English
## I. Two Major Categories of Hot-dip Galvanizing Processes
Principle of Hot-Dip Galvanizing: Pretreated steel workpieces (cleaned and activated) are immersed in molten zinc liquid. Iron-zinc alloy coatings form via iron-zinc diffusion reactions. The process falls into two types: continuous hot-dip galvanizing and batch hot-dip galvanizing.
### (I) Continuous Hot-dip Galvanizing (High-speed Mass Production on Assembly Lines; Products Include Steel Strips, Steel Tubes and Steel Wires)
1. Continuous Galvanizing of Steel Strips
(1) Four Mainstream Processes Classified by Pretreatment: Modified Sendzimir Process, United States Steel Process, Cook-Nortemen Process and Selas Process. Their core differences lie in the arrangement of pre-cleaning and annealing procedures, with respective advantages and disadvantages in production line length, operating speed, acid resistance issues and applicable steel types.
(2) Coating Thickness Control Technology: The traditional roll coating method limits production speed and delivers poor coating thickness precision. The upgraded gas jet (air knife) method produces uniform, adjustable-thickness coatings compatible with high-speed production lines and automatic control. Finished galvanized steel strips feature excellent ductility and stamping formability.
2. Continuous Galvanizing of Steel Tubes: Two pretreatment procedures (solvent method and hydrogen reduction method) are adopted. Gear conveying and internal-external air blowing for zinc liquid recovery enable uniform galvanization on both inner and outer tube walls.
3. Continuous Galvanizing of Steel Wires: Low-carbon steel wires undergo recrystallization annealing to eliminate work hardening, while medium and high-carbon steel wires skip annealing to retain high tensile strength. Coating thickness is determined by wire withdrawal posture: vertically discharged wires gain thick, uniform coatings, whereas obliquely discharged wires form thinner coatings with poorer uniformity. The mechanical properties of finished wires are constrained by carbon content as well as annealing and galvanizing temperatures.
### (II) Batch Hot-dip Galvanizing (Separate or Scattered Workpieces Processed in Batches)
1. Basic Procedures: Degreasing & Rust Removal → Water Washing → Solvent Fluxing → Galvanizing → Water Cooling → Passivation. Divided into dry process (the industry mainstream, featuring low zinc slag generation and strong coating adhesion) and wet process based on fluxing methods.
2. Application Advantages: Applicable to workpieces ranging from screws to large steel structures. Double dipping or progressive dipping enables processing of oversized components. Complex cavity workpieces with vent holes can obtain full zinc coverage on inner cavities. Thicker workpiece walls, extended dipping time and sandblasting pretreatment all lead to increased coating thickness.
## II. Comprehensive Performance of Hot-dip Galvanized Coatings
1. Corrosion Resistance
(1) Conventional corrosion resistance follows the performance rules of zinc coatings mentioned above. Fully iron-zinc alloy coatings with dull appearances do not compromise anti-corrosion service life. Red spots on coatings of high-silicon steel only affect surface appearance without impairing substrate corrosion protection.
(2) Maximum Heat Resistance Temperature: Stable long-term corrosion resistance below 200°C. When temperatures exceed 200°C, the surface pure zinc layer peels off, yet the underlying alloy layer still protects the steel substrate.
2. Adhesion and Machinability: Metallurgical bonding between iron and zinc guarantees strong coating adhesion. Excessively thick coatings form brittle alloy phases prone to cracking and peeling under bending. For stamping materials, aluminum and nickel are added to molten zinc to suppress the growth of brittle phases.
3. Hardness and Wear Resistance: Iron-zinc alloy phases boost surface hardness and deliver outstanding wear resistance.
4. Weldability: Spot welding and seam welding are feasible. Spot welding strength is close to that of low-carbon steel, while welding current needs to be increased by 10%~15%.
## III. Overall Technical Advantages of Hot-dip Galvanizing
1. Long Service Life and Low Maintenance Costs: Service life reaches 20–40 years in industrial and marine environments, and 50–100 years in clean ordinary atmospheres with nearly zero long-term maintenance requirements.
2. Triple Anti-corrosion System: Physical isolation blocks corrosive media; corrosion products seal coating gaps to slow corrosion rates; zinc acts as a sacrificial anode to provide cathodic electrochemical protection when coatings are damaged.
3. Complete and Firm Coverage: Metallurgical bonding resists collision damage and coating peeling during machining, fully covering structural gaps and inner blind cavities of components.
4. Convenient Quality Control: The process is easy to regulate. Coating thickness compliance can be quickly verified via visual inspection and magnetic thickness gauges.
## IV. Industrial Practical Applications
1. Application Scenarios by Industry
Transportation: Highway guardrails, traffic signs, street light poles, bridge steel structures, vehicle body sheets, subway structures.
Construction Industry: Steel structural components, scaffolding, roof enclosures, water supply and drainage pipes, fences and louvers.
Power and Telecommunications: Transmission towers, microwave towers, substation components, cable casings.
Petrochemical Industry: Oil transmission pipelines, heat exchange equipment, equipment housings.
General Machinery: Home appliance casings, instrument cabinets, solar support frames, chemical plant structural frames.
2. Classic Engineering Cases
Sydney Opera House: Under marine corrosive conditions, galvanized steel rebars combined with surface concrete provide long-term resistance to salt spray erosion.
Stainsby Hall Bridge in the United States: The galvanized steel girders required zero maintenance for 25 years after completion, drastically cutting operation and maintenance expenses.
The Sydney Olympic Tennis Stadium, various urban transportation infrastructure and petrochemical station components all rely on galvanization for long-term corrosion protection.