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Several alloys are used in marine applications. Alloy 18-8 (same as table utensils) will show rust spotting. Alloy 316 which contains some molybdenum is superior to most other Stainless alloys and is used for propeller shafts. Type 302 could deteriorate rapidly.
Normally, Stainless Steel protects itself from corrosion by a naturally generated oxidized surface. This condition is called its passive state. When this film is removed and unable to replace itself, the Stainless Steel will become active and will deteriorate rapidly. Lack of oxygen in cracks or scratches in the surface bring Stainless to an active state. Since crustacean growth, chlorides, pollution, stagnant water, damp wood and crevices will turn Stainless from passive to active, it is not recommended for underwater use.