Stuart R.J. Saunders

Oxidation growth stresses in an alumina-forming ferritic steel measured by creep deflection

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 (<1 μm), the planar stress within the oxide layer is everywhere compressive. Average values are approximately 850 MPa after 0.5 hr oxidation but reduce to <200 MPa after 6.5 hr. These values are very much less than would be expected under conditions of elastic deformation.

Hot salt corrosion test procedures and coating evaluation

Abstract There is considerable uncertainty regarding the relevance to practical applications of the various test procedures used to assess the resistance of superalloys to hot salt corrosion. A description is given of work to evaluate the applicability of the results obtained from current test methods in comparison with the performance of components in service. The test methods used were (i) burner rigs, (ii) crucible tests with and without electrochemical acceleration and (iii) various furnace tests such as the Dean test and those using a synthetic ash. The morphology of the corrosion product obtained was compared with that formed on blades and other parts during service exposure. The results indicate that burner rig testing gives the most realistic simulation of service behaviour and that some of the other tests can be modified to be more relevant. A highly significant aspect is the influence of contaminants on corrosion behaviour, which can vary depending on test method and must therefore be understood in detail when a particular test procedure is defined. It is shown that the deposition rate of sulphate from the vapour phase is an important factor controlling corrosion rates in burner rigs and Dean tests. In burner rigs the obvious correlation with contaminant concentration was not found, and a more important parameter is believed to be the contaminant mass flux. Tests in which the flux was close to that in operating gas turbines gave the best simulation of corrosion. The corrosion resistance of coatings can be assessed using similar procedures, but more care is required in the interpretation of results since failure of a coated component may result from a single defect. A statistical approach to the interpretation of behaviour is therefore discussed.

Current Understanding of Steam Oxidation - Power Plant and Laboratory Experience

This review discusses key papers presented at an EPRI sponsored Workshop on “Scale Growth and Exfoliation in Steam Plant” that was held at the National Physical Laboratory (NPL) in September 2003 [1]. Additionally, some more recent developments on modelling both scale growth and exfoliation are described. Scale exfoliation in the steam circuit of power plant boilers leads to tube blockages and, further downstream in the power plant, to erosion of the steam turbine blading; and this can have serious consequences for plant performance. Factors controlling this behaviour are reviewed. These include the thermochemistry of oxide formation as a function of operating conditions, scale microstructure and scale growth rates. It is well known that the oxidation rate of steels in steam is about an order of magnitude greater than that in air or oxygen, but the mechanism responsible for this increased rate is still unclear. Various hypotheses, which consider transport of volatile species through cracks and pores, diffusion of OH - or protons and direct access of steam to the metal oxide interface, are proposed to account for the increased rates of reaction in steam compared with air. Modelling exfoliation of thick oxide scales is considered in a number of ways. The basis of the original model by Armitt et al [2] has been extended and further developed. A popular approach is to assume that an oxide layer develops through-thickness cracks when a critical tensile stress (the oxide strength) or strain (the oxide strain to failure) is encountered. Another approach applies fracture mechanics principles to defects that are assumed to exist in the oxide layer, although there is great uncertainty regarding the relevant defect size distributions that control behaviour. A third lower bound (and conservative) approach is to consider the energetics of steady state through-thickness cracking that involves the fracture energy for through-thickness cracking and avoids the difficult issue of needing to know the defect size that initiates through-thickness cracking. Additionally, the need to incorporate kinetics of scale growth into the developing exfoliation models is briefly discussed.

Anin situ investigation of the tensile failure of oxide scales

A small four-point-bend jig has been used in a scanning electron microscope (SEM) to monitor the tensile fracture processes in iron and nickel oxide scales in situ. The scales were 3–40 μm thick, the strain rate was 4×10−5 sec−1 and acoustic emission (AE) was used to correlate signals with specific cracking events. The technique provided detailed information of the failure processes, and several micrographs were taken as the crack pattern developed during testing. Failure started with short random through-scale cracks. These cracks soon formed a regular pattern. Spallation only occurred at much higher strains and resulted in very energetic AE signals. These signals were used together with the SEM observations to determine the strain to cause spallation. Measurements of the crack spacing as a function of strain showed that plastic stress relaxation by interfacial slip and/or substrate yielding processes affected cracking. Thus, the oxide remained attached to the substrate longer than would be expected from purely elastic behavior. An analysis based on the observed crack spacing at the onset of spallation gave ratios of 0.8–1.9 and ∼0.9 between tensile and interfacial strength for iron and nickel oxides scales, respectively.

Fracture and spallation of oxides

Mechanical damage to thin, protective oxide layers arises principally in service from differential strains produced by temperature changes. For typical alumina- or chromia-forming alloys, in-plane tensile stresses are developed within the oxide layer during upward temperature transients and compressive stresses are produced during cooling. This paper reviews the fracture of oxide scales due to these imposed stresses, under both tensile and compressive loading conditions. Imposed strain rates can vary over many orders of magnitude and, since these are applied at high temperatures, the possibility exists that stress relaxation processes (generically termed ‘creep’) will reduce the development of high stresses within the oxide or at the oxide/metal interface and, thus, have a beneficial influence on the processes of scale failure. This aspect is considered in detail both by reviewing published data and by presenting new numerical results on the influence of metal creep strength on the growth characteristics ...

Experimental data on oxide fracture

Protective oxide scales on high temperature materials often fail in service leading to increased oxidation rates. Thus, a knowledge of oxide failure behaviour and failure strains is required. This paper reviews the measured failure strains of chromium oxide and aluminium oxide scales at room and oxide growth temperatures under applied strain. Data on iron and nickel oxide scales are also included for modelling purposes. Failure strains in tension show a power-law dependence on the measured void size within the layer, as expected from fracture mechanics models. Residual tensile or compressive growth and cooling strains prior to loading can increase or decrease the measured failure strains depending on the loading mode, i.e. tension or compression. At growth temperature, lateral oxide growth and creep may act to increase the failure strains, leading in some cases to measured failure strains in excess of 2%. However, the effective failure strains in tension, after taking account of the residual strains, oxid...

Standardisation of test methods for the mechanical properties of thin coatings

Abstract The need for standardisation is put in context, and the possibly conflicting views of the researcher and the test-house discussed. It will be shown that there is no conflict since both types of activity require reliable measurements which can only be obtained through the implementation of a proper quality system supported by internationally agreed standard test procedures. The work of relevant national and international standards committees is reviewed, and pre-normative and co-normative research needs are summarized. Prenormative research is, in general, well supported in Europe through the Standards Measurement and Testing Programme, while on a wider international scale the VAMAS organisation offers potential for this activity in the field of test methods for thin coatings. This review defines the needs for future work in this area, and it is surprising that many well established methods of test do not have appropriate standards. Examples of recent and current standards work are given, together with their validation by carrying out international intercomparisons.

Measurement methods for the determination of fracture strain

Consideration is given to methods currently used for the determination of tensile and compressive fracture strains of thermally formed protective oxide layers at ambient and elevated temperatures. The methods discussed are four-point bend testing in both tension and compression and direct tensile and compression tests. Detection of failure of the oxide is usually by acoustic emission, but in some cases, where the acoustic signal is too small, direct observation of failure can be made by in situ methods using scanning electron microscopy or acoustic microscopy. Details of the tests are described together with the limitations of the different techniques. An attempt is made to estimate errors and consider the effects of important test variables such as strain rate, temperature and dwell periods.

Use of the mechanical properties microprobe for characterization of oxide scales

A mechanical properties microprobe (MPM) based on low-load, depth-sensing submicrometre indentation testing is an extremely powerful instrument for examining surface mechanical behaviour and offers several advantages for determining the elastic and plastic properties of protective oxide scales. Its high spatial and force resolutions allow measurements on thin scales and, for thicker surface oxides, determination of gradients in properties. Under optimal conditions, hardness and modulus values can be determined with an overall accuracy of about 10%. A key factor in analysis of the load-displacement data is an accurate determination of the indenter contact area. Because of this, companion indentations into bulk oxides using the same indenter reduce uncertainties associated with comparing the elastic and plastic properties of such materials with their corresponding scales. Use of a standard MPM is currently limited to near room temperature. More effective depth-sensing indentation testing of oxide scales at ...

Application of thermography in the evaluation of early signs of failure of thermal barrier coating systems

Abstract For a number of years piezospectroscopy (Cr fluorescence) has been used to monitor the stress levels in the thermally grown oxide (TGO) that forms between the bondcoat and the thermal barrier coating (TBC) in TBC systems. The purpose of that work has been to observe early signs of failure and thus allow operators to schedule service intervals before failure of the TBC system occurred. This paper reports the use of thermography as an additional tool that can be used to assess the “health” of TBC systems. The technique consists of imaging the surface of the TBC coated specimen with a high spatial resolution infra-red camera while the specimen is heated, and monitoring the temperature of the outer surface of the TBC. Conductive heating through the substrate and radiative heating incident on the TBC have been studied. Early results are encouraging, revealing a clear correlation between thermograms obtained using the conductive and radiative forms of heating, some of the stress maps obtained using piezospectroscopy and direct metallographic evidence. Examples of electron beam physical vapour deposited (EB PVD) and air plasma sprayed (APS) TBC systems have been studied as they were progressively aged. Cracking and disbonding associated with the TGO and/or TBC have been observed in places where thermography showed differential heating.