I would have advised going thru some basic metallurgy book (it may be towards middle of the book)
__________________
Fantastic ideas for a Fantastic World, I make the illogical logical.They put me in cars,they put me in yer tv.They put me in stereos and those little radios you stick in your ears.They even put me in watches, they have teeny gremlins for your watches
Not knowing your level of technical know-how I'l be basic.
Most metal alloys are made up of mixtures of differing crystals in their microscopic scructure. Some times in the course of casting, welding or heat cycling the metallic crystals can have differing chemical compositions. When this occurrs then a galvanic cell can be created between neighboring crystals since one may have more of the alloying material that inhibits corrosion than the other. The result of this galvannic corrosion is called intergranular corrosion or oxidation as you say. Bad welds or prolonged exposure to high temperatures are the most common causes.
Another cause of this is when metals are heavily worked the alloying materials can actually be worked out of the crystalline structure since the crystals become so elongated through the mechanical working you actually see corrosion products form between these long thin crystals. More commonly seen in non-ferrous extrusion applications such as aluminum alloys.
The third way this happens is when the material is exposed to a chemical that specifically attacks one of the alloying metals. Stainless steels are particularily sensitive to chloride attack since the chlorine ion is extremely electronegative. Again the galvanic corrosion occurrs because the crystal matrix is not uniform in the metal, so some crystals have more chromium in them than others. As the weaker crystals are depleated of their alloying metals, then you begin to see the intergranular attack occur and once a galvanic cell forms, that area rapidly corrodes.
For stainless steel materials, this is due to "sensitization" or carbide precipitation. This happens when the material is exposed to high temperature during fabrication, welding or in exercise. When the material is exposed to high temperature, the chrome causes precipitation of Chromium carbide causing a deployment of Cr along the grain boundary.
When in exercise and in contact with an aggressive, corrosive environment, the grain boundary with lower corrosion resistance due to the lower content of Cr will be corroded easily, preferentially causing the intergranular corrosion.
grain bounddary oxidation
Re: grain bounddary oxidation
