The seven-tank process of Hot-dip galvanizing
Hot Dip galvanizing (HDG) is the oldest form of zinc metallic coating and by far the most widely used galvanizing process to make a metal rustproof. Galvanized steel is widely put into applications where corrosion resistance is needed. The process of HDG is normally carried out as per International Standards mentioned in IS 2633, IS 2629, IS 6745, IS 4759, & ASTM 123A.
HDG process involves seven-steps action. A material must pass through each stage for creating a uniform layer of zinc deposition. These steps are clearly mentioned in all the International Standards mentioned above.
Degreasing
Pickling
Rinsing
Flux Solution
Hot Plate
Zinc Bath
Quenching
The surface of the steel is completely cleaned at each process, i.e, decreasing, pickling, rinsing, fluxing, and hotplate. The prepared steel is galvanized by fully immersing in a molten zinc bath at 450°C - 460°C, to produce a uniform coating of zinc and zinc-iron alloy layers. The thickness of the coating is determined principally by the mass of the steel being galvanized.
The molten zinc in the galvanizing bath covers corners, rivets, seals edges, and seams to penetrate in recesses to give complete protection to areas that are potential corrosion spots. The HDG coating is slightly thicker at the corners and narrow edges, providing increased protection to the surfaces.
The visual inspection of the Hot-Dip Galvanized products speaks about the quality of work and the excellence of the galvanizer.
Degreasing:
The rolled steel surface is first immersed in a caustic solution to clean up the oil, paint, rust, scale, and other surface contaminants that may be present. A chemical reaction occurs to break down the oil/grease, dirt, and paint so that a clean surface of a metal is available for galvanizing. The material is then shifted to the rinsing tank to remove the leftover chemical from the degreasing tank.
Pickling:
The next step is to remove the mill scale present on the rolled steel surfaces. The material is immersed in the sulfuric acidic solution for 5 to 10 minutes, depending on the material for the pickling process. The material is then shifted to the rinsing tank to remove the leftover chemical from the pickling tank.
Flux Solution:
The acid-cleaned rolled steel is then fully immersed in a solution flux. This flux solution is prepared by using a 60/40 per cent zinc chloride / ammonium chloride and it is maintained at a temperature of 60°C. This flux solution helps in removing the oxide film which usually occurs on the highly reactive steel surface after acidic cleaning, and prevents further oxidation before hot-dip galvanizing.
Hot-Plate:
The next step is to dry the ready material for the hot-dip galvanizing process. The hot plate is maintained at a temperature of 70°C.
Zinc Bath - Hot-dip Galvanizing:
On immersion in the galvanizing bath, the steel surface is witted by the molten zinc and reacts to form a series of zinc-iron alloy layers. To allow the formation of the coating the work remains in the bath until its temperature reaches that of the molten zinc,i.e, in the range of 450°C to 460°C. The work is then withdrawn at a controlled rate and carries with it an outer layer of molten zinc which solidifies to form the relatively pure outer zinc coating. The period of immersion in the galvanizing bath varies from 2 minutes for relatively light articles to 10 minutes for massive structural members.
The resulting galvanized coating is tough and durable, comprising relatively pure zinc and zinc-iron alloy layers bonded metallurgically to the underlying steel, completely covering the article and providing unmatched resistance to abrasion. An important advantage of the hot-dip galvanizing process is that visual inspection shows that steel is completely protected from corrosion under any circumstances.
Quenching:
Items are quenched in a mild sodium dichromate solution to prevent the undesirable reaction of the newly formed coating with the atmosphere. Also, note that the heavy solid structures are first immersed in the water tank before the quenching tank to cool down the treated material and to avoid the yellow stains of dichromate solution.
Average Coating Thickness applied during HDG
Galvanization refers to creating zinc-iron alloy layers on the steel. When a material is immersed in the hot zinc bath, layers of zinc-iron alloys grow rapidly on the surface of the steel. The rate of alloy layer growth then diminishes and is finally very slow. When the steel is withdrawn from the bath an outer layer of relatively pure zinc is also carried out. This pure form of zinc solidifies on reacting with the outer atmosphere, giving the highest level of protection to the steel. The total zinc coating mass applied depends mainly on the mass and thickness of the steel being galvanized.
As per International Standards: IS 2633, IS 6745, IS 4759, IS 2629, & ASTM 123 A, one can follow the minimum average coating thickness and maintain the same during the hot-dip galvanizing process.
NOTE: 1 g/m² coating mass = 0.14 μm coating thickness.
Article Thickness (mm) | Local Thickness - minimum g/m² | Local Thickness - minimum μm | Average coating of mass g/m² | Average Coating thickness μm |
Steel > 5 mm | 505 | 70 | 610 | 85 |
Steel > 3 mm to < 5 mm | 395 | 55 | 505 | 70 |
Steel > 1.5 mm to < 3 mm | 325 | 45 | 395 | 55 |
Steel < 1.5 mm | 250 | 35 | 325 | 45 |
Casting > 6mm | 505 | 70 | 575 | 80 |
Casting < 6mm | 430 | 60 | 505 | 70 |
(AS PER IS 1461)
As indicated the total coating mass on heavier steel sections normally contains a minimum of 610 grams of zinc per square meter of surface area, (g/m²) equivalent to about 85 μm thickness. The coating thickness is slightly greater at the corners.
Galvanized coatings are slightly thicker at corners and edges is an important advantage
over most organic coatings which thin out in these critical areas.
The corrosion rate of a galvanized coating is generally linear, less than 1 micron per year.
The structure of the galvanized coating and the relative thickness of its zinc-iron alloy layers have little or no effect on the protective life of the coating. Always remember, the Protective life of the steel depends completely on the total mass of the coating.
On most commonly galvanized steels, the relatively pure outer zinc layer of the galvanized coating solidifies to give the typical bright grey crystal or ‘spangle’ finish. Certain steel compositions may cause the zinc-iron alloy layer to grow through to the surface of the galvanized coating producing a matt grey finish sometimes also known as ‘grey bar’.
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