Corresponding author: Konstantin D. Ivanov (ivanovkd100@yandex.ru)
Academic editor: Yury Kazansky
The authors consider examples of using the thermal cycling method for heavy lead- and lead-bismuth-based coolants with simultaneous measurement of the thermodynamic activity (
As a result of the experiments, it was found that iron oxides previously formed in the coolant, as such or in the form of an oxide film on the structural steels, are not inert phases in heavy coolants, but can exchange iron and oxygen with the melt. The nature of this exchange depends both on the actual coolant impurity state and on external conditions, including, first of all, temperature.
The experimental data obtained by thermal cycling of lead- and lead-bismuth coolants were analyzed both in the region of relatively high oxygen
It is concluded that the proposed coolant thermal cycling method is informationally significant and can be recommended for further use, for example, to obtain quantitative data on the content of iron impurities in the coolant.
The direct entry of structural steel components into the coolant in the presence of protective passivation coatings on the surface of these steels, in contrast to liquid metal corrosion processes, where the steel components dissolve in the coolant, is a complex multistage process of diffusion and interaction of steel and oxygen components. This process takes place simultaneously in the steel itself, on the inner boundary of the oxide film, in the volume occupied by the oxide coating, on the outer border between the oxide layer and the coolant and directly in the coolant itself. This is a complex interconnected process (
In the region of sufficiently low oxygen
It is much more difficult to distinguish between these two states in the region of partial oxygen pressures, which are close to the release of the excess solid oxides from the coolant. At present, it is generally accepted that the “isoconcentration” temperature dependence of the sensor readings in this region is fulfilled, and when the temperature decreases, the PbO phase is released (
At the same time, it often turns out that in the region of ~ 400–600 °C, the sensor readings stabilize at a temperature drop of
This contradicts thermodynamic calculations, according to which the potential of the {Pb}-<PbO> system has a small negative temperature slope and stabilizes in the considered temperature range at a level of
At one time, attempts were made to explain the observed deviations of the sensor readings at the equilibrium level of the coolant with the oxide phase by various factors (the influence of the thermoelectric power, pollution of the sensors, etc.). However, the data on the level of equilibrium in the “coolant-oxide phase” system (120–125 mV) were systematic and recorded by various sensors. At the same time, steel was always present in the system. Meanwhile, a previous analysis of the observed differences in the readings of two
Indicative in this regard is also the nature of EMF changes of the same sensor in the experiments with a rotating disk, carried out when EI852 steel samples were tested in lead- and lead-bismuth coolants. In the initial period (after the sample rotation was established), characterized by an increase in the interaction of the liquid metal melt with atmospheric oxygen, a regular decrease in the sensor readings was recorded: in lead, at 620 °C, from 124–122 mV to ~ 110 mV and in lead-bismuth, at 620 °C, from ~ 125 mV to ~ 77 mV. As slag accumulated on the surface of the melt (which was observed visually), the
Thus, the data presented indicate a qualitative change in the coolant condition in the presence of an iron source not only in the “deoxidized” region but also in the range of oxygen activity close to unity.
To confirm this thesis, experiments were carried out to identify the nature of changes in the potential of the melt in the state of its equilibrium with the resulting solid phase. For this purpose, the lead melt was periodically cycled during the exposure of EP823 steel samples in it at a base temperature of 620 °C.
Before the steel samples were immersed, the lead melt was previously refined in air, and the resulting slag was removed from its surface. The surface of the samples was relatively small (37 cm2), and a ceramic cup (Al2O3) was used as a lead container. All this allowed us to hope that, at the beginning of the exposure of the samples, the melt did not contain metallic impurities in appreciable amounts.
Figure
Temperature dependence of the
A much more rapid coolant “ferruginization” was observed in the experiments with EP302 steel, in which the contact surface of the steel with the coolant was 189 cm2, and the amount of lead was approximately two times less (Fig.
EMF temperature dependence of the
The experiments show that, over time, the liquid metal is saturated with iron, and the solid phase formed during the cooling process is not pure lead oxide but, probably, is complex oxide with one or another iron content.
It is logical to assume that the impact of iron impurities in a varying degree should also be manifested in the region of intermediate oxygen activities in the coolant (relatively far from the equilibrium line <Fe>-<Fe3O4> and the equilibrium line {Pb}-<PbO>) and, moreover, in the area of activities directly adjacent to the line <Fe>-<Fe3O4>.
To analyze this effect, it is more convenient to use coordinates, where the lg
lg (
where –20609/
Figure
Calculated temperature dependences of a constant oxygen concentration for the pure {Pb}-<PbO> system.
The experimentally observed deviations from the temperature slope indicated for a pure system should characterize the degree of the impact of iron on the lead melt oxidation potential. The iron available in the coolant or coming from the source consumes part of the oxygen available in the melt. As a consequence, the temperature slope in such a system should increase. If dissociation or conversion of any oxides with the release of free oxygen into the melt proceeds in the system, then the temperature coefficient should decrease in absolute terms.
Figure
Temperature dependence of the oxygen partial pressure in the melt cooling process in the experiments with EP823 steel.
The results obtained confirm that, under appropriate conditions, iron oxides in heavy lead-containing coolants can dissociate. This is apparently due to the lack of iron in the coolant necessary for the further oxide formation. The decomposition of iron oxides by this mechanism is accompanied by the release of oxygen in accordance with the stoichiometry of these oxides.
Thus, it was found that iron oxides previously formed in the coolant, as such or in the form of an oxide film on the structural steels, are not inert phases in heavy coolants, but can exchange iron and oxygen with the melt. The nature of this exchange depends both on the actual coolant impurity state and on external conditions, including first of all, temperature.
The authors considered examples of using the thermal cycling technique for heavy lead- and lead-bismuth-based coolants in order to obtain additional information about their actual physicochemical state based on the analysis of the oxygen thermodynamic activity behavior.
It is shown that the proposed coolant thermal cycling method is informationally significant and can be recommended for further use, for example, to obtain quantitative data on the content of iron impurities in the coolant.
* Russian text published: Izvestiya vuzov. Yadernaya Energetika (ISSN 0204-3327), 2020, n. 1, pp. 98–106.