The hot dip galvanized zinc coating
Coating thicknesses are normally determined by the steel thickness and are set out in EN ISO 1461. There are three exceptions to this rule, the first produces a slightly thinner coating, the other two increase it.
Galvanized Coating Appearance
Specifiers and end users often anticipate that batch hot dip galvanized steel will have a uniform bright colour and appearance, a view often formulated from having seen pregalvanized steel product (sheet) which is produced using a carefully controlled automated process.
In practice, when fabricated articles or individual steel sections are galvanized by the batch process to EN ISO 1461 some variation in coating appearance might occur. See link below.
base steel for A hot dip galvanizEd Coating
Chemical composition of the base steel for successful hot dip galvanizing
- The formation of the zinc-iron alloy coating depends principally on the chemical composition of the steel that is galvanized. All common steels and irons can be hot dip galvanized, but steels with particular silicon contents may produce a very fast reaction between the iron and the zinc (read below about Galvanizing reactive steel).
- For best results it is just as important for the customer to ensure careful preparation of the steel components as it is to ensure that the construction and design are suitable for galvanizing.
The Life of a Zinc Coating is Directly Proportional to its Thickness.
Although zinc coatings can be applied by several different processes, only hot dip galvanizing provides a continuous, tough, metallurgically bonded coating of substantial and uniform thickness inside and outside.
The bar chart below shows how widely the thicknesses of zinc coatings can vary and emphasises the need to be aware of those which may not give the lifetime and performance required. It is also important to ensure that all parts of a steel structure have equivalent protection, since premature failure of the coating on small parts (e.g. fasteners) can result in major structures requiring maintenance of the metal finishes long before it would otherwise be necessary.
How to Obtain Thicker Galvanized Coatings
Thicker coatings may be produced by one of the following:
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Surface roughening
- Grit blasting to Sa 2½ (EN ISO 8501) 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. Most steel articles can be treated in this way, providing it is thick enough to withstand blasting.
Microstructure of a thick coating obtained by grit blasting steel prior to galvanizing.
Galvanizing reactive steel
- This process, covered in 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 EN ISO 1461 and in 7371 Part 6.
Centrifuged galvanized coating
- 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.
Microstructure of the thick coating obtained using a silicon-rich steel
Thickness of the zinc coating in relation to the silicon
Approximate thickness of the zinc coating in relation to the silicon (Si) content of the steel. (This is a schematic representation, please contact Galvanizers Association for detailed information).
For illustrative purposes only.
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