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Effect of steel composition on zinc coating

Creation and composition of zinc coating

When steel is submerged in the zinc bath containing molten zinc at a temperature of 450ºC, iron-zinc (Fe-Zn) diffusion on the surface results in three hard, dark-grey Fe-Zn alloy layers. When the steel is removed from the sink bath, a film of solidified zinc with a silvery and shiny appearance forms on these three Fe-Zn layers. The minimum thicknesses are stated in the international NEN-EN ISO-1461 norm.

Reactivity of steel to molten zinc

Generally speaking, all steel alloys can be hot-dip galvanised. However, the composition of the steel can affect the structure of the zinc layer, the thickness, the appearance (shine, uniformity, roughness) and local mechanical resilience (impact strength).

At high silicon (Si) and/or phosphorus (P) concentrations in the steel, the reaction between the liquid zinc and the steel can be very strong. In these cases, the steel is described as "reactive". The more reactive the steel, the faster the Fe-Zn alloy layers are formed. That means that the proportion of alloy layers in the galvanising coating as a whole is higher than when the steel is less reactive or not reactive at all. In some cases, the coating can actually be made up exclusively of Fe-Zn alloys. The galvanising coating will be thicker in the presence of reactive steel and it is possible that the appearance will not be as smooth because of the presence of both light and dark grey zones. A disadvantage of such "oversized" thick zinc layers with a gray appearance or erratic structure is that they are sometimes extra brittle with an open structure on the surface. This has only limited impact on the corrosion resistance, but can affect the appearance of an applied duplex system.

Steel compositon extremely important

The composition of the steel is therefore extremely important in terms of the galvanising result. Silicon in steel has been found to promote growth of the alloy layers at levels between 0.03% and 0.14% (the so-called Sandelin area) and above 0.25%.

NEN-EN-ISO 14713-2 states the expected result for steel with particular Si or P levels. See Table Category A to D. These data can be used to determine parameters when ordering steel.

Category (informative) Typical coating properties Typical percentages of reactive elements Additional information
A The coating has a shiny appearance with a fine texture ≤0.03% Si and < 0.02% P Steel types with a composition in accordance with Si + 2.5P ≤ 0.09% will very probably have these properties. It is generally expected that these properties will also apply to cold-rolled steel, on condition that the composition of the steel is in accordance with Si + 2.5P ≤ 0.04%.
B The outer zinc coating is a part of the structure of the coating. 0.14% Si to 0.25% Si Fe-Zn alloy may extend to the surface of the coating The thickness of the coating increases with silicon concentration. Other elements may also affect the reactivity of the steel. In particular, a phosphorus concentration above 0.035% results in an increase in reactivity.
C The coating is darker and has a rougher structure. >0.03% Si to ≤ 0.14% Si Excessively thick coatings may form.
D Fe-Zn alloys strongly determine the structure of the coating and often extend to the surface of the coating, reducing resilience. > 0.25% Si Excessively thick coatings may form.

Ordering steel

The composition of the steel is extremely important in terms of the galvanising result. The same categories A, B and D are used in the standard for technical delivery conditions for non-alloy structural steel (NEN-EN-10025-2). Category C concerns the Sandelin area and does not appear in this standard as it is undesirable to choose these steels for hot-dip galvanising.

To ensure uniform appearance, it is important to order steel from one steel trader only and to require the ordered steel components to come from the same batch in order to increase the probability of a uniform galvanising result.

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