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334 CONSEQUENCES OF CORROSION
Figure 5.6 Hydrant bolt from base flange. (David Raymond, City of Ottawa, Public Works
& Services.)
TABLE 5.6 Cost of Cathodic Protection and Replacement of Water Main
1 km Replacement ($) Cathodic Protection Savings ($) Percent Cost
6-in. water main 800,000 $20,000 × 3 cycles 740,000 7.5
12-in. water main 1,100,000 $37,500 × 3 cycles 987,500 10.5
5.3.7.4.3 Internal Corrosion of Aluminum Compressed Air Cylinders The safety
of aluminum compressed air cylinders has been an important problem in recent years.
Safety procedures regarding their use and maintenance are necessary to ensure public
safety. Sustained-load cracking (SLC) is a common failure mode of these cylinders.
SLC is a metallurgical anomaly that develops occasionally in high-pressure cylinders
made of aluminum alloys.
Visual inspection of the air cylinder made of 6161 aluminum alloy that failed a
safety test because of high moisture content (>200 ppm) was done. An optical probe
inspection showed internal corrosion. Figure 5.7 shows an out-of-service compressed
air cylinder. The cylinder was cut open (Fig. 5.8) and the internal surface was cor-
roded. The corrosion pattern appeared to be caused by condensation. Figure 5.9 shows
the bottom portion of the cylinder, and the corrosion pattern appears to be uniformly
distributed inside the cylinder. Figure 5.10 is a close-up view of the corrosion surface,
which shows white spots. These spots are probably corrosion pits. Pitting corro-
sion is more like corrosion-induced cracking. Pitting seems to grow intergranularly
(Fig. 5.11) and deepens along the boundary of the aluminum grains. Figure 5.12 is
an enlarged cross section of a corrosion pit. The oxygen/aluminum ratios were deter-
mined at various locations in the cracks shown in Figure 5.12. The oxygen/aluminum