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RESEARCH PAPERS

# Determination of Corrosion Layers and Protective Coatings on Steels and Alloys Used in Simulated Service Environment of Modern Power Plants

[+] Author and Article Information
Hubertus Nickel1

Institute for Materials and Processes in Energy Systems, Research Centre Jülich GmbH, D-52056 Jülich, Germany, and University of Technology, Aachen, Germanyh.nickel@fz-juelich.de

Institute for Materials and Processes in Energy Systems, Research Centre Jülich GmbH, D-52056 Jülich, Germany, and University of Technology, Aachen, Germany

1

Corresponding author.

J. Pressure Vessel Technol 128(1), 130-139 (Oct 06, 2005) (10 pages) doi:10.1115/1.2137769 History: Received July 15, 2005; Revised October 06, 2005

## Abstract

The development of modern power generation systems with higher thermal efficiency requires the use of constructional materials of higher strength and improved resistance to the aggressive service atmospheres. In this paper, the following examples are discussed. (i) The oxidation behavior of 9% Cr steels in simulated combustion gases: The effects of $O2$ and $H2O$ content on the oxidation behavior of 9% Cr steels in the temperature range $600–800°C$ showed that in dry oxygen a protective scale was formed with an oxidation rate controlled by diffusion. In contrast, that in the presence of water vapor, after an incubation period, the scale became nonprotective as a result of a change in the oxidation mechanism. (ii) The development of NiCrAlY alloys for corrosion-resistant coatings and thermal barrier coatings of gas turbine components: The increase of component surface temperature in modern gas turbines leads to an enhanced oxidation attack of the blade coating. Considerable efforts have been made in the improvement of the temperature properties of MCrAlY coatings by the additions of minor elements, such as yttrium, silicon, and titanium. The experimental results show the positive, but different influence of the oxidation behavior of the MCrAlY coatings by the addition of these minor elements. (iii) The development of lightweight intermetallics of TiAl-basis: TiAl-based intermetallics are promising materials for future turbine components because of the combination of high-temperature strength and low density. These alloys, however, possess poor oxidation resistance at temperatures above $700°C$. The experimental results showed that the oxidation behavior of TiAl-based intermetallics can be strongly improved by minor additions of $1–2at.%$ silver. (iv) The oxide-dispersion-strengthened (ODS) alloys provide excellent creep resistance up to much higher temperatures than can be achieved with conventional wrought or cast alloys in combination with suitable high-temperature oxidation/corrosion resistance. The growth mechanisms of protective chromia and alumina scales were examined by a two-stage oxidation method with $O18$ tracer. The distribution of the oxygen isotopes in the oxide scale was determined by secondary ion-mass spectroscopy and SNMS. The results show the positive influence of a $Y2O3$ dispersion on the oxidation resistance of the ODS alloys and its effect on growth mechanisms.

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## Figures

Figure 1

Corrosion of 9–12% Cr and austenitic steels in simulated flue gas at 650°C (30mgcm2 weight gain equivalent to 0.1mm reduction in component thickness)

Figure 2

SIMS-Cs+ depth profile of oxide scale on the 9% Cr steel (P91) after 30hr oxidation at 650°C in N2−1 vol. % O216−1 vol. % H2O18

Figure 3

Influence of water vapor on the rate of oxidation of 9% Cr steel (P91) in N2 with 1 vol. % O2+x vol. % H2O mixtures between 600 and 800°C(ptotal=1bar)

Figure 4

Metallographic cross section photograph of specimen oxidised 5hr at 15l∕hr gas flow in Ar∕50%H2O at 650°C(14)

Figure 5

Raman spectra comparing standard spectrum of hematite and measured spectrum of outer oxide (point 1 in Fig. 4) of specimen oxidized for 5hr at 15l∕hr gas flow in Ar∕50%H2O at 650°C(14)

Figure 6

Proposed mechanism for the enhanced oxidation 10% Cr steel in argon-water vapor mixtures. Times t1,t2,t3,t4,t5 represent subsequent time steps during the oxidation process (14).

Figure 7

Cyclic oxidation behavior at 1100°C of Ni-20Cr-10Al model alloys with different yttrium additions

Figure 8

SNMS depth profiles of alumina scale on NiCrAlY+2 mass % Si after oxidation at 1000°C in Ar+20 vol. % O2

Figure 9

Oxygen isotope distribution in alumina scales on Ni-20Cr-10Al-0.4Y with 0.4 and 2 mass % Ti after two stage oxidation in air∕air+O218(16hr∕32hr) at 1000°C

Figure 10

Weight change as a function of exposure time for oxidation of two Ag-containing alloys in air at 800°C, compared to results from Ti-48Al and Ti-48Al-2Re

Figure 11

XRD patterns (a) and {012} diffraction lines of a-Al2O3 phase (b) in oxide scale of Ti-48Al-2Ag after 400hr oxidation in air at 800°C

Figure 12

SIMS-Cs+ analysis of scale and Al-depletion layer of Ti-48Al-2Ag (a) and Ti-45Al-10Nb (b) after 25hr oxidation at 800°C in air

Figure 13

SEM/BSE image showing microstructure of Ti-Al-Ag coating on alloy Ti-46.5Al-4(Cr,Nb,Ta,B) after 250hr exposure in air at 800°C

Figure 14

X-ray diffraction results of MA 754 and Ni-25Cr after 300hr isothermal oxidation at 1000°C

Figure 15

Distribution of the oxygen isotopes O18 and O16 in the oxide layer of alumina formers after 3hr oxidation in O18-enriched air followed by 9hr of oxidation in normal air at 1000°C.

Figure 16

RBS analysis of the oxide scale on alloy MA 956 after 4hr oxidation in air at 1100°C

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