Galvanic corrosion between CS and Duplex

[Mechapower]

Dear All
I am having a question from my client whether i need to take consideration of galvanic corrosion between CS and Duplex.

Front channel(tubeside) of HExchanger is CS and tubesheet-tube & shell is 31803. Water is thorugh tubes & front channel and H2S gas is on shell.

There is a gasket between tubeside flange and tubesheet.

Should i consider the galvanic corrosion.

According to NACE CORROSION ENGINEER'S REFERENCE BOOK (3 edition) Galvanic cooriosn happens with two dissimilar materials with corrosive electrolyte.

In this case H2O is the electrolyte which has contact with both CS and duplex and H2O is not corrosive. The H2S doesnt contact CS.

If yes, what shall i can do to prevent it.

Please advice.

thanks

 

[gr2vessels]

Talk to a corrosion engineer or materials engineer to explain you the corrosion mechanism. Then you ask one of the cathodic protection suppliers to calculate for you the required amount of zinc or magnesium you need in the channel for protection. By the way, the water is not quite electrolite, unless contains sufficient impurities.
Another way is to use insulating kits for the flange bolting, to disrupt the electrical continuity and then no more galvanic corrosion, simple as that.
Cheers,
gr2vessels

 

[MikeMet]

We have had that combination in service for years with no galvanic effect. However, you can still get crevice corrosion of the carbon steel gasket surface of the channel even with no galvanic effect. So pay attention to gasket type and bolting make up.

 

[0707]

I suggest you to line your tube sheet with Belzona or similar product. This way you will minimize galvanic corrosion between tube sheet and tube ends.

REGARDS

LM

 

[21121956]

Hello everybody:

Mechapower, in this site

http://www.euro-inox.org/pdf/map/Contact_with_Other_EN.pdf

section #5, Preventing galvanic corrosion, you can find some additional information related to your query.

 

[strider6]

what kind of water in the tubeside? seawater, production, freshwater?

S

Corrosion Control

 

[gr2vessels]

Well, indeed, not enough info. I assumed a normal water cooler for the H2S gas. However, if it were a corrosive water, like sea water or production water, I doubt it they would have selected carbon steel on the tube side and the galvanic corrosion would have been the least of their concerns. Therefore I assume some normal cooling water, with not much of the corrosive components. In that case, the channel side would have a lot less exposed area than the Duplex, hence the cathode is protected against galvanic corrosion. However, due to the significant potential difference between the two materials, the galvanic corrosion of the carbon steel channel may be an issue, hence the channel should be protected with a sacrificial anode or insulating kit for the bolts. Obviously, the anode has to be calculated by professionals and installed by professionals. The insulating kit require only a trip to your friendly 'hardware store' and not much of rocket science to install. Perhaps Mechapower will have the courtesy of replying to the answers provided and will clarify for us the extent of his/our problem.
Cheers,
gr2vessels

 

[Mechapower]

Hi All
Thank you very much for your valuable suggestions. I was outside of country so couldn't reply you back.

I have talked with few heat exchanger vendors reagrdign this galvanic corrosion and surprisingly they are not aware of it.

The cooling water for the Heat exchanger will be PH controlled. And i read in many placed that pure water is not an electrolyte. But i have some confusion about it.

I have decided to use isolation kit to the bolts.

Again, thanks all for your suggestions.

 

[Jmoore1]

Mecha, be wary if you don't need the isolation kits, at my site they expressly forbid use of them, even in corrosive environments as they can cause more problems than they prevent.