When to use Inconel, Ferralium, Hastelloy in Seawater?

[ChrisWest8]

I am reviewing a design that uses an Inconel 625 spring made from 0.008" wire stock to hold some small Ferralium 255 cogs in place. The spring is required to stay intact for 25 years, submerged in Gulf of Mexico seawater. I am concerned that even a single pit could cause the spring to fail.

Proponents of the design claim that because Inconel 625 has a PREN > 40, it is immune to general, crevice, and pitting corrosion in seawater. They further claim that the Ferallium 255 will act as an anode because the spring is in close contact with it.

I have argued against the design as much as I know how. It has been agreed that the spring will be replaced with a 0.012" diameter Hastelloy C-276 spring on later models, but that the current design will stay in place for now. I do not agree with either.

To add to this, there is the possibility that low concentrations of cuprous oxide may be present in the seawater, and I am concerned about its effects on the oxide film.

I have several questions:

1. Is Ferralium 255 generally accepted for long term (20-30 year) marine applications?
2. What, if any is the galvanic reaction between Inconel 625 and Ferralium 255? Same for C-276 and F-255?
3. What does PREN > 40 really mean in seawater environment? Immunity to corrosion?
4. Would you accept a 0.008"d Inconel 625 or a 0.012" d Hastelloy C-276 wire to make a spring that you expect to last 25 years in seawater, even if it were coupled to a lmuch larger mass of Ferralium 255?

I would truly appreciate prompt and thorough responses.

Thanks and Best Regards,

 

[metengr]

ChrisWest8;
A suggestion. Alloy behavior in sea water applications can be found from several good resource documents that can be downloaded for free on the internet, see the web site below;

http://www.specialmetals.com/technical.htm#highperformance

In particular is the document from Special Metals titled "High Performance Alloys for Resistance to Aqueous Corrosion". This will give you a good idea on the crevice corrosion and general corrosion resistance of your listed alloys.

Ferralium 255 is a copper-containing, super duplex stainless steel, with corrosion behavior listed in various internet resources, for comparison. You can Google Ferralium 255 for general technical information. I found this out because we one of the AE firms we hired was considering this material to build an FGD absorber vessel.

For sea water applications and galvanic concerns in sea water, check the web site below for excellent technical papers on this subject;

http://www.nidi.org/index.cfm/ci_id/80.htm

 

[ChrisWest8]

Thank you for the resources. I have already read everything available in those places. It was actually the Special Metals literature that got me so worried in the first place.

I really need some more input on this, if anyone else has insight to offer.

 

[SJones]

1. The Mo content of Ferralium is a little on the low side when compared with other super duplex stainless steels which may not render it so resistant to pitting. If you get the PREN of a super duplex above 40, it is usually considered a good candidate for seawater below 30 Celsius. PREN is, perhaps, not such a relevant factor for nickel alloys. However, if the seawater is stagnant within the device you are considering, other corrosion mechanisms may also come into play.

2. For passive alloys, the potential difference will not be a significant driver for bimetallic corrosion. Should one of the alloys go active (more likely the Ferralium) - it's a different story and surface area ratio comes into play.

3. PREN is not a guarantee of immunity. It is only a coarse guide for materials selection.

4. Yes - if the fErralium corrodes it will cathodically protect the nickel alloy!

Steve Jones
Materials & Corrosion Engineer

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

 

[ChrisWest8]

The contact with the Ferrallium is not electrically sound, it is simple contact. Isn't crevice corrosion and pitting of the spring in other areas still possible if the connection to the ferallium is not good? I know that we always check anode connections on our pipelines to ensure extremely low resistance. Doesn't the oxide film insulate to some extent?

What about corrosion of either material directly at the points of contact due to oxide film damage (mechanically or by the induced current) which could initiate pitting or crevice corrosion.

What about cuprous oxide's effect on the passivity of the metals?

I have reviewed technical sources that cite depths of crevice corrosion attacks on Inconel 625 as high as 0.15 mm/180 days and C-276 as high as 0.1mm/180 days. I have also reviewed multiple technical papers that warn that no metal is "immune" to corrosion in seawater, and that every application, especially critical ones, must be evaluated separately and thoroughly, and that all variables must be taken into account (environmental pollution, biological presence, galvanic compatibility, etc.) Is this a pretty standard case or would trials be called for if integrity is extremely critical?

--Chris

 

[ChrisWest8]

By the way, the water is stagnant. There are numerous crevices as well. There is a particular abundance of crevices on the Ferralium part itself, but also some on the 625.

I have just realized that fouling of either part will also hampber the function of the device. I am concerned about biofouling or simple particlate fouling on the I-625. However, with the Ferrallium, there is also the risk of oxidation fouling right? Oxidation fouling in the crevices mentioned above would be especially delterious to the device's proper functioning.

SJones---any chance I could send you a better description of the device (drawings, pictures) to give you a better picture of what I am dealing with?

 

[EdStainless]

What concerns me are a couple of points.

There may not be enough galvanic difference between the F255 and the 625 to protect the 625.

If the F255 suffers crevice corrosion at a contact crevice with the 625, then the corrosion products will lower the local pH and put a lot of metal ions in the local solution.
While the active pitting of the F255 will protect the 625, once the pit growth stops, or gets farther away, then the local environment may lead to pitting of the 625.

How good is the 625 spring? Any local surface defects (scratches or nicks) could be real issues.

The F255 does have a good track record in seawater, but I don't know about 25 years.
Tools such as PREN or CCCT are used for rankinghte relitive preformance fo alloys. They tell you nothing about actual service conditions. You must determine that by comparison to known applications.

I like the move to a heavier C-276 spring. The better alloy and lower stress are both good.



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