Galvanic Corrosion of 316 Thread

[Lee.k]

I have a Duplex plate (exact grade unknown) that will have a tapered thread tapped through it. A tapered 316 st st adaptor will be screwed into it. Both components will feature on an earthed skid, in a salt water environment (offshore application). No loads will be experienced.

My questions are:

Is an earthed assembly good enough to avoid galvanic corrosion of the 316 St St adaptor?

If crevice corrosion, in the 316 St St adaptor, might be an issue, what precautions that can taken? Note - welding is not an option, as the 316 adaptor will be fitted and removed occasionally.

 

[EdStainless]

316 and sea water? So you only expect this to last for a few weeks right?
If this is longterm then superduplex (25-27%Cr) should be used for both parts.
Galvanic is the least of your worries.
The 316 will crevice corrode unless you absolutely seal the threads.
I have seen silicone sealants used as well as non-hardening thread lock compounds.
However you will then get crevice corrosion at the edge of your sealant.


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[TugboatEng]

This might work if you can attach sacrificial anodes to it. Silicon bronze would probably provide the best cost/performance ratio. However, if this is a large structure you would have to attach a whole lot of bronze to protect one fitting.

 

[Lee.k]

Points taken. I'm trying to source a suitable adaptor in Duplex, as 316 st St isn't the best choice (hence me writing my concerns).

What does crevice corrosion dependant on, other than material, e.g temperature and pressure? The operating temperature will be 0degC-40degC.

 

[TugboatEng]

Crevice corrosion depends on a lack of oxygen. Anywhere water can enter but not flow freely is subject to crevice corrosion. Threads are especially problematic because their tight clearances support capillary action which draws water in and holds it. Crevice corrosion can occur at any contact point including loosely adhered paint, under dirt, even under grease.

 

[EdStainless]

The tighter the crevice the worse that it is.
Once corrosion starts the local chemistry gets worse (lower pH, more metals, less oxygen) and progresses faster.
This is the same as pitting corrosion, but a tight crevice just starts corroding sooner.
Oxygen matters because SS needs it in order to re-passivate and stop corroding.
All of this corrosion is electrochemical so temp, pH, and chemistry (Cl and conductivity) are the keys.
With temp and pH being the major ones.
I have a photo where an inspector wrote "OK" on a part in paint pen.
After a few months there is a beautiful series of pits outlining the letters OK.

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P.E. Metallurgy, consulting work welcomed