Corrosion is a chemical reaction. So it is dependent upon the concentration of the impurities - which make the water conduct, the oxygen - which is essential for the reaction to proceed and the temperature which determines the rate.
Deep fresh water is generally one of the best 'preserving' mediums - preserving in that the rate of deterioration is lowest. So some of the aircraft coming out of the deeper parts of the Great Lakes look in amazing condition - compare those to the wrecks in Truk lagoon which have largely disintegrated.
Burying the object in soft mud really helps for the same reason - some of the artifacts from the Wasa (Swedish 16th century warship) look almost usable.