Hot-dip galvanization is the process of coating iron and steel with a layer of zinc by immersing the metal in a bath of molten zinc at a temperature of 815-850 F (435-455 C). When exposed to the atmosphere, the pure zinc (Zn) reacts with oxygen (O₂) to form zinc oxide (ZnO), which further reacts with carbon dioxide (CO₂) to form zinc carbonate (ZnCO₃), a usually dull grey, fairly strong material that protects the steel underneath from further corrosion in many circumstances.

Galvanized steel is widely used in applications where corrosion resistance is needed without the cost of stainless steel, and can be identified by the crystallization patterning on the surface (often called a "spangle"). Like other corrosion protection systems, galvanizing protects steel by acting as a barrier between steel and the atmosphere. However, zinc is a more electronegative metal in comparison to steel. This is a unique characteristic for galvanizing, which means that when a galvanized coating is damaged and steel is exposed to the atmosphere, zinc can continue to protect steel through galvanic corrosion.

A. Surface preparation

Surface preparation is a critical step in the application of any coating. In most instances where a coating fails before the end of its expected service life, it is because of incorrect or inadequate surface preparation. The galvanizing process has its own built-in means of quality control because zinc will not react with an unclean steel surface. Any failures or inadequacies in surface preparation will be immediately apparent when the steel is withdrawn from the zinc bath because the unclean areas will remain uncoated, and immediate corrective action can be taken.

1. Caustic cleaning
   • A hot alkali solution, mild acidic bath, or biological cleaning bath removes organic contaminants such as dirt, paint markings, grease, and oil from the metal surface. Epoxies, vinyls, asphalt, or welding slag, which cannot be removed by degreasing, must be removed before galvanizing by grit-blasting, sand-blasting, or other mechanical means.
2. Rinsing to remove the caustic
3. Acid pickling (alternatively abrasive blasting)
   • A dilute solution of heated sulfuric acid or ambient hydrochloric acid removes mill scale and iron oxides (rust) from the steel surface. As an alternative to or in conjunction with pickling, this step can also be accomplished using abrasive cleaning or air blasting sand, metallic shot, or grit onto the steel.

4. Rinsing to remove the pickling acid
5. Fluxing
   • The final surface preparation step in the galvanizing process, a zinc ammonium chloride solution, serves two purposes. It removes any remaining oxides and deposits a protective layer on the steel to prevent any further oxides from forming on the surface prior to immersion in the molten zinc. The flux is allowed to dry on the steel and aids in the process of the liquid zinc wetting and adhering to the steel.

B. Application

1. Dipping in molten zinc bed
   • During the galvanizing step of the process, the material is completely immersed in a bath of molten zinc. The bath chemistry requires at least 98% pure zinc maintained at 815-850 ^oF (435-455 ^oC).
   • While immersed in the galvanizing kettle, the zinc reacts with the iron in the steel to form a series of metallurgically bonded zinc-iron intermetallic alloy layers, commonly topped by a layer of impact-resistant pure zinc.
   • The steel is held in the molten zinc bath and until the temperature of the steel equilibrates with that of the bath.
   • Once the fabricated items’ coating growth is complete, it is withdrawn slowly from the galvanizing bath, and the excess zinc is removed by draining, vibrating, and/or centrifuging.
   • The metallurgical reaction will continue after the materials are withdrawn from the bath, as long as it remains near bath temperature.

2. Cooling
   • Galvanized articles are cooled in a quench tank with a passivation solution to reduce its temperature and inhibit undesirable reactions of the newly formed coating with the atmosphere.

3. Inspection
   • Inspection of hot-dip galvanized steel is simple, because zinc will not react with unclean steel. Therefore, visual inspections are very accurate and easy to perform. Coating thickness is an important requirement as it directly relates to the effectiveness of hot-dip galvanizing as a corrosion protection system because zinc thickness is linear to the life of the coating. However, measuring coating thickness is only one of the requirements in the inspection process, other key items include adhesion, appearance, and finish.

C. Post treatment

1. Chromate solution
   • Uses a chromic acid (more commonly hexavalent chromium). Will passivate zinc to form pleasant sheen with improved corrosion resistance performance.
2. Phosphating
   • Uses a dilute solution of phosphoric acid and phosphate salts. Will form a layer of insoluble, crystalline phosphates on zinc surface. Improve corrosion resistance, lubricity.
3. Light rolling
   • Use centrifugal force to distribute wet zinc evenly on surface. Roll the article when it is taken away from zinc clave.
4. Roller levelling
   • Use a metal roller to level excessive molten zinc on surface.

Service life of zinc layer is directly linked to its thickness, so measuring the thickness of HDG can be critical. There are various ways to measure single spot reading and average DFT, such as:

• Magnetic gauge (single spot measurement)
• Weight by mass (average thickness):
  • Clean, weigh and dip a test piece in the molten zinc, let it cool and weigh again. Convert the increase in weight to grams zinc per square metre surface (g/m²). 1 g/m2 equals a zinc thickness of 0.14 micr, so if the weight increase was 750 g/m2, the hot dipped zinc thickness will be 105 micr. (750 x 0.14).

Prior to coating.

There are some important considerations that need to be done when inspecting the complete HDG (hot-dip galvanizing) process.

Construction and design.
Safety.

Any closed hollow structures must be ventilated by drilling holes to avoid explosion when immersion in zinc bath. Pockets, narrow crevice, gaps, pores and cavities in welding can acts as acid traps if not rinsed. Any residue will destroy the HDG coating over time time.

Thermal cutting heats the steel and produces surfaces that react differently when galvanized. The coating thickness may be lower than expected and the adherence to the steel surface may be poor. The cut surfaces should therefore be grinded to remove the heat affected zone before galvanizing.

The surface must be checked for contamination such as grease, oil, tar, paint, temporary marking and welding slags before the HDG process.

After hot-dip galvanizing coating.

Coating thickness.

Electromagnetic gauges are normally used for measuring the coating thickness. Mean thickness according to ISO 1461 is 85 µm, minimum 70 µm for > 6mm steel.

Surface appearance.
When the surface of a hot-dip galvanized object is studied visually, the surface should be smooth and free of visible faults such as blisters, barbs, zinc ash, flux residue and uncoated areas. Lumps, drops and thick runs are not permitted where they can affect the use of the hot-dip galvanized object. Any defects must be repaired. The HDG object should be clean and undamaged.

Adherence between coating and steel.
Measuring the adhesion on batch galvanizing objects are normally not necessary. However, on continuous galvanized products it might be important on object which are to be exposed for mechanical loads such as bending, turning or striking.

Storing and transport.

Objects should be stored and transported in a way that water and condensation can evaporate quickly. Wet-storage stain (white rust) can be formed if object is packed closely with narrow gaps.

Hot dip galvanizing and paint (duplex system).

A duplex system which consists of HDG followed by coating generally has a much longer service life than only coating by itself.

Sweep blasting as surface preparation method will give the best result regarding cleanliness and roughness. Use mineral blasting materials, nozzle pressure 0,2-0,3 MPa, with angel of impact 30-60 °. In this case about 10 µm of the zinc layer is removed. Assure that no more zinc than the top layer is removed to maintaing corrosion resistant properties.

It is recommended to apply the tie coat as soon as possible after sweep blasting, preferrably within 20 -25 minutes, but by latest 24 hours (providing acceptable climatic conditions and no exposure to humidity). Ambient condition requirement is maximum 50% RH and steel temperature minimum 3°C above dew point temperature.

Paint system on hot dip galvanized steel can be found in ISO 12944-5, table D.1