Page 412 - Corrosion Engineering Principles and Practice

380 C h a p t e r 9 A t m o s p h e r i c C o r r o s i o n 381

300

250

Service life (Years) 200 Suburban Rural

150

100

Temperate marine
50
Tropical marine
Industrial
0
0 20 40 60 80 100 120 140 160 180 200
Zinc thickness (µm)

FIGURE 9.51 Service life of hot-dipped galvanized coatings as a function of
zinc thickness and specific environments.

therefore important to know the specific corrosion rate in a given
application environment in order to effectively use zinc-coated steels in
outdoor structures. A widely used method for corrosion life estimation
of the various types of galvanized steels provides generalized corrosion
rate values for five predetermined atmospheric environments as a
function of zinc coating thickness (Fig. 9.51) [33]. Service life in Fig. 9.51
is defined as the time to 5 percent rusting of the steel surface. This
method may be used to estimate the service life of a given coating
thickness or to specify a coating for a given environment.
This graphical method is applicable to zinc-coated steel produced
by batch or continuous galvanizing, including hot-dip, electrogalva-
nized, and thermal sprayed coatings. However, it does not apply to
coatings containing more than one percent alloying elements. The
method also assumes that the galvanized product is free of significant
defects that could accelerate corrosion. Additionally, the service life
prediction does not consider issues of water entrapment that can create
severe crevice chemistry as shown earlier in Fig. 9.6.

9.7.6 Polymeric Materials
Essentially all polymers freely exposed to the elements will change in
some manner. The active rays of the sun become potent agents of
change in the organic materials. Further polymerization of the resin
can occur to produce embrittlement. Other types of new bonding can
be triggered to make polymers more crystalline. Any volatile component
of the material, such as a plasticizer, can be evaporated. The polymer
chains may be simply oxidized and broken up to destroy the product.