Galvanic Corrosion

[roniprabowo]

Dear all,
I have a heat exchanger with carbon steel tubesheet.
There is stainless steel weld overlay on tubesheet. The tube is carbon steel. Tube to tubesheet joint is strength weld. My question is there any possibility for galvanic corrosion between CS tube with SS weld overlay ??

Thanks a lot

 

[rmw]

A lot of the answer to your question depends upon your fluid. Does it have conductivity?? And, if so, how much?? If not very much, I would not worry about it.

Can you give any paritculars on your fluid??

You state that the tube to tubesheet overlay is a strength weld??? Normally the strength comes from the tube expansion into the tubesheet, and the weld is a seal weld only. Please explain.

Tube to tubesheet joints like the one you describe are very common in high pressure feed water heaters, which last years, and few of which fail from galvanic corrosion. But, then boiler feed water is not very conductive.

rmw

 

[roniprabowo]

Sorry the fluid is pure liquid ammonia in tube side and steam in shell side.
And about the joint type, there are expand only, heavy expand and seal weld and light expand with strength weld.

 

[PVRV]

Sorry,, why was SS welding consumables used for a CS - CS joint. Under what premise was the WPS and PQR qualified.

 

[metengr]

If one were to assume that liquid ammonia has some electrical conductivity, I would think that having the stainless steel weld overlay on the tubesheet would not be a problem because the surface area of the stainless steel tubesheet (cathode) is greater than the surface area of the tubes (anode).

If the heat exchanger shell is carbon steel, I would periodically inspect the weld interface between the weld overlaid tube sheet and the carbon steel shell to check for knife line corrosion attack.

 

[Metalguy]

*If* the fluid really is conductive, gal. corr. is made worse by having a large cathodic area wrt the anodic area. But I'm really not familiar with ammonia corrosion problems.

 

[metengr]

Metalguy;
Good catch. Actually, the anode area will be greater than the cathode area because the total surface area of the carbon steel tubes will be greater than the surface area of the overlaid ss tubesheet. Thus, there should be no galvanic corrosion problem. For some reason I had flip flopped the areas.