Calculation of Reactive-evaporation Rates of Chromia

Abstract

A methodology is developed to calculate Cr evaporation rates from Cr 2 O 3 with a flat planar geometry. Variables include temperature, total pressure, gas velocity, and gas composition. The methodology was applied to solid oxide fuel cell conditions for metallic interconnects and to advanced steam turbines conditions. The high velocities and pressures of the advanced steam turbine led to evaporation predictions as high as 5.18 × 10 -8 kg/m 2 /s of CrO 2 (OH) 2 (g) at 760°C and 34.5 MPa. This is equivalent to 0.080 mm per year of solid Cr loss. Chromium evaporation is expected to be an important oxidation mechanism with the types of nickel-base alloys proposed for use above 650°C in advanced steam boilers and turbines. It is shown that laboratory experiments, with much lower steam velocities and usually much lower total pressure than found in advanced steam turbines, would best reproduce chromium evaporation behavior with atmospheres that approach either O 2 +H 2 O or Air+H 2 O with 57% H 2 O.

Effects of water vapor on protectiveness of Cr2O3 scale at 1073 K

“…Its presence is also increases the evaporation loss of Cr 2 O 3 [10]. Significant loss of Cr 2 O 3 in wet condition is due to several factors.…”

Chromia Evaporation in Advanced Ultra-Supercritical Steam Boilers and Turbines

“…Unprotective scales can lead to rapid metal loss, or "breakaway" oxidation. A detailed methodology for calculating evaporation rates in a variety of environments was presented in earlier work for gas flow over a flat plate (Holcomb, 2008). Two basic equations were developed: Eq.…”

“… Rapid, or "breakaway" oxidation from the depletion of Cr in the scale resulting from reactive evaporation of Cr 2 O3 in the presence of O 2 and H 2 O (Essuman et al, 2008;Holcomb, 2008Holcomb, , 2009Young & Pint, 2006). Clear links to evaporation were shown by how increased gas flow rates led to faster breakaway oxidation and increased Fe:Cr ratios within the oxide scale (Asteman et al, 2000).…”

“…In prior work, a methodology to calculate the expected chromia evaporation rate as a function of temperature, pressure, gas velocity, and steam chemistry was developed for chromia forming alloys (Holcomb, 2008). The methodology was experimentally validated at low pressures and gas velocities by how well its kinetics www.intechopen.com…”

“…At 760 °C and 340 atm, evaporation rates as high as 5.18 × 10 -8 kg/m 2 /s of CrO 2 (OH) 2 (g) were predicted for rotating high pressure (HP) turbine blade components (Holcomb, 2008). This is equivalent to 0.08 mm per year of solid Cr loss.…”