Alternatives for 316 Crevice Corrosion 3

[nickel188]

I am attempting to solve a crevice corrosion problem on an instrument that my company manufactures. The instrument was designed for use in fresh water up to 250 ft deep and is made of 316 stainless steel. Ideally it should have a life of 5 years or more. The design of the case has several o-ring grooves and one press fit. It works great in fresh water, but in salt water it is ideal for crevice corrosion and my testing has demonstrated failure (leaks) after a few months in moving salt water with moderate bio-fouling.

I am very price sensitive so I am very reluctant to switch to titanium. Can you recommend an alternative (machineable) material that will not suffer the effects of crevice corrosion but is a bit less expensive? Marine bronze? Certain grades of aluminum? or do I need to switch to something like Inconel 625 or AL-6XN?

Alternatively, is it possible to protect the device by attaching a sacrificial metal such as a zinc? Does cathodic protection work for crevice corrosion?

Thank you,
Nick

 

[EdStainless]

Ti or AL-6XN should work fine. But the price will kill you. Can you redesign to use less metal? 6XN is a lot stronger than 316 or Ti. Reducing the thickness would help offset some of the material cost.
Don't waste money on 625 unless you need high temp strength. For simple Cl corrosion resistance 622 or 59 willb e the same price and have better crevice corrosion resistance.

Could you use a non-metalic? I don't see good options for metals if you can't afford something that is 3x the price.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[nickel188]

Non-metalic is not really a viable option because I would need to go through a fairly extensive re-qualification process.

What about adding a zinc? Does that protect from crevice corrosion or only general surface corrosion?

 

[shifter]

Tiodize and General Magnaplate both have anti-corrosion processes for 316 that'll prevent this type of corrosion. In fact, Tiodize has a process that mimics anodizing in a sense. (X-?) It (possibly) pulls chromium to the surface for added protection. I've also used Tiodize's Tiolon 1000, which is a coating. The exposed surfaces may wear off, but the critical areas which in essencially anaerobic environments remail well-protected.

 

[mshimko]

From a past project whose sole purpose was to minimize or eliminate the risk of crevice corrosion under o-rings, I've reached a few "common sense conclusions" (actually, ideas I remember from others):

1 - all materials are subject to crevice corrosion, given a sufficiently suvere crevice.

2 - galvanic corrosion (or the inverse - cathodic protection) is ever present.

With these in mind, here's a suggestion that may or may not be cost effective (you'll need to decide): consider cladding a thin layer of a more crevice-corrosion resistent material where the o-ring seats. This new material will also be cathodically protected by the bulk 316-CRES in the area.

 

[EdStainless]

The only problem that I have with coatings/surface treatments in general is that when you have a failure, and it will always happen, the area ration between active and passive is so unfavorable that failure is assured.
When I used to coat equipment for continious exposure like yours, we assembled it then sprayed with Monel, and then sealed with epoxy or urethane (depending on temp).
The sealer had a very nice rough surface to bond to, and if the surface was damaged all that was exposed was Monel, not the underlying stainless.
This makes units un-servicable, but it works. You can't coat of seal only a portion becuase the edges will be subject to crevice attack.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[shifter]

You might want to investigate Tiodize's X-40. From my experience, it is unlike any normal "coating" - more like a hard-anodizing for stainless. We have specified this process for military connectors and have not had any problems for many years. It was specified particularly because we wanted to protect o-ring surfaces.

 

[mshimko]

For EdStainless - I agreee 100% with the surface area concern if/when failure occurs, which is why I recommended coating ONLY the area in contact with the o-rings.

I also have a similar concern as "we" gradually shift piping materials to more corrosion resistent materials; we are aimultaneously increasing the problems of galvanic corrosion of the "last piece" of the previous material.

 

[mlafond]

You might want to check out an older posting by stevenmon. It talks about crevice corrosion and using lubricants to seal out the salt water. Just do a search for stevenmon and you will come across it.

 

[WayneAlexander]

Have you looked into Silicon Bronze? With some bronzes, dezincification is a major concern, but not this this one, and I don't think crevis corrosion should be a concern with this. For vessel thru hull fittings, this is a choice alloy. Studies have shown that "sometimes" anodes have helped with crevis corrosion in SS applications. The problem is defining sometimes.

 

[marine101]

Nick,
Do you have any inforamtion regarding depth in fresh water when stainless steel crevice corrosion would occur?
thanks,
bob

 

[nickel188]

marine101, I do not have information about depth in fresh water/vs crevice corrosion. My understanding is that crevice corrosion with 316 SS is not an issue in fresh water.

What I really need is a better understaning of the relative resistance of various materials to crevice corrosion. What is the relative resistance of 316 SS versus titanium versus AL6XN versus silicon bronze? There must be a standard test for crevice corrosion in salt water with a table listing the relative resistance. Can anyone point me towards this information?

 

[EdStainless]

Other options that come to mind are NiAl Bronze, 70/30 Cu/Ni (one of these might be your best bet), and Monel, but at todays Ni prices Monel would be very dear.

There are a number of publications available from the Ni Institute concerning material selection for saline waters, brines and such. I have a few of them and none that I have list crevice corrosion limits for all of the potential alloys. I am sure that it is there somewhere, just not in my sources.

One reason that this info is not common is htat each alloy system (stainless, Ti Cu based) have different weaknesses. Because of this they tend to have different standard tests.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[mshimko]

Relative CC resistance of various metals?

I don't know of a single listing; but I know the first place I would look for data: the Handbook of Corrosion Data by ASM International.

If you want a relatively decent yet concise description of CC testing, I recommend ASM Handbook Volume 13 - Corrosion.

 

[unclesyd]

While we are looking here is one that is being used more and more.

http://www.hpalloy.com/2551.htm

 

[RPstress]

In an example I read of, in a severe chloride environment (a swimming pool building) they were having bad cracking problems, and they wound up using 317LMN. This didn't crack. (They needed the LMN; 317, 317L, etc., weren't good enough.)

Are you sure it's crevice corrosion and not SCC?

 

[rolledalloy]

Comparisons of AL-6XN to 316L and 625 are available in Allegheny Ludlum's website at titanium is not included.

http:/

http://www.alleghenyludlum.com/ludlum/documents/al6xn.pdf

It may be of some assistance.

 

[EdStainless]

If cracking is the problem then 2205 (or even a lean grade like 2101 or 2003) will be better than any 317 variant.
2205 will even give 317LMN a run in terms of pitting resistance.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

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

 

[geoffthehammer]

We had a smiliar problem with 316 stainless but we were immersing it in saline solution - much higher salt concentrate than sea water. 316 would show considerable corrode within days.
We changed the critical parts to either Ti or NiAlBronze. Both to good effect.