Physical Electronics Dept.

You are invited to attend a lecture

By

Prof. Emeritus Hubertus Nickel

(Former Institute of Materials and Processes for Energy Systems, Research Center Juelich GmbH and University of Technology Aachen, D-52425 Juelich, Germany)

On the subject:

Determination of Corrosive Layers and Protective Coatings on Steel and Alloys in Simulated Service Environment in Advanced Power Stations

The development of modern power generation systems with higher thermal efficiency requires the use of construction materials with higher strength and improved resistance to aggressive service atmospheres. Our research supporting this development includes: 1) Studying the effect of water vapour on the oxidation behaviour of 9%Cr steels in simulated combustion gases. 2) Developing NiCrAlY alloys for corrosion resistant coatings and thermal barrier coatings for gas turbine components. 3) Studying the oxidation resistance of TiAl-based intermetallics, which are promising materials for future turbine components because of the combination of high temperature strength and low density, at temperatures above 700°C, and 4) studying the use of oxide dispersion strengthened (ODS) alloys, which can have excellent creep resistance up to much higher temperatures than can be achieved with conventional wrought or cast alloys as well as high temperature oxidation/corrosion resistance. The effects of the oxidation behaviour as well as the microstructure stability of protective coatings was determined by combining results of kinetic studies with extensive analytical investigations using among other techniques, SNMS; SIMS; SEM; TEM; RBS; LRS, XRD, as well ¹⁸O tracer analysis.

It was found that:

1) O₂ and H₂O content in 9%Cr steel in the temperature range 600-800°C in dry oxygen produces a protective scale with an oxidation rate controlled by diffusion in the scale. In the presence of water vapour, after an incubation period, the scales become non-protective, as a result of a change of the oxidation limiting process. The destruction of the protective scale by water vapour does not only depend on H₂O content but also on the H₂O/O₂ ratio.

2) Minor elements (e.g. Y, Si, Ni) added to MCrAlY (M=Co and/or Ni) coatings (used for protecting turbine blades), had a positive effect on their oxidation behaviour and microstructural stability.

3) 1-2 at.% Ag added to TiAl-based intermetallics allows transforming these alloys to alumina by stabilizing the ternary Z-phase (Ti₅Al₃O₂) in the sub-scale depletion layer.

4) Y₂O₃ dispersion positively influences the oxidation resistance of the ODS alloys having protective chromia and alumina scales.