Connecting duplex, monel and aluminum / bronze in seawater service

[samoht]

A flanged Aluminium Bronze valve is connected to a Duplex (22%cr) vessel. Our client has specified the gasket material to be Monel. The media is Seawater. The SW temperature may vary from 15degC to 45degC. Is galvanic corrosion a concern with this configuration?

 

[EdStainless]

I am concerned about the AlBrz and dealloying. The duplex/monel should be fine.
Just remember a 22% Cr duplex will not fully resist cevice corrosion in seawater.

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

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

 

[SJones]

At 45 deg C, your client may be replacing the vessel a little earlier than planned.

Steve Jones
Materials & Corrosion Engineer

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

 

[mshimko]

I would be concerned about galvanic corrosion of the bronze. Small amount of bronze, large amount (area) of stainless, I'd be concerned.

 

[samoht]

mshimko,
Since there is a chance for galvanic corrosion, would you recommend using a non-conducting material in the gasket i.e. rubber?

Sjones,
What is your main concern at this temperature? FYI customer plans to use some kind of corrosion inhibitor (chemicals) there is also a 3mmm corrosion allowance on the vessel. Oxygen level at normal operation will be approx 300ppm. Design life is 25years.

 

[mshimko]

If you can reliably electrically isolate the bronze vale, at EVERY connection, and verify same using impedance measurements, then its an option to consider.

WHat I would recommend, (but maybe not practical at this point in time) would be to install a duplex stub about 12 incges long off the vessel, then connect the valve, This would at least provide an equal ratio.

 

[joeswoes]

...or use composite valves and don't worry about it.

 

[SJones]

A 3mm corrosion allowance on a duplex stainless steel vessel plus the use of inhibitor - somebody is having a laugh. The PREN of 22Cr duplex is not sufficient to withstand seawater at 45 deg C. A 40 minimum PREN is required taking it straight to 25Cr. Why pay for a duplex vessel and then pay again for corrosion allowance and chemical corrosion control?

Steve Jones
Materials & Corrosion Engineer

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

 

[samoht]

Since our scope does not include corrosion control I have to make a guess that it is there to protect the seawater piping system as well as the vessels. Why pay for all this? That is a question to which I do not have the answer.

 

[SJones]

I suppose the client knows best eh?

Steve Jones
Materials & Corrosion Engineer

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

 

[mshimko]

I saw an article a few years back in NACE Materials Performance. The point was one of the primary aspects of being a corrosion engineer is SALES. Sales in that we need to convince someone (typically the boss or the customer) that it may be smart to spend more money now and avoid a corrosion problem later, as corrosion is almost always a probem "tomorrow" rathr than "today".

When dealing with cutomers, I'd just just suggest that you (a) answer the mail; that is, provide the answer to the question the curtomer asks, but then (b) provide the answer to the question he didn't ask.

It may not help as often as we would like, but I've seen customers change course based on this type of feedback (most recently just 2 weeks ago, in fact). Occasionally I even submit responses to solicitations that essentially suggests (in nice terms, of course) the customer is asking the wrong question.

 

[EdStainless]

The other thing to remember is that there is no such thing as a corrosion allowance in stainless steel exposed to a chloride environment. Since the failures will be localized, either crevice or pitting attack, the time to penetration is limited by corrosion initiation. Once it starts it will penetrate the full wall quickly, adding metal will have little effect.

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

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