I. Pretreatment of raw materials: Laying the foundation for the adhesion of the galvanized layer
Raw material pretreatment is the primary step in the production of galvanized wire. Its core objective is to remove impurities from the surface of the steel wire or iron wire to ensure that the subsequent galvanized layer can adhere closely. This stage consists of three key steps: pickling, water washing and flux plating. Each step requires precise control of parameters.
The pickling process mainly aims to remove the oxide scale, rust and oil stains on the surface of the steel wire. During the rolling process of steel wire, an oxide layer will form. If it is not thoroughly removed, it will directly lead to defects such as bubbles and peeling of the galvanized layer. Usually, a 15%-20% hydrochloric acid solution is used as the pickling medium, with the temperature controlled at 40-60℃. The soaking time is adjusted according to the degree of rust on the surface of the steel wire, generally ranging from 30 to 60 minutes. To prevent excessive acid washing from corroding the steel wire matrix, corrosion inhibitors (such as ureopine) should be added to the acid solution at a ratio of 0.5% to 1%, which can control the corrosion rate of the steel wire within 0.1g/m² · h. After pickling is completed, there will be residual acid on the surface of the steel wire, which needs to be immediately transferred to the water washing process.
The water washing process is divided into two steps: cold water washing and hot water washing. Cold water washing involves rinsing with running water to remove the residual acid on the surface. The water temperature should be maintained at room temperature, and the rinsing time should be no less than 3 minutes to ensure that the pH value reaches the neutral range of 6-7. Hot water washing should use water at 80-90℃, which can not only further remove residual impurities but also preheat the steel wire, creating temperature conditions for the subsequent electroplating process. After hot water washing, the steel wire needs to be dried by hot air to completely evaporate the moisture on its surface, avoiding the moisture from being brought into the flux solution and affecting the stability of its composition.
Flux plating is the final step of pretreatment. By forming a uniform protective film on the surface of the steel wire, it prevents secondary oxidation before entering the zinc bath. The flux solution is usually prepared by mixing zinc chloride and ammonium chloride in a ratio of 3:1, and the solution temperature is controlled at 60-80℃. After the steel wire is immersed in the flux solution, a thin layer of chloride film will form on its surface. This film not only isolates the air but also promotes the fusion of the zinc bath and the steel wire substrate during the subsequent galvanizing process, significantly enhancing the adhesion of the galvanized layer.
Ii. Implementation of Galvanizing Process: The core step determining anti-corrosion performance
Galvanizing process is the most crucial step in the production of galvanized wire. Currently, the industry mainly adopts two processes: hot-dip galvanizing and electro-galvanizing. There are significant differences between the two in terms of principle, equipment and performance.
Hot-dip galvanizing process (also known as hot-dip galvanizing) involves immersing pre-treated steel wire into molten zinc liquid to form a zinc-iron alloy layer and a pure zinc layer on the surface of the steel wire. The key point of the process lies in the control of the zinc bath temperature, which usually needs to be maintained at 440-460℃. This temperature range can ensure that the zinc bath maintains good fluidity and at the same time prevent the steel wire matrix from being overly softened due to high temperature. The time for the steel wire to be immersed in the zinc bath needs to be adjusted according to its diameter. For fine steel wires with a diameter of 1-3mm, the immersion time should be 3-5 seconds, while for thick steel wires with a diameter of 8-12mm, it should be extended to 8-12 seconds to ensure that the zinc bath can fully penetrate into the tiny pores on the surface of the steel wire. The thickness of the hot-dip galvanized coating is usually between 60 and 150μm. Its structure, from the inside out, consists of a zinc-iron alloy layer (accounting for approximately 10% to 20% of the total thickness) and a pure zinc layer. This double-layer structure endows the hot-dip galvanized wire with extremely strong anti-corrosion ability, enabling it to withstand 10 to 15 years of corrosion in outdoor environments.
The electro-galvanizing process (also known as cold galvanizing) utilizes the principle of electrolysis to deposit a layer of pure zinc on the surface of steel wire. The core equipment is an electrolytic cell, where the electrolyte is composed of zinc oxide, zinc sulfate and boric acid. The pH value is controlled between 3.5 and 5.0, and the temperature is maintained at 20 to 40 degrees Celsius. Steel wire is connected to the circuit as the cathode, and zinc plate serves as the anode. When current passes through, zinc ions gain electrons on the surface of the steel wire and are reduced to metallic zinc, forming a galvanized layer. The thickness of the electro-galvanized coating is relatively thin, usually ranging from 5 to 30μm, and the uniformity of the coating is higher with good surface gloss. However, due to the lack of a zinc-iron alloy layer, its adhesion and corrosion resistance are weaker than those of hot-dip galvanized wire. It is generally suitable for indoor dry environments and has a service life of 3 to 5 years.
Whether it is hot-dip galvanizing or electro-galvanizing, the tension of the steel wire needs to be controlled after galvanizing. A stable pulling force is applied through a traction device to prevent the steel wire from bending or deforming during the cooling process, ensuring that the straightness error of the finished product does not exceed 0.5mm/m.
Iii. Post-treatment process: Enhance product stability and applicability
The post-treatment process is the final step in the production of galvanized wire, mainly including cooling, passivation, stretching and straightening, and inspection and grading. These steps directly affect the storage stability and ease of use of the product.
The cooling process is particularly crucial for hot-dip galvanized wire. The temperature of the steel wire just taken out of the zinc bath can reach over 400℃, and it needs to undergo staged cooling treatment: first, the temperature is reduced to around 200℃ through air cooling, and then it is rapidly cooled to room temperature in a water cooling tank. This gradient cooling method can prevent the steel wire from generating internal stress due to sudden cooling and avoid cracks in the galvanized layer. However, electro-galvanized wire, due to its relatively low temperature after plating, only needs to be naturally cooled.
Passivation treatment is a key supplement to enhance the corrosion resistance of galvanized coatings. By forming a passivation film on the surface of the galvanized coating, it isolates zinc from direct contact with air and moisture. Commonly used passivation solutions include chromate passivation solutions (hexavalent chromium or trivalent chromium) and chromium-free passivation solutions (such as silane treatment agents). Chromate passivation can form a dense oxide film, enhancing salt spray resistance by 3 to 5 times. However, due to environmental protection requirements, the industry is gradually shifting towards chromium-free passivation technology, and the salt spray resistance time after treatment can reach 24 to 48 hours. The passivated steel wire needs to be dried again to ensure that there is no residual liquid on the surface.
The stretching and straightening process mainly aims to correct products with diameter deviations and excessive straightness. By using a drawing die to slightly stretch the steel wire (with an elongation controlled at 1%-3%), it can not only adjust the diameter accuracy to ±0.02mm but also improve the straightness. Finally, the inspectors will use an eddy current flaw detector to check the continuity of the galvanized coating and A coating thickness gauge to measure the thickness. Each batch of products will be graded: For grade A products, the galvanized coating is required to have no missed plating or pinholes, and the thickness deviation should not exceed ±5μm. Grade B products are allowed to have a few pinholes, but they must meet the basic usage requirements.
Through this series of refined production processes, galvanized wire can possess excellent anti-corrosion performance and mechanical strength, meeting the usage requirements of different fields such as construction, agriculture, and animal husbandry. The strict control of every process parameter is the core guarantee for ensuring the stability of product quality.