Effect and role of alloyed yttrium on the fireside corrosion behaviour of Ni-Fe based alloys for 750 °C ultra-supercritical boiler applications

The influence of alloyed yttrium (0.15-0.3 wt.%) on the fireside corrosion behaviour of Ni-Fe based alloy was investigated at 750 degrees C. Results show that external scales of Cr2O3 and Al oxide, as well as internal Ti sulphide form during the corrosion regardless of the chemical composition of alloys. However, the scale thickness and corrosion rate are reduced by the alloyed-yttrium, e.g. the mass gain of 0.3 wt.% yttrium-contained alloy after 50 h corrosion is similar to 60% the value of the yttrium-free alloy. The yttrium doping in Cr sulphide particles in oxide scales most likely accounts for such a positive effect.