Cast Iron waste water pipe corrosion. Pipe cracks along top of pipe

[Airglider]

The cast iron horizontal pipe runs in my building for wastewater are all corroding and cracking along the top edge of the pipe. We do not use acid drain un-cloggers and our tenants consist of law firms so no unusual waste is being disposed of. The raged looking crack, many times almost the full length of a horizontal pipe section is large enough so that it causes sewer odor problems. The cracks occur in high use pipe, low use, and all sizes between 2" and 14" diameter pipe. Occasionally this causes leaks when a pipe is clogged. The pipe is located in a thirty-story class "A" high rise building that is 16 years old. I chair a group of building engineers in Phoenix Arizona and all are reporting the same problems.

What exactly causes this type of failure?

What kind of pipe can prevent this type of corrosion failure that can be mounted in an HVAC air-plenum that is fire resistant and cost effective to install?
(The perfect waste water pipe!)

 

[stanier]

I would suggest that the problem may be the result of hydrogen sulphide formation. This then dissolves in condensation forming overnight to create sulphoric acid. The acid weakens the grain structure in the CI until a crack appears.

Many buildings use themoplastic materials for wastewater piping. I am unsure how these meet the "fire resistant" tag you have used. Fire retardants can be used. GRP can be formulated with fire retardants. Otherwise you may be stuck with a lined or FBE coated steel. Cement lined ductile iron with a coating seal over the cement lining is common for wastewater mains, grivity or pressure. These give good service and are as cost effective as other materials.

 

[metengr]

I believe stanier is right on, in that just plain wet hydrogen sulfide gas will corrode ductile/CI piping. Your problem has more to do with how the piping was installed. If sewage is sitting in the horizontal lines that means there was little if any pitch used for drainage, and in a three story building you could develop head pressure that is causing the cracks.

I have seen ductile/CI drain piping used in my previous homes with no problems provided you have proper drainage. The drain piping should not have sewage sitting and decomposing overnight to form hydrogen sulfide gas that preferentially corrodes the piping. This is the root cause of your problem. Upgrading to alternate materials, other than PVC piping (which is not allowed in certain building codes), is not a solution.

Instead, I would replace the damaged piping with ductile/CI and make sure you have adequate pitches in the horizontal runs to assure complete drainage. Also make sure you have adequate venting in the drain lines.

 

[SMF1964]

Is there corrosion associated with the ID surface all around the pipe (i.e., not just at the top)? A possibility is you have a general corrosion mechanism occurring in a cast iron that has a very high residual stress. When pitting reaches critical depth (can be very shallow) the pipe will 'spontaneously' crack. Depending how the pipe was cast, you can end up with a very long crack or a whole lot of fragments.

Alternative solution would be a ductile iron (if you are sticking with metal).