Additional Thoughts on Leica's Corrosion Problem

I posted yesterday on the news that Leica was experiencing corrosion issues with the IR filter glass used in its CCD sensor cameras, starting with the M 9.

I was pressed for time yesterday, so I didn't post anything beyond reporting the problem.  I have some additional thoughts today.


I'm having a hard time understanding how damaging the coating on the IR filter glass could result in corrosion.  Glass is an inert material.  It normally doesn't corrode, even when subjected to corrosive materials.  That's why it's used to contain acids.


The filter does apparently have a coating, but that coating should be inert as well.  (The fact that the filter only corrodes when damaged seems to indicate that the coating normally is inert.)

Somehow, a filter made with two inert materials stops being inert if the surface becomes damaged.


That doesn't seem to make sense.


There are a couple of possible explanations that I can come up with.
  (Discalimer: Based on limited information.)


First, the filter might have more than one coating.  Having a layer that reacts when exposed to air that is covered by a second protective layer would explain what is happening with the Leica filter.  The filter is fine as long as the protective layer is undamaged, but damaging that layer results in the second layer corroding.

Second, there is something in the coating that corrodes the glass when exposed to the environment.  There is only one thing I know of that can damage glass this way: Hydrofluoric Acid.  Unlike other acids, hydrofluoric acid has the ability to dissolve glass.  (One of it's uses is etching glass.)

I suppose it's possible for the glass itself to be damaged if the coating contains a fluorine compound.


Both possibilities have implications.

The first one would suggest that someone overlooked a fairly obvious design flaw.  That doesn't bode well for whatever company is responsible for the filter design.  (I assume Leica, but that's not necessarily correct.)

The second would have implications when it comes to the use of fluorine compounds in cameras.  This would include lenses as well as internal filters.  If damaging a fluorine containing coating can result in the underlying glass being damaged, that would make those coating ill-suited for use in cameras and lenses. 

Nikon recently developed a fluorine coating for its lenses.  It's extremely effective at repelling dust, grease and dirt.  Hopefully it doesn't include a risk of corrosion as well.


There is another issue that needs to be addressed as well.

That has to do with Leica's fix.  They are offering to replace any sensor effected by the corrosion problem.  The key here being "replacing the sensor".

The sensor itself s not actually affected by corrosion, only the filter.  Simply replacing the filter would seem to be a much easier and less expensive fix.  In fact, replacing the filter with one that doesn't use the faulty coating is the only permanent solution to the problem.

Instead, Leica is replacing the entire sensor and replacing it with one that uses the same faulty filter.  That means the corrosion problem could re-occur on any "fixed" camera. 


I'm not sure which is harder to understand, how the problem occurred in the first place or Leica's response.