Near-threshold corrosion fatigue crack propagation studies have been conducted using compact tension specimens of Mil S-24645 HSLA steel and a compatible weld metal in air, ASTM seawater at the free corrosion potential, and ASTM seawater with −0.8V and −1.0V (SCE) cathodic protection. Tests were conducted at 10 Hz and a stress ratio, R = 0.1. The weld metal exhibited slower crack growth rates and higher threshold stress intensity ranges in both air and ASTM seawater than the base metal. However, the weld metal itself exhibited similar behavior in air, ASTM seawater and under cathodic protection, giving a threshold stress intensity range of 10 to 12.7 ksi in1/2 (11 to 14 MPa m1/2 ). With the exception of the weld metal tested in air, after correction for crack closure, the effective threshold stress intensity range was reduced to the range 3.2 to 5.0 ksi in1/2 (3.5 to 5.5 MPa m1/2 ). Metal wedges, approximately 5 to 10 μm (1–2 grain sizes), which developed during the tests, were observed to contribute to crack closure in the −1.0V (SCE) cathodically protected specimens.