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Neural networks can be a useful tool to analyse the oxidation and corrosion behaviour of materials at high temperature. Examples are given of the use of neural network models to analyse datasets of material behaviour after exposure to combustion, gasification and steam atmospheres. The use of networks to identify changes in mechanism, additional significant experimental parameters and the onset of spallation is demonstrated.The limitations of neural network modelling are briefly discussed. Although they can be trained to fit any existing dataset, care must be taken in using the networks to predict a time sequence of events.

The excellent steam oxidation resistance of iron aluminide coatings on ferritic steels at 650 °C has been demonstrated both by laboratory tests and field exposure. These coatings are formed by the application of an Al slurry followed by diffusion heat treatment at 700 °C for 10 h. The resulting microstructure is mostly composed of Fe2Al5 on top of a much thinner FeAl layer. This coating exhibits perpendicular cracks due to thermal expansion mismatch between coating and substrate. However, these stress relieving cracks do not seem to have an effect on the mechanical properties of the substrate. Cyclic oxidation, creep resistance and TMF testing of these coatings at 650 °C indicate excellent performance.

International audience; A novel mechanistic approach is proposed for the prediction of the life of thermal barrier coating (TBC) systems. The life prediction methodology is based on a criterion linked directly to the dominant failure mechanism. It relies on a statistical treatment of the TBC's morphological characteristics, non-destructive stress measurements and on a continuum mechanics framework to quantify the stresses that promote the nucleation and growth of microcracks within the TBC. The last of these accounts for the effects of TBC constituents' elasto-visco-plastic properties, the stiffening of the ceramic due to sintering and the oxidation at the interface between the thermally insulating yttria stabilized zirconia (YSZ) layer and the metallic bond coat. The mechanistic approach is used to investigate the effects on TBC life of the properties and morphology of the top YSZ coating, metallic low-pressure plasma sprayed bond coat and the thermally grown oxide. Its calibration is based on TBC damage inferred from non-destructive fluorescence measurements using piezo-spectroscopy and on the numerically predicted local TBC stresses responsible for the initiation of such damage. The potential applicability of the methodology to other types of TBC coatings and thermal loading conditions is also discussed.

Low velocity burner rigs have been used to perform cyclic oxidation tests. Due to the nature of the equipment there are limitations on the cycle frequency that can be used and there are also several other parameters that must be defined. The importance of these parameters on metal wastage in the martensitic steel, P91 has been investigated. Salt deposition rates are independent of fuel and salt composition under the conditions investigated. However the observed metal wastage in the martensitic steel P91 is influenced both by the fuel and salt compositions. Intermittent washing of the specimens has little influence whilst the corrosion product remains intact but causes additional loss of corrosion product after spallation has been initiated.

A survey of existing testing practice and pre-existing data was conducted to provide a starting point for the COTEST project on cyclic oxidation testing. The main parameters within the test that need to be controlled were identified as: control of test environment; temperature stability during hold periods; heating and cooling rates; specimen preparation; and post test evaluation. Existing experimental facilities were surveyed to establish the full range of variability within these parameters.

This study has used thermogravimetric, acoustic emission and finite-element modelling techniques to examine the tensile cracking of a chromia layer on a 20Cr25Ni steel during thermal cycling from an oxidation temperature of 900 °C. It is found that the process is highly sensitive to the temperature amplitude of the thermal cycle and that hold periods at the bottom temperature result in enhanced oxide cracking whereas hold periods at the peak temperature can inhibit the onset of cracking. For standard saw-tooth cycles, the finite-element analysis shows that these effects can be explained by a creep hysteresis effect. This arises from stress relaxation during the cooling half-cycle, which results in the development of in-plane tensile stresses within the oxide layer on the return to temperature. Low-temperature hold periods in the creep regime increase the in-plane tensile stress in the oxide on the return to temperature. High-temperature hold periods tend to reduce the peak tensile stress produced in subsequent cycles but, for the example examined here, the effect is small.

Important efforts to develop new steels or to protect high creep strength steels in order to allow operation of steam turbines at 650 °C are being carried out world-wide to increase efficiency. Within the European Project “SUPERCOAT” (Coatings for Supercritical Steam Cycles), work has been concentrated in the development of coatings to withstand 50,000–100,000 h of operation at 650 °C under high pressure steam. Aluminide coatings on ferritic–martensitic steels produced by applying an Al slurry followed by a diffusion heat treatment, have shown to be protective at 650 °C under steam for at least 32,000 h of laboratory steam exposure under atmospheric pressure. Although the “as diffused” coatings present through thickness cracks, these do not propagate during exposure to steam or thermal cycling and no new cracks seem to develop. Moreover, no changes in residual stresses could be observed after thermal cycling. Microstructural characterization of samples at different periods of exposures has been carried out by SEM-EDS and XRD. The principal mechanism of coating degradation is loss of Al at the surface due to inwards diffusion. Microhardness as well as Young's modulus and fracture strength were measured using well established techniques. The coatings show reasonable ductility (∼1.6%) when stressed in tension between room temperature and 400 °C which further increases at higher temperatures providing evidence that the coatings should withstand the mechanical conditions likely to be encountered in service.

The fracture and adhesion properties of five different commercial high velocity oxygen fuel (HVOF) coatings have been evaluated as a function of temperature. Tests were carried out on coatings in the as-received, aged and densified conditions. Under all conditions the adhesion of the coatings was excellent. The strain to cracking was generally low for coatings in the ‘as-coated’ condition, however this was improved by densification or thermal ageing. It was observed that in some cases the crack path changed with treatment.

Laboratory exposures cannot reproduce all the features present in service conditions. The experimentalist is faced with the conflict between increasing the complexity of laboratory tests to replicate service more closely and keeping testing costs low by maintaining a simple procedure. The influence of various experimental parameters, which can be controlled in the laboratory, on the steam oxidation response of materials is reviewed and recommendations for best practice are proposed.

Within the European COST522 programme, there are extensive project groupings investigating the degradation of materials, both alloys and coatings, in a wide variety of power plants. Two project groupings are addressing materials degradation in (a) gas turbines and (b) the hot gas paths/heat exchangers of boilers and gasifiers. In both of these broad topics there is a need to generate and compile quantitative information on materials degradation that is appropriate to components operating in systems using new/dirtier fuels, as well as to components operating at higher metal temperatures.The data generated from research into both of the topics are being compiled into databases. For the boiler/gasifier heat exchanger fireside, the database contains corrosion damage information generated from: (a) a range of plant and pilot plant operations (using biomass, waste and coal fuels), (b) laboratory tests (targeted at particular environments to investigate different specific degradation effects in more deta...

9 Cr martensitic steels are widely used in high temperature steam environments for their combination of creep strength and oxidation resistance. These materials are pushed to their limit in the quest for higher efficiencies in boilers and steam turbines and loss of the protective oxide scale may result in premature failure of critical components.The major cause of oxide scale spallation is a temperature drop. The mechanism behind the spallation process is the generation of internal stresses in the oxide scale due to the mismatch of thermal expansion coefficients between the substrate and the haematite, magnetite and spinel layers in the oxide scale. These scales can be partially relaxed due to creep processes if the cooling rate is sufficiently slow.Specimens of P92 have been oxidised in flowing steam for periods up to 2000 h and cooled at a linear rate of 100°C h–1. Acoustic emission (AE) of the samples was monitored during the oxidation and cooling periods. Specimens were sectioned and prepared ...

Three methods have been used to measure the adhesion of oxide scales and coatings on materials used in turbines. All three methods were suitable for oxide scales grown on 9- and 12-Cr steels, whereas for coated materials the methods were not universally applicable. The data generated by each method were not directly applicable to current predictive models for oxide scale failure. However, a route has been proposed that would allow future workers to correlate data obtained by testing with the input required by the modelling approaches. This could be achieved by an extensive assessment of finite element modelling coupled with critical experiments.

Deflection tests have been used to estimate the stresses developed in the alumina layer formed during short-term oxidation of a Fe-22Cr-5Al-0.3Y Fecralloy steel at 1000°C. Elastic analysis of the deflecting specimen is inappropriate under these test conditions because of the low creep strength of the alloy. Accordingly, a recent creep analysis has been used in this work using currently determined creep properties of the alloy substrate. The results of the analysis show that for the thin oxides produced (