Corrosion is the loss of material through environmental degradation. Two primary forms of corrosion experienced in a marine environment are oxidization (exasperated by saline environments) and galvanic corrosion.
Oxidization is the reaction of the material due to exposure to oxygen. Oxidization presents as rust in steel and a coating of aluminum oxide (which prevents further degradation) in aluminum. Copper is often alloyed with nickel to decrease corrosion due to oxidization.
Galvanic corrosion is caused by a current induced by having two dissimilar metals in close proximity. In marine environments, the water acts as an electrolyte to complete the circuit.
Due to the iron in steel, it is ferric and susceptible to rust. It is also susceptible to galvanic corrosion.
Aluminum is protected from oxidization by a layer of aluminum oxide (shows as a cloudy grey discolouration), however turbulent flow can strip this layer and it is susceptible to galvanic corrosion.