More on rust

I haven't read all of this, but thought you might be interested in another viewpoint.

Pete Stanaitis

--------------------------- 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 snipped-for-privacy@cc.com

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