MadSci Network: Chemistry

Re: what type of corrosion is the rusting of steel?

Date: Tue Aug 30 12:06:11 2011
Posted By: James Griepenburg, , Chemical consultant, Chemmet Services
Area of science: Chemistry
ID: 1311154472.Ch

Corrosion basics:

some research: id=NACE-08332&soc=NACE&speAppNameCookie=ONEPETRO

The requirements for corrosion to take place in aqueous conditions are:

  1. An anode location
  2. An anode reaction [oxidation] that takes place on or at the anode
  3. A cathode location
  4. A cathode reaction [reduction] that takes place on or at the cathode
  5. There must be an electrochemical difference between the anode and cathode reactions; the system must not be at chemical equilibrium such as a completely discharged battery.
  6. Electrical connection between the anode and cathode
  7. Ionic conduction between the anode and cathode sites
The same conditions are necessary for electroplating, battery cell operation and similar operations which are simply controlled corrosion.

Interrupting, inhibiting, removing etc. any one of the seven requirements will prevent [or at least considerably slow because in actual conditions several mechanisms may be operating simultaneously] corrosion.

So-called uniform corrosion is rarely uniform unless conditions are carefully controlled in procedures such as electropolishing or control of anode dissolution in electroplating, There are two common examples of uniform corrosion. The first is when there are cathode and anode areas and the anode area is physically large. Since corrosion occurs over the entire anode area it will appear uniform, but careful examination usually shows increased corrosion rate closer to the cathode. The second common example is that of an ostensibly pure metal that corrodes all over when immersed in water. Since corrosion appears somewhat uniform over the surface the cathode and anode sites must be miniscule and evenly distributed over the material. These sites can be crystal imperfections, atoms or domains of alloying elements, even machining or casting imperfections. Microscopic or electron microscopic examination of such corrosion usually reveals pitting and sometimes even shows cathodic sites of impurites in the pitted areas. "Uniform" corrosion is sensitive to slight liquid motion or stirring, temperature and chemical concentration variations and physicak interfaces.

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