Interface formation between steels and alumina coatings in corrosive atmospheres
Authors: Dörfel Ilona , Nofz Marianne , Sojref Regine , Feijoo Maria Mosquera , Schulz Wencke , Wollschläger Nicole , Hesse Rene , Kranzmann Axel 
Published: December 2016

A way to increase application range and 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 CO
2
emission by firing the coal in oxygen and recirculated flue gas.

Substrate materials were two steels of the POEMA project: P92 (9% Cr), and HR3C (25% Cr), and the steel K44X (19% Cr), not used in POEMA. It is interesting to examine the protection effect of the coating in this case, because this steel is developed to be used in air up to 1000° C. 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 heat treated for 30 min 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 300 hrs for the steel P92, the same conditions for 2000 hrs for the steel HR3C. K44X was tested at 900° C in laboratory air for 500 hrs. After the first heat treatment of 30 min as well as after long‐term testing, TEM characterizations were performed. The microstructure of the interfaces between steel and coating is of special interest to detect failure mechanisms, and identify diffusion and crystallization processes with the aim to understand the principle 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 first heat treating 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, only P92 presents an intact interface, while K44X and HR3C showed breakaway oxidation. One can conclude: The positive impact of the coating and a high chromium content of a substrate are essential for the formation of protective chromium oxide layers. For long service times a renewal effect of the Cr‐oxide layers is necessary, originated by the possibility of continuous Cr diffusion to the interface without forming Cr depletion. In this context the interaction with the other alloying elements of the particular steel is of importance.