Eli5: Why is black box (from plane crash) put in a container filled with water when retrieved from the ocean?
Salt really accelerates corrosion, but the corrosion will really only progress when exposed to oxygen and there’s a lot more oxygen in air than water
By taking the box out of the ocean and putting it into a bucket of water they keep any innards which may have been exposed to salt water from being exposed to air which will provide the oxygen needing for the corrosion to progress. When it gets to its final destination it will be removed from the water and then cleaned, but you don’t want it to spend a few days traveling and degrading because its exposed to the air.
Primarily to prevent dissolved salts to dry onto components within the device.
Water is not the big, bad enemy here but things dissolved in the water are. Once it’s retrieved, it’s kept in water to keep deposits in suspension, then bathed and carefully extracted and dried.
The flight recorders usually get badly damaged by the crash. So the recovery crew would be unable to read out the data but have to ship it back to labratories that can carefully disassemble it and repair it to recover the data. However the recovery crew needs to package it in such a way as to prevent further destruction. If the flight recorders have been exposed to ocean water then we know that anything that does not handle water is already damaged. But there are things that can not be exposed to air, it can be that it rusts or it can dry up and become brittle. So keeping it in water is much safer then drying it out in air.
Furthermore the salt in the ocean water can be very corrosive and cause chemical reactions over time. If you leave the ocean water in place or even if you let it dry then the salt will continue to do damage. But flushing the flight recorders out with fresh water will remove the salt. This leaves the flight recorders in their most stable conditions for transport to the labs. Recovery crews will even try to bring destilled deoxygeneted water which is extra stable. This is not that different from the water at the bottom of bogs which can preserve bodies for millenia. So the flight recorders should be safe in the few hours it takes to get it back to a proper lab.