Browsing by Author "Chapman, Neil"

Now showing 1 - 6 of 6
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    An approach to evaluating the impact of contaminants on flux deposition in gas turbines
    (Springer, 2023-08-18) Mori, Stefano; Mendil, Mathilde; Wells, Jonathan; Chapman, Neil; Simms, Nigel; Wells, Roger; Sumner, Joy
    Gas turbines are a key part of many countries’ power generation portfolios, but components such as blades can suffer from hot corrosion attack, which can decrease component lifetimes. Corrosion is driven by impurity levels in the fuel and air (e.g., species containing sulphur and/or alkali metals) and depends on environmental conditions (e.g., air pollution, seawater droplets), that can lead to formation of harmful species in the gas. Understanding and determining the deposition flux of such contaminants is crucial for understanding the problem. Thermodynamic simulations were used to determine types and amounts of potentially corrosive contaminants, this was followed by deposition fluxes calculations. An operating scenario, based upon an offshore platform was evaluated. The effectiveness of different filtration systems has been evaluated. The impurity levels of alkali metals, such as sodium, greatly impacts the calculated deposition flux of species linked to corrosion attack. The presence of Na2SO4, and K2SO4 was found, at temperature representative of stage 2 nozzle guide vanes. Lowering sulphur input (from fuel or air) can be an efficient way to decrease deposition, attention must also be paid to lowering the amount of alkali metal entering the gas turbine, which can be lowered by the filtration systems’ correct use.
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    Corrosion fatigue testing: the combined effect of stress and high temperature corrosion
    (Taylor & Francis, 2017-10-20) Chapman, Neil; Brooking, Laurie; Sumner, Joy; Gray, Simon; Nicholls, John R.
    A corrosive environment can have a detrimental effect on the fatigue life of a material due to a change in failure mechanism. Attempts have been made to replicate this change on nickel-base superalloy CMSX-4 cast in the <001> orientation. Fatigue testing in air, of this material typically produces a fracture on an angle of approximately 55° which is consistent with the fracture having propagated on a {111} slip plane. The aim of the research was to fatigue test in a corrosive environment with the purpose of producing a crack/fracture which deviated from the typical angle and thus confirm that the corrosive environment had affected the fatigue mechanism. It was concluded that the change in mechanism to high temperature corrosion fatigue was associated with a reduced load application rate together with precorroding the test specimens to trigger the initiation of the corrosion fatigue mechanism.
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    Mechanical stress effects on 550°C hot corrosion propagation rates in precipitation hardened Ni-base superalloys: CMSX-4, CM247LC DS and IN6203DS
    (Springer, 2021-12-10) Chapman, Neil; Gray, Simon; Sumner, Joy; Nicholls, John R.
    Combinations of temperature, stress and hot corrosion may cause environmentally-assisted cracking in precipitation-hardened Ni-base superalloys, which is little understood. This research aims to increase current understanding by investigating the effects of mechanical stress on the hot corrosion propagation rate during corrosion-fatigue testing of CMSX-4, CM247LC DS and IN6203DS. The parameters used during the tests included a high R-ratio, high frequency, and a temperature of 550 °C. The results showed CMSX-4 experienced a predictable increase in the hot corrosion rate, CM247LC DS also experienced increased rates, but no obvious trend was apparent; whilst IN6203DS showed no evidence of an increased rate. These different behaviours appear to be a result of an interaction between the mechanical stress and microstructural features, which include gamma-prime volume fractions in both the matrix and eutectic regions, along with the distribution of the eutectic structure. The different behaviours in the hot corrosion propagation rate subsequently affected the respective corrosion fatigue results, with both CMSX-4 and CM247LC DS experiencing fracture but with significantly more scatter involved in the CM247LC DS results. All IN6203DS corrosion-fatigue specimens completed the respective tests without fracture and showed no evidence of cracking. It, therefore, appears that precipitation hardened Ni-base superalloys, which are susceptible to environmentally-assisted cracking, also experience increased hot corrosion propagation rates.
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    Stress corrosion testing of CMSX-4, CM247LC DS and IN6203DS Ni-base superalloys
    (Springer, 2020-11-23) Chapman, Neil; Gray, Simon; Sumner, Joy; Nicholls, John R.
    The combination of stress and hot corrosion may result in Ni-base superalloys experiencing stress corrosion cracking, of which, the mechanisms are little understood. The aim of this research was to enhance the understanding by performing a series of stress corrosion exposures, at temperatures of 550, 500 and 450 °C, on: CMSX-4, CM247LC DS and IN6203DS superalloys. After completing the exposures, the superalloys were ranked with respect to the severity of the cracking experienced (CMSX-4 showing the worst severity, followed by CM247LC DS and then IN6203DS which showed no evidence of cracking at all) and the ranking appeared to be correlated to the gamma prime volume fraction. This suggests the gamma prime volume fraction is associated with the crack mechanism with lower values increasing the resistance to stress corrosion cracking. From the findings of this research, a new crack initiation/propagation mechanism is proposed which is based on a summation of stresses that includes those associated with the gamma prime
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    Surface roughness evolution to identify incubation time for hot corrosion of nickel-base superalloys: CMSX-4, CM247LC DS and IN6203DS at 550 °C
    (Springer, 2020-09-24) Chapman, Neil; Gray, Simon; Sumner, Joy; Nicholls, John R.
    In the absence of protective scales, nickel-base superalloys have an extremely limited hot corrosion incubation period before increased rates of attack are experienced. This paper reports on the nickel-base superalloys: CMSX-4, CM247LC DS and IN6203DS subjected to 550 °C hot corrosion exposures of durations ranging from 0 to 800 h, during which none of the superalloys developed a fully protective scale. The aim of the research was to identify the incubation period of each superalloy and this was achieved by means of surface roughness evaluations. A metrology exercise was performed on the cross section of test specimens which produced Cartesian data points which were subsequently converted to Ra and Rz data. Statistical analysis of the results suggested the incubation period lasted approximately 400, 500 and 200 h, respectively, for each superalloy. It was concluded that refractory metal phases within the microstructure were associated with the relatively short IN6203DS incubation period. This paper demonstrates that monitoring the changes in surface roughness provides a plausible method to identify the transition from incubation to propagation when studying 550 °C hot corrosion attack
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    The use of APS thermal barrier coatings in corrosive environments
    (Springer, 2017-02-25) Wells, Jonathan; Chapman, Neil; Sumner, Joy; Walker, Paul
    Thermal barrier coatings (TBC) can be used to reduce the metal temperature of gas turbine blades enabling higher Cr alloys (lower strength) to be used when gas turbines are to be used in corrosive environments (where hot corrosion resistance is required). However, the TBC must also be resistant to the corrosive environment and remain attached to the blade. A 1000 h test to evaluate air plasma-sprayed (APS) TBC adhesion to a low-pressure plasma-sprayed CoNiCrAlY bond coat (with and without through thickness cracking) under hot corrosion conditions at 850 °C has been carried out. The APS TBC significantly reduced the hot corrosion rate of the CoNiCrAlY; however, delamination cracking occurred with a thinner thermally grown oxide than would be expected from isothermal and cyclic oxidation testing.