As soon as painting and gritblasting stops steel starts to oxidate away - in a few months in marine ambient the hull is gone if the tight maintainance shedule for steel is neglected.
compare a concrete submarine hull after 20 years with no maintenance in the water... not the slightest sign of deterioration - the concrete is even better and stronger than it was the day of the launch - as concrete gains strenght over the years due to a ongoing slow curing process.
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