Introduction To Hot Dipped Galvanising
From
Bins-n-Benches

1.Hot Dip Galvanising is a modern and technically advanced product that protects iron and steel from rust by providing a thick metallic zinc envelope that completely covers the steel surface and seals it from the corrosive action of its environment. Its high quality zinc coating is welded to the steel surface by an alloying reaction between zinc to form a tough, durable and impervious coating to protect the steel by a remarkable electrochemical system. Electrochemical protection is sometimes known as sacrificial protection because where there is damage or minor discontinuity in the sealing coat of zinc, the zinc sacrifices itself to protect the steel to which it is alloyed. It will go on doing almost the last atom of zinc and makes sure that corrosion does not attack to steel that you fabricate or use. Unlike organic paint coatings that require frequent renewal, the galvanised coating cannot come of as it is alloyed to the surface of the steel, coating every surface of the product, inside and out. Although Hot Dip Galvanising is simple and effective in use, the metallurgy that goes on during the application process is quite complicated. The galvanised coating forms because iron and zinc react together to form an alloy. But the surface of fabricated steel is often contaminated with rust and oil, and alloying takes place only if the surface of the steel is clean enough to be wetted by liquid zinc. The Galvaniser cleans the steel by degreasing and pickling to obtain a chemically clean surface. The galvanising reaction between zinc and steel takes place in liquid zinc, usually at 445-460°C. At this temperature, iron and zinc react quickly. Typically, the steel may be in the zinc for only a few minutes. Fabrication is taken out of the zinc when the reaction is complete. Although the zinc coating has already formed, its internal structure continues to change until the steel returns to normal temperature. There is no cheating in Hot Dip Galvanising, as zinc will not alloy with the steel if surface contaminants are incompletely removed. In these areas, the alloying reaction cannot take immediate action and reprocess the work following a simple visual inspection. This abrasion resistant and aesthetically pleasing coating is then able to last for years and little or absolutely no maintenance is required. Galvanising does have a finite life and its effectiveness as a barrier depends directly on the thickness of the coating and the environment

2. Lowest long term cost.

3. The economic case for galvanising is still improving.

4. Long life. Even in cases where the initial cost of galvanising is higher than a comparable alternative, galvanising is almost invariably cheapest in the long term. There is a high labour cost component inherent in the application of most other coatings and their cost directly reflects increases in the cost of labour. The graph below shows relative cost trends since 1980. The standard 610gm² galvanised coating* has a typical life without maintenance of more than 50 years in rural environments and 20 to 25 years in urban and coastal conditions.

5. Reliability. The galvanised coating is bonded metallurgically to the base steel so that it becomes part of the steel surface it protects. Coating life and performance are reliable and predictable. * British Standard BS: 729-71 specifies a minimum coating mass of 610g/m² on steel over 5mm thick.

6. Coating toughness.

7. Elimination of maintenance for long periods.

8. Automatic protection for damaged areas. A galvanised coating has a unique metallurgical structure which gives outstanding resistance to mechanical damage to mechanical damage in transport, erection and service. The use of galvanising defers maintenance for very long periods compared to other coatings for steel. Remember, maintenance can be extremely expensive when it entails difficult access, or when structures are located in remote areas. Galvanised coatings corrode preferentially to steel, providing sacrificially or cathode protection to small areas of steel exposed through damaged to the coating. Unlike organic coatings no touch up is needed.

9. Complete Protection.

10. Ease of inspection.

11. Time savings in factory, and on site. Every part of galvanised article is fully protected, even recesses, sharp corners and inaccessible areas.No coating applied to a structure or fabrications after completion can provide the same protection. Galvanised coating thickness is controlled automatically by steel thickness and a standard, minimum coating mass results. Soundness and continuity coatings are judged readily by eye. Galvanising is faster than painting, and elimination of site painting, can save substantial time in site erection or assembly.

12. Mechanical-properties Galvanising has no effect on the mechanical properties of commonly galvanised structural steels.

Steps to galvanising

1 Soil and grease removal: Hot alkaline cleaner is used to remove oil and grease soluble paints.
2 Rinsing: To prevent alkaline from reacting with pickling solution, preventing unnecessary acid waste.
3 Scale removal or pickling: Subsequently the steel passes through an acid bath to remove surface rust and mill scale to produce a clean metallic surface.
4 Rinsing: To remove acid and iron salt.
5 Prefluxing: The cleaned steel is next immersed in hot flux solution ( usually zinc ammonium chloride) to prevent oxidation and ensure that the surface is chemically clean before its immersion in molten zinc.
6 Hot Dip Galvanising: The dried fabrication is next immersed in molten zinc where it immediately reacts to form the zinc-iron alloy layers on its surface. The period of immersion depends solely on the zinc and weight of the steel product.
7 Finishing: The work is then withdrawn at a controlled speed. In addition to the zinc/iron alloy layers, a coating of relatively pure zinc solidifies at the surface when chilled in water. This total zinc coating is metallurgical bond to steel, completely covering the whole article.
8 All materials will be transferred to the deracking section for final inspection and touch up of any bare spots and removal of sharp edges.