Ilona Dörfel

Microstructure, smoothening effect, and local defects of alumina sol-gel coatings on ground steel

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-saving and cost-saving protection type for commercial steel P92.Graphical Abstract

Microstructural origin of time-dependent changes in alumina sol–gel-coated Inconel 718 exposed to NaCl solution

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 sol–gel 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.Graphical Abstract

Alumina Coatings as Protection against Corrosive Atmosphere

Oxidation protection layers were deposited by spin coating on substrates of Ni based superalloy SC 16 [1]. The protection effect of that type of coatings during heat treatment in dry air at 800°C for 4000 hours was investigated by Dressler [2]. In order to develop protection coatings also effective in a steam containing corrosive atmosphere, different types of multilayered coating systems were tested in an environment of a flowing mixture of CO2 and steam at 500 °C for 120 hours and subsequently at 700 °C for another 120 hours. Here the results of the investigations at two different types of coating systems will be presented: (i) two layers of a mixture of ethanolic boehmite sol with a suspension of submicron corundum powder and (ii) the two layers of (i) covered by a third layer of an aqueous sol. Before testing in corrosive atmosphere both types of samples were heat treated at 500 °C for 0.5 hours and at 800 °C for 2 hours in air.