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For high oxygen activity, alloys with a high chromium content present a better behaviour than<br />

low chromium alloys. This is due to the different characteristics of the protective oxide layer based on<br />

formation of oxide rich in chromium, which is hindered for the low chromium alloys. At the same<br />

time, free and low chromium alloys are able to form thicker oxide layers, which are accepted to be less<br />

protective [14]. For low oxygen activity in the melt, alloys with low chromium content present a<br />

higher corrosion resistance [4], since chromium has a higher solubility than iron in lead-bismuth. This<br />

seems to be the case of Q, R (2¼ Cr-Mo) specimens comparatively to B, C (F82H) specimens<br />

although the behaviour of both steels is not conclusively different.<br />

Fedirko et al. [8] also found that spinel rich in chromium can protect iron-chromium alloys in<br />

molten lead. After 1 000 hours of testing, four different zones could be observed on the transverse<br />

micro-sections of Fe-16Cr-1Al. The first zone was a porous oxide layer with a high concentration of<br />

iron and lead and a low concentration of chromium and aluminium. This zone was formed by<br />

magnetite and lead. Lead concentration decreased with distance from the surface, but on the interface<br />

of the first and the second zone lead was present. The second zone was a continuous oxide film<br />

enriched with chromium and aluminium. The composition of this film was a Fe 3<br />

O 4<br />

-FeCr 2<br />

O 4<br />

solid<br />

solution with the spinel structure. Lead was not present in the interface between the second and third<br />

zone. A process of internal oxidation with chromium and aluminium oxides mainly along grain<br />

boundaries was detected in the third zone. Finally, the fourth zone had the matrix composition but<br />

grain boundaries also contained oxide compounds. On the contrary to the observed in oxygen<br />

saturated melts, spinel rich in chromium is not permeable to lead in solutions with low oxygen<br />

activity.<br />

In addition, in static tests performed in oxygen saturated molten lead at 520ºC, Benamati et al. [9]<br />

observed the formation of continuous layers of reaction products with an average thickness of 20 µm<br />

after 2 000 hours and of 40 µm after 3 700 hours in martensitic steel F82H mod. specimens. The<br />

reaction product layers were formed by two distinct sub-layers of Me 3<br />

O 4<br />

. Only the inner one contained<br />

chromium in addition to iron. Lead was detected in the outer sub-layer of all the product layers. These<br />

authors point out that oxide layers formed in oxygen saturated liquid lead containing an additional<br />

source of oxygen in the form of lead oxide are not protective, since the oxide layer thickness increases<br />

along with the exposure time.<br />

On the other hand, Müller et al. [10] found oxide layers of similar characteristics to the observed<br />

in type A specimens in samples of a martensitic steel with 9.99 Cr (OPTIFER) tested in stagnant liquid<br />

lead at 550ºC under controlled Ar-H 2<br />

/H 2<br />

O atmosphere containing 8 × 10 –6 at% oxygen. Müller<br />

observed a corrosion attack with three different zones. The outer layer consisted of magnetite without<br />

appreciable chromium concentration. The middle layer was formed by Cr-Fe spinel, with a chromium<br />

concentration lower than the detected in the A 3<br />

specimens. Finally, an internal oxygen diffusion zone<br />

in which oxides precipitate along the grain boundaries could be observed. After 3 000 hours, the<br />

thickness of magnetite and spinel were 20 and 15 µm respectively whereas the depth of diffusion zone<br />

was 10 µm.<br />

According to Fedirko et al. [7], in liquid lead with low activity of oxygen the formation of the<br />

external oxide layer based on Fe 3<br />

O 4<br />

is very low and significant amounts of active alloying elements<br />

are accumulated in more internal layers. The lack of mobility of chromium through the magnetite<br />

lattice explains the formation of the spinel layer. Faster diffusing elements like manganese and iron<br />

will pass through to the outer layer, while slower diffusing elements like chromium will be oxidised<br />

without movement and remain in the inner layer. Both magnetite and spinels M 3<br />

O 4<br />

are based in a<br />

close-packed cubic sub-lattice of oxygen ions in which the metals ions are the faster moving species<br />

and determine the oxidation rates. Thus, in martensitic steels, chromium moves more slowly than iron.<br />

The outer layer is formed by magnetite and the inner layer consists of iron, chromium spinels. The<br />

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