[en] Considerable research has been performed by AEA Technology on the corrosion of steels by hot sodium melts containing sodium hydroxide and sodium oxide. This research has principally been in support of understanding the effects of sodium-water reactions on the internals of fast reactor steam generators. The results however have relevance to sodium fires. It has been determined that the rate of corrosion of steels by melts of pure NaOH can be significantly increased by the addition of Na₂O. In the case of a sodium-water reaction jet created by a leak of steam into sodium, the composition of the jet varies from 100% sodium through to 100% steam, with a full range of concentrations of NaOH and Na₂O, depending on axial and radial position. The temperature in the jet also varies with position, ranging from bulk sodium temperature on one boundary to expanded steam temperature on the other boundary, with internal temperatures ranging up to 1300 deg. C, depending on the local pre-reaction mole ratio of steam to sodium. In the case of sodium-water reaction jets, it has been possible to develop a model which predicts the composition of the reaction jet and then, using the data generated on the corrosivity of sodium melts, predict the rate of corrosion of a steel target in the path of the jet. In the case of a spray sodium fire, the sodium will initially contain a concentration of NaOH and the combustion process will generate Na₂O. If there is sufficient humidity, conversion of some of the Na₂O to NaOH will also occur. There is therefore the potential for aggressive mixtures of NaOH and Na₂O to exist on the surface of the sodium droplets. It is therefore possible that the rate of corrosion of steels in the path of the spray may be higher than expected on the basis of assuming that only Na and Na₂O were present. In the case of a pool sodium fire, potentially corrosive mixtures of NaOH and Na₂O may be formed at some locations on the surface. This could lead to significant corrosion of any steel structures penetrating the surface, or steel surfaces forming the boundary of the pool. In this paper, the data on the corrosivity of sodium melts containing NaOH and Na₂O, as a function of temperature, concentration of each species and velocity of the melt, will be presented for ferritic and austenitic steels and the possible relevance of these data for sodium fires will be discussed. (author)