Are you sure that you didn't get the materials backwards?
Why isn't the bolting stainless?
And yes, it looks like there is some damage, but what does the inside look like?

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P.E. Metallurgy, Plymouth Tube

Looks like galvanic corrosion of the steel bolts in contact with the stainless bellows.

And of iron embedded in the surface from tool impacts.
It is possible that the corrosion products from the rusting iron then lead to pitting.
I presume that the bellows is 321, which does not have enough corrosion resistance for any marine application.

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P.E. Metallurgy, Plymouth Tube

As with Ed, it does look like tool impacts leading to contamination of the bellows surface.

Hi Keith
Bellow Materials is :EN 1.4662 is Lean Duplex LDX 2404, UNS-S82441, Flange is :EN-1.4501,SDSS-S32760.In a corrosive environment like marine environment.Hence using Carbon steel bolt would definite accelerate the process of galvanic corrosion.That would be the first step to avoid.Use of Duplex S.S bolting would definitely be advisable.You may see the details at: http://www.ssf.co.uk/duplex-bolt.html.

As per the picture attached there're numerous corrosion sites on the SDSS flange away from the C.S bolt touched area.Do you have this scenario all over the flange.Zeron 100(UNS 32760)is extremely proven for sea water applications. However if the exhaust temperature is too high (60 Deg C or Up) pitting is possible.
How was the supply condition of the flange, solution annealed, machined & Pickled or Not No Pickeled, those are important considerations also.

Appreciate your thoughts.

Thanks.

Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299
All provided answer are personal opinions or personal judgements only. It's not connected with any employers by any means.

acide condensate in the botton of the line is a piting corrosion possibility-

lm

KO...

NASA tested 321-A CRES in a marine environment and discovered a tendency for this kind of penetrating corrosion. I have also seen this peppered-pitting corrosion in acft ECS duct exterior surfaces [no bellows as-of-yet], but have NOT been able to determine the cause... especially when adjacent pieces of 321 sheet metal never seem to pit like these few/specific sheet metal sections.

NOTE.
Bellows formed from Inconel [nickel alloy] 600 or Inconel [nickel alloy] 625 would likely perform much better in this marine environment, especially with complimentary [similar material] fasteners.

NOTE.
Aerospace bellows often have an internal sleeve seam-welded on the up-stream end of the bellows intake, spanning most/all the convolutions up-to max extension, for fluid-flow stability. IE: the separated convolution roots tend to introduce high frequency turbulence in the duct flow that the long-sleeve minimizes/eliminates. IF this sleeve is oriented so flow is upward [sleeve is fixed on lower internal end of bellows], then even minor back-flow [condensation, carbon residues, etc] could easily be trapped/stagnated between the sleeve and convolutions. In this case a discrete pattern of 'large-enough' bleed holes, positioned just above the sleeve attachment, could be useful in flushing-out acidic-gas/fluids.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
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321 is very sensitive to surface condition when it comes to pitting. It is also impacted by the level of Ti and morphology of the TiCN particles. It is a pain to get lab tests to repeat.
In a seawater environment the only real options are a 6%Mo superaustinetic, a superduplex, or a high Cr+Mo Ni alloy (625 or C276).
I don't think that the bolts have anything to do with this. The spots are either from installation damage or they are where there are welds (bellows to flange or the internal liner).

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P.E. Metallurgy, Plymouth Tube

Many thanks for all of your inputs... it is well appreciated and hopefully our designer and manufacturer can take your comments and suggestions to progress this issue.

Thanks again

Keith Olsen
Full Ahead Marine Engineers LLC