sea water corrosion 316 vs 17-4

[mecoman]

Hello,

Looking for a little advice for gaining strength in a sea water (salt water) environment.

Presently using 316SS (no passivation) on a vertical post 1/2" diameter threaded in at the bottom with shoulder stop. Part is breaking above shoulder with large fillet applied. I know I need to at least passivate since I think corrosion is pitting away and weaking the part at stress points. I am also think to change to 17-4H1150 to increase yeild strength, but am a little concerned about corrosion and wanted to know if other options may be available. I believe toughness is an issue.

Part may be dry, wet or cycle between the two. Rod is being use to hold an oar assembly.

Thanks for you feedback and ideas.

Kind Regards,
MECOMAN

 

[EdStainless]

I wouldn't expect either material to survive. You might consider the duplex 2205 for this. It isn't fully resistant to corrosion seawater, but it is a lot better than either of your mentioned alloys.

Passivation will not give you meaningful improvement in corrosion resistance. 316 simply will not stand up in a seawater environment. But yes, it is good practice in order to make sure that you get all that you can from an alloy.

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Rust never sleeps
Neither should your protection

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[TVP]

In addition to what EdStainless mentioned, this part will be quite susceptible to stress corrosion cracking (SCC). You really need to change to something more appropriate, such as a duplex grade like 2205 or a nickel- or titanium-based alloy if you want complete immunity from corrosion.

 

[mecoman]

Thanks Ed,

I am still not sure if the failure was from corrosion or excessive load. The part does experience a large amount of repeated shock loading with a good bending moment.

Regarding passivation, I have seen none passivated thread corrode in 316 in 1-3 months and then apply passivation and get 3 years with part removed for other reasons/service.

How about the 17-4, added yeild strength should improve toughness, any thoughts on it?

Regarding the duplex 2205, how does it compare yield strength wise with 316 and 17-4 H1150?

Thanks Again,

MECOMAN

 

[EdStainless]

The standard annealed properties for 2205 are 65ksi min yield and 95ksi min UTS with a 25% min elongation. So it is roughly 50% stronger than 316L and much more corrosion resistant.

If you have parts with significant amounts of Fe int eh surface then passivation makes a huge difference. If a part is exposed to abrasion or wear then passivation doesn't really matter since you will be damaging the surface.

Your failures are probably some combination of corrosion and load. How much of each would take some work to figure out.

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

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

 

[stanier]

SAF 2205 may also gives problems with a threaded part and you may need to consider SAF 2507. A local environment where sea water is stagnant may compromise the SAF 2205. I would expect this to occur in a threaded component.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

http://www.waterhammer.bigblog.com.au

 

[mcguire]

The experience of undersea oil producers is that 2205 will not pit in seawater unless there are crevices, in which case 2507 is necessary to avoid all traces of corrosion.
You may get away with 2205, or even 2003, if you ensure
that the locus of failure has full exposure without a crevice. 2003 will pit, but it's a lot better than 316 and as strong as 2205. If 316 can last 3 years, then the added corrosion resistance of 2003, and moreso for 2205, should be a sufficient for a good engineeering solution, if not total immunity.
2205 will be the easiest to obtain if not the cheapest.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/