I have noticed that of all of the copper blocks, copper doors take by far the longest time to oxidize.  I suspect that the game thinks that since copper doors occupy 2 blocks, the game treats it as if there are two copper blocks next to each other, therefore causing the oxidization process to occur at that rate rather than the rate you get when the copper is spaced out.  this is what causes the oxidization rate to be much slower than the other copper blocks.  I was aging a bunch of copper grates and doors for some builds I'm doing in a survival world, and I was getting fully oxidized grates before the doors had even reached the 'exposed' stage.  I doubt this is an intended feature, so I decided to report it here.