The corrosion of Fe-15 wt% Y and Fe-30 wt% Y in sulfidizing-oxidizing atmospheres at 600-800 degrees 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 mixtu res 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 sul fidizing-oxidizing atmospheres of high sulfur potential. The presence of yttrium is only slightly beneficial, since even a yttrium c ontent of 30 wt% can only reduce the corrosion rate of iron to a limited ex tent 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 mo re rapidly than pure yttrium at short times, but presumably more slowly at long times. On the contrary, at 800 degrees C, when yttrium behaves protect ively, 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 t he two metals and an innermost zone of internal attack containing compounds of yttrium with both oxygen and sulfur, Thus, iron can still diffuse throu gh the intermediate region at nonnegligible rates to form the outer FeS lay er. The scaling behavior of these alloys is discussed by taking into accoun t the limited solubility of yttrium in iron as well as the presence of inte rmetallic Fe-Y compounds. (C) 1999 Elsevier Science Ltd. All rights reserve d.