1.What are the common methods for degreasing (deoiling)?
Common methods:
Alkali degreasing: Immerse the steel in an alkaline solution (such as a mixture of sodium hydroxide and sodium carbonate, at 60-80°C). This removes the oil through a saponification reaction between the alkali and the oil (the oil becomes water-soluble soap and glycerin). This method is suitable for batch processing.
Solvent degreasing: If the oil is heavy or has a specific type (such as mineral oil), first wipe or soak the steel with an organic solvent (such as gasoline or alcohol) to quickly dissolve the oil before proceeding with alkaline washing to improve degreasing efficiency.
2.What are the common methods for pickling (rust/scale removal)?
Hydrochloric acid pickling: The most commonly used method involves immersing the steel in a 15%-20% hydrochloric acid solution (temperature 20-40°C). The hydrochloric acid reacts with scale and rust (e.g., Fe₂O₃ + 6HCl = 2FeCl₃ + 3H₂O), dissolving impurities. Hydrochloric acid pickling is fast and less corrosive to the steel substrate (but the pickling time must be controlled to avoid over-pickling).
Sulfuric acid pickling: If hydrochloric acid is more expensive, a 10%-15% sulfuric acid solution (temperature 60-80°C) can be used. However, sulfuric acid reacts more slowly and can easily cause "hydrogen embrittlement" on the steel surface (requiring the addition of a corrosion inhibitor, such as methenamine). Therefore, it is currently less commonly used than hydrochloric acid.
3.What are the commonly used processes for assisting plating (activating the surface and promoting zinc bonding)?
First, immerse the steel in a tank of clean water to rinse away any residual acid. Then, immerse it in the flux solution (typically a mixture of zinc chloride and ammonium chloride, 10%-20% in water, at 60-80°C) for 1-3 minutes before removing it.
The flux solution forms a thin film of zinc-ammonium complex on the steel surface, which not only isolates it from air and prevents oxidation, but also rapidly decomposes upon entering the zinc bath, preventing air bubbles and ensuring a continuous coating.
4.What are the steps for drying and putting into zinc pot?
Drying: The steel surface after flux plating contains moisture from the flux solution and must first be dried in a drying furnace (100-120°C). If moisture enters the zinc pot, it will instantly vaporize and explode upon contact with the high-temperature zinc solution, causing zinc splashing (a dangerous process) and creating bubbles that can affect the coating.
Entering the zinc pot: The dried steel is slowly immersed in the molten zinc solution. The zinc temperature must be strictly controlled between 445-460°C (zinc's melting point is approximately 419°C, which allows for good fluidity and a full reaction). During immersion, ensure that the steel is completely submerged in the zinc solution and avoid contact with the bottom or sides of the zinc pot (to prevent the ingress of impurities).
5.How to control zinc dipping time and temperature?
Galvanizing time: This should be adjusted based on the thickness of the steel. Thin items (such as wire and sheet metal) require 1-3 minutes of galvanizing (a rapid reaction can lead to an excessively thick alloy layer and brittle coating). Thick items (such as pipes and steel sections) require 3-10 minutes of galvanizing to ensure the zinc solution fully penetrates corners and avoids leaks.
Temperature Effects: If the temperature is too low (<440°C), the zinc solution will have poor fluidity, and the coating will be prone to "sagging" (localized accumulation). If the temperature is too high (>470°C), the iron and zinc react too quickly, resulting in an excessively thick alloy layer (even exceeding the pure zinc layer), making the coating hard and brittle, and prone to cracking (e.g., the coating will fall off when bent).