We know that the magnetic plates have a thin layer of carbon that serves to prevent corrosion, since the magnetic layer is quite corrosive.

In fact, when there are floods, it is often possible to recover the information in specialized centers, but it is thanks to the fact that the carbon layer remains intact. Does anyone know what would happen if, due to bumps or scratches, the surface penetrates that layer of carbon?

An outdoor dish with a layer of carbon in poor condition and with cracks ... How long would it last without corrosion and therefore loss of data?

The corrosion happens when an oxidant (e.g water) comes into contact with the metal. When the outer layers corrode or oxidise the the water can then come into contact with the other metals forming an electrolytic cell as in a battery and a voltage passes between the different metals which accelerates what is now electrolytic corrosion.

The oxidant can be as something as simple as flood or rainwater, or even water vapour in air. I could go into more depth on this subject, however there are many articles on the web on electrolytic corrosion and the chemical series. The latter show the voltage differential between two metals in an electrolyte.

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If you can keep your head when all about you are losing theirs, you probably don't fully understand the situation. ... Mr Kipling

https://www.mjm.co.uk/

I must understand that if water / moisture penetrates the magnetic layer ( where the data is ), a chain reaction starts and it completely corrodes in a short space of time?

The time will depend on the volume and corrosiveness of the electrolyte. Sea water will be more corrosive than uncontaminated river water.

_________________
If you can keep your head when all about you are losing theirs, you probably don't fully understand the situation. ... Mr Kipling

https://www.mjm.co.uk/