Abstract: The cathodic protection inhibitions of 10 Ni5 CrMoV steel and its weldment in seawater were studied by potentiostatic cathodic polarization, weight loss method, slow strain rate tensile test(SSRT) and fracture morphology analysis. The results showed that hydrogen embrittlement susceptibility of 10 Ni5 CrMoV steel and its weldment obviously increased as the potential negative shifted to-0.94 V. The reasonable cathodic protection potential was suggested to be-0.77 V~-0.94 V(vs. Ag/AgCl).