Materials Science Forum Vols. 595-598

Abstract: Thirteen boiler steels were exposed in pure water vapour at 650°C for 32 weeks (5,376h). To assess their oxidation kinetics, four current discontinuous measurement methods were used. The results show that, for the T91 steel, the different investigated methods give similar results; for the other steels, the evaluation of corrosion rates depends on the method used. Thus their lifetime prediction is often over- or undervalued. This is mainly the case for steels with scales that spall off and/or steels with a non-uniform scale growth (localised oxide nodules for a rather long exposure time). This is particularly true in the steam environments which lead to oxidation processes with oxide scales different from those in dry air or oxygen. So the use of steam oxidation data and/or relations among them has to be done with caution because it can lead to a wrong prediction of performance.

Abstract: The scale growth on two austenitic alloys, Alloy 310 and Sanicro 28, under KCl deposits was examined. This is relevant to the long term corrosion of superheater tubes in biofuel combustion. Coupons were encapsulated in tablets so that 1 mm of KCl with a relative density of 91% covered the metal. Samples were tested at 500°C in 5%O2-10%H2O-N2 for 24, 168 and 672 h. After exposure the salt was broken off and the scale was characterised by using SEM-EDX and AES. After 24 h a 50 nm thick oxide surrounded 500 nm thick chromates on the surface. No oxide layer was detected under the chromates and no Cl was found under either layer. The chromate growth requires lateral transportation of Cr along the surface. This reduces the protectiveness of the oxide and accelerates the formation of less protective Fe rich oxides. The formation of chromates also releases HCl inside the KCl tablet. The chromates did not grow significantly between 24 and 168 h, but the oxide grew equally thick beneath and between them and Cl was enriched around the metal oxide interface. After 672 h the oxides were about 5 μm thick and only few chromates were seen. Crystals of KCl formed in areas with thick porous and Fe rich oxides on both alloys.

Abstract: Volatile species released during the initial stages of oxidation of a P92 ferritic steel, with and without an aluminized coating, at 650°C in Ar+20%H2O for 150h were studied. TG-MS experiments were conducted in a closed steam loop in order to obtain information about the oxyhydroxides formation as reaction between coatings and steam. From those results, the role of the different coating element could be established and optimized for the coating durability. An oxidation mechanism based on the TG-MS results is given. The morphology/composition and structure of the oxidized samples were also studied using SEM/EDS and XRD techniques.

Abstract: Ferritic steels are usually used in boiler or supercritical steam turbines which operate at temperatures between 600-650°C under pressure. Protective coatings are often applied in order to increase their oxidation resistance and protect them against degradation. In this study new Al-Mn protective coatings were deposited by CVD-FBR on P92 ferritic steel. The initial process parameters were optimized by thermodynamic calculations using Thermo-Calc software. Then, those parameters were used in the experimental procedure to obtain Al-Mn coatings at low temperature and atmospheric pressure. Co-deposition was achieved at moderate temperatures in order to maintain the substrates` mechanical properties. The coatings` microstructure and phase constitution was characterized. Fe-Al intermetallic coatings containing Cr and Mn were obtained. The phase constitution is discussed with reference to the Fe-Al-Mn ternary phase diagram. The effect of diffusion heat treatment on the phase transformations as well as the steam oxidation resistance of these coatings at 650°C and 800°C was investigated.

Abstract: The steels with chromium contents between 9 and 12%wt are used for power plants with advanced steam conditions. These steels possess good creep properties similar to the 9% Cr steels as well as good creep and good oxidation resistance at temperatures between 500-600°C. In the last years efforts have been made to develop coatings for protection against oxidation in order to allow operation of steam turbines at 650°C. In this study, Al-Hf protective coatings were deposited by CVD-FBR on the ferritic steel HCM-12A followed by a diffusion heat treatment, and were shown to be protective at 650°C under steam for at least 3000 hours of laboratory steam exposure under atmospheric pressure. The morphology and composition of the coatings were characterized by techniques, including scanning electron microscopy (SEM), electron probe microanalysis, and Xray diffraction (XRD). The results showed a substantial increase of steam oxidation protection afforded by Al-Hf coating deposited by the CVD-FBR process.

Abstract: The influence of KCl, K2CO3 and K2SO4 on the initial stages of corrosion of 304-type (Fe18Cr10Ni) stainless steel was investigated at 600°C in 5% O2 + 40% H2O. Small amounts of salt (1.35 .mol K+/cm2) were added before exposure. The exposures were carried out in a thermobalance. Exposure time was 24 hours. Reference exposures were carried out in 5% O2 and in 5% O2 + 40% H2O. The oxidized samples were analyzed by SEM/EDX, XRD and IC. KCl and K2CO3 are very corrosive towards 304L, producing thick non-protective scales. Corrosion is initiated by the reaction of the potassium salts with the protective, chromium-rich oxide forming K2CrO4. This depletes the oxide in chromia and converts it into iron-rich non-protective oxide. In contrast, K2SO4 does not accelerate corrosion significantly.

Abstract: There is growing concern over the effects of global warning. In response the power generation sector is having to consider a wider range of systems and fuels for use in generating heat and power. One of the classes of solid fuels that is being increasingly developed is biomass, which is regarded a both sustainable and carbon neutral. In fact, the term biomass covers a wide range of fuels from waste products, such as straw, forestry wastes and sawdust, through to purpose grown energy crops, such as coppiced willow and miscanthus. To maximise combustion plant efficiency it is necessary to use high temperature/pressure steam turbines. However, to generate such steam conditions, the high heat exchanger surface temperatures can interaction with the various potential products of biomass combustion to cause excessive deposition and corrosion of these surfaces. This paper considers the range of heat exchanger operating environments that can be produced by the combustion of different potential biomass fuels, especially the effects of the higher K and Cl contents of the faster growing biomass fuels. This paper reports the results of a series of laboratory corrosion tests that have been carried out to assess the effects of various types of biomass on the corrosion of high temperature heat exchanger materials in combustion plants. The corrosion tests have been carried out using the deposit recoat method in controlled atmosphere furnaces. Six 1000 hour tests have been carried out at typical superheater / reheater and evaporator conditions (450-600°C) using simulated deposit and gas compositions, which have been selected on the basis of potential biomass fuel compositions. The five metals exposed in this study are widely used in power plant heat exchangers: 1% Cr steel, 2.25% Cr steel (T23), 9% Cr steel (T91), X20CrMoV121, TP347HFG and alloy 625. During the course of the tests, the material degradation was monitored using traditional mass change measurements. In order to produce statistically valid data on the actual metal loss from the materials, the performance of the materials in these tests was determined from dimensional metrology before and after exposure: pre-exposure measurements were made using a micrometer; post-exposure measurements were made using an image analyser system. SEM/EDX and XRD analyses have been used to confirm corrosion mechanisms and their association with corrosion damage levels. For each material, the dimensional metrology data have been used to determine the sensitivity of the corrosion damage to changes in the exposure conditions (e.g. deposit composition, gas composition) to generate models of the corrosion performance of the materials. The corrosion data and model outputs have been compared with data available from power plants operating on coal, straw or wood fuels.

Abstract: In this study, the foundation is being developed for the numerical simulation of the processes that determine the oxide scale exfoliation behavior of the steam-side surfaces of superheater and reheater tubes in a steam boiler. Initially, the assumptions concerning the base state for calculating oxide strains also were critically examined. The state of stress-strain of an oxide growing on the inside surface of an externally-heated tube was considered for the conditions experienced in a boiler during transition from full- to partial-load operation. Since the rate at which the oxide grows is an important consideration, it was necessary to determine the appropriate temperature to use in the oxidation rate calculations. The existence of a temperature gradient through the tube, and the cyclic nature of the boiler operation (temperature and pressure) were considered; the growth temperature of the oxide was taken to be the oxide surface temperature. It was determined that the commonly-used approach for accounting for geometrical effects when calculating stress-strain development in a growing oxide scale of using the analogy of an infinitelylong flat plate gave sufficiently different results than when using a cylindrical geometry, that the latter was adopted as the preferred calculation procedure. Preliminary calculation of strains developed in multilayered oxides formed on alloy T22 as a function of boiler operating conditions indicated the magnitude of the strains in each layer; the large strain gradients between the layers inferred the importance of the detailed scale morphology in determining the mode of exfoliation.

Abstract: The combustion chamber of slagging coal gasifiers is lined with refractories to protect the steel shell of the gasifier from elevated temperatures and corrosive attack of the coal slag. Refractories composed primarily of Cr2O3 have been found most resistant to slag corrosion, but they continue to fail performance requirements. Post-mortem analysis of high-chromia refractory bricks collected from commercial gasifiers suggests that slag penetration and subsequent spalling of refractory are the cause of the short service life of gasifier refractories [1]. Laboratory tests were conducted to determine the penetration depth of three slags representative of a wide variety of coals in the United States into chromia-alumina and two high-chromia refractories. Variables tested were refractory-slag combinations and two partial pressures of O2. Slag penetration depths were measured from spliced images of each refractory. Samples heated to 1470°C for 2 hrs had maximum penetration depths ranging from 1.99±0.15 mm to at least 21.6 mm. Aurex 95P, a highchromia refractory containing 3.3% phosphorous pentoxide (P2O5), showed the least slag penetration of all refractories tested. P2O5 likely reacts with CaO and MgO in the slag, forming an immiscible Ca-Mg phosphate phase. The extraction of basic components from slag causes an increase in slag viscosity restricting the molten slag penetration into the refractory.

Abstract: Grain boundary engineering (GBE) was employed to improve the oxide exfoliation resistance and mitigate oxide growth by optimizing the grain boundary character distribution. Studies were performed on alloys of Incoloy 800H and Inconel 617. Alloys 800H and 617 were selected due to their potential applications for the Generation IV nuclear power systems. The effect of GBE on the corrosion response was evaluated using supercritical water exposure tests and cyclic oxidation tests. The microstructure of the tested samples was analyzed by means of optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, electron backscatter diffraction, and gravimetry. The effects of thermal expansion mismatch and Cr volatilization on the corrosion response are discussed.