Liquid metals properties degradation

In most cases the occurrence of mechanical properties degradation, and in particular embrittiement in the Uquid metal, is a cause of concern and needs to be carefully investigated (not only on the bulk material but also on the welds), primarily if alternative materials not suffering embrittlement are not considered or are not available. In fact, for several cases the embrittlement and degradation of mechanical properties induced by the liquid metal can be prevented and mastered through specific design criteria and through a close control of the liquid metal chemistry. 

In general, it has been observed that neutron irradiation does not impact the corrosion mechanism and the rate of reference structural materials exposed to liquid Na, Pb and Pb-Bi. On the contrary, neutron irradiation and liquid metal synergetic effects on the degradation of the mechanical properties of structural materials have been observed. As an example, ferritic/martensitic steels when neutron-irradiated and in contact with liquid Pb-Bi show degraded mechanical properties attributable to both the irradiation defects and the Uquid metal impact. As already reported before, from the reactor operation point of view, the design criteria of reactor components have to include these phenomena. 

A comprehensive literature review on the degradation of mechanical properties of structural materials exposed to liquid Pb and Pb-Bi is given in Ref. [16]. As reported in this reference most of the experiments have been performed in liquid Pb-Bi and the extension of these experimental findings to pure Pb cordd lead to incorrect estimations, since liquid Pb-Bi seems to be more aggressive than liquid Pb. Moreover, the main structural materials tested were the 9Cr ferritic/martensitic steel 791 and the austenitic steel AISI316. However, a generalization of the residts obtained in order to predict the behavior of other 9Cr ferritic/martensitic and austenitic steels is not feasible, since as, for instance, minor alloying elements in the steel impact the materials behavior in these liquid metals. 

A continuous oxide layer without defects is considered as protective and hinders direct contact between the bare T91 steel and the liquid metal, therefore reducing the risk for a degradation of the mechanical properties of the steel. 

As discussed previously, the mitigation of stmctural materials corrosion and its mechanical properties degradation can be done through the control of the oxygen potential in the liquid metal, resulting in the growth of a native oxide layer on the steel... 

As discussed in this chapter, the corrosion phenomena, as well as the degradation of the mechanical properties in liquid Na and liquid Pb/Pb-Bi, are driven by the solubility of the steel elements in the liquid metals and/or the solubility of nonmetaUic elements as oxygen and carbon. In particular, the solubility of oxygen is very important for corrosion phenomena in both liquid Na and liquid Pb/Pb-Bi, while the solubility of carbon is relevant for liquid Na only. 

Strength, brittleness, and solvent permeability properties are limited because of lack of control of the ceramic composition on a macro- and microlevel. Even small particle sizes are large compared with the molecular level. There have been a number of attempts to produce uniform ceramic powders including the sol-gel synthesis in which processing involves a stable liquid medium, coprecipitation in which two or more ions are precipitated simultaneously. More recently, Carraher and Xu have used the thermal degradation of metal containing polymers to deposit metal atoms and oxides on a molecular level. 

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