P. Hancock

Particle-surface interactions during the erosion of a gas turbine material (MarM002) by pyrolytic carbon particles

Abstract A single-impact technique was used to study particle-surf ace interactions during the erosion of a typical turbine blade material, MarM002, by pyrolytic carbon particles. It was shown that carbon particles as small as 50 μn can cause severe erosive damage and that the predominant mode of material loss is by the removal of surface oxide. The erosive response of MarM002 was considered at 700, 750, 850 and 950 °C and is shown to be a function of the temperature and the oxide thickness, with lower temperatures and thicker oxide scales favouring brittle erosion behaviour. This behaviour can account for the reported increase in corrosion of superalloys at lower temperatures, provided that an erosive component is present in the system, and suggests that optimum conditions for erosion-corrosion resistance are established at a temperature in the region of 850 °C for the conditions under investigation.

The use of laboratory and rig tests to simulate gas turbine corrosion problems

Abstract Reasons why the actual gas turbine engine environment is difficult to characterize are discussed. Factors which influence the form, chemical composition and temperature, of contaminant species present in the engine, are considered and related to laboratory and rig experiments. The “Contaminant Flux Rate” is shown to be an important factor in determining why many laboratory and rig tests do not relate to engine performance. The relevance of the high velocity and ingestion of small particles in the gas stream on corrosion effects are considered and related to laboratory and rig experiments.

Particle-surface interactions during the erosion of aluminide-coated MarM002

Abstract Particle-surface interactions during the erosion of a nickel aluminide coating were assessed using a single-impact technique. It is shown that the erosive response is a function of the surface scale thickness and the temperature, with the temperature not only influencing the surface scale plasticity but also determining the contribution of the coating substrate to the impact process. In this respect the ductile-to-brittle transition temperature of the coating is of particular importance. Under a wide range of conditions typical of those found in gas turbines the erosion of aluminide coatings is shown to be controlled by the formation and removal of surface scales. This implies that the use of aluminide coatings will increase the erosion resistance of typical turbine blade materials because of the superior oxidation and corrosion resistance of this coating. This increase in erosion resistance will be particularly significant at higher operating temperatures, above 900 °C.

The mechanical properties of surface oxides on nickel-base superalloys—I. Oxidation

Abstract The oxidation behaviour of a range of nickel-base, high temperature alloys, including Brightray C, Brightray S, Nimonic 90, Nimonic 105, IN 713C, IN 587 and IN 597, has been studied over the temperature range 850–1050°C. The mechanical integrities of the oxides formed on the alloys at each temperature have been measured, and it has been shown that the alloy ranking order obtained from such oxide integrity measurements more accurately predicts known high temperature service performance, in industrial power plant environments, than thermogravimetric data.

THE EROSION OF GAS TURBINE BLADE MATERIALS BY SOLID SEA SALT

Abstract A single impact technique has been used to study the erosion of turbine blade materials IN738C, IN739 and IN792 mod 5A, by sea salt particles. It has been shown that sea salt particles can cause severe erosive damage to these materials and that the predominant mode of material loss is by removal of oxide scale. The erosive response of these materials differs significantly at 700 and 850°C, the type of response being a function of the oxide scale thickness and plasticity, with lower temperatures and thicker oxides favouring brittle erosion behaviour of the surface scale. This behaviour can account for the reported increase in corrosion of super alloy materials at 700°C, provided that an erosive component is present in the system.

The mechanical properties of surface scales on nickel-base superalloys—II. Contaminant corrosion

Abstract The effect of both sodium chloride and sodium sulphate contaminant corrosion on the same range of nickel based alloys as tested in oxidation is reported over the temperature range 650–950°C. It is shown that the onset of corrosion by both NaCl and Na 2 SO 4 is determined by the surface oxide integrity and that the ranking order for corrosion initiation by both these species is identical to that observed with respect to the mechanical stability of the surface oxide scales measured during isothermal oxidation. This correlation is shown to be significant in terms of the fundamental mechanisms of hot corrosion initiation.

The mechanism of high temperature erosion of coated superalloys

Abstract Diffusion and overlay coatings are now widely used for modern gas turbine parts such as nozzle guide vanes and turbine rotor blades. Problems are still encountered, however, when such components operate in environments where significant levels of particulates are present in the hot gas stream. The nature of these particulates depends on the service environment and may include sand-silicates, fly ash, sea salt and pyrolytic carbon. This paper highlights the significant parameters that control the mode of degradation and, in particular, the influence of particle loading, impact energy, temperature, scale morphology and composition on the deformation characteristics of both the surface oxide scale and coating. The erosion degradation mechanisms have been identified and a Monte Carlo model of the erosion-corrosion process developed, which is applicable to typical gas turbine service environments.

Vapour phase alloy design of corrosion-resistant overlay coatings

Abstract This paper examines the design of hot corrosion-resistant NiCrAl-based coatings by co-deposition from multiple sources. Initial studies examined the deposition of a wide range of ternary alloy compositions by magnetron sputtering or co-evaporation from multiple EB sources. In the magnetron sputtering work a segmented target, consisting of pure Ni, Cr and Al, was used. For the co-evaporation studies, binary and pure element source materials were used. In both deposition procedures new coating compositions were produced by mixing these source materials in the vapour phase. The new coatings were deposited onto pure Al2O3 substrates to ensure that no interaction between the coatings and substrate would occur during the corrosion tests. Low temperature hot corrosion of the novel coating compositions was assessed using an ‘ash recoat’ test procedure. Samples were coated in sodium chloride (at 0.75 mg cm−2) every 20 h and exposed in a flowing air-1 % SO2 equilibrated to an air-SO2-SO3 environment, at 750°C for times up to 100 h. Pack aluminised IN738 and uncoated IN738 were included in the test as reference materials and these exhibited classic type II hot corrosion morphologies, as did some of the new coatings tested. Other new coatings resulted in similar deposit chemistries but showed a much lower tendency to form corrosion pits. By modelling the corrosion behaviour along quasibinary sections drawn through the NiCrAl ternary diagram, it has been possible to construct an iso-corrosion contour map at 750°C. These studies have highlighted a new coating composition capable of resisting type II hot corrosion, which on the basis of short time tests offers a six-fold improvement in hot corrosion resistance over conventional NiCrAlY overlay coating compositions as measured using this ‘ash recoat’ test procedure.

Effects of SO2/SO2 on the efficiency with which MgO inhibits vanadic corrosion in residual fuel fired gas turbines

Abstract The use of MgO as a fuel additive to combat vanadic corrosion is widely accepted practice for boilers and turbines operating on residual or low grade fuels. MgO has the ability to form high melting stable vanadates but the presence of SO 2 /SO 2 can react with the magnesium and reduce its effectiveness. This paper aims to quantify the effectiveness of MgO as an inhibitor in the presence of increasing amounts of SO x . A method of determining permissible levels of SO x to satisfy predetermined corrosion rates or additive efficiencies is described. Examples of the loss of additive efficiency with volume of SO x at temperatures of 750 and 850°C are given for both nickel and cobalt base alloys.

The influence of imposed strain rate on fracture of surface oxides

Abstract The mechanical properties of chromium rich scales formed on 304 stainless steel have been investigated as a function of deformation rate and operating temperature. At 900°C at slow strain rates −6 s −1 no cracking was observed at strains up to 10%. At rapid strain rates in excess of 10 −4 s −1 oxide cracking was found to be independent of strain rate and controlled by the fracture toughness of the oxide. In the intermediate region, with strain rates between 10 −4 and 10 −6 s −1 in the temperature range 700–950°C, the behaviour is determined by the creep deformation and fracture mode of the oxide. The mechanism of surface oxide failure is examined and an equation to predict cracking density over the full range of both monotonic tensile and creep fracture modes is suggested.