ICMCTF2011 Session A1-3: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

One straightforward solution to improve the efficiency of coal-fired power plants, and thereby reduce CO₂ emissions, is to increase the operating temperature and pressure. To ensure the durability of components at the ambitious goal of the ultrasupercritical steam program, 760°C and 350 bar, new material solutions must be developed. Oxidation resistant coatings have been shown to effectively reduce reaction rates in steam at this temperature. However, the impact of the coating on the alloy mechanical properties needs to be evaluated. Creep testing is being conducted on as-fabricated, pre-oxidized and aluminized Fe- and Ni-based specimens to gain a better understanding of how mechanical properties are affected. For comparison, in-situ steam testing of coated and uncoated specimens is being conducted in a new environmental creep rig.

In the case of supercritical steam turbines, may attemps have been made in terms of micro-structural coatings design, mainly based in aluminides.

In order to consider another alternatives to minimize those problems, nano-structured coatings, applied by HIPIMS-PVD based in Cr and Al design, have been applied onto high temperature structural materials in order to analyze their high temperature oxidation resistance in steam environments.

The gravimetric results obtained have been analysed upto 2.000 hours, jointly with the evaporation behavior analysed by TG-Mass spectromnetry. Excellent results have been achieved for the nano-structured coatings tested. Those results are comparables with the results obtained for micro-sctructured coatings, and in some case better for nano-structured coatings.

According to the results obtained, the nano-structured coatings have a potential application as protective systems in high temperature, for some applications.

Due to the demand for higher efficiencies and the use of lower quality fuels (process residues, bio-fuels, waste, etc) in thermal energy conversion plants, a need arises for material surfaces resistant to highly aggressive high temperature environments. The most critical types of such environments exhibit low oxygen partial pressures with high activities of sulphur, carbon, vanadium or halogens. Frequently, conventional metallic alloys are pushed to their limits concerning high temperature corrosion resistance under such conditions so that the need for highly resistant coatings arises.

In this paper recent developments are presented which show coating solutions for high carbon or high chlorine environments. In the first case coking and metal dusting can lead to dramatic and unexpected failure cases, in particular in the petrochemical industries. In other industrial applications even in the case of the use of expensive highly resistant nickel-base alloys coking and metal dusting can also present serious problems. An innovative coating concept based on poisoning of the catalytic effect of metallic surfaces completely suppresses the mechanisms of coking and metal dusting. Several poisoning elements can be used but a particularly effective element turned out to be Sn in combination with a Ni-precursor. For high chlorine containing environments a thermodynamics-based concept led to the development of a Ni-Mo-Al coating with superior corrosion resistance compared to high alloy nickel-base-materials that are usually used as expensive high-end materials in Cl-environments.

The thermodynamic background for the development of the two types of coatings is discussed and the manufacturing procedures are described. Furthermore, examples for the resistance in different highly aggressive environments are presented and analysed. The paper closes with an outlook on other types of coatings for severe environments and the necessary R+D work.