Corrosion Behaviour of Steels and Nickel Alloys in High-Temperature Sodium

Our findings are summarized on the corrosion behaviour of low-alloy steels, austenitic steels and nickel alloys when exposed to sodium under flowing conditions in a pumped loop. The effects of altering various parameters on the rate of mass transfer from steel specimens in the highest temperature portion of the loops have been studied. These include the effect of temperature in the range 450 to 725°C. the effect of oxygen content of the liquid metal up to a level of approximately 30 ppm by weight (by adjustment of cold-trapping temperature), the effect of sodium velocity in the range 0.5 to 40 ft/s, the effect of specimen position, i.e. any downstream effects, and material variables such as the effect of alloying content in stainless steels and metallurgical conditions, i.e. cold work. A discussion is included of the nature of complex oxide films formed on the surface of steels on initial exposure to sodium, and indications are given of selective loss of alloying elements from the surface of austenitic steels on exposure to liquid metal, as determined by microprobe, chemical and X-ray analysis. Differences between the behaviour of iron-rich and nickel-rich alloys are discussed, particularly with respect to different responses to the effect of liquid metal velocity and oxygen level in the sodium. This leads to a general discussion of possible mechanisms of mass transfer and likely rate -determining processes, in particular why the mass transfer rate of iron should be sensitive to the level of oxygen in the the sodium. Also discussed are possible reasons for the very sharp fall in mass transfer rate of steel above a certain limiting velocity in the loop system in use. Much more remains to be done before a full understanding of the process is gained, and possible experiments are outlined. (author)