Pretty much no suitable metal is safe forever around salt and water. Stainless steel is a marketing term. The technical term is corrosion resistant steel and the more resistant versions tend to gall in contact with similar or identical alloys. Aluminum parts requires surface treatments to protect them but damage to that surface treatment can leave the aluminum open to corrosion; I would guess that there are bits of grit in the air stream that would blast those surface treatments off eventually. Aluminum also expands more with temp than steels do, which would contribute to the next problem.
One problem in making these valves is that if the flaps have clearance in the hinges they will rattle and rub and wear against the hinge pin in the very high-frequency vibration environment, so the gaps need to be very small. But with small gaps any corrosion at all can bridge the gap and bind the hinge. In normal use any corrosion will be at some small location and operation will wear it down as if forms. Sitting stationary does not cause this wear. Unlike most locations this one cannot be lubricated; oil would be blown off and grease, which consists of oil+ thickener, would see depletion of the oil leaving the thickener to bind up the mechanism.