Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 89

Table 1.6

to first maintenance

Mass g/m 2 Coastal Industrial

including and Urban

both surfaces

275

35O

45O

6OO

After Galvatite Technical manual

(British Steel Strip Products.)

Hot dip galvanized steel-typical period (years)

Suburban

and Rural

2-5 2-5 5-10

2-5 2-5 5-10

5-10 2-5 10-20

10-20 5-10 20-50

3. The need for paint protection in internal applications if the conditions are wet
   and polluted.

When zinc-coated products are stored under wet conditions or where conden-
sation can occur, white rust can form on the surface. This is due to the formation
of zinc carbonate and detracts from the appearance of smooth zinc coatings.
However, although it may form in large amounts, the appearance of white rust
does not necessarily indicate severe degradation of the zinc coating and it will
generally convert to a protective layer. As indicated earlier, the formation of white
rust can be retarded by immersing galvanized steel in chromic acid solutions to
passivate the surface.
Care must be taken when fixing zinc-coated steels to other metals in order to
avoid galvanic corrosion. In particular, copper or brass should not be coupled
directly to galvanized steel since the coating can fail rapidly in wet and polluted
atmospheres.
In aqueous media, the performance of zinc-coated steels is affected by a number
of factors:

1. pH of solution - the corrosion rate is generally low in the pH range 6-12 but
   can be rapid outside this range.


2. Hardness of water - hard water precipitates carbonates on zinc surfaces which
   reduce the rate of corrosion.


3. Water temperature - whereas zinc provides sacrificial protection to the steel
   substrate at ambient temperature, the reverse situation occurs at temperatures
   above 60-70~ Therefore, zinc-coated steels should not be used in hot-water
   systems.


4. Chlorides -soluble chlorides can produce rapid attack, even within the pH
   range 6-12.

Cold.forming behaviour

The cold-forming behaviour of zinc-coated strip is governed primarily by the

forming characteristics of the substrate. As indicated earlier, in hot dip galva-

nizing, cold-reduced strip is subjected to rapid heating and cooling cycles, similar