Corrosion fatigue

Corrosion fatigue is the cracking of a component under combined cyclic stress and a corrosive environment, where the two together cause failure at stresses far below the dry fatigue limit. In propeller shafting, seawater reaching the shaft surface through a worn stern seal removes the protective oxide at each load reversal, so a fresh metal surface corrodes and cracks initiate at pits. Unlike plain fatigue, corrosion fatigue shows no true endurance limit; the S-N curve keeps falling. Tail shafts crack most often at the propeller-keyway forward end or the aft liner edge. Defenses are continuous shaft liners, oil-lubricated stern tubes, and class-mandated periodic shaft surveys.