adhered during production and storage.
Acid cleaning (rust/scale removal): After degreasing, transfer the tube to a pickling tank (typically 15%-20% hydrochloric acid or sulfuric acid) and soak for 15-30 minutes (adjusted depending on the degree of rust). This dissolves the surface scale (Fe₂O₃, Fe₃O₄) and rust, revealing a fresh, off-white iron surface. Rinsing (Neutralization): After pickling, any residual acid on the pipe surface must be immediately rinsed in a clean water tank, then transferred to a weakly alkaline neutralization tank (such as sodium carbonate solution) to neutralize the residual acid and prevent subsequent corrosion of the substrate. (If residual acid remains, zinc slag will form during zinc dipping, affecting the quality of the zinc layer.)
Activation: After rinsing, the round pipe is immersed in a "plating flux" (usually a zinc chloride-ammonium chloride aqueous solution). After drying, a uniform salt film forms on the pipe surface. This serves the following purposes: 1. It isolates the pipe from air, preventing secondary oxidation of the substrate before entering the zinc pot; 2. It reduces the surface tension of the zinc solution, promoting its penetration into the pipe surface. 2. Hot-Dip Galvanizing: High-temperature zinc immersion to form a composite coating (core stage)
Drying/Preheating: After flux galvanizing, the round tubes are placed in a drying oven (approximately 100-120°C) to remove surface moisture and prevent water from entering the hot zinc pot, which could cause zinc splashing (a safety hazard). The tubes are also preheated to 80-120°C to minimize the temperature difference with the zinc bath and reduce the generation of zinc slag caused by rapid boiling.
Zinc Dipping: The preheated round tubes are slowly immersed in molten zinc (440-460°C). The immersion time is adjusted based on the tube wall thickness (approximately 30-60 seconds for thin-walled tubes and 1-2 minutes for thick-walled tubes). Two key reactions occur during this process:
① A chemical reaction occurs between the iron substrate and the zinc bath: Fe + Zn → FeZn₇ (a zinc-iron alloy layer). This layer is tightly bonded to the substrate and is the core of corrosion protection.
② The zinc bath physically deposits on the alloy layer, forming a pure zinc layer (typically 70%-80% of the total coating thickness). Zinc Control: After zinc dipping, the round tube is slowly removed from the zinc pot. Compressed air is blown away (or a special "zinc knife" is used to scrape off excess zinc liquid from the surface of the tube) to control the coating thickness (avoiding localized excessive zinc thickness, forming "zinc nodules") while ensuring uniform coating.
3. Post-Processing: Optimizing Appearance and Performance
Cooling: The zinc-controlled round tube enters a cooling tank (usually using cold water or weakly alkaline cooling water) for rapid cooling to room temperature. This stabilizes the zinc coating structure and prevents oxidation and discoloration at high temperatures.
Passivation (Optional): In some applications (e.g., requiring higher corrosion resistance), the cooled round tube is immersed in a chromate passivation solution to form an extremely thin passivation film (colored or military green) on the zinc surface, further enhancing its resistance to atmospheric corrosion and improving its appearance.
Inspection: Check coating thickness (usually using a magnetic thickness gauge), adhesion (cross-cut test or bend test; the zinc layer should be free of any peeling), and appearance (no defects such as nodules, missing plating, or black spots).