Iron, chromium and especially nickel are soluble in liquid lead. The solubilities are shown in Figure 3.18. In lead-bismuth eutectic (44.5Pb, 55.5Bi) the solubilities are about a factor of 10 higher. Unprotec­ted steel surfaces corrode very rapidly by dissolution, not only of the metal surface but also beneath the surface as the liquid penetrates into defects and along grain boundaries.

The corrosion rate can be reduced substantially by a protective oxide layer on the surface of the steel. At temperatures in the range of 300-470 °C and oxygen concentrations of 10-9-10-8 by atoms (~10-8- 10-7% by weight) an oxide layer a few tens of qm thick is formed that protects the steel from direct contact with the liquid metal. Corrosion continues by oxidation as oxygen diffuses through the layer. The layer grows in thickness until an equilibrium is reached, with oxide being formed at its inner surface and being lost by spalling from the outer surface. Figure 3.19 shows the composition of the oxide layer formed

on a ferritic steel after exposure to flowing lead-bismuth eutectic at 470 °C. The outer part of the layer consists of porous Fe3O4 whereas the inner part is a compact (Fe, Cr)3O4 spinel. The protection is only partially effective, however, in that corrosion by oxidation continues at rates of 50 |Fm per year or more. Similar but thinner protective layers are formed on austenitic steels.

The steel surface is protected only while the oxide layer is in place. It can be disrupted mechanically by the impact of particles carried in the coolant, or even by the turbulence of the coolant itself. The pressure fluctuations in turbulent fluid flowing with velocity v are of the order of pv2/2, and the high density of lead and lead-bismuth eutectic (for both of which p ~ 10 500 kgm-3) implies that the velocity has to be limited if the oxide layer is not to be eroded. In practice coolant velocities have to be kept below about 3 ms-1 to control the rate of corrosion.

Temperatures have to be limited as well. Above about 470 °C oxide is laid down in an increasingly thicker but less compact and more unstable layer that is much more susceptible to erosion, and above 550 °C it offers little protection so that both austenitic and ferritic steels corrode rapidly by dissolution.

There is some evidence that the presence of additional materials can result in the formation of more stable protective layer. A few percent of silicon or aluminium in the composition of the steel, for example, are beneficial in this respect. Alternatively it may be possible to aluminise the surface of the steel. Oxide-disperse steels (see section 3.3.8) may also be resistant to corrosion at higher temperatures. But although it is possible that such materials may provide a solution to high-temperature corrosion they require testing over extensive periods before they can be validated for use in a power reactor. Until this has been done corrosion effectively limits the maximum temperature in a reactor cooled by lead or led-bismuth eutectic to 470 °C.