Electrode Potential of Duplex Stainless

[BLaroux]

I've been searching for good numbers on the electrode potential of 2205 duplex stainless, but keep comming up dry. Conditions/specifics are:

Flowing Seawater (8-13 fps)
50 to 80 degrees F
Sat. Calomel reference electrode

I've found one reference to it in these forums as being "between that of 430SS and 3XX series SS" (~-0.1 to -0.3V) under these conditions, but I'm in real need for more concrete #'s.

On other sites some of the super duplexes (ferralium 255 in particular) are decribed as having similar potentials to titanium. (~0.1 to <0V) This is a pretty big difference when compared to the above info on 2205... im wondering if this increase in potential is typical with the addition of more Cr and Cu?

The application involves galvanic compatibility with nickel aluminum bronze, ... primarily I'm attempting to determine if 2205 is a closer match to it than 316SS.

My company has a serious lack of reference materials on this subject...any suggestions for a source that would contain this kind of info would be appreciated!

 

[EdStainless]

What do you have for the NAB valuse? -0.150 to -0.200 V is what the NiDI ref shows.
316 Passive is 0 to -0.100 V
Ti is +0.050 to -.050 V

I know that the high alloy 6% Mo stainless grades are 0 to -0.050 V
Given it corrosion resistance 2205 would have to be in the same range.

The books from NI (now called the Nickle Institute) on corrosion in seawater and brines (No 11003 is one) are great sources, and they are free.

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[kenvlach]

Go to ASSIST and get MIL-STD-889B DISSIMILAR METALS.
"This standard defines and classifies dissimilar metals, and establishes requirements for protecting coupled dissimilar metals, with attention directed to the anodic member of the couple, against corrosion."

http://assist.daps.dla.mil/quicksearch/basic_profile.cfm?ident_number=36032

The table of materials & their compatibilities in Notice 3 may be all you need.

 

[BLaroux]

Yes, I have the same values for NAB.

Thanks for the references, they have already helped alot. I had been working off of an old revision of 889 without the notice 3 chart and was finding it to be a bit sparse.

So it sounds like I should be comparing corrosion resistance when trying to estimate a metal's electrode potential. Good to know... I was trying to compare composition instead, which wasn't really making a whole lot of sense. (I still see a few exceptions to this new rule of thumb though...)

Although I'd prefer a experimentally determined potential for 2205, it looks like the 0 to -0.05 approximation will have to do. If that the case then 2205 + NAB is borderline ok. (200 millivolt max. difference) The NAB is a much larger wetted mass anyway.

I have the same trouble finding the potentials of other alloy systems (various stainlesses, Stellite, etc.)... is experimentation my only option if I want exact #'s for individual alloys?

 

[EdStainless]

For most alloys the particular process condition and surface finish will have a significant impact on the potential. Exact numbers are only applicable in the exact conditions that they were measured.

If you need values for stainless grades, find other grades with similar pitting resistance and use those values.
For 'odd' alloys you may need to make educated guesses. Often old alloy lit. will have such information.

As I see it the 2205 vs NAB difference is no more than 0.15 V. Given the larger amount of NAB, and that both alloys are corrosion resistant in the environment that would not bother me at all.

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Rust never sleeps
Neither should your protection

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