Use of galvanized ductile iron grooved fitting

[Slowzuki]

Good afternoon,

I've been asked by a client to consider a substitution in my design of a wet pipe sprinkler system. The system will have an oxygen depleted condition when in service and there are no know MIC problems in the surrounding areas.

The system was designed using normal plain steel sprinkler piping and victaulic grooved fitting factory painted with enamel. This combination normally gives good service in wet pipe sprinkler use. We have restricted the use of small galvanized pieces in the system as they normally get attacked as an anode.

The client has asked us to consider the use of galvanized vic elbows and tee's as they don't want to paint the normally orange fittings.

Despite the insulation of the rubber gasket, the fittings are somewhat electrically connected via the coupling and I believe the small area off the galvanized fitting will be attacked. It would not be a problem if only the galvanizing were removed but I believe the underlying ductile iron may become pitted in the process and subject to a higher corrosion rate than the rest of the system.

The clients engineer feels this will not impact the 30 year design life however I feel there is a good chance it will. Does any one have any information one way or the other? I have asked Vic to weigh in on it but I have not heard from anyone in their engineering department yet.

 

[moltenmetal]

If there's no oxygen, how is it that the galvanized pieces are "attacked as an anode"? What's the other half reaction? It sounds like you've witnessed this event, so I want to know the circumstances of the failure in particular.

I'm having a hard time imagining a water service where bare carbon steel and/or ductile iron is suitable but galvanized components are not.

 

[Slowzuki]

The systems are tested annually at which point fresh oxygen bearing water is introduced. During this time corrosion occurs until the oxygen is consumed.

If say a galvanized pipe plug is installed in the system the zinc is rapidly stripped off the pipe plug and pitting starts on the plug leading to more rapid corrosion at that point in the future.

I do not have experience with mixing galvanized VIC fittings into this service, threaded plugs have a better connection to set up a cell so I'm at least cautious.