Why is corrosion produced?
The corrosion process implies the tendency for a metal to revert to its natural, primitive state.
Most metals (except for Cu, Pt, Ag, Au, etc.) are found in nature in chemical combination with one or more nonmetallic elements (oxides, sulfates, carbonates, silicates, etc), in the form of ore. This is their natural, lower energy state.
Significant energy, applied via metallurgical or chemical means, must be used to reduce the ore to pure metal. This is a superior and unstable energy level, which is why metals show a tendency to revert to its natural state in contact with certain environments.
In other words, the result of corrosion is metal as it is found in its natural state when in contact with the atmosphere.
Noble metals remain inalterable in such environments. These two facts confirm the theory that corrosion is the tendency of metals to return to the lower energy balance in which they are found under natural conditions.
We may therefore state that a metal in contact with a certain environment, under certain conditions of pressure and temperature, will be subject to corrosion, if its reaction corresponds to sigma G < 0.
If sigma G > 0 under certain circumstances, then we may say that the reaction will not take place, and the metal will remain unharmed.
If sigma G < 0, the reaction may take place, but we have yet to establish the speed of such reaction.