Hot-dip galvanizing technology and processes, galvanizing technology

1. Brief description
Hot-dip galvanizing, also known as hot-galvanizing, is a process in which steel prefabricated components are immersed in molten zinc to form a metallic coating. In recent years, with the rapid development of high-voltage power transmission, transportation, and telecommunications, the requirements for the corrosion protection of steel components have become increasingly stringent, and consequently, the demand for hot-dip galvanizing has continued to rise.
2. Safety and Protective Characteristics of Hot-Dip Galvanized Coating
Generally, the thickness of a phosphating treatment layer ranges from 5 to 15 μm, whereas the thickness of a hot-dip galvanized layer typically exceeds 65 μm, and can even reach up to 100 μm. Hot-dip galvanizing offers excellent coverage, with a dense coating free of any organic impurities. As we all know, zinc's resistance to atmospheric corrosion relies on both mechanical protection and electrochemical protection. Under atmospheric corrosion conditions, the surface of the zinc layer forms a protective film composed of ZnO, Zn(OH)2, and basic zinc carbonate, which to some extent slows down the corrosion of zinc. When this protective film—also known as "white rust"—is damaged, it will naturally reform a new protective layer. However, once the zinc layer is severely damaged to the point where the iron substrate is significantly exposed, zinc begins to provide electrochemical protection for the substrate. With a standard electrode potential of -0.76 V for zinc and -0.44 V for iron, when zinc and iron form a micro-battery, zinc acts as the anode and dissolves, while iron serves as the cathode and remains protected. Clearly, hot-dip galvanizing provides better resistance to atmospheric corrosion for the underlying ferrous metal than phosphating does.
3. The entire process of hot-dip galvanizing layer formation
　The entire process of hot-dip galvanizing involves the formation of an iron-zinc alloy layer between the iron substrate and the outermost pure zinc layer. During hot-dip coating, an iron-zinc alloy layer forms on the surface of the workpiece, enabling excellent bonding between the iron substrate and the pure zinc layer. This process can be simply described as follows: When the iron workpiece is immersed in molten zinc, a solid solution of zinc in α-iron (body-centered cubic structure) initially forms on the surface. This is a crystalline phase formed by the dissolution of zinc atoms into the solid iron matrix. The bond between the two metal atoms is relatively weak, with only modest attractive forces acting between them. Once the zinc concentration in the solid solution reaches saturation, zinc and iron atoms begin to diffuse mutually. Zinc atoms that have diffused into the iron substrate rearrange themselves within the substrate’s crystal lattice and gradually form an alloy with the iron. Meanwhile, iron atoms that have diffused into the molten zinc react with zinc to form the intermetallic compound FeZn13, which settles at the bottom of the galvanizing bath as zinc slag. When the workpiece is removed from the zinc bath, a pure zinc layer—characterized by a hexagonal crystal structure—forms on its surface. The iron content in this pure zinc layer is no more than 0.003%.
4. The entire process and related documentation of hot-dip galvanizing technology

4.1 The entire processing procedure
Workpiece → Degreasing → Water Cleaning → Acid Pickling and Passivation → Water Cleaning → Immersion in Organic Solvent for Pre-coating Assistance → Air Drying and Heating → Hot-Dip Galvanizing → Combing → Cooling → Passivation Treatment → Rinse Bath → Drying → Inspection
4.2 The entire process of related manufacturing technologies indicates
(1) Degreasing
Oil removal can be performed using organic chemical degreasers or water-based metal material degreasing cleaners, ensuring that the workpiece is thoroughly wetted by water. (2) Acid pickling and passivation: Either a solution of 15% H2SO4 and 0.1% thiourea at 40–60℃, or a solution of 20% HCl and 1–4 g/L of hexamethylenetetramine at 20–40℃ can be used for acid pickling and passivation. The addition of a desulfurizing agent can prevent over-etching of the substrate and reduce hydrogen absorption by the iron matrix.
　If degreasing and acid pickling/passivation are not properly addressed, the adhesion of the coating may be poor, making it difficult to apply zinc or causing the zinc layer to peel off.
(3) Immersion Plating Agent
Also known as a flux, this agent helps maintain certain specific properties on the workpiece prior to immersion plating, thereby enhancing the adhesion between the coating and the substrate. Composition: NH5Cl 15%–25%, ZnCl2 2.5%–3.5%, temperature 55–65℃, duration 5–10 minutes. To reduce the evaporation of NH5Cl, a moderate amount of petroleum jelly can be added.
(4) Air-drying and heating
To better prevent workpieces from deforming due to a significant temperature rise during immersion plating and to remove residual moisture, thereby avoiding zinc explosions that could cause the zinc bath to spatter, the heating temperature is typically maintained between 120°C and 180°C.
(5) Hot-dip galvanizing
　It is crucial to carefully control the temperature of the zinc bath, the dipping time, and the rate at which the workpiece is removed from the zinc bath. If the temperature is too low, the zinc bath will have poor fluidity, resulting in a thick and uneven coating that is prone to dullness and poor surface quality. On the other hand, if the temperature is too high, the zinc bath will flow more readily, making it easier for the zinc to detach from the workpiece, thereby reducing dullness and skinning defects. This also leads to stronger adhesion, thinner coatings, superior surface appearance, and higher production efficiency. However, excessively high temperatures can cause severe iron loss in both the workpiece and the zinc pot, generating large amounts of zinc dross, compromising the quality of the zinc coating, increasing zinc consumption, and even rendering the plating process unfeasible. At the same temperature, a longer dipping time will result in a thicker coating. When aiming for the same coating thickness at different temperatures, higher-temperature dipping will require a longer dipping time. Typically, manufacturers choose a temperature range of 450–470°C and a dipping time of 0.5–1.5 minutes to better prevent high-temperature deformation of the workpiece and reduce zinc dross formation caused by iron loss. Some factories use higher temperatures for large workpieces and cast iron parts, but they carefully avoid temperature ranges where iron loss peaks. To improve the fluidity of the hot-dip galvanizing bath at lower temperatures, prevent excessive coating thickness, and enhance the surface appearance of the coating, 0.01% to 0.02% of pure aluminum is often added. The aluminum should be added in small, frequent doses.
(6) Sort out
After plating, the primary purpose of brushing the workpiece is to remove excess zinc and zinc nodules from the surface; this can be done either by vibration or manual methods.
(7) Passivation treatment
The purpose is to enhance the resistance of the workpiece surface to atmospheric corrosion, reduce or extend the time before white rust appears, and maintain an excellent appearance of the coating. All are treated with chromic oxide passivation, using a solution containing 80–100 g/L of Na2Cr2O7 and 3–2 ml/L of hydrochloric acid.
(8) Refrigeration
Tap water is generally cold, but its temperature must not be too low, as excessively rapid cooling can cause thermal shock and lead to cracking in the matrix structure of workpieces—especially castings.
(9) Testing
The coating exhibits a bright, fine surface with no signs of dripping or wrinkling. For thickness measurement, a coating thickness gauge can be used—a method that is relatively simple. Alternatively, the coating thickness can also be calculated based on the amount of zinc adhesion. To assess the coating’s tensile strength, a bending tester can be employed: bend the sample at an angle of 90° to 180°; the coating should show no cracks or flaking. Another method involves using a heavy hammer to perform impact testing.
5. Generation and Handling of Zinc Dust and Zinc Slag
5.1 The Formation of Zinc Dross and Zinc Slag: Zinc dross and zinc slag not only seriously compromise the quality of the zinc-plated layer, leading to an uneven coating and the formation of zinc nodules, but also significantly increase the cost of hot-dip galvanizing. Typically, each ton of workpiece plated consumes between 80 and 125 kg of zinc; however, if zinc dross and slag accumulation is severe, zinc consumption can rise to as much as 140 to 200 kg. To control zinc carbon formation, it’s crucial to maintain proper temperature control and minimize surface oxidation of the zinc bath, which gives rise to sludge and dross. Some Chinese manufacturers use fire-resistant sand or carbon ash for covering purposes. Abroad, ceramic or glass beads—materials with low thermal conductivity, high melting points, low specific gravity, and no chemical reaction with the zinc bath—are employed as coverings. These spherical particles are easily pulled away by the workpieces and do not adhere to them at all.
　The formation of zinc dross in the molten zinc bath is primarily caused by a deviation in fluidity that occurs when the iron content dissolved in the zinc bath exceeds its solubility at the given temperature. The zinc content in the zinc dross can reach as high as 95%, and this is the root cause of the increased cost of hot-dip galvanizing. As shown by the solubility curve of iron in molten zinc, the amount of iron dissolved—that is, the iron loss—varies depending on both the temperature and the duration of thermal insulation. Around 500°C, the iron loss increases significantly with rising temperature and longer insulation time, exhibiting an almost linear relationship. Below or above the range of 480–510°C, the increase in iron loss with time slows down considerably. Consequently, the temperature range of 480–510°C is referred to as the "worsening dissolution zone." Within this temperature range, the molten zinc exhibits more severe corrosion of both the workpiece and the zinc bath. Above 560°C, the iron loss continues to rise sharply, and once the temperature reaches over 660°C, the zinc causes devastating corrosion of the iron substrate, leading to a dramatic increase in zinc dross and making further plating impossible. Therefore, plating operations are currently carried out predominantly within two temperature ranges: 450–480°C and 520–560°C.
5.2 Control of Zinc Slag Quantity
To reduce zinc slag, it is necessary to lower the iron content in the zinc melt—this means starting with the factors that influence iron dissolution:
⑴ During plating and thermal insulation, avoid the iron’s melting peak range—specifically, do not carry out operations when the temperature is between 480°C and 510°C.
⑵ The raw materials for the zinc pot should, as far as possible, be thick steel plates welded by electric arc welding, with low carbon and silicon contents. High carbon content accelerates the corrosion of the frying pan by the molten zinc, while high silicon content promotes the erosion of iron by the molten zinc. At present, 08F high-quality alloy steel plates are widely used. These plates have a carbon content of 0.087% (ranging from 0.05% to 0.11%), a silicon content of no more than 0.03%, and contain elements such as nickel and chromium that can inhibit iron corrosion. Carbon structural steel must not be used, as it would lead to excessive zinc consumption and a shorter service life for the zinc pot. Some people have also proposed using carbon-carbon composites to fabricate molten-zinc tanks. Although this approach can reduce iron loss, the design and manufacturing process itself presents significant challenges.
⑶ Regularly skim off the dross. First, raise the temperature to the upper limit of the processing temperature range to facilitate separation of the zinc dross from the molten zinc. Then, lower the temperature below the processing temperature range so that the zinc dross settles to the bottom of the tank and can be scooped out using a ladle. Parts immersed in the molten zinc should also be promptly removed.
⑷ To prevent iron compounds from the flux agent from being carried into the zinc bath along with the workpieces, the flux agent should be filtered regularly once it has been used for a certain period, as light-brown iron-containing compounds will form over time. It is best to maintain the pH of the flux agent around 5.
⑸ When the aluminum content in the plating bath falls below 0.01%, the formation of residues will accelerate. Proper amounts of aluminum not only enhance the fluidity of the zinc bath and improve the color tone of the coating, but also help reduce zinc dross and zinc ash. A small amount of aluminum floating on the liquid surface is beneficial for minimizing oxidation; however, too much aluminum can compromise coating quality and lead to pinholes.
⑹ Heating and temperature increase should be uniform to avoid overheating and localized overtemperature.