Here is the particular thread:
http://www.bladeforums.com/ubb/Forum54/HTML/002267.html
Corrosion is nothing more than the spontaneous destructive oxidation of metals. In reality, corrosion begins when the manufacturing process begins. All metals corrode; in our normal atmosphere of 21% oxygen, all metals except gold, platinum, and palladium corrode spontaneously. Corrosion takes place through an electrochemical cell. The two half-reactions use the corroding metal itself as both the electrodes and the electronic conductor of the external circuit. The ionic conductor is usually a film of aqueous solution on the surface of the metal.
All of us know that iron rusts, and that iron pipes corrode in wet soil. This reaction of corrosion is an electrochemical process. In many corrosion reactions the metal is oxidized to a soluble form. The cathodic reduction reaction of the cell is the reduction of the hydrogen ion, of oxygen, or of water.
The destructive corrosion actually occurs beneath the paint film at the edges of the crack, in the region of lower oxygen partial pressure. The mechanism of corrosion here is differential aeration; the greater amount of oxygen in one place as compared to another sets up a concentration cell -- a galvanic cell driven by a concentration difference for one or more species.
There are two basic methods of protecting metals from corrosion (after the manufacturing process). One is to slow down the process by reducing the rate at which the reaction can take place. The other is to reduce the electromotive force of the corrosion cell, or transfer the electrons by electronic conduction to a location where the corrosion can occur without harm. Slowing down the corrosion process is done with protective coatings. These help to keep out oxygen, water, and electrolyte salts. Electrolyte salts assist corrosion by reducing the electrolyte resistance in the corrosion cell; the presence of small salt crystals in the air is the major reason why metal corrodes more rapidly at seacoasts. Protective coatings are effective in preventing corrosion as long as they adhere firmly to the surface of the metal to be protected. There is a third process (alloying method). An example, when chromium or chromium and nickel are alloyed with the iron; this alloy is not only absolutely rustproof but will even resist the action of such corrosive chemicals as hot, concentrated nitric acid. The alloying method is probably the most satisfactory but also the most expensive form. Here is some info on the "less expensive" forms.
Cathodic protection from corrosion occurs when a metal to be protected is forced to be the cathode of the corrosion cell by coupling it to a metal more easily oxidized than itself. Metal fences and sheets made of iron can be protected by galvanizing them, which consists of coating them with zinc. The galvanized metal will not corrode until after the zinc coating does, since zinc corrodes more readily than does iron. For example, protection of an oil pipeline with a magnesium sacrificial anode. The anode will corrode but the pipeline will not. The magnesium anode can be replaced by an inert or iron anode if a continuous external power source is used to hold the iron anode at a magnesium potential.
In
anodic protection from corrosion, the metal to be protected is briefly made positive to form a stable oxide film on its surface. The stable oxide film then protects the underlying metal from corrosion. This is effective for some steels. Other metals such as aluminum form a protective adherent oxide film by themselves. Stainless steels form a protective film of nickel/chromium oxides since they have a high content of these metals. Iron oxide does not form a coherent and adherent film on iron metal so iron oxide does not protect iron from corrosion. Aluminum metal is highly reactive to oxygen, but in the presence of air it spontaneously and rapidly forms a tough resistant film of aluminum oxide which protects the bulk of the metal from corrosion. The oxide film can be enhanced, and its properties changed, by anodization.
Anodization is a process in which the aluminum is made the anode of an electrolysis cell using an aqueous acid electrolyte for a few minutes at comparatively low current densities. The resulting oxide film makes the aluminum more resistant to corrosion. The film can also be dyed with coloring matter since it is more porous and absorbent. A wide variety of anodized finishes are found on the aluminum parts of small appliances and household siding. Anodization is also used with magnesium, zinc, and titanium.
There was an actual test performed that tested "Corrosion Produced by Leather in Contact with Metal." Here is the link:
http://www.astm.org/DATABASE.CART/PAGES/D1611.htm
But, it'll cost ya $20+ to get a copy.
[This message has been edited by GigOne (edited 10-09-2000).]
