Abstract The electrochemical behaviour of a highly alloyed steel Sanicro 28 and a more common grade SR60 were examined in aqueous chloride solutions using long term (>60 min) potentiostatic tests. Special attention was paid to ensuring crevice free electrode assemblies. Lowering the pH to 1.0 or raising the chloride ion concentration to 3 M was found to have very little influence on the stability of passive films formed on Sanicro 28. For 5R60 on the other hand, lowering the pH (11.0 to 2.0) at a fixed chloride level (0.5 M) was found to alter quite markedly the stability of the passive films formed as evidenced by a large increase in the number of current fluctuations appearing on the current decay profiles. Reduction of the temperature of the test solution or chemical treatment of the test specimens in HNO 3 or HNO 3/HF solutions prior to testing leads to a considerable improvement in passive film stability. For Sanicro 28, temperature increases or decreases (5–65°C) and chemical pretreatment had little or no effect on film quality. There is little variation found in the magnitude of 24 h passive current values recorded for the steels; however, they are more than two orders of magnitude lower than those obtained from potentio-dynamic polarization plots.