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Thin Sol-Gel Alumina Coating as Protection of a 9% Cr Steel Against Flue Gas Corrosion at 650 °C
(2018)
Samples of sol-gel alumina coated and uncoated P92 steel were exposed to flue gas at 650 °C for 300 h. As result of this treatment a 50 µm thick bi-layered oxide scale had formed on the surface of the uncoated sample. Below the scale a 40 µm thick inner oxidation zone was detected. In contrast, the porous, micron thick alumina coating enabled the formation of a chromium oxide scale with a thickness of some nanometers at the interface between steel substrate and coating. In this case high temperature corrosion of the steel was prevented so far.
The combustion of coal in CO2-reduced Oxyfuel power plants requires creep resistant and corrosion
resistant materials, which can withstand high temperatures up to 600°C and CO2 rich atmospheres.
Among the heat resistant materials, the 9-12% chromium steels are proven to resist high wall
temperatures in conventional power plants and are suitable as membrane wall, superheaters and steam
piping.
During Oxyfuel combustion a flue gas is generated, which consists mainly of H2O (30 mol %) and
CO2 (70 mol %). The present paper is focused on the corrosion of 9-12% chromium steels under
oxyfuel conditions in a temperature range between 550 and 625°C.
Depending on Chromium content of the 9-12%chromium steels, carburization of the base material,
perlite formation and carbide formation were observed. Alloys with lower chromium content form a
non protective oxide scale with perlite at the scale-alloy interface. Steels with 12% chromium have a
small growing oxide scale with enlarged M23C6-particles at the scale-alloy interface. The carburization
of the base material is found to be increased for the 9% Cr-steel. Higher pressure of the flue gas results
in the formation of less resistant scales and cause accelerated carburization of the base materials.
However, the carburization has an impact on the mechanical properties at the surface and leads to an
embrittlement, which is deleterious during thermal cycling.
Oxidation kinetics, phase analysis of the scale (transmission electron microscope) and carburization
depths (microprobe) of the base materials are presented.
The resistance of structural steels against corrosion in CO2-H2O and CO2-H2O-O2
atmospheres at temperatures above 550°C was observed with high magnification methods.
The test atmosphere is the basic composition of an oxyfuel coal combustion atmosphere. The
analytical information base was applied to develop reaction schemes for the oxidation of
steels. It was demonstrated that the formation of phases and the kinetics of oxidation reactions
in this atmospheres is affected by the formation of intermediate iron hydroxide and a high
carbon activity. In the temperature regime above 550°C the thermal equilibrium between H2O,
CO2 and Fe reacted and formed CO. CO triggers the formation of FeO and carbides and
increases the reaction kinetics. Porosity at the interface is established by gaseous phases
such as Fe(OH)2 or H2O. H2 formation is not limited to the steel - oxide interface but can occur
in the whole oxide scale. Examples of the experimental results were compared with a
thermodynamic approach. The objective of the work was to derive qualitative rules of the basic
corrosion reactions.
To increase efficiency, modern steam plants are pushing their operational regime from super-critical (600 °C/300 bar) to ultra-super-critical (740/760 °C/350 bar) stretching existing turbine materials to their limits. The focus is on new generation functional materials and technologies which complement the inherent properties of existing materials.
Current work proposes a novel High Power Impulse Magnetron Sputtering (HIPIMS) Deposition technology, for the first time, for deposition of a ceramic based CrN/NbN coating with a nanoscale multilayer structure (bi-layer thickness Δ = 1.9 nm) with superior adhesion (LC2 = 80 N) to protect low Chromium P92 steel widely used in steam power plants. Thermodynamic calculations predict the equilibrium phases and aggressive gaseous compounds generated by the interaction of steam with the coating. CrN/NbN coated P92 steel samples oxidised at 600 °C in a high pressure (50 bar) 100% steam atmosphere for up to 1000 h reveal the coating's superior oxidation resistance and protective mechanisms, especially against the detrimental effect of Hydrogen. High temperature (650 °C) Tensile Strength, Low Cycle Fatigue and Creep tests confirm that, unlike other state-of-the-art PVD technologies, HIPIMS is not detrimental to the mechanical properties of the substrate material. Water droplet erosion tests confirm no measurable weight loss after 2.4 X 10⁶ impacts.