Page 353 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION DAMAGE, DEFECTS, AND FAILURES 331

can be major if the failure is not detected and corrected in time. In other words, the
consequence of any hidden function failure is increased exposure to the consequences
of a multiple failure.

5.3.7.4 Selected Examples of Failure and Their Prevention Cathodic protection
of water mains: Here two examples of cathodic protection: (i) ductile iron main and
(ii) cast-iron-lined main are discussed.

5.3.7.4.1 Ductile Iron Main The valve was buried in 1995 in the soil area of
800–900 Ω cm resistivity thus providing active conditions. The 8-in. valve was
epoxy-coated with a zinc anode attached to the top body by an exothermic weld.
The valve is a two-part casting fabricated with a sealing ring and 5/8-in. diameter
bolts around the flange. The larger bolts create a continuous electrical circuit. This
1
valve failed as a result of a combination of / 2-in. diameter bolts holding the top
1
body instead of 5/8-in. bolts and the corrosion affecting the undersized / 2-in. bolts
that held the valve together. The bolts were uncoated and made of carbon steel. As
the valve was epoxy-coated, the corrosion was mainly on the 12-in. coupling bolts
1
and on the undersized / 2-in. bolts that held the valve together. These were uncoated
1
carbon steel bolts. The / 2-in. bolts did not provide electrical continuity between the
top body and the valve as with larger bolts, resulting in the lower part of the valve
being unprotected (Fig. 5.2).
In the valve system, bolts are generally the first to corrode. In chambers, the cause
of corrosion is easy to recognize and corrective action can be taken in early stages.
The chambers are generally filled with water and soil that run into them from the
road. Further water from the ditches and along the water main enters the manhole.
Leaks can be reduced, but with traffic and shifting of the chamber in the frost cycles

Figure 5.2 Twelve-inch bolt from coupling. (David Raymond, City of Ottawa, Public Works
& Services.)