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Anyone know what will be at 3.900 meters dept the percentage of oxygen ?
The oxygen content is irrelevant, with sea water the main cause of corrosion is electrochemical. Two dissimilar metals with a conducting path between them (and sea water is conductive) form a battery, and the least noble metal will just dissolve away. Since circuit boards contain copper tracks and tin/lead solder, the tracks disappear in a disappointingly short time if exposed to sea water. This isn't too big a deal if the chips are still intact, as they can be removed and re-soldered to a new circuit board.
However, the same thing happens if water gets into the chips themselves, typically along the interface between the package and the leads: the aluminium metallization on the chip just rots away. Putting chips in a pressure cooker is a standard quality-control test for hermiticity, you typically start getting failures after a few weeks, and that's at only 1 bar (although that's at 121 degrees C- I've no idea how this translates to a higher pressure and lower temperature).
There are techniques used in failure analysis where you can use the beam of an electron microscope to "read out" an unconnected circuit, but I don't know whether this will scale up to the megabits of data in the recorder chips.
Basically it's all down to how well the internal and external packaging has managed to keep the sea water out, after a year under water at umpteen bar and following a high-velocity impact. There is, unfortunately, a good chance the data will be unreadable.
Not that the conspiracy theorists will believe the BEA if that's what it ends up announcing.