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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.
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.