Immersion test at 650C

that soluble substance is mixture of carbonate and K,Cr04. The ratio of Li/Fe of residuals are from 0.94 to 1.42. This fact indicate that main part of insoluble substance is LiFeO,. The insoluble substance was removed oxide formed on surface of materials. >,,

Effect of Fe content in alloys on Li/(Li+K) in salt after corrosion test shown Fig. 2. Original value of this ratio is 0.62-0.64. After corrosion test, Li in salt on type 304 and 316L decreased, because Fe ion was formed solid LiFeO,. In coating test, as the amount of salt was limited, the decrease of Li was drastic for type 304 and 316L stainless steels. On the developed alloy (24%Fe), Ii/(Li+K) ratio was not reduced in case of coating and immersion tests. This results suggested that 30%Cr-45%Ni alloy scarcely fixed Li ion in corrosion.

Effect of Cr content on the. concentration of dissolved Cr and Fe ions in molten salt were shown in Fig. 3.~ The concentration of Fe ion was small and independent from alloy composition and test duration. After 205 hours immersion test, the concentration of dissolved Cr ion was almost same for all alloys tested. After 500 hours corrosion test, Cr concentration increased with Cr content of alloy up to 25%, and become small at 30%Cr alloy. -

The structure of oxide films after corrosion tests were examined by SEM. The cross-sectional view of oxide films is shown in Fig. 4. For all alloys tested, oxide films had same structure. From EPMA analysis, inner layer is Cr rich oxide. The outer layer is LiFeO, determined by X lay diffraction. 30%Cr alloy has thin oxide layer and it is.mainly Cr rich oxide. Type 304SS and 316L SS has thick oxide layer and its main component was LiFeO,.

Summary

After corrosion test, distribution of metal ions (Fe, Ni, Cr, Li and K) in surface oxide and molten salt was determined.

1. Fe ion is fixed in outer oxide layer formed on surface as LiFe02.

2. Cr ion is fixed inner oxide layer and dissolved into salt as CrO,2 ion.

3. Type 304 or 316 stainless steel show poor corrosion resistance and fix Li as LiFeO,. The developed 30%Cr-45%Ni-1 %A1-0.03%Y-Fe alloy scarcely fix Li ion, because its high corrosion resistance and small Fe content.

Metal

Fig. 4 Cross-sectional view of surface of 30%Cr-45%Ni alloy after coating test for 700hours at 650°C

Metal

Fig. 4 Cross-sectional view of surface of 30%Cr-45%Ni alloy after coating test for 700hours at 650°C

4. The developed alloy with 30%Cr has high corrosion resistance and its dissolved Cr is as same as type 304 and 316. Amount of dissolved Cr into salt by corrosion is determined by corrosion rate, Cr content of alloy and structure of oxide films. Type 31 OS Stainless steel containing 25%Cr is not sufficient in view of corrosion resistance and Cr dissolution.

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