To increase efficiency, modern steam plants are pushing their operational regime from supercritical (600°C/300 bar) to ultra-super-critical (740/760°C/350 bar) stretching existing turbine materials to their limits. The focus is on new generation functional materials and technologies which complement the inherent properties of existing materials. Current work proposes a novel High Power Impulse Magnetron Sputtering (HIPIMS) deposition technology, for the first time, for deposition of a ceramic based CrN/NbN coating with a nanoscale multilayer structure (bi-layer thickness ∆ = 1.9 nm) with superior adhesion (L C2 = 80N) to protect low Chromium P92 steel widely used in steam power plants. Thermodynamic calculations predict the equilibrium phases and aggressive gaseous compounds generated by the interaction of steam with the coating. CrN/NbN coated P92 steel

MIcrostructure and cheMIcal coMposItIon of fe-cr alloy teMpered In hIgh teMperature and atMosphere contaInIng ar-so2The paper presents the microstructure, chemical and phase composition of thin scale, obtained as a result of hightemperature corrosion of X20Cr13 stainless steel. Samples were exposed to gas atmosphere of the following composition: 0.25 vol.% of SO2 and 99.75 vol.% of Ar at 600 °C for 5 h. As a consequence, thin compact scale was formed on steel surface. This scale consisted of three different zones. An amorphous zone was formed close to steel surface. Then, nanocrystalline zone could be observed. Finally, larger grains were formed during the corrosion process. The analysis of the chemical composition revealed higher concentration of chromium near steel surface. In contrast, to chromium, the content of iron, increased near the scale surface. It was found out that the (Cr, Mn, Fe)5O12 phase appeared in the thin scale.

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.

The steam side oxidation of ferritic–martensitic VM12-SHC steel was investigated under thermo-cyclic conditions in water steam at 620/320 °C and 30 bar with a focus on assessing the influence of pre-oxidation time, specimen geometry and surface finish. The specimens were pre-oxidized under isothermal conditions in water steam at 620 °C and 30 bar for 500 h or 1500 h. After pre-oxidation treatment, all specimens were subjected up to 258 thermal cycles. Three different geometries—rectangular coupons, U-shaped ring segments and ring samples—were investigated to evaluate the influence of open/closed shape, and flat/curved surface on corrosion rate. At the same time, two types of surface finish were considered: “as received” and “ground.” The formation of a protective scale by pre-oxidation was investigated. EBSD and ESMA analyses revealed that the Cr-content of the alloy appeared to be insufficient for obtaining a protective oxide scale under studied conditions, at the same time the anayses confirmed that initial oxidation depends on presence of minor alloying elements as Si and Mn, strong oxide formers which can alter the kinetics and morphology of the corrosion reaction. Moreover, rectangular coupons with small wall thickness and flat surface exhibited the highest corrosion rate, while “ground” curved samples showed only local oxidation. This indicates that for same pre-oxidation time, oxidation kinetics is controlled by curvature.