Technical Info Sheet #12
Pure zinc is a relatively soft metal. Yet a thermal zinc coating applied to steel is almost always harder than organic coatings (paint systems). So why is it that hot-dip galvanized steel is highly scratch and impact resistant? To answer that question, we must first explain something about how a zinc coating is created.
METALLURGICAL ADHESION
When immersed in a zinc bath, the steel surface takes on the temperature of the molten zinc fairly quickly (about 450°C). As soon as the temperature of the steel rises above about 200°C, a diffusion reaction occurs between the two metals. In the process, zinc-iron alloy layers form on the steel surface. This metallurgically anchors the zinc to the steel surface. The result is the best possible adhesion. When the steel is removed from the zinc bath, the steel takes the hot, liquid zinc with it from the bath. This layer of pure zinc solidifies on top of the zinc-iron alloy layers. The thermal zinc layer is thus composed of 3 zinc-iron alloy layers and a top layer of pure zinc (see Figure 1).
Figure 1:
A micro shot of the characteristic layer structure of a zinc layer on a steel object
THE HARDNESS OF ZINC
Despite their very high zinc content, zinc-iron alloy layers are 3 to 5 times harder than the top layer of pure zinc. They are also usually more than 50% harder than the steel substrate (see Figure 2). The galvanizing layer is thus composed of particularly hard zinc-iron alloy layers and a relatively much softer top layer of pure zinc.
As a result, the soft, zinc-iron top layer attenuates the mechanical stress caused, for example, by blows or knocks, the force on the alloy layers. As a result, the risk of damage is relatively small. Only in the event of large, very localized point loads, where the steel and thus softer substrate deforms, can the hard alloy layers crack and loosen.
The great hardness of the alloy layers also ensures excellent scratch resistance. It is not for nothing that stairs, wheelbarrows, loading containers, dump trucks and stables, for example, are hot-dip galvanized. It is also true that the thermal treatment (immersion in zinc at 450°C) does not affect the mechanical properties of the steel.
Figure 2:
This figure shows the hardness of the steel, the zinc iron alloy layers
and the pure zinc layer (also called solidification layer)
EN ISO 1461
Coatings applied by hot-dip galvanizing to iron and steel objects – Specifications and test methods.
