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deposited at the cold zone, and the elements concentration in the liquid metal represents a steady–state<br />

balance between the rates of solution and precipitation in different zones of corroding systems [3].<br />

For austenitic stainless steels, it is well documented (for example, Gorynin et al. [2]) that corrosion<br />

resistance is determined by oxygen thermodynamic activity in lead and lead alloys. For oxygen<br />

concentrations lower than the concentration at which the dissolved oxygen is in equilibrium with the<br />

spinel formation on the steel surface materials, dissolution occurs. This equilibrium value is about<br />

5 × 10 -8 wt% at 550ºC. For concentrations higher than this value, materials oxidation appears. Steel<br />

composition has also a significant influence in the corrosion/protection of structural materials. In<br />

general, it is accepted that in absence of oxygen, carbon steels present a higher corrosion resistance<br />

than high chromium steels, which suffer attack [4]. However, results obtained by Gorynin [13] point<br />

out that at 460ºC, in flowing Pb-Bi with oxygen concentration less than 10 -7 wt%, 1Cr-MoV alloy<br />

presents higher corrosion rate than 18Cr-10Ni-Ti alloy, whereas 1Cr-2Si-Mo is more resistant to<br />

corrosion than both the previous ones. For high oxygen activity the corrosion of iron–chromium alloys<br />

is higher than for iron whereas for low oxygen activity the alloy presents higher resistance corrosion.<br />

Formation of oxide layer on the structural materials can prevent alloy dissolution.<br />

A general consideration of the results seems to point out that the oxygen content in the loop is not<br />

enough to form, in general, protective oxide layers on the specimens placed in the hot zone of the<br />

loop. In fact, specimens B, C (F82H) and Q, R (2¼ Cr-Mo) present material dissolution and no oxide<br />

layers. However, in the type A (F82H) and P (2¼ Cr-Mo) specimens tested from the beginning of the<br />

operation, an appreciable growth of the oxide layer was detected. It seems that A and P samples are in<br />

oxidation condition whereas the rest of the specimens would be in dissolution conditions. This<br />

observation would mean that, although the gas is bubbling during all the operation time with the same<br />

flow rate and the same oxygen content (10 ppm), the oxygen concentration in the liquid lead-bismuth<br />

varies along the operation time and so does the corrosion behaviour of the samples.<br />

Type A specimens (F82H) inserted into the loop at the beginning of the operation are covered by<br />

an homogeneous double oxide layer. The thickness of this oxide layer reaches 20 µm after<br />

3 022 hours. The growth of this oxide layer is almost inappreciable between 340 and 1 030 hours, and<br />

then it grows significantly up to 3 022 hours. These results seem to be in accordance with the general<br />

behaviour accepted for the oxidation of iron-chromium alloys and stainless steels [5]. After an initial<br />

protective period, a sudden rate increase occurs once the break-away time has been reached. This stage<br />

is often followed by a further rate reduction by a self-healing process. This last step has not been<br />

observed in our tests, probably due to their short duration. The oxide layers of type A specimens show<br />

similar characteristics. The inner oxide layer, Fe (Fe 2-x<br />

)Cr x<br />

O 4<br />

, seems to be protective enough to prevent<br />

lead-bismuth penetration. No eutectic is incorporated to this layer contrary to the observed in iron<br />

oxide outer layer. Eutectic concentration in the outer layer decreases and disappears in the interface<br />

outer/inner oxide layer.<br />

Type P specimens (2¼ Cr-Mo) inserted into the loop at the beginning of the operation are covered<br />

by a non-adherent oxide layer with a wrinkled aspect. Labun et al. [6] describe a layer with these<br />

characteristics for a 3Cr-Fe steel oxidised in dry oxygen at 700-800 ºC. They say that the wrinkling is<br />

due to a substantial growth of this layer in the lateral direction relative to the underlying layers. In our<br />

case, lead-bismuth penetrates through the external oxide layer and is place between it and the steel.<br />

The layer formed on 2¼ Cr-Mo specimens is much thicker in all the cases that the formed on type A<br />

specimens. Fedirko et al. [7] tested Armco iron, Fe-16Cr and Fe-16Cr-1Al in stagnant liquid lead at<br />

600ºC with some amount of lead oxide to get a concentration of 10 -5 wt% oxygen and they mention in<br />

this work that the formed oxide layer is thicker for the Armco iron. The information on the oxidation<br />

behaviour of stainless steels in liquid lead-bismuth is very scarce. However, it is accepted than the<br />

available information on stainless steels oxidation in molten lead can be useful to analyse the materials<br />

behaviour in lead-bismuth.<br />

406

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