The corrosion of unirradiated and unsensitized Advanced Gas-cooled Reactor cladding material, 20/25/Nb stainless steel, was studied as a function of temperature and [Cl-] typical of those found in interim spent fuel storage pond waters. With respect to preventing corrosion, it found to be advantageous to dose the ponds to pH≈11.4. At pH lower than 7, the initiation of pitting is observed at ∼0.4V vs Ag/AgCl, an undesirable condition as pits are considered to be initiators of stress corrosion cracking (SCC) which may contribute to loss of cladding integrity during storage. Such pits are not seen at a pH 11.4 for the expected and projected pond operating temperatures of 24-60°C. There generally appears to be no localised corrosion threat to cladding as the temperature is increased in this range, although substantial pit formation is observed at the extreme maloperation temperature of 90°C at pH 11.4 indicating a loss of protection.