材料期刊网

The corrosion of Co-15 wt.% Y has been studied at 600-800 degrees C in H-2-H2S-CO2 mixtures providing a sulfur pressure of 10(-8) atm at 600-800 degrees C and of 10(-7)atm at 800 degrees C and an oxygen pressure of 10(-24)atm at 600 degrees C and of 10(-20)atm at 700-800 degrees C. The corrosion rates in such sulfidizing-oxidizing atmospheres were compared,,with those of pure cobalt and yttrium. The addition of yttrium to cobalt is only slightly beneficial, since Soi a yttrium content of 15 wt.% the corrosion rate is reduced quite significantly with respect to pure cobalt at 800 degrees C under 10(-7)atm S-2, only to a limited extent at 600 degrees C, and even slightly increased at 700 degrees C. Moreover, the alloy corrodes considerably more rapidly than pure yttrium at 800 degrees C, when the latter. behaves protectively.,At 600 and 700 degrees C, yttrium exhibited breakaway behavior, while the alloy corroded more rapidly than yttrium at short times, but more slowly at long times. Under all conditions, except at 800 degrees C under 10(-8) atm? Sr, the alloy forms ail external layer of cobalt sulfide overlying an intermediate region of very complex composition containing a mixture of the compounds of the two metals and ail innermost region of internal attack containing compounds of yttrium with both oxygen and sulfur. Thus, cobalt can still diffuse through the intermediate region to form the outer cobalt-sulfide layer at nonnegligible rates. The scaling behavior of rite Co-15% Y alloy is discussed by taking into account the limited solubility of yttrium in cobalt as well as the presence of an intermetallic Co-Y compound in the alloy.