The numbers you are looking for are standard electrode potential
<http://en.wikipedia.org/wiki/Standard_electrode_potential_%28data_page%29>.
Their use depends on environment, so the may not be as easy to use as
the standard implies. They are of even less help when getting into
alloys and coatings.
A discussion is at Wikipedia on galvanic corrosion
<http://en.wikipedia.org/wiki/Galvanic_corrosion> which also has a
simpler table. Few of our products however are in a moist environment
which enhances galvanic corrosion. It's probably time to replace the
table in the standard with more appropriate advice.
Bob
Galvanic corrosion
On 03/25/2014 06:30 AM, John Woodgate wrote:
In message <[email protected]>, dated Tue,
25 Mar 2014, Doug Powell <[email protected]> writes:
Thanks for your insights, I believe we are in agreement. Possibly the
reason the table was never revisited is because either the potentials
listed were taken from an undocumented source or some sort of an ad
hoc test method was used and never documented.
It was taken from a physical chemistry textbook, but by someone who
was not a chemist and didn't understand what it really means. A
similar thing happened with the definition of 'petroleum spirit',
which took about 30 years to fix.
No one today wants to risk upsetting years of usage based on this
table. I see no reference for the data source, although I have not
tried going back through earlier editions. I am currently viewing IEC
60950-1: 2005 and 2001.
Well, they are seriously out-of-date in many other respects than that
table. But the table dates from before TC108 was formed; it was in the
TC74 safety standard, whose number I don't have.
The two errant points indicate 0.5 V. Unfortunately, this table is
normative. Interesting that silver, gold and platinum are listed in
elemental form. When used as plating on another base metal, I am not
so certain these potentials will be the same.
The potentials don't have any great significance anyway, in terms of
corrosion in the real world where there are chlorides, sulphates and
all sorts of other stuff present.
I also noticed, the way the numbers are set up there are no negative
potentials as you would find in a anode-cathode system referenced to
hydrogen. In reality electrochemical potentials, or more correctly,
electrode potentials must have a standardized point of reference.
In the 60950 table it is apparent the reference was reassigned as the
negative electrode of every combination.
Yes, those are differential voltages.
My guess is someone did an experiment with commonly available
metallic samples of the day and placed them in an aqueous solution.
Simple measurements of galvanic potentials of every combination were
taken.
Right, but it's unrealistic, as there are no anions present except H+.
I expect that the work was done properly and written up as a
peer-reviewed paper, maybe in the early 20t century.
My hope was to reproduce the experiment that generated these
numbers and expand the table to include other alloys commonly used in
today's world. The problem of course is all the unknowns: ambient
temperature, atmospheric pressure, electrode profile,
instrumentation, and number of samples averaged.
I wouldn't bother, since the potentials are not a reliable guide to
corrosion. What would be ideal would be to get information on how the
aerospace connector manufacturers treat the subject, but they wouldn't
tell me. I suppose there is a lot of proprietary information involved.
-
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