A few months ago I attended the Titanic exhibit at the San Francisco Metreon. (Highly recommended!) It included the "Big Piece", a 20' by 25' piece of the Titanic's hull. The piece was preserved by Conservation Solutions; their Titanic case file describes how the piece of hull was protected as it was brought to the surface.
The steel piece was desalinized by soaking in a pool of solution with aluminum/magnesium blocks. I tried to figure out why this works by comparing aluminum, magnesium, and iron (the main component of steel) in a reactivity series and confirmed that magnesium and aluminum are both more reactive than iron. These metals displaced the iron in reactions with sodium chlorides. Gradually the salts moved out of the hull piece to the alloy blocks. This process took almost two years. Then the piece was cleaned and coated with wax to protect it from new contaminants.
The Titanic exhibit explained that items that have been in salt water for so long are fragile when brought to the surface because molecules of the original item have been replaced by microorganisms and salts, and when a piece is cleaned and dried out, particles are removed which are now instrumental in the structural integrity of the item. The restored pieces are injected with wax to stabilize them.
Another fascinating thing about the Titanic exhibit: each visitor is given a boarding pass with information about a passenger on the ship. The exhibit includes a passenger list so you can see whether your passenger made it or not. My ticket was for a French model traveling as the companion (presumably the mistress) of San Francisco filmmaker. She didn't survive the sinking. The body of her travel companion was recovered with her purse clutched in his hand.