The discovery of a severely cracked mixing tee in the process gas cooling section of a hydrogen plant showed a form of cracking that appeared to have features typical of both fatigue and chloride stress corrosion cracking of 304L stainless steel. However, the conditions were significantly less severe than what would be anticipated for either process alone. It was speculated that a synergism between these processes occurred causing this failure. A laboratory program was undertaken that confirmed that cracking could occur in environments with as little as 1 ppm chloride and 1 ppm NaSCN when both cyclic stressing and anodic polarization were present. Conventional slow strain rate tests on 304L stainless steel in this environment with anodic polarization did not cause environmentally assisted cracking confirming the importance of cyclic stressing. Similar cyclic tests on duplex stainless steel 2205 and nickel alloy 625 did not show cracking regardless of the applied potential up to 600 mV versus Ag/AgCl reference electrode.