Impressed current cathodic protection can result in hydrogen embrittlement, which can cause trouble with high-strength steels, particularly at welds. Therefore, the limiting potential for hydrogen embrittlement should be examined in detail as a function of the cathodic protection potential. This study investigated the effects of post-weld heat treatment (PWHT) on marine structural steels from an electrochemical viewpoint. In addition, the slow strain rate test (SSRT) was used to investigate both the electrochemical and mechanical effects of PWHT on impressed current cathodic protection. According to the SSRT, the optimum cathodic protection potential was -770 mV [with a saturated calomel electrode (SCE)]. SEM fractography analysis showed that the fracture morphology at an applied cathodic protection potential of -770~-850 mV (SCE) was a dimpled pattern with ductile fractures, while a transgranular pattern was seen at potentials below -875 mV (SCE). Therefore, the cathodic protection potential range should be -770~-850 mV (SCE).