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THE COATING When the reaction between iron and zinc has virtually ceased and the article taken out of the galvanizing bath complete with its outer coating of free zinc, the process is complete. In reality there is no demarcation between steel and zinc but a gradual transition through the series of alloy layers which provide the metallurgical bond.
Schematic section through a typical
hot
dip galvanized coating
Microstructure of a typical hot dip
galvanized coating
COATING THICKNESS
Coating thicknesses are normally determined by the steel thickness and are set out in BS EN ISO 1461. There are three exceptions to this rule, the first produces a slightly thinner coating, the other two increase it.
CENTRIFUGED GALVANIZED COATINGS
- This process, covered in BS EN ISO 1461, is used for galvanizing threaded components and other small parts. The parts, after preparation, are dipped in the molten zinc in a perforated basket. After the coating has formed this is centrifuged or spun at high speed, to throw off surplus zinc to ensure a clean profile. Minimum average coating weights for centrifuged work are identified in BS EN ISO 1461 and in BS 7371 Part 6.
Thicker coatings may be produced by one of the following:
THICKER COATINGS BY SURFACE ROUGHENING -
This is the most common method of achieving thicker coatings.
Microstructure of a thick coating obtained
by grit blasting steel prior to galvanizing
Grit blasting to Sa 2½ (ISO 7079) the steel surface prior to immersion, using chilled angular iron grit of size G24, roughens and increases the surface area of steel in contact with the molten zinc. This generally increases the weight per unit area of a hot dip galvanized coating by up to 50%. Any steel article can be treated in this way, providing it is thick enough to withstand blasting. It may not be possible to grit blast the inside surface of hollow sections and fabrications, but these are almost always the areas least prone to corrosion. Thicker coatings than those required by BS EN ISO 1461 should only be specified following consultation with the galvanizer or Galvanizers Association.
GALVANIZING REACTIVE STEELS
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A thicker zinc coating will be obtained if the article to be galvanized is manufactured from a reactive steel. The constituents in steel that have the greatest influence on the iron/zinc reaction are silicon, which is frequently added to steel as a deoxidant during its production, and phosphorous. Silicon changes the composition of the zinc-iron alloy layers so that they continue to grow with time and the rate of growth does not slow down as the layer becomes thicker. To a lesser degree phosphorus exerts a similar influence on the formation of the coating.
Microstructure of the thick coating obtained
using a silicon-rich steel
When an article made from a reactive steel is removed from the zinc bath, a zinc layer adheres to the alloy layer as with any other steel article. However, the reaction rate in these steels can be so high that this pure zinc layer is transformed completely to zinc-iron alloy before the article has had time to cool. The result is a coating of equal or increased thickness that can be much darker in appearance. The change in appearance does not alter the corrosion resistance of the coating.
Approximate thickness of the zinc coating in relation to
the Silicone (Si) content of the steel
(This is a schematic representation, please contact
Galvanizers Association for detailed information)
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