Galvanic Corrosion question for the experts

[UMDpru]

I have been asked to review the corrosion potential of a bolted joint. The fastener material is MP35N. This fastener is being used to bolt together two pieces of Monel K-500. The environment will be in seawater and the flow rates around the fastener head are likely to be very very low if any at all.

I have been reading a great deal and have come to the conclusion that MP35N is susceptible to Galvanic corrosion when used in the preceeding fassion. The question for me now is how much corrosion can I expect and over what time frame.

I am primarily a Mechanical Engineer and I have no background in Corrosion Engineering so I was wondering if someone more experienced could provide some insight or direction...

My Mechanical group really has no one experienced with corrosion either so I am alone with this.

any help would be appreciated...

Adam

 

[EdStainless]

I can't imagine that there is enough galvanic diff between K500 and MP35N to cause galvanic problems. You may need to get some samples and make some measurements. There are people that can do it for you if needed.

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Plymouth Tube

 

[UMDpru]

some references that I have read have suggested that you need a greater than .15V difference in potential before you start to see galvanic problems...

I have pretty much come to the same conclusion as you - the two metals, although different, are very close on the galvanic series and thus shouldnt pose a problem...

any other opinions out there in the eng-tips world?

 

[chief]

Use a good quality marine paint to give the bolt/fastener extra protection

Offshore Engineering&Design

 

[gr2vessels]

Can you use bolting insulators (needed only for your peace of mind)?

 

[EdStainless]

I would prefer to not use isolators or coatings. Minor failure in these can lead to accelerated crevice corrosion. Just let the alloys do their job.

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Plymouth Tube

 

[UMDpru]

this connection is of the higher pre-load variety. one part of the joint will be painted to maintain low visibility - the anode alloy.

I have found in MIL-STD-889B Notice 3 that Nickel-Copper and Nickel-Cobalt alloys are incompatible in a seawater environment anyhow.

I'm going to have to look at different alloys with mechanical properties close to MP35N.

Thanks for the help everyone!