Galvanic corrosion bewteen 316L st st and 6Mo st st subsea

[robwyatt]

Hello all,
I have an application where I need to connect a 316L st steel component to a 6Mo st steel component.
I have been prompted to assess the potential (no pun intended) for galvanic corrosion between these two components.

I would appreciate your thoughts and advice.
Here are my thoughts:
1. They are both austenitic st steels, so should be close on the galvanic series? (need to be <-0.15V potential difference).
2. Would adding sacrificial anodes (zinc) immediately adjacent to the joint prevent the galvanic corrosion?
3. Separate the compnents with an intermadiate material that would be compatible with the other two, e.g. Inconel625 / Titanium? All materials must be non-magnetic due to high electrical current passing within the items.
4. Components cannot be (electrically) isolated from each other as a metal-metal seal ring must fit between the two items.

Many thanks in anticipation of your advice,
Rob

 

[SJones]

If the assembly is hooked into a cathodically protected structure, the risk of galvanic interaction is minimised. Coating with thermal spray aluminium could be an option to achieve the same result. Examine the area ratios - if they are favourable to the 316 (i.e. the 6Mo has significantly less area) no other measures may be needed but 316L is not ideal as a subsea material without CP.

Steve Jones
Materials & Corrosion Engineer

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

 

[EdStainless]

I would suggest some galvanic protection. You will probably get some crevice corrosion of the 316 at the connection.

There should be "dissimilar metal crevice corrosion" data in the AL-6XN source book at the Allegheny Ludlum web site that shows 6XN-316-6XN tested in seawater for one year.

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Plymouth Tube

 

[strider6]

it seems that there could be a galvanic corrosion and some caution in design should be used.. the final decision also depends on the type of component we are talking, if it's accessible for inspection/maintenance, criticality of the service...

this doc can give you some advice, but not much..

http://www.csidesigns.com/PDFs/AL-6XNCatalog.pdf

this doc can also help you, look at page 67-69.

http://www.corrdefense.org/Key Docu... Management Guide for Selecting Materials.pdf

hope this help

S.

Corrosion Protection & Corrosion Control

 

[corrosionman]

Robwyatt
If two dissimilar metals are immersed in seawater and a potential exist between them, then , even if the assembly is subject to a cp system, that same potential will still exist, and , presumeably there will be massively increased corrosion on the least noble of the immersed metals.
You can minimise corrosion by coating both of the immersed metals. you can eliminate it by electrically insulating all metals from each other (but you say this is not possible ). Best of luck.
CM

 

[SJones]

Corrosionman,

That 'same potential' may not exist if an external anode is added. The resultant potential difference between the two alloys will be dependent upon their polarisation behaviour in seawater and not their individual rest potentials in seawater.

Steve Jones
Materials & Corrosion Engineer

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