PREN, Inconel 625, Ferralium 255, C-276 and 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,

 

[EdStainless]

I'll give you a call.
ed

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