Sodium has the effect of removing nickel and chromium from the sur­face layers of austenitic stainless steel. The chromium forms sodium chromite and the nickel is removed by dissolution in the sodium. Figure 3.16 shows the nickel and chromium concentrations near the surface of stainless steel exposed to flowing sodium. In this region the nickel concentration may be reduced to about 1% and chromium to 5-8%, and as a result a surface layer of ferrite is formed some

because in all cases the controlling process is the dissolution of ferrite. Figure 3.17 indicates that, with an oxygen concentration of around 10 ppm, which can be attained in practice in a fast power reactor, cor­rosion rates up to 10 pm per year are to be expected in the hottest parts of the coolant circuit where the temperature is around 600 °C. Higher oxygen concentrations carry a risk of excessive corrosion.

Corrosion or, more accurately, wear can be made worse by the relative motion of surfaces in contact, usually called “fretting”. In a reducing environment such as sodium there is a tendency for surfaces in contact to become welded together at high spots. If the surfaces are forced to slide over each other, for example by some source of mechanical vibration, the welded high spots are sheared, and material may be transferred from one surface to the other. Fretting damage is a danger in parts of the core subject to vibration due to turbulent flow of the coolant. Self-welding is a problem for items of equipment such as fuel subassemblies that have to be removed from the reactor from time to time.

Depending on the activity of dissolved carbon, sodium can transfer carbon either to or from steels immersed in it. This process is known as carburization or decarburization depending on whether carbon is gained by the steel or lost from it. The rate of loss or gain of carbon depends on temperature because both the carbon activity in sodium and steel and the diffusion rates vary with temperature.

The carbon activity in austenitic steel is relatively low so it tends to be carburized even if the carbon concentration in the sodium is as low as 5 parts per million. Carbide is precipitated in the surface layers of the steel, reducing the ductility at low temperatures. Low-alloy ferritic steels, however, such as 2.25 Cr 1 Mo with about 0.1% carbon, tend to be decarburized and lose strength as a result.

Carbon activity is higher at low temperature, so there is a tend­ency for decarburization to be particularly important if a sodium cir­cuit incorporates ferritic steels in a low-temperature region such as a heat exchanger. If excessive carburization of austenitic steels in the

high-temperature regions is to be avoided, careful control of the car­bon activity in the sodium has to be maintained.