Page 354 - Challenges in Corrosion Costs Causes Consequences and Control(2015)
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332                                        CONSEQUENCES OF CORROSION




























           Figure 5.3 Aluminum ladder rungs. (David Raymond, City of Ottawa, Public Works &
           Services.)



           it becomes very difficult to eliminate the inflow of water. Chambers in these areas
           are found with totally corroded bolt heads. Bolts with stems reduced to pencil thick-
           ness are also noted. Rungs of aluminum ladders for access to the chambers have
           completely disappeared.
              Corrective action consists of replacing mild steel bolts with 304 stainless steel
           bolts coated with a wax-based primer to reduce the corrosion of bolts in flooded
           chambers.
              In the chambers, crevice corrosion, galvanic corrosion, and general corrosion of
           the aluminum ladder were observed (Fig. 5.3). Crevice corrosion was also observed
           on couplings, bolts, and valves. The aluminum steps in some valve chambers were
           attacked by chlorides/phosphates.


           5.3.7.4.2  Cast-Iron-Lined Main A chamber in service for a 10-year period in
           which rebuilt valves were present was found to be saturated with chlorides and
           phosphates. The chlorides caused the corrosion of bolts to such an extent that the
           chambers were leaking. The chambers housed 24- and 16-in. valves along with air
           drain-out. All the bolts on the valves needed replacement and the aluminum rungs
           were completely corroded.
              The corrosion prevention strategy consisted of wrapping the valve with wax tape,
           along with magnesium anodes. The chamber was thoroughly cleaned and valves
           wrapped in wax tape as shown in Figure 5.4. As the chamber was entered for a limited
           number of times, the corroded ladder was left intact and the chamber entered with the
           restraining device and/or portable ladder.
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