Crevice corrosion on seawater exchanger 2

[mthakur]

Any ideas or experiences of major crevice corrosion combine with minor pitting on the tube sheet face made in 254 SMO, especially on the gasket sitting areas, medium seawater at moderate temp of 40 deg C. could this be due to using gasket material having inconel 625 impregnation within?

further will a duplex and super duplex SS for the tubesheet will help in improving the situation?

 

[EdStainless]

1. Changing alloys will only help if you go to a more pitting resistant material (higher PreN). Don't compare PREN for duplex and autenitic, they only can be compared within an alloy group. Look at Critical Crevice Corrosion Temperatures, but be aware that there is a lot of vairability in the test.

2. I doubt that galvanic corrosoion played a role. Sound like simple crevice. Thought, what is the gagket material?

I have seen units made from 6%Mo superaustenitics that had C-276 weld overlay in the gasket regions.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[SJones]

30 deg C is usually considered the limit for these alloys in chlorinated, aerated seawater. Higher temperatures may require C-276 or titanium.

http://www.nace.org/nacestore/assets/ConferencePapers/1998/98707.pdf

and

NORSOK M-001



Steve Jones
Materials & Corrosion Engineer

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

 

[mthakur]

Thanks EDstainless for the valuable input.
The gasket material is compressed non asbestos fiber wire impregenated with inconel wire.

The PRE number for SMO 254 is higher compared to that of Dupelex, in what way duplex SS are considred better except their tensile strength which is better than SMO.

Is it standard practice to have overlay of another material on gasket region of SMO 254? In this case this problem is common in nature.

You experience with coating the tube sheet wtih epoxy coating such as Belzona etc for the protection against pitting/crevice?

 

[EdStainless]

Let me clarify my PREN comment. A duplex with a PREN of 40 and an austenitic with a PREN of 40 will not have the same crevice corrosion resistance. Of course you also have to be careful how the PREN is calculated, different companies use different formulas.

That aside, the 254SMO is at the low end of superaustenitic and superduplex alloys. The range isn't huge and I wouldn't expect an alloy only a few points higher to work significantly better.

Overlay is common, but not for gaskets. Gaskets are uncommon in seawater service because of the crevices formed. A welded joint would be more typical.

What is your tube material? Coating with epoxy is common when you are using low alloy tubesheets. It might work here. Just remember that the edges or the coatings are crevices also. At least if they are in a flow area they are a lot less likely to begin pitting than in a stagnant area.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[mthakur]

The tubes are made in sanicro 28, infact Belzona/epoxy actually helped to stop the further deterioration as experienced in three years of operation. But then the epoxy is not a accepted solution in terms of industrial standard. Our experience so far with epoxy is fairly good provided its applied with a good surface preparation. Does seawater velocity also plays a major role for pitting corrosion, but even with low flow there will never be a situation where some areas will experience stagnation.

 

[EdStainless]

Epoxy coated tube sheets are very accepted in the power generation industry. Many condensers would not function without tubesheet coatings.
Yes, you are right, sample prep is everything with coatings. A good coater is work every penney (I know one).

My concern about the flow is that I wouldn't want the edges of the coating to to be in a stagnant corner.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[bmoorthy]

M Thakur

Is this exchanger at BPCL?

 

[mthakur]

Mr Moorthy,
Its in the middle east, Does BPCL use seawater as cooling medium for heat transfer?

 

[bmoorthy]

M Thakur

I am not sure about that. But i have heard of failure (Severe Pitting and Crevice) of 32750 Over lay on Flange faces Especially below the gasket faces, despite the fact that the PQR was tested for G48 tests.

There is one more thing you may want to check, whether the flange is distorted (however slight may it be).

This is because during welding, if the flange got distorted, then while machining the gasket face, there could be regions where material is removed in excess than required (Especially towards the outer edge, where the gaskets are located, if it where confined type). It is very difficult to determine the actual overlay available after machining.

When you bolt the flange and the Tube sheet, if you observe the difference in OD then the above reason could be attributed to failure.