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The solid-state diffusion in binary and ternary systems is overall treated and modeled using the bi-velocity method. The mathematical formulation is given, and the boundary conditions discussed. The model allows the calculation of concentration profiles, spatial distribution of the entropy production rate, and diffusion path. The last two can be used to identify the phases present in the diffusion zone of the ternary system, and determine the thicknesses of the respective layers. The model is applied for the first time to simulate diffusion during aluminization of nickel and its super-alloys, MAR-M200+Hf and CMSX-4, and the predictions are compared with the results of the experiments performed by the authors. The results give a new quantitative explanation of the high-active and low-active regimes of aluminization. It is, in particular, confirmed that the high-active and low-active regimes of nickel aluminization can be carried out by a variation of the HCl/H2 flow ratio. The most important, and a brand new, result concerns the entropy production rate. It is shown that its maxima coincidence with the two-phase zone boundaries in real space, which gives new insight into the modeling of the diffusion in ternary systems. [ABSTRACT FROM AUTHOR]

Combined cycle power systems offer increased efficiency of electricity generation and lower environmental emissions of CO2, SO x and NO2, as well as being adaptable to most fossil/biofuels. Industrial gas turbines are at the heart of such power stations and are being developed to perform at higher firing temperatures and pressures to achieve even greater efficiencies, with lower emissions. Fuel gases derived from renewable fuels, such as biogases from digestors or syngases from solid fuel gasification, may contain contaminants that are extremely corrosive to the gas turbine components (e.g. blades and vanes) located in the hot combusted gas path. Such damage can result in a gradual loss of turbine efficiency and reliability. Therefore, it is of paramount importance that the materials used for gas turbine components that operate in these environments provide acceptable and predictable in-service life times. Single crystal superalloys (e.g. CMSX-4) were developed to have improved mechanical properties (creep and fatigue) at increasing component operating temperatures, especially in relatively clean aero-engine operating environments. This paper describes work carried out to investigate the development of hot corrosion processes on CMSX-4 (uncoated and Pt-Al coated) in a range of potential environments for blade materials in industrial gas turbines fired on biomass derived fuel gases. A series of laboratory tests has been carried out using the 'deposit recoat' technique, with exposure conditions covering: deposits of 80/20 and 50/50 (Na/K)2SO4, with additions of lead, a gas composition of 100 vpm SO x, 100 vpm HCl in simulated combustion gases, deposition flux of 15 µg/cm2/h, temperature of 700 °C, for periods up to 1000 h. During their exposure the materials were monitored using traditional mass change methods. However, quantitative damage data in terms of metal loss was obtained using dimensional metrology, pre-exposure contact measurements combined with post-exposure measurements of damage observed by optical microscopy on polished cross-sections. These measurement methods allowed the distribution of damage to be determined and the material sensitivity to such hot corrosion processes to be quantified. [ABSTRACT FROM AUTHOR]

CMSX-4 and IN738LC, industrial gas turbine (IGT) aerofoil alloys with low (6.5 wt%) and high (16 wt%) chromium contents respectively, were exposed to type II hot corrosion conditions (700 °C, SO X in air, alkali sulphate/chloride deposits) for up to 1000 h. By comparing detailed pre- and post-exposure dimensional metrology from the samples, the fraction of each sample's surface undergoing the incubation stage (lower levels of sample damage due to the presence of a protective scale) or the propagation stage (higher levels of sample damage due to the direct attack of the base alloy) of type II hot corrosion damage was determined. Corrosion pit development under type II conditions may be described as an extreme event. As such, the transition from incubation to propagation may be modelled using Weibull statistics, which were found to give a good fit to the spread of incubation lifetimes in the exposed IN738LC samples. Much shorter incubation lifetimes were found for the lower chromium content alloy (CMSX-4). Propagation rates for CMSX-4 were found not to be constant over time. Instead, when exposed to the higher deposition fluxes, the propagation rates fell after long exposure times as the resultant deposit/corrosion product inhibited the transportation of reactive species. This improved, quantitative understanding of the transition from incubation to propagation damage under type II hot corrosion conditions will assist in the development of quantitative hot corrosion damage models, enhancing IGT component lifetime prediction. [ABSTRACT FROM AUTHOR]