Steels_ Metallurgy and Applications, Third Edition

88 Steels: Metallurgy and Applications

Corrosion resistance

The life of a zinc coating on steel is roughly proportional to its thickness, and
therefore, galvanized strip is produced in a range of coating thicknesses to satisfy
different conditions and end uses. In the hot dip product, the amount of zinc on
the steel is expressed in g/m 2, the figures after the letter Z (plain zinc) and
ZF (iron-zinc alloy) in Table 1.4 indicating the minimum mass per unit area,
including the coatings on both sides of the strip.
In electro-galvanized strip, the coatings are significantly lighter, as shown in
Table 1.5. In this table, it should be noted that the coating mass relates to a
single surface rather than the combined mass for the two surfaces, which is the
convention used for the hot dip product. Electro-galvanized material can also be
supplied with single-sided (e.g. ZE 25/00) or differential (e.g. ZE 75/25) coatings.
As with other steel products, the atmospheric corrosion resistance of zinc-
coated steel is affected greatly by the nature of the environment and it is common
to differentiate between the performance in rural, industrial and marine locations.
The performance of hot dip galvanized steel, in terms of life to first maintenance,
is illustrated in Table 1.6. This indicates the following features:

1. The detrimental effect of sulphur dioxide (industrial) and chloride ions
   (marine) in the atmosphere.


2. The beneficial effect of increasing the thickness of the zinc coatings.

Table 1.4 Use of hot dip coatings

Light coating Z 100
Z 200

Medium coating Z 275
Heavy-duty coating Z 350
Z 450
Z 600
Iron- zinc coating ZF 100
ZF 180

Where corrosion conditions are mild or where

forming requirements preclude heavier coatings

Standard coating

For longer life requirements

Fe-Zn coatings for good painting and welding

characteristics

Table 1.5 Coating thickness/mass for electro-galvanized coatings

Coating

designation

Nominal zinc coating

values for each surface

Minimum zinc coating

values for each surface

Thickness Mass Thickness Mass

~tm g/m 2 ~tm g/m 2

ZE 25/25 2.5 18 1.7 12

ZE 50/50 5.0 36 4.1 29

ZE 75/75 7.5 54 6.6 47

ZE 100/100 10.0 72 9.1 65

After BS EN 10152: 1994.