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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.
Alumina coatings with differing phase compositions were deposited on Inconel-718 (IN-718) Ni-base
superalloy substrates using solgel processing. Mass gain measurements served for studying the oxidation
behavior of coated metal surfaces if exposed to 800°C and 900°C for up to 4000 h in static air. Itwas found, that
alumina coatings significantly reduce the oxidation related mass gain of IN-718 even after heating to 900°C
for 4000 h. Transmission electron microscopy (TEM) studies revealed diffusion of elements from the metal
substrate (IN-718) into the alumina coatings and to a much lower degree from the coating into the
substrate. These diffusion processes are greatly influenced by the phase composition of the coatings, especially
by the presence of alpha-alumina.
Inconel 718 was spin coated twice and fourfold with a sol to obtain 200- and 400-nm-thick transition alumina films on the surface. Bare and solgel alumina-coated Inconel 718 samples were exposed to NaCl solution to study their corrosion behavior by means of electrochemical impedance spectroscopy. In combination with scanning electron microscopy, it was shown that bare Inconel 718 is after initial passivation prone to pitting corrosion. For the coated Inconel 718 samples, an improvement in the protective effect of the coatings with time was observed. This observation is in contradiction to the results of other authors who observed degradation of alumina coatings exposed to NaCl solution with time. Transmission electron microscopy revealed a formation of (1) a compacted region at the coating surface as well as (2) a chromium-rich region at the interface between coating and alloy during the contact with a NaCl solution. The last one mainly contributes to the observed chronological sequence of electrochemical characteristics of the samples.
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.
Microstructure, smoothening effect, and local defects of alumina sol-gel coatings on ground steel
(2017)
Porous alumina films with thicknesses of a few microns were prepared via a dip-coating technique on steel P92. The coating is shown to protect the steel against massive corrosion, which is typical in the hot reactive environment of coal fired power plants. To mimic real conditions ground steel plates were coated with a boehmite-sol. This leads to an overall smoothing of the formerly rough surface. In the following short annealing step the inner porous construction with worm-like particles consisting of nano-crystallites and amorphous alumina is formed. Due to the simultaneous diffusion of chromium and iron ions out of the bulk steel material into the porous alumina coating, a dense interface with satisfactory adhesion is formed. However, the film exhibits few local defects like cracks or dense alumina nodules caused by steep edges in the ground surface or agglomeration of boehmite-sol components, respectively. Cracks especially have to be avoided. This problem can be overcome so far by slight modifications in the sol preparation process and surface treatment of the substrates. Nevertheless the results demonstrate the potential of sol-gel based alumina coatings as a time- and cost-saving protection type for commercial steel P92.