Galvanic corrosion 3

[Nicknackwhack]

Hi

What possible condition would be required to enable galvanic corrosion in a deaerated environment?
I'm looking at a join between duplex and carbon steel which has condensed water with dissolved CO2 and H2S.
Is there a possibility of the combination of the two dropping the pH to make galvanic corrosion a credible threat through hydrogen evolution off of the Duplex?

Conditions

90C
10 barg

 

[CoryPad]

You already have described conditions which could initiate galvanic corrosion: dissimilar materials in contact with each other and an electrolyte (the water solution) which is corrosive (water acidified by carbon dioxide and hydrogen sulfide).

 

[metengr]

Surface area of the anode and cathode must be considered as part of the galvanic equation.

 

[EdStainless]

Oxygen is not the critical factor, conductivity is.
Is this a mechanical joint? Like a tight crevice? The steel will corrode, just how fast is the question.

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Plymouth Tube

 

[Nicknackwhack]

Thank you all for your input. I know there will be a galvanic circuit in place my query is whether it will be negligible in effect. I am investigating a rather large cathode (tubes) and small anode (tubesheet)
The CO2 is only 0.12 mol% and the h2s is 100 ppm.

 

[metengr]

A large cathode is not recommended for galvanic corrosion. You need a large anode to cathode area ratio.

 

[moltenmetal]

You do need something to take up the electrons resulting from the oxidation of the anodic metal for galvanic corrosion to be sustained. That does not need to be oxygen, but there must be something that can be reduced. Your system, being an open loop rather than a closed one, may still have something in there which can accept the electrons. Reducing conditions alone are no guarantee of corrosion resistance. That something is neither H2S nor CO2 unless I'm missing something, but both of those species reduce pH and increase ionic strength of the electrolyte (both bad), and the S-2 can disrupt the passive film on many alloys too. If the pH is low enough, the something accepting the electrons could be H+ which can be reduced to H2. Whereas in a closed loop system the pH will rise as H2 is evolved and leaves, in an open loop this never happens.

In summary, yes you have some reason to be concerned, even though it's deoxygenated.

 

[OGMetEngr]

Excellent answer moltenmetal. Hit the nail on the head on that one.