Page 376 - Corrosion Engineering Principles and Practice

344 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 345

thus been confined to areas where corrosion could cause critically
important damage such as for airport winter maintenance. It can
therefore be expected that the road environment will likely remain
corrosive well into the future.
The impact of salts on protective coatings is widely recognized.
Any breach or holiday in the coating will let salts reach the metallic
substrate and initiate a very aggressive environment that in turn will
force the coating to blister and peel off (Figs. 9.16 and 9.17), sometimes
until complete perforation of the metal (Fig. 9.18).
The effect of deicing salts extends much beyond the immediate
vicinity where the salts are spread because these salts can travel as aerosol
particles generated by the traffic circulation. Figure 9.19 summarizes the
results of a study using standard corrosion coupons [11] deployed on a
pedestrian walkway across a well-traveled road during the winter
months of a moderately cold Canadian city where rock salt is the deicing
agent of choice. These results clearly indicate that the corrosion rates
(percent mass loss) while being highest closer to the ground (at the bottom
of the pillars) are still appreciable many meters above the traffic level. For
comparison, similar measurements made in non-trafficked areas of the
same city typically showed corrosion rates fifty times smaller during the
same exposure period, that is, 0.2 percent mass loss.
In another study, it was demonstrated that high corrosion rates
could be measured more than 100 m downwind of a major highway in
similar winter conditions Fig. 9.20 [12]. The corrosion trends measured

FIGURE 9.16 Blistering and peeling of protective coating subjected to regular
deicing salt application.