Pipe Corrosion - Adding A53/135 Black Steel Pipe to Existing Galvanized Pipe System 1

[PEDARRIN2]

I have a client who prohibits the use of galvanized pipe on their sprinkler systems. This is a new requirement and existing buildings have galvanized pipe for dry systems.

I am designing an addition to an existing building and I want to know if there are any concerns with connecting A53/A135 black iron pipe (standard sprinkler pipe) to a galvanized pipe.

I don't think there should be any issue, but wanted to pose the question.

 

[TomDOT]

If it's actually dry, not "dry" - I wouldn't be too concerned. If there is moisture, the galvanizing will be consumed faster at the connections.

 

[PEDARRIN2]

The piping will likely not be totally free of some water or moisture. That is the nature of the beast.

I am assuming the increased rate of loss of the galvanizing would be due to galvanic corrosion between the zinc and the black steel?

I would then assume we should install a dielectric break at each connection.

I really do not want to tell the client they have to remove their existing galvanized pipe, just because their new standards do not allow it in new construction.

 

[TomDOT]

Yes, if there is moisture you will have selective corrosion of the galvanizing near the joint due to galvanic corrosion. Potential difference is about a third of a volt.

 

[Compositepro]

Why be concerned? Galvanizing is designed to be sacrificial. When the zinc is gone you have bare steel that will rust just like black iron. Then you can paint it or replace it at some point many years from now. It makes no sense to replace any pipe now.

 

[PEDARRIN2]

Compositepro,

It is the interior of the piping where the concern lies. The exterior of the pipe will be painted so there would be no moisture to cause galvanic corrosion.

And something else I just thought of - the proportion of the "area" of the zinc(anodic) to the steel(cathodic) would be about 3:1 - we are adding about 30% on to the existing system. So this could help mitigate some of the corrosion potential wouldn't it? Or is the ratio of areas not large enough to counteract the separation in the galanic chart?

 

[Ripz]

What Compositepro is saying, and I agree with him, is that the internal galvanised layer will be consumed at the connection, as it is designed to, albeit faster than usual.

But what remains is your carbon steel. Which will react to the system in the very same manner as the new Black Iron pipe you're about to install.