The Electromotive Series
How Electrochemistry Affects Corrosion.
by
Steve Smith
Caveat: It's been a long time since high school chemistry. If someone
out there thinks that I've made a mistake, please let me know.
Corrosion in metal exposed to the weather is primarily oxidation (I'm
NOT talking here about the effect of acids or bases on metals).
Oxidation is greatly accelerated in the presence of another metal due
to the difference in potential (voltage) caused by the contact of the
two metals. This brings up the best method of prevention: Put an
insulator between all dissimilar metals!
Electrochemical oxidation will only occur in the presence of some kind
of electrolyte; in our case dirty water, or moisture and dirt. This
suggests the second line of defense against corrosion: Coat the metals
so that moisture cannot get in. Note that the insulator approach will
NOT work if water puddles over it and bridges between two dissimilar
metals.
A lot of the time, you cannot do either of the above. The tables below
will help you figure out what will happen.
Two (or more) dissimilar metals, in electrical contact with each other
through an electrolyte (dirty water, damp earth, ..) will produce a
voltage. This is a simple battery. If you put a voltmeter from one
metal to the other, you can measure this voltage. ***Only one metal
will corrode***! The metal which corrodes is the one with the lower of
the voltages (from the tables below). The metal which has the larger
voltage will not be corroded (until the metal with the lower voltage
is all gone, or loses contact with the electrolyte). This is how you
can predict corrosion; this is also a third way of protecting your
metal work called Anodic Protection.
Anodic Protection (=sacrificial anode) means that you attach a lump of
metal to your sculpture which has a lower voltage than any other metal
in your sculpture. The attached lump (anode) WILL corrode away, but as
long as contact is maintained and some part of the lump is still
there, your sculpture will not corrode (a regular maintenence program
is needed, of course). This is how galvanized steel works! There is
also a magnesium anode in your water heater. When it's gone, the water
heater starts rusting and fails quickly. (funny how they don't tell
you to replace the anode...)
Note that for a piece of iron stuck in the ground, you don't know what
other lumps of metal might be corroding it. Maybe there's a dime
somebody dropped nearby...
So here's the first table:
____________________________________________________
| |
| Electromotive Series of the Metals |
| from Lange's Handbook of Chemistry, Eighth edition,|
| Handbook Publishers Inc., Sandusky, Ohio, 1952. |
| |
| Metal Voltage |
| |
| Magnesium -2.34 volts |
| Beryllium -1.70 |
| Aluminum -1.67 |
| Manganese -1.05 |
| Zinc -0.76 |
| Chromium -0.71 |
| Iron -0.44 |
| Cadmium -0.40 |
| Nickel -0.25 |
| Tin -0.14 |
| Lead -0.13 |
| Copper +0.34 |
| Silver +0.80 |
| Palladium +0.83 |
| Platinum +1.20 |
| Gold +1.42 |
|____________________________________________________|
Given the above discussion, it is easy to see why gold doesn't
corrode. If gold is connected to any metal on the list, the other
metal (all having a lower voltage) is the one that corrodes. In
galvanized iron, the zinc corrodes instead of the iron, since zinc is
more negative than iron (zinc corrosion is much less noticeable than
rust).
Other examples:
-A sculpture made from silver (+0.8) and nickle (-0.25) would
show corrosion on the nickle.
-A sculpture made from aluminum (-1.67) and nickle (-0.25)
would show corrosion on the aluminum.
-A sculpture made from copper (+0.34) and silver (+0.80) would
show corrosion on the copper
Alloys have their own voltages, which I do not have a table of.
However, Lange's also lists the following:
___________________________________________________________________
| |
| An Electromotive Series of Metals and Alloys |
| |
| Metals in the same group below will experience little or no |
| corrosion. Metals in different groups (separated by a line space) |
| will corrode according to which group occurs first in the table. |
| |
| Magnesium, Magnesium alloys |
| |
| Zinc, Galvanized Steel, Galvanized Wrought Iron |
| |
| Aluminum 52SH, 4S, 3S, 2S, or 53S-T, Aluminum clad |
| |
| Cadmium |
| |
| Aluminum A17S-T, 17S-T, 24S-T |
| |
| Mild Steel, Wrought Iron, Cast Iron |
| |
| Ni-Resist |
| |
| 13% Chromium Stainless Steel, type 410 (active) |
| |
| 50-50 Lead-Tin Solder |
| |
| 18-8 Stainless Steel, type 304 (active) |
| |
| 18-8-3 Stainless Steel, type 316 (active) |
| |
| Lead, Tin |
| |
| Muntz Metal, Manganese Bronze, Naval Brass |
| |
| Nickel (active), Inconel (active) |
| |
| Yellow Brass, Admiralty Brass, Aluminum Bronze, Red Brass, Copper,|
| Silicon Bronze, Ambrac, 70-30 Copper-Nickel, Comp. G-bronze, Comp.|
| M-bronze. |
| |
| Nickel (passive), Inconel (passive) |
| |
| Monel |
| |
| 18-8 Stainless Steel, type 304 (passive), 18-8-3 Stainless Steel |
| (passive). |
|___________________________________________________________________|
Unfortunately, I don't know enough chemistry to explain what they mean
by active and passive.
Steve Smith
steve@cc.com
Archived by The ArtMetal Project
http://www.artmetal.com/project/----------------------------------------------------
