Differences Between Electrogalvanizing (Cold Galvanizing) and Hot-Dip Galvanizing

2025-03-06 13:12:34

The protection mechanism of zinc against corrosion operates through two main principles: mechanical protection and electrochemical protection. When exposed to corrosion, the zinc layer forms a protective film on the surface, consisting of basic zinc carbonate, ZnO, and Zn(OH)₂, which helps reduce the degree of corrosion. This protective film, commonly referred to as "white rust," can regenerate if damaged. If the zinc coating is severely damaged, exposing the steel substrate to corrosion, zinc will provide electrochemical protection to the steel. Since zinc has a standard electrode potential of -0.76V, while iron has -0.44V, zinc acts as the anode in a galvanic cell and dissolves preferentially, while iron remains as the cathode and is protected.

1. Fundamental Differences

• Hot-dip galvanizing (HDG), also known as hot galvanizing, is a method where steel components are immersed in molten zinc to form a zinc coating. The process involves degreasing, pickling, fluxing, drying, and then dipping the steel into molten zinc for a specific time before removal.

  
  

  
  

• Electrogalvanizing (EG), also known as cold galvanizing, uses electrochemical deposition to apply zinc to the workpiece. After degreasing and pickling, the workpiece is placed in a solution containing zinc salts and connected to the cathode of an electrolytic system. A zinc plate is placed on the anode, and when an electric current passes through, zinc ions migrate to the workpiece, gradually forming a zinc layer on the surface.

2. Appearance Differences

• Hot-dip galvanized coatings are thicker, and the surface may have air bubbles, giving it a duller and slightly rough texture compared to electrogalvanizing. The coating usually appears silver-white and may show flow marks or small zinc lumps. Hot-dip galvanized pipes generally have a blue ink ring at both ends and include markings for standards and specifications.

• Electrogalvanized coatings are smoother, brighter, and more uniform. If a color passivation process is used, the coating appears mainly yellow-green with a rainbow-like sheen. White passivation produces a bluish-white or white layer with a slight greenish tint, which may also show a rainbow effect under certain angles of sunlight. Electrogalvanized pipes have smooth ends, lack zinc lumps, and do not display standard markings on the outer surface.

3. Zinc Coating Thickness Differences

• Hot-dip galvanizing results in a thicker zinc layer, typically 30-60 microns.

• Electrogalvanizing produces a thinner zinc layer, usually 5-30 microns.