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Increase in steel weight during hot-dip galvanising

Increase in weight

Hot-dip galvanising can result in a weight increase of “black” (ungalvanised) steel of up to 20%. The increase in weight is determined by the combination of a series of factors, the main one being the composition of the steel in the material to be galvanised.

Factors affecting weight increase during hot-dip galvanising:

  • steel composition
  • wall thickness
  • composition of zinc bath
  • surface condition of the steel

Steel composition

Most types of steel can be hot-dip galvanised without difficulty. However, the concentrations of particular elements in the steel, particularly silicon (Si) and phosphorus (P), as well as the combination of the different elements, determine and strengthen the reaction between the steel and the molten zinc. Steel with these properties is known as "reactive" steel.

Steel composition

Most types of steel can be hot-dip galvanised without difficulty. However, the concentrations of particular elements in the steel, particularly silicon (Si) and phosphorus (P), as well as the combination of the different elements, determine and strengthen the reaction between the steel and the molten zinc. Steel with these properties is known as "reactive" steel.

The Sandelin effect (reactive steel)

The level of Si in the steel or the welding material can cause a reaction during hot-dip galvanising, with the zinc layer becoming three times thicker than usual. This "Sandelin effect" results in spots that are darker, mat and rough in appearance. The Si concentration required to prevent the Sandelin effect sometimes conflicts with the percentage required given other material properties. The Sandelin effect is an unavoidable risk of hot-dip galvanising.

Si concentration in steel and hot-dip galvanising

As soon as the steel goes into the zinc bath, it starts bonding with the zinc. If the steel has an Si concentration of 0.12 - 0.25%, the reaction in the zinc takes place within the first few minutes. This process slows down over time while the product is in the zinc bath. When the steel emerges from the zinc bath, the alloy acquires a pure zinc film.

However, at high Si concentrations of 0.03 - 0.12% and in excess of 0.25%, Si acts as a catalytic converter: the reaction rate continues rather than falling off. This results in a very thick zinc coating and therefore a larger increase in weight. The layer of zinc can be three times thicker than on steel with the same wall thickness that is less reactive. Furthermore, the resulting layer can be brittle and the bonding with the steel can be relatively poor, so that the zinc layer may be damaged more easily.

Average wall thickness

The thinner the wall of the steel, the larger the surface per unit weight of steel that requires galvanising, and the larger the increase in weight. At the same time, when treating steel with the same composition but with a thicker wall, one can expect a thicker zinc layer and therefore a larger increase in weight. The specifications in the European Galvanising Norm NEN-EN ISO 1461 take this specifically into account.

Composition of the zinc bath

To reduce the effect of reactive steel on the thickness of the zinc layer, zinc alloys have been developed containing elements such as nickel, aluminium, tin and/or bismuth. Using these zinc alloys rather than unalloyed zinc limits the extra increase in weight in reactive types of steel.

Surface condition of the steel

As a rule, any rust and mill scale on the black steel is removed by pickling. If blasting is used rather than pickling, the steel surface will have a larger specific surface area. That results in the zinc coatings that can be up to 20 ųm thicker than layers on the same type of steel that has undergone pickling only.