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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.
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.
The suitability of an Al2O3 coating for corrosion protection on X20Cr13 was evaluated in various artificial geothermal brines, focusing on the influence of different pH (4, 6 and 8) and their chemical compositions on the coating properties. All experiments were performed in the laboratory using autoclaves at 150 ◦C and 1 MPa in deaerated condition for 1 and 7 days. Results showed that the pH of geothermal waters is the most detrimental factor in the transformation of ɣ-Al2O3 and its protective abilities. Delaminations were found in the Coating exposed to geothermal brines with pH 4. FTIR spectra indicated a transformation of ɣ-Al2O3 to boehmite AlOOH after exposure to pH 4 and 6, and bayerite Al(OH)3 was formed after exposure to pH 8. Different Crystal structures of the hydrated Al2O3 also contribute to the stability of the coatings, observed by the SEM- EDX of the surface and cross-section of coatings. This study indicated that ɣ-Al2O3 sol-gel coating presents a promising aspect of corrosion protection in geothermal environment with a neutral pH.
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.