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Organisationseinheit der BAM
Up to ~ 3 µm thick alumina coatings on corundum ceramic, sodalimesilica glass and Inconel718 were produced from mixtures of boehmite sols and corundum suspensions.
Transmission electron micrographs in combination with electron diffraction and energy dispersive X-ray spectroscopy served to identify crystallographic phases and to characterize the microstructure of the coatings. Using corundum ceramic as substrate the initially deposited boehmite transforms via transition aluminas to corundum while heating to 1200 °C. In the cases of glass and Inconel718 thermal treatments up to 520 °C and 1100 °C, respectively, cause diffusion of ions from the substrates into the coatings. Thus additional oxide phases were formed.
All coatings are free of cracks or delaminations and do not show any directed crystal growth.
Sol-Gel-Aluminiumoxidschichten für den Oxidationsschutz von Hochtemperaturlegierungen, Teil 2
(2009)
Sol-Gel-Aluminiumoxidschichten für den Oxidationsschutz von Hochtemperaturlegierungen, Teil 1
(2009)
Alumosols in combination with well dispersed corundum were successfully used to form up to 2.5 ìm thick coatings on the Ni-base alloy SC16. These coatings withstood heat treatments at 700 °C in a water vapour containing atmosphere. The heat treatment caused formation of delta-alumina in the coating and diffusion of chromium and titanium into the coating.
Improved wetting of bare and pre-coated steels by aqueous alumina sols for optimum coating success
(2010)
Alumina coatings are a promising candidate for
the protection of metals prone to high temperature corrosion.
If applied via sol–gel process, especially by using so
called aqueous modified Yoldas-sols, the deposition is
hardened by differences in surface free energy of substrate
and sol. In this paper the apparent surface properties of
samples to be coated and sols were examined by contact
angle measurements, IR-spectroscopy and tensiometry.
The results imply, after consideration of possible uncertainties,
the use of surface tension reducing 2-butanol
already present during hydrolysis of the sols to obtain an
easy and successful coating process.
In order to reduce CO2 emissions fossil fuelled power stations with high combustion efficiency are being developed. The increase of the operating temperature, a common way to improve combustion efficiency, leads to enhanced corrosion of heat exchange steel tubes in the power plants. Within the framework of the European project “Production of Coatings for New Efficient and Clean Coal Power Plant Materials” (POEMA)", high temperature corrosion protection coatings are now under investigation.
Thin porous sol-gel alumina films are promising candidate coating materials. Coatings were prepared by applying boehmite sols on grinded steel P92 and subsequent heat treatments at temperatures up to 650 °C. Thus a porous layer of worm-like particles was formed consisting of nano-crystallites and amorphous alumina. A dense interface with satisfying adhesion resulted from diffusion of chromium and iron ions out of the steel into the porous coating. However, the film locally exhibited some cracks caused by steep edges in the grinded steel surface. An appropriate substrate pretreatment should avoid this problem.
The protection of the steel relies on the barrier function of the alumina coating and the formation of a dense chromia layer at the steel surface.
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.
Technically relevant P92 steel (9% Cr) was coated with a micron-thick porous alumina layer prepared by sol-gel technique and treated with flue gas (60 CO2-30 H2O-2 O2-1 SO2-7 N2 (mole fraction in %)) at 650 ° to mimic an oxyfuelcombustion process. Local defects in the coating were marked using focused ion beam (FIB) technique and were inspected after exposition to hot flue gas atmosphere at 300, 800, and 1300 h, respectively. Local defects like agglomerated alumina sol particles tend to spall off from the coating uncovering the underlying dense chromia scale. Re-coating was found to restore the protection ability from oxidation when repeatedly treated with hot flue gas. Cracks and voids did not promote the local oxidation due to the formation of crystalline Mn/S/O species within and on top of the coating. The protective character of the steel-coating system is a result of (i) the fast formation of a dense chromia scale at the surface of sol-gel alumina-coated P92 steel bars in combination with (ii) the porous alumina coating acting as diffusion barrier, but also as diffusion partner in addition with (iii) fast Mn outward diffusion capturing the S species from flue gas.
Though Ni-base superalloys show a high oxidation and corrosion resistance, coatings could still improve these properties, especially if used at temperatures up to 1000 °C. Here, a coating was prepared by applying a boehmite-sol via dip-coating and a subsequent heat treatment at 600 °C for 30 minutes. To evaluate the coating, the oxidation behavior of bare and alumina coated Ni-base alloy Inconel 625 in air at 900 °C was studied for up to 2000 h. Electron microscopic studies of sample surfaces and cross-sections showed that (i) in the 3.5 µm – 6.3 mm thick scale formed on the bare alloy, Fe and Ni are located as fine precipitates at the grain boundaries of the chromia-rich scale, (ii) Ni and Ti are concentrated to a minor degree at the grain boundaries of the scale, too; and for the coated sample: (iii) the only 1.8 µm thick sol-gel alumina coating slows down the formation of chromia on the alloy surface and reduces the outward diffusion of the alloy constituents. The protective effect of the coating was evidenced by (i) diminished chromium diffusion at grain boundaries resulting in less pronounced string-like protrusions at the outer surface of the coated IN 625, (ii) formation of a Cr-enriched zone above the alloy surface which was thinner than the scale on the uncoated sample, (iii) no detectable Cr-depleted zone at the alloy surface, and (iv) a narrower zone of formation of Kirkendall pores.