Alumina sol-gel coatings for corrosion protection of steel P92

The work to be presented is part of the POEMA project which has the goal of developing new coatings for steels and alloys used in supercritical steam power plants for efficient and clean coal utilization. From the required high temperature mechanical properties perspective (in particular creep strength) steel P92 is one of the chosen materials. It can be used at temperatures up to 650°C. Sol-gel alumina coatings are among the coatings to be produced and tested. Even though such coatings are comparably thin (in the range of few micrometers) they are able to effectively diminish high temperature corrosion. The behavior of the new coatings for protecting materials both from steam oxidation and fire-side corrosion will be studied in steam and oxy-fuel combustion atmospheres. Coatings were applied by dipping the substrates into ethanolic boehmite sols. After heat treatment at 650°C for 30 min up to ~2.5μm thick and well adherent transition, alumina coatings were formed. Coated and uncoated samples were exposed in steam and oxy-fuel atmosphere (60 CO2 - 30 H2O - 2 O2 - 1 SO2 - 7 N2, all in vol. %; flow rate 3*10-2 m/s, 1 bar,) at 650°C for up to 300 h whereby the influence of ash deposits was also regarded. Samples are characterized by using optical microscopy, scanning and transmission electron microscopies, and electron probe microanalysis. Additionally, the mass gain was recorded. The results proved the protective abilities of the thin sol-gel alumina coatings, which did not spall, had no cracks, did not show larger corroded areas, and exhibited a very low mass gain. Hence, such coatings are promising candidates for protection of steel P92 used in power plants.