Refractory Metal Coatings on High Creep Strength Steel for Oxyfuel Coal Combustion

Oxyfuel coal combustion, where coal is burned in oxygen instead of air, offers the potential for simplified CO₂ sequestration, since the flue contains little N₂ and is mainly CO₂.  However, the widespread use of flue gas recycling to dilute the pure oxygen feed stream exacerbates corrosion problems in the furnace and boiler tubes by increasing the concentrations of SO₂, SO₃, HCl, and H₂O.  In addition, the dew point temperature is raised, making condensation of H₂O, H₂SO₄ and H₂SO₃more likely during downtime.

P91 steel, which contains ~8.4% Cr, 0.9% Mo and 2.2% V, is are widely utilized for high temperature applications in the power industry due to its increased creep strength.  However, Cr-containing steels are susceptible to failure in H₂O-containing oxidizing environments due to formation and volatilization of compounds such as CrO₂(OH)₂.  We report the formation of Ti and Zr diffusion coatings atop high creep strength steels to improve their corrosion resistance.  Ti coatings 10-30 µm thick can be formed on P91 steel by pack cementation at 1000°C in flowing Ar.  Similarly, Zr coatings 10-30 µm thick can be formed by pack cementation at 1080°C. 

The composition as a function of position for both Ti and Zr diffusion coatings is studied as a function of position by scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDX).  This demonstrates formation of an inter-diffusion zone, with no sharp interface exists between coating and substrate, which may help limit delamination.  Results will also presented for thermogravimetric analysis (TGA) of both Ti and Zr coatings on P91 steel in gas mixtures that contains 5% N₂, 70% CO₂, 2% CO, 0.3% SO₂, 21.7% H₂O, 1% O₂, and trace levels of HCl.