Technical Info Sheet #15
The terms “Galvanizing” or “Galvanizing” refer to a range of different methods of protecting steel using zinc coatings. Hot-dip galvanizing is one of these methods. Within this we must make an important distinction between continuous hot-dip galvanizing of steel sheet (coils) and discontinuous hot-dip galvanizing of finished steel products (objects) or constructions.
See also “Hot-dip galvanizing – Speech confusion in galvanizing – Different technique to galvanize” and TECHNICAL INFOBLAD 11: ZINKAPPLICATION METHODS.
IMPORTANT DIFFERENCES
Continuously galvanized steel leaves the steel mill with a certain zinc coating thickness indicated by a weight per surface area (gr/m2).
This steel sheet is then machined in workshops to produce the final product. In the case of continuous hot-dip galvanizing (or Sendzimir galvanizing) of steel sheets/steel coils, all the operations to make the final structure (such as cutting, edging/setting, punching and welding) are therefore carried out after galvanizing.
Characteristic of continuous galvanizing is the relatively thin zinc coating thickness. This thin coating is necessary to allow for operations (in particular settling) after galvanizing without damaging the zinc layer on site too much. Other elements are also often added to the zinc in the zinc bath.
When processing (cutting, drilling and welding) continuous galvanized steel sheet, the cut edges, drill holes and welds are no longer protected by a zinc layer which, despite the cathodic protection of zinc, can cause corrosion problems. Especially in an outdoor environment and conditions where condensation may occur or when there is high humidity. In the case of edging/setting, the zinc layer at the site of setting is also pulled apart, reducing the barrier effect.
Because continuous hot-dip galvanized items have parts where no protective zinc layer is left and the zinc layer thickness is remarkably lower than that of discontinuously galvanized items, continuous hot-dip galvanized items are used almost exclusively in air-conditioned indoor applications. Continuously hot-dip galvanized steel sheet is suitable for an outdoor environment only when a high-quality (powder) paint system is applied on top of the zinc layer (automotive, cladding).
In discontinuous galvanizing, on the contrary, all the operations are done before galvanizing. After the workpiece is manufactured in a construction workshop, it is delivered to a hot-dip galvanizing plant where
the entire object is dipped in a 450°C zinc bath. A completely sealed, fairly thick coating is formed on the surface. Virtually all steel products can be galvanized including construction profiles.
Discontinuous hot-dip galvanized objects are used almost exclusively in outdoor environments, although architects are increasingly choosing to use them in indoor spaces as well for aesthetic reasons.
After all, galvanized steel gives an industrial look, a tremendously long maintenance-free life and is 100% circular.
CLASS DIVES
For discontinuous hot-dip galvanizing, coating thickness is expressed in micrometers (1 μm = 10-³ mm). For continuous galvanized steel strip, on the other hand, coating thickness is expressed in weight per unit area (g/m²). Usually this then refers to two-sided. EN 10346 is the standard in which technical delivery conditions are described.
Conversion factors (according to 7.9 of EN 10346) are:
In continuous hot-dip galvanizing of steel sheets, the desired thickness can be set very precisely. The extremes that occur in practice amount to:
For discontinuous hot-dip galvanizing, the coating thickness to be obtained depends on steel wall thickness, steel composition (Si, P, etc.), surface roughness of the steel and immersion time. Therefore, standard EN ISO 1461 prescribes only the mandatory minimum coating thicknesses (see Table 1).
CORROSION PROTECTION
As you can read in Technical Information Sheet 10: “Corrosion resistance of hot-dip galvanized steel”, the duration of corrosion resistance depends on the aggressiveness of the atmosphere and the applied zinc coating thickness.
The duration of protection is almost directly proportional to the zinc coating thickness. With continuously galvanized sheet, it is possible to apply zinc alloys such that it results in improved corrosion protection at some atmospheres, but at a zinc coating thickness significantly less than that applied by discontinuous galvanizing.
Manufacturers of continuous galvanized sheet with product names such as MagiZink® and Magnelis®, claim superior corrosion resistance, especially in chloride-loaded environments. Standards and independent studies do not exist to substantiate these claims.
| SECTIEDIKTE VAN HET VOORWERP (INDIEN NIET GECENTRIFUGEERD) | PLAATSELIJKE DEKLAAGDIKTE IN MICROMETER (μm) | GEMIDDELDE DEKLAAGDIKTE IN MICROMETER (μm) |
|---|---|---|
| STAAL > 6MM | 70 | 85 |
| STAAL > 3MM TOT ≤ 6MM | 55 | 70 |
| STAAL ≥ 1,5MM TOT ≤3MM | 45 | 55 |
| STAAL < 1,5MM | 35 | 45 |
| GIETSTUKKEN ≥ 6MM | 70 | 80 |
| GIETSTUKKEN < 6MM | 60 | 70 |
EN ISO 1461
Coatings applied by hot-dip galvanizing to iron and steel objects – Specifications and test methods.
ISO 4998
Continuous hot-dip zinc-coated and zinc-iron alloy-coated carbon steel sheet of structural quality
EN 10346
Continuous immersion-coated flat steel products – Technical conditions of delivery
TECHNICAL DATA SHEET 3
Thermal deformation due to galvanizing
TECHNICAL DATA SHEET 10
Corrosion resistance of hot-dip galvanized steel
TECHNICAL DATA SHEET 11
Zinc application methods
