Stress corrosion cracking (SCC) is a documented threat to the integrity of carbon steel radioactive waste storage tanks at the Hanford Site. Historically, halides have been thought to have a minimal effect on SCC susceptibility in nitrate-rich, alkaline wastes. However, much of the historical work that formed the basis for this conclusion was performed under short term open circuit potential (OCP) conditions. Previous work has demonstrated that the short-term OCP of simulated waste can be more negative than in-tank waste values and that, typically, laboratory OCPs drift 100’s of mV in the positive direction before stabilizing. The positive drift in OCP can increase SCC susceptibility of tank steels. The focus of more recent work has been to define the potential necessary to cause SCC susceptibility in waste simulants in the temperature range between 50 and 75°C. This is referred to as the critical cracking potential (CCP). A negative shift in the CCP when increasing hydroxide concentration necessitated an expanded test program to understand the cause of the unexpected results. A SSR test matrix in simulants consisting of varied levels of hydroxide, fluoride, and chloride concentration was performed. The results of the investigation showed that halides, both chloride and fluoride, cause a negative shift in CCP for a given simulant. This result has important implications on the assessment of SCC susceptibility for all tanks, not just those that are operated in excess of 50°C.