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- Corrosion protection (3) (entfernen)
A way to increase the application range and the lifetime of metallic substrates is their protection by coatings. Here the effect of alumina coatings on common steel substrates having different Chromium content is examined. The background of the investigations is to expand the application possibilities of mechanically good steels to higher temperatures and corrosive atmospheres in the framework of the European project POEMA (“Production of Coatings for New Efficient and Clean Coal Power Plant Materials”) which was introduced to identify materials that can withstand the aggressive conditions arising during the oxyfuel process in modern coal power plants. This process is one possibility to reduce the CO2 emission by firing the coal in oxygen and recirculated flue gas.
Substrate materials were P92 (9% Cr), K44X (19% Cr), and HR3C (25% Cr). The alumina coatings were deposited by a sol-gel-process using boehmite, this is relatively simple and offers application possibilities for a wide technical range without special surface preparations. All samples were dried for 30 minutes at 650° C to remove the organic components and to start the crystallization process of the alumina. For P92 and HR3C the following conditions simulated the oxyfuel process: 650°C in wet flue gas for 300h for the steel P92, the same conditions for 2000 hours for the steel HR3C. K44X was tested at 900°C in laboratory air for 500 hours. As well after drying as after testing TEM characterizations were performed, where the interfaces between steel and coating are of special interest to characterize the microstructure, detect failure mechanisms, and identify diffusion and crystallization processes and so to understand the mode of action of the alumina coating.
The TEM samples were prepared by the lift-out-technique using a FIB Quanta 3D and were investigated in a STEM JEM2200FS at 200kV.
After the coating and drying process all steels show chromium oxide layers directly at the steel surface, they vary in thickness and are island shaped in the case of the steel K44X. Chromium depletion arose more or less in the superficial zones of all samples. After the long term tests K44X showed breakaway oxidation while P92 and HR3C presented intact interfaces. This reveals: Beside the positive impact of the coating not only the Chromium content of a substrate is essential for the formation and self-healing of protective chromium oxide layers during the service time of steels. One has to consider the possibility of the steel to allow a continuously Cr diffusion to the interface and so e renewal of the Cr-oxide layer.
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.