A detailed explanation of hot-dip galvanizing and electrogalvanizing (cold galvanizing)

The full name of hot-dip coating is hot dip coating, which refers to the method of immersing the coated metal material into other liquid metals or alloys with lower melting points for coating. The main substrate material for hot-dip coating is steel, so the melting point of the metal in the coating material must be much lower than that of steel, otherwise it will affect the substrate material.

Coating material
Common coating metals include zinc (melting point 419.5 ℃), aluminum (melting point 658.7 ℃), tin (melting point 231.9 ℃), and lead (melting point 327.4 ℃).

Hot-dip galvanizing, also known as galvanizing, is a method of obtaining a metal coating on steel components by immersing them in molten zinc.

The process of hot-dip galvanizing

This process involves the formation of an iron-zinc alloy between the iron substrate and the outermost pure zinc layer. The formation of this iron-zinc alloy layer on the surface of the steel component during hot-dip galvanizing ensures a strong bond between the iron and the pure zinc layer. The process can be briefly described as follows:

When the iron workpiece is immersed in molten zinc, a solid solution of zinc and α-iron (body-centered cubic) is first formed at the interface. This is a crystal formed by the dissolution of zinc atoms in the solid state of the base metal iron; the two metal atoms are fused together, and the attractive force between the atoms is relatively small.

Therefore, when the zinc reaches saturation in the solid solution, the atoms of the two elements, zinc and iron, diffuse into each other. Zinc atoms that diffuse into the iron substrate migrate in the base lattice, gradually forming an alloy with iron.

The iron that diffuses into the molten zinc forms the intermetallic compound FeZn13 with zinc, which settles at the bottom of the hot-dip galvanizing pot, forming zinc dross.

When the steel component is removed from the molten zinc, a pure zinc layer is formed on the surface. This layer is a hexagonal crystal with an iron content of no more than 0.003%.

The actual process of galvanizing the surface of steel pipes (taking steel pipes as an example)

Solid iron dissolves ->Iron combines with zinc to form an iron zinc alloy compound, producing an iron zinc alloy layer ->The outer side of the iron zinc alloy layer is connected to a pure zinc layer. After cooling, the pure zinc layer crystallizes and is connected to the steel substrate on the inner side of the zinc layer. Therefore, the hot-dip galvanizing process is mainly the process of forming a galvanized layer due to diffusion.

The protective effect of the galvanized coating on the steel surface

Complete coverage, long-lasting protection
When the galvanized layer is intact, the corrosion rate of zinc itself is extremely slow, which can extend the service life of steel by 15-30 times. The galvanized layer is metallurgically bonded to the substrate, covering comprehensively and firmly, and can resist mechanical damage.

Sacrifice anode protection when there is partial damage
If the coating is partially damaged, the exposed steel base and zinc will form a primary battery in a humid environment. Zinc, as the anode (with a more negative potential), preferentially corrodes to protect the steel as the cathode, which is known as "sacrificial anode" protection.

Adhesion is more important than thickness
The galvanized layer often corrodes from the weak joint. If the adhesion is poor, even if the coating is thick, it will corrode and fail early, similar to the "short board effect of wooden barrels". Therefore, overall adhesion is a key factor affecting the protection life.

The hot-dip galvanized coating provides the steel substrate with the following three-fold corrosion protection:

The hot-dip galvanized layer can provide the following triple anti-corrosion protection for the steel substrate:

Isolation layer protection
The two important properties of the isolation protective layer are its adhesion to metals and its own wear resistance. In this regard, hot-dip galvanizing provides a hard, metal bonded isolation layer that can completely cover the surface of steel and isolate it from corrosive environments.

Protection of corrosion product layer
The corrosion products formed on the surface of hot-dip galvanized layer will cause volume expansion, blocking the discontinuous gaps caused by selective dissolution of the coating, thereby hindering further corrosion of the coating and reducing the corrosion rate of the galvanized layer in environmental corrosive media.

Electrochemical protection
For any small areas exposed by accidental damage, such as bumps or scratches, due to the negative potential of zinc compared to iron, the hot-dip galvanized layer is preferentially corroded as a sacrificial anode, providing cathodic protection for steel.

Advantages of the Hot-Dip Galvanizing Process

Compared with other metal anti-corrosion methods, hot-dip galvanizing has unparalleled advantages in terms of electrochemical protection, coating density, coating durability, coating maintenance free, adaptability to the shape and size of steel parts, and high production efficiency.

Hot-dip galvanized steel has advantages such as formability, weldability, paintability, and good ductility. Hot-dip galvanizing technology is increasingly developing towards large-scale, low-cost production.

Galvanized Layer Thickness and Lifespan

Since the lifespan of the galvanized layer mainly depends on its thickness, it is easy to visually inspect its surface for continuity and brightness, and a magnetic thickness gauge can be used to conveniently and accurately measure whether the thickness meets the standard requirements.

Atmospheric exposure tests show that a hot-dip galvanized layer with a thickness of 86 μm has a service life of 13 years in heavy industrial areas, 50 years in marine environments, 104 years in suburban areas, and 30 years in urban areas.

Generally, a galvanized layer with a zinc coating of 600 g/m² has a maintenance-free service life of 20-50 years. For ungalvanized color-coated steel sheets used in construction, the service life is 3-5 years, while color-coated steel sheets with a galvanized substrate can last 20-30 years.

Electrogalvanizing, commonly known as "cold galvanizing" or "water galvanizing"; It uses electrochemical methods to use zinc ingots as anodes, where zinc atoms lose electrons and dissolve into the electrolyte in an ionic state. Steel materials act as cathodes, where zinc ions are reduced to zinc atoms and deposited on the surface of the steel, forming a uniform, dense, and well bonded metal or alloy deposition layer.

Zinc Electroplating Process Flow (Silver-white / Blue-white)

Different Coating Thickness

Hot-dip galvanizing generally produces a thicker zinc layer, typically above 40 μm, and even up to 200 μm or more. The hot-dip galvanized layer is generally 10 to 20 times thicker than the electrogalvanized layer.

Electrogalvanized coatings are very thin, approximately 3–15 μm, with a coating weight of only 10–50 g/m².

Different Zinc Coating Weight

The zinc coating weight of hot-dip galvanized sheets cannot be too low, generally a minimum of 50–60 g/m² on both sides, and a maximum of 600 g/m². Electrogalvanized sheets can have a very thin zinc layer, with a minimum of 15 g/m², but achieving a thicker coating requires a very slow production line speed, which is not suitable for the process characteristics of modern production lines. The maximum is generally 100 g/m². Because of this, the production of electrogalvanized sheets is significantly limited.

Different Coating Microstructure

Hot-dip galvanized sheets have a slightly brittle compound layer between the pure zinc coating and the steel substrate. The pure zinc coating mostly forms zinc spangles during crystallization, and the coating is uniform and non-porous.

In electrogalvanizing, zinc atoms are only deposited and precipitated on the surface of the steel sheet, and are attached to the steel strip surface by physical forces. There are many pores, making it highly susceptible to pitting corrosion caused by corrosive media. Therefore, hot-dip galvanized sheets are more corrosion-resistant than electrogalvanized sheets.

Different Heat Treatment Processes

Hot-dip galvanized sheets generally use cold-rolled sheets as raw materials. The annealing and hot-dip galvanizing are carried out continuously on the galvanizing line. The steel strip is heated and then cooled in a short period, so its strength and plasticity are affected to some extent. Its stamping performance is inferior to that of similar cold-rolled sheets that have undergone degreasing and annealing on a professional production line.

Electro-galvanized sheets use cold-rolled sheets as raw materials, basically ensuring the same processing performance as cold-rolled sheets. However, its complex process also increases production costs.

Different Appearance

The surface of the hot-dip galvanized layer is rough and bright, and in severe cases, there are zinc spangles;

The electro-galvanized layer is smooth and dull (dirty).

Different scope of application and process

Hot dip galvanizing is suitable for large components and equipment; Hot dip galvanized steel sheet is first pickled to remove iron oxide on the surface of the steel pipe. After pickling, it is cleaned in an ammonium chloride or zinc chloride aqueous solution or a mixed ammonium chloride and zinc chloride aqueous solution tank, and then sent to the hot-dip plating tank.

Electrogalvanizing is suitable for small components. Using electroplating solution, apply positive and negative electrodes to the steel plate and electroplating solution respectively.

review

Hot dip galvanizing has good coverage ability, dense coating, and no impurities. It has the advantages of uniform coating, strong adhesion, and long service life. Hot dip galvanizing has better resistance to atmospheric corrosion than electroplating galvanizing on the base metal iron.

The electroplating method for manufacturing galvanized steel sheets has good processing performance, but the coating is thinner and the corrosion resistance is not as good as hot-dip galvanized steel sheets; The amount of zinc attached to electroplated galvanized sheet is very small, only galvanized on the outer pipe wall, while hot-dip coating is plated on both the inside and outside.