The purpose of the work is to investigate the dual effect of cathodic protection regarding fatigue of steel in seawater. Based on crack growth rate tests carried out under various conditions, the detrimental effect of hydrogen embrittlement is detected. Also the good effect of cathodic protection is seen. Results from tests in air are compared with tests under free corrosion and under cathodic protection at various R- ratios and loading frequencies. Included are tests under pulsed cathodic
protection where the cathodic current is switched on and off during each mechanical load cycle. These results are used to explain diffusion rate effects of hydrogen in steel. The beneficial effect of cathodic protection was found to be due to the calcareous deposits. Effects of very low potentials (-1.5 V SCE) and protection in deep water are briefly discussed with respect to fatigue engineering.