Originally Posted by
Neil Miller
Both are types of metal corrosion. However, steel that has accumulated a thin coating of corrosion, ie a 'patina', over many years is not the same as metal that has corrosive pits in it. In fact the thin patina acts like a protective layer - it is stable, and with moderate care the blade will last for many, many years to come. With pitting we have corrosive elements or particles that attach at random intervals to the steel blade, forming small differences in potential between themselves and the underlying layer (blade) that spreads both outwards and downwards, as Bruno said it oftern travels just under the surface and when the surface is abraded pits are revealed that are more extensive than first thought.
You could call the bad stuff an 'unstable patina' that gives rise to pitting. The following gives an idea of the electro-chemical processes involved:
Regards,
Neil