The corrosion of Fe-15 wt% Y and Fe-30 wt% Y in sulfidizing-oxidizing atmospheres at 600-800°C

The corrosion of two iron-based alloys containing 15 and 30 wt% yttrium and of pure yttrium has been studied at 600-800 degrees C in H-2-H2S-CO2 mixtures under a sulfur pressure of 10(-8) atm and an oxygen pressure of 10(-24) atm at 600 degrees C and of 10(-20) atm at 700-800 degrees C to establish the effect of yttrium additions on the resistance of pure iron to mixed sulfidizing-oxidizing atmospheres of high sulfur potential. The presence of yttrium is only slightly beneficial, since even a yttrium content of 30 wt% can only reduce the corrosion rate of iron to a limited extent at 600 and 800 degrees C, but not at 700 degrees C. More precisely, at 600 and 700 degrees C, when yttrium shows breakaway, the alloys corrode more rapidly than pure yttrium at short times, but presumably more slowly at long times. On the contrary, at 800 degrees C, when yttrium behaves protectively, the two alloys corrode considerably more rapidly than pure yttrium. Under all conditions the two alloys form an external FeS layer overlying a very complex intermediate region containing a mixture of the compounds of the two metals and an innermost zone of internal attack containing compounds of yttrium with both oxygen and sulfur, Thus, iron can still diffuse through the intermediate region at nonnegligible rates to form the outer FeS layer. The scaling behavior of these alloys is discussed by taking into account the limited solubility of yttrium in iron as well as the presence of intermetallic Fe-Y compounds. (C) 1999 Elsevier Science Ltd. All rights reserved.