304 vs 316 for below waterline marine applications on boats

[EricatNordic]

We are very cautiously investigating a switch from 316ss to 304ss on less critical below waterline marine hardware applications where some corrosion could be tolerated over a 10-20 year service life. The applications would mainly be skegs and rudder blades fabricated by welding waterjet cut plate. All the rudders would still have nautiloy or aquamet shafting only the plates are being considered for 304. Everything is bonded and fitted with sacrificial zinc.

Anyone made a first hand comparison between the two alloys in applications like this? Did the change shorten the usefull life of the parts or were they still being replaced at the same inteveral for other reasons?

The main reason for the switch is that 304 is available more consistently.

 

[kenvlach]

Above waterline, where salt can concentrate as spray dries, 304 is much worse. Below waterline, maybe OK if very cold water & use 304L. See
Corrosion Resistance in Marine Environments

http://www.assda.asn.au/asp/index.asp?pgid=17991&cid=34299&id=56951

 

[EdStainless]

I presume that you are trying to save money? Look at lean duplex grades, Outokumpu LDX2101 and Allegheny AL2003. They have as good or better pitting resistance than 316L, they are much stronger (make thinner), and they cost less.
I see 304 as taking a risk for this application.

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Rust never sleeps
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[EricatNordic]

Thank you both. Helpful articles.

How are the duplex alloys for welding? Do they still require low carbon welding rod? Do they behave predictably between physical test and FEA models?

-Eric

 

[EdStainless]

Yes, you need low carbon filler any time you are welding ot stainless.
There are special weld rod grades for welding duplex-to-duplex. If you are welding duplex to austenitic stainless then 309L would be the preferred filler.
The Outokumpu web site (is difficult to navigate) has some great welding guides on it.
These grades have been used extensivly for structures and equipment. The properties are predictable.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[EricatNordic]

You have all been very helpful. Thank you.

-Eric