Dielectric union requirements 2

[WinniPEng]

I will typically specify a dielectric union to be installed whenever dissimilar metals are joined. When copper and steel pipe are joined, the steel pipe will corrode due to the galvanic action. But, when stainless steel is joined to copper (in a domestic water system) will a dielectric union still be required? Does anyone know of a reference guide which indicates which common piping materials require dielectric unions at their joints, and which metals can be joined safely without dielectric unions?

Thanks.

 

[PEDARRIN2]

If you go to the corrosion engineering forum, you will get a lot of information.

In general, you use the galvanic series chart. When metals are close on the chart, the corrosion is less. When metals are farther apart, the corrosion is more.

That is a fairly simplistic way to look at it.

If you google corrosion and galvanic series, you can find a lot of info on the net.

 

[MJCronin]

Winni..

Here is a pretty good reference:

http://www.corrosion-doctors.org/Definitions/galvanic-series.htm

Generally speaking, I would use the DI union on any copper system (copper desperately wants to corrode)especially where you connect to a galvanized system.

I do not believe that a DI union is required on the joint between any carbon steel to SS system, they are "too close" on the galvanic series.

-MJC

 

[WinniPEng]

Thank you both for the leads.
The website was very helpful, and it appears as though the joint between copper pipe (V = 0.35) and stainless steel pipe (304 SS has +/- 18% chromium content, so V = 0.50) will NOT require a dielectric union, since the differnece is only 0.15.

However MJCronin, this same table leads me to believe that the joint between carbon steel (V = 0.85) and stainless (V = 0.50) may actually need a DI union, with an index difference of 0.35, no?

 

[moltenmetal]

Unless it's a corrosive service where carbon steel is already marginal, galvanic units are not usually used between stainless and carbon steel components. The lower electrical conductivity of stainless steels tends to reduce the galvanic currents, reducing the galvanic corrosion rate.

 

[SJones]

Moltenmetal - nothing to do with conductivity; it's the poor cathode efficiency for the reduction reaction.

Winni - you may wish to refer to the suite of EN standards: 12502 Guidance on the assessment of corrosion likelihood in water distribution and storage systems

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[moltenmetal]

SJones: thanks for correcting my misperception!