Fused salts corrosion rates

It is not worth while, therefore, to give a digest of experimentally determined corrosion rates , but Table 2.21 indicates some sources of such data and their nature. (Some references to data on compatibility of fused salts with non-metallic materials have been included for the sake of completeness.) It should be remembered, that in the case of alloys, failure usually arises from selective attack which causes porosity of the container, even though the wall may appear on casual inspection to be quite sound... 

The obvious method of reducing corrosion in fused salts is to choose a system in which either the metal can come to equilibrium with the melt, or else truly protective passivity can be attained. In most cases in industry neither of these alternatives is used. In fact, fused salt baths are usually operated in air atmosphere, and the problem is the prevention of excessive corrosion. This can be done in two ways, (a) by reducing rates of ingress of oxidising species (mainly O2 and H2O) from the atmosphere, and rates of their diffusion in the melts, and (b) by keeping the oxidising power (redox potential) of the melt low by making periodic additions to the bath. 

High temperature (Type 1) hot corrosion (HTHC) is normally observed in the temperature range of about 825-950°C when the condensed phase is clearly liquid. The typical microstructure for HTHC shows the formation of sulphides and a corresponding depletion of the reactive components in the alloy substrate. The external corrosion products consist of oxide precipitates dispersed in the salt film. The presence of the pore, crevice or crack across a protective film can lead to the sulphidation of the alloy substrate. This results in a significant shift in the basicity of the salt film. Once the fused salt contacts the alloy substrate, the rate and duration of the rapid corrosion kinetics are decided by the magnitude and gradient of salt basicity relative to the local solubilities for the oxide scale phases (Rapp and Zhang, 1994). 

In general, it is fair to state that one of the major difficulties in interpreting, and consequently in establishing definitive tests of, corrosion phenomena in fused metal or salt environments is the large influence of very small, and therefore not easily controlled, variations in solubility, impurity concentration, temperature gradient, etc. . For example, the solubility of iron in liquid mercury is of the order of 5 x 10 at 649°C, and static tests show iron and steel to be practically unaltered by exposure to mercury. Nevertheless, in mercury boiler service, severe operating difficulties were encountered owing to the mass transfer of iron from the hot to the cold portions of the unit. Another minute variation was found substantially to alleviate the problem the presence of 10 ppm of titanium in the mercury reduced the rate of attack to an inappreciable value at 650°C as little as 1 ppm of titanium was similarly effective at 454°C . 

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