Basically as long as the ceramic coating is intact, it will do a good job protecting the magnesium. If it rubs off due to physical contact, that will be the real trouble. Then any additional moisture will cause some corrosion. There are two main common points where this happens. In the original release, the stems would over close and hit the orbitals. Later gen's put a piece of plastic in the hinge so it would stop in the proper location.
The second point is under the ear socks. When they are coating, they have to be held somehow, so they chose the ear stem where it would later be covered by the socks. But as the unobtainium absorbs sweat, it deposits the moisture right into that unprotected spot, causing tumors. My brothers are really rough.