Crack tip shielding by crack closure mechanisms is well known to retard near-threshold fatigue crack growth in structure steels. When high-strength low-alloy steels are immersed in aqueous chloride or marine environments, they must be cathodically protected to prevent excessive corrosion and degradation of fatigue resistance. This paper examines two competing mechanisms that may contribute to near threshold corrosion fatigue crack growth in cathodically protected marine structural steels. The crack propagation rates are compared with microstructural observations of the corrosion fatigue crack paths in order to identify possible mechanisms contributing to near-threshold crack growth.