Materials Science Forum Vols. 595-598

Abstract: The depth distribution of the residual stress in graded (Ti, Al) N coating deposited on steel by arc ion plating was measured by the Stripping Layer Substrate Curvature Technique, and the effect of graded (Ti, Al) N and mono-layered (Ti, Al)N coating on the fatigue properties of 1Cr11Ni2W2MoV stainless steel were investigated. The depth distribution of the residual stress in mono-layered Ti70Al30N and TiN were also measured for comparison. The results show that the residual stresses in the coatings are compressive, which increase gradually from the coating/substrate interface and reach a maximum value at the middle region, then decrease until the surface. Compared with TiN and Ti70Al30N, the stress maximum value in the graded coating is nearer to the coating surface. It is also shown that the fatigue strength of the graded (Ti, Al)N and Ti70Al30N coated samples are superior to that of the uncoated substrate. The improvement of the fatigue properties for the coated samples is thought to be attributed to the hard coatings with high compressive stress.

Abstract: The specimens of O-phase Ti-22Al-25Nb (at%) intermetallics coated with silica-based enamel received mass gains of about 1 mg/cm2, after 300 h of oxidation or hot corrosion at 800 °C. These rates were much faster than the growth rates of silica films at the same temperature. To understand this phenomena, the specimens were analyzed using SEM, XRD, EPMA and TEM. An oxide layer with thickness of several μm was observed at the enamel/substrate interface of the coated specimens after either oxidation or hot corrosion. XRD and TEM analysis revealed the newly formed oxide layer was composed of α-Al2O3, Al2SiO5, Al2TiO5, rutile-TiO2 and NbO2. It was shown by EPMA profiling that an Al-depleted zone was located just beneath the oxides. It was proposed that the solid reactions between the enamel coating and the O-phase Ti-Al-Nb played important roles for the oxidation and hot corrosion behavior of the coated specimens.

Abstract: Many high-temperature coatings rely on the formation of a continuous and adherent thermally grown oxide (TGO) scale of α-Al2O3 for extended resistance to degradation. For instance, the durability and reliability of thermal barrier coating (TBC) systems in gas turbines are critically linked to the oxidation behavior and stability of an alumina-forming β-NiAl-based bond coat. This study focuses primarily on the development of unique Pt+Hf-modified γ′-Ni3Al+γ-Ni coating compositions that form highly adherent, slow-growing TGO scales during both isothermal and cyclic oxidation at high temperature. Recent findings on the isothermal and cyclic oxidation behavior of γ′+γ alloys and coatings will be discussed, with particular emphasis on the effects of Pt, Al and Hf contents and distributions. Inferred reasons for the observed “Pt effect” will also be presented.

Abstract: Slurry iron aluminide coatings are very resistant to steam oxidation at 600-650º C. These coatings can be used to protect new generation Ultra Super Critical (USC) steam power plant ferritic/martensitic steel components. The microstructure of the initially deposited coating changes as a function of time, mainly due to coating-substrate interdiffusion, going from mostly Fe2Al5 to FeAl, causing the precipitation of AlN in those substrates containing a minimum content of N and moreover, developing Kirkendall porosity at the coating-substrate interface. Steam oxidation at 650º C causes the formation of a protective thin layer of hexagonal χ-Al2O3 phase along with some α- and γ-Al2O3 after the first few hours of exposure. However, despite the relatively low temperature, and after several thousands hours the protective layer was mostly composed of α-Al2O3. A study of the evolution of the microstructure of slurry aluminide coatings deposited on P92 and exposed to steam at 650º C has been carried out by scanning and transmission electron microscopy and X ray diffraction.

Abstract: The aim to reduce the CO2-emissions has triggered the evaluation of new cycle concepts for power plants. For the coal-fired power plants, the oxy-fuel firing is a promising option for CO2- emission reduction. Here, the combustion takes place in a nitrogen-free atmosphere. The oxygen is separated from the air and burned in near-stoichiometric conditions with the fuel. The gas composition is significantly changed, when the combustion is changed from air-fired to oxy-fuel fired condition. For lignite, the carbon dioxide content is raised from 15 to 59vol% and the watercontent from 10 to ~32%. For the same fuel, the SO2-content in the flue gas increases by a factor of 3-4 to ~0.5%. These changed environmental boundary conditions will affect corrosion life of the materials especially on the water walls and the heat exchanger surfaces. Considering the significant changes in the combustion gas, the composition and the occurrence of the corrosive deposits has been evaluated with the thermodynamic modelling program ‘FactSage’. The chemical compositions of the deposits have been modelled for dried lignite from Germany. The results exhibit that the oxy-fuel firing will give a significant change in the atmosphere as well as in the deposit composition. Consequently, the corrosion rates of current used materials in air fired boilers need to be evaluated for the application in oxy-fuel fired boilers.

Abstract: The efficiency of Waste-to-Energy (W-t-E) boilers is affected by fireside corrosion of the heat exchangers that involve unexpected shutdown of facilities for repairs and limit the increase of steam conditions used to produce electricity. The parameters governing fireside corrosion are various and mechanisms are very complex, nevertheless, they are relatively well documented in the literature. In this paper, a laboratory-scale corrosion pilot, which reproduces MSWI boilers conditions, is described. The specificity of our approach includes simultaneous simulation of the temperature gradient at flue-gas/tube interface, the velocity of flue-gas and ashes. Corrosion rates obtained on Tu37C carbon steel at a metal temperature equal to 400°C and a flue gas temperatures of 650°C and 850°C (1100 ppm HCl, 110 ppm SO2 and synthetic ashes free of heavy metals) are respectively around 1.6 2m/hour and 5.6 2m/hour. Preferential metal loss, attributed to erosion-corrosion phenomena, is also observed at low flue-gas temperature (T=650°C) on the face exposed at 90° to the flue-gas. The analysis of corrosion scales demonstrates the reproducibility of results and the reliability of corrosion mechanisms determined from experiments, with degradation observed similar to superheater tubes from EfW facilities. Thus, the corrosion pilot developed can be used as an accurate simulator of the environment encountered in MSWI.

Abstract: Fireside corrosion is since a long time the main limitation to increase efficiency of energy recovery boilers of waste to energy (W-t-E) facilities. Nevertheless, the increase of steam conditions in addition with the variation of feeding fuels composition imply greater risks of fouling and corrosion along with heat exchanger failure, loss of plant availability and high maintenance costs. Fireside corrosion mechanisms had already been widely treated in the literature and this paper will review the main critical factors that enhance fireside corrosion of superheater. Recent failure cases will be developed in regards with recent studies that provide interesting routes to predict corrosion failure and develop maintenance strategy.

Abstract: Corrosion field tests have been carried out in the superheater region of a commercial waste-fired 75MW CFBC boiler using air cooled probes. Exposure time was 24 and 1000 hours. The effect of adding sulphur to the fuel on the corrosion of two high alloyed steels and a low alloyed steel was studied. The fuel consisted of 50% household waste and 50% industrial waste. The exposed samples were analyzed by ESEM/EDX and XRD. Metal loss was determined after 1000 hours. Both materials suffered significant corrosion in the absence of sulphur addition and the addition of sulphur to the fuel reduced corrosion significantly. The rapid corrosion of the high alloyed steel in the absence of sulphur addition is caused by the destruction of the chromiumcontaining protective oxide by formation of calcium chromate. Adding sulphur to the fuel inhibited chromate formation and increased the sulphate/chloride ratio in the deposit. Iron(II) chloride formed on the low alloyed steel regardless of whether sulphur was added or not.

Abstract: Power generation from coal using ultra supercritical steam results in improved fuel efficiency and decreased greenhouse gas emissions. Results of ongoing research into the oxidation of candidate nickel-base alloys for ultra supercritical steam turbines are presented. Exposure conditions range from moist air at atmospheric pressure (650°C to 800°C) to steam at 34.5 MPa (650°C to 760°C). Parabolic scale growth coupled with internal oxidation and reactive evaporation of chromia are the primary corrosion mechanisms.

Abstract: Chlorine gas is widely encountered in chemical industries, as well as in waste incinerators and plastic/polymer decomposition mills. The presence of chlorine may significantly reduce the life-time of the components. Under chlorine-based atmospheres, the process of scale formation may be considerably affected and the presence of chlorine usually impedes the formation of a long term protective dense oxide scale. Based on thermodynamic calculations and previous investigations, NiAl and NiAlMo APS-coatings were produced to be used as protection for conventional steels against chlorine corrosion. Indeed, thermodynamic diagrams showed that molybdenum should have a positive behaviour in “reducing”-chloridizing atmospheres, whereas aluminium has a positive behaviour in “oxidizing”-chloridizing atmospheres. Coatings of approximatively 300 μm thickness were thermally sprayed on Armco Iron and on a commercial ferritic 18 Cr steel. This work presents the corrosion behaviour of NiAl and NiAlMo APS-coatings under chlorine-based atmospheres at 800°C. In addition, metallographic characterisation as well as EPMA investigations of the coating cross sections were carried out before and after the corrosion tests.