HOT DIP GALVANISING

What is Hot Dip Galvanising (HDG)?

Hot-dip galvanizing (HDG) is the process of dipping fabricated steel into a kettle or vat containing molten zinc. The process is inherently simple which provides a distinct advantage over other corrosion protection methods. While the steel is in the kettle, the iron in the steel metallurgically reacts with the molten zinc to form a tightly-bonded alloy coating that provides superior corrosion protection to steel. durability, longevity, cost, and sustainability applies only to the hot-dip galvanizing. HDG is the best known method to protect steel.

At the end of the process the surface of the metal is coated with armor plate in micron level. This armor plate prevents the metal from rusting under many different atmospheric conditions. A product, which had hot-dip galvanization, can remain for many years without requiring any maintenance or repair.

Hot-dip galvanizing is used throughout various markets to provide steel with unmatched protection from the ravages of corrosion. Myriad applications of steel products in the harshest environments benefit from the use of hot-dip galvanizing. The uses of hot-dip galvanized steel continue to evolve, and new markets are emerging all the time. Similarly, the decision to galvanize has matured beyond the customary corrosion protection to an array of other reasons. Though corrosion resistance is inherent any time HDG is utilized, more and more specifiers select hot-dip galvanized steel for other reasons including lowest initial cost, durability, longevity, availability, versatility, sustainability, and aesthetics.

Hot Dip Galvanising (HDG) Coating
The coating that develops during the galvanizing process is metallurgically bonded to the steel – virtually becoming a part of the steel itself. During the reaction in the kettle, the zinc interacts with the iron in the steel to form a series of zinc-iron alloy layers. The photomicrograph below is a cross section of the galvanized steel coating, showing a typical microstructure comprised of three alloy layers and a layer of pure metallic zinc.

Steel is an abundant, efficient building material that provides specifiers design freedom. However, for projects exposed to the atmosphere and other harsh environments, it is critical to coat the steel for corrosion protection. Often large construction projects target a 50-100 year design life, highlighting the need fordurable, long lasting corrosion protection. There are countless examples demonstrating the proven protection of hot-dip galvanizing in some of the harshest environments.

The reason for the extensive use of hot-dip galvanizing (HDG) is the three-fold protective nature of the coating. As a barrier coating, it provides a tough, metallurgically-bonded zinc coating that completely covers the steel surface and seals the steel from the corrosive action of the environment. Additionally, zinc’s sacrificial behavior protects the steel, even where damage or a minor discontinuity in the coating occurs. Finally, the natural weathering of the coating results in the development of an additional layer of protection on the surface.

 

Hot Dip Galvanising (HDG) Process

The hot-dip galvanizing (HDG) process consists of three basic steps:

  1. Surface Preparation

  2. Galvanizing

  3. Inspection



Surface Preparation
Surface preparation is the most important step in the application of any coating. 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.

Surface preparation for galvanizing consists of three steps:

a. Degreasing/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.

b. Pickling
A dilute solution of 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.

c. 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.

Galvanising
During the true galvanizing step of the process, the material is completely immersed in a bath of molten zinc. The zinc kettle dimensions 7.5 * 3.2 * 1.8 Which operates at MICHA. The cranes are provided by the INGENIA automatic system which is necessary for the process.

The bath chemistry requires at least 99.99% pure zinc maintained at 435-455 C. Surface cleaning chemicals and zinc bath routinely analyzed and compliance with the standards is provided on laboratories at MICHA.

While immersed in the kettle, the zinc reacts with the iron in the steel to forma series of metallurgically bonded zinc-iron intermetallic alloy layers, commonly topped by a layer of impact-resistant pure zinc.

Once the fabricated items’ coating growth is complete, it is withdrawn slowly from the galvanizing bath, and the excess zinc is removed by vibrating. The metallurgical reaction will continue after the materials are withdrawn from the bath, as long as it remains near bath temperature. Galvanized articles are cooled either by immersion in a passivation solution or water or by being left in open air.

Inspection

At MICHA, Coating control is carried out in accordance standards (ASTM A123, EN ISO 1461). Tests made with micrometer or non-destructive testing measuring instruments and standards compliance are compared.

Why Hot Dip Galvanising (HDG)?

  • Low Cost

  • Low Service Cost

  • Longevity

  • Application Speed

  • Covering Toughness

  • 3 Way Protection

  • Quality Control Convenience
     

Application of Hot Dip Galvanized Steel

Unprotected steel corrosion is a major problem and galvanizing is an effective solution for  this problem. HDG is made in all applications and high quality at MICHA.

You can get more detailed information from us about galvanizing applications for protection against corrosion.

  • Power Generation and Power Transmission

  • Infrastructure Development

  • Telecommunication Towers

  • Building and Construction

  • Mininng

  • Oil and Gas Production

  • Agriculture and Animal Husbandry

  • Farming