Browsing by Author "Gray, Simon"

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    Advancing the synergy between models and experiments to investigate environmentally and mechanically driven crack propagation
    (Cranfield University, 2023-09) Elsherkisi, Mustafa; Castelluccio, Gustavo M.; Gray, Simon
    Aero-gas turbine running temperatures are rapidly increasing in order to improve their efficiency, and as a consequence components are subjected to more extreme environ- ments. With higher operational temperatures and improved reliability, there is an in- creased chance of both corrosion and mechanical degradation. In addition to operational temperatures, the environment in which an aircraft flies has a significant effect on the material life. Many contaminants are ingested by the engine and deposited on the turbine blades, which often leads to surface degradation. Depending on the ingested contami- nants, temperature, and applied stresses, cracking can be initiated and propagated rapidly. This is particularly evident in the lower-shank regions of single-crystal nickel-based su- peralloy blades, which have recently experienced significant cracking. This study aims to understand the mechanisms behind crack propagation in single- crystal nickel alloys exposed to intermediate temperatures, and when this propagation is either mechanically or chemically driven. This research started by assessing crack inter- action mechanisms that were hypothesised to be both stagnating and accelerating crack growth, depending on specific length scales and crack formations. This was performed by integrating available experimental data to calibrate a phase field model that could predict the extension of cracks for different crack separations and layouts. The modelling results clearly characterised the length scales needed to encourage crack shielding, and which crack formations would see a stress intensification and consequently crack coalescence. These results informed the decision to revisit the experimental setup to optimise which experiments were performed. Using this newly developed methodology, the salt deposi- tion method was amended with the aim of isolating the deposition sites to minimise crack interaction mechanisms. The hypothesis was that significantly longer cracks would be ob- ii served if this could be achieved. This was performed for both the C-ring (at 550°C), and corrosion-fatigue (at 700°C) tests. In the case of CMSX-4, the results were striking, with the C-ring seeing cracks as much as ten times the size of those previously seen. CMSX-10 however, did not show a significant difference, as such, a microstructural characterisation analysis was conducted, whereby the γ/γ′ structure for the two alloys was replicated from microscopy data and further phase field models were run. The results showed that a more regular structure was more resistant to crack propagation owing to the misalignment of γ′ , which caused stress relaxation in the γ channel and at the interface. Finally, this thesis shows how the model, once calibrated for one material and species, can be used to approximate the response expected for another single-crystal nickel alloy or a change in the embrittling species, while accounting for a degree of uncertainty. This is not to say that modelling can or should replace experiments but rather to highlight that preliminary modelling results can be used to build a test matrix that can reduce the number of experiments that are run. It should be noted that this thesis does not focus on the chemical/corrosive aspects in much detail, but rather investigates the importance of stress. This thesis summarises the importance of integrating modelling, microscopy, and experiments to set and answer hypotheses more efficiently.
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    Analysis of combined static load and low temperature hot corrosion induced cracking in CMSX-4 at 550°C
    (Elsevier, 2019-10-18) Brooking, Laurie; Gray, Simon; Dawson, Karl; Nicholls, John R.; Simms, Nigel J.; Sumner, Joy; Tatlock, G. J.
    A CMSX-4 3-point bend specimen was statically loaded under hot corrosion conditions and SEM, (S)TEM and EDX techniques were used to analyse the cracking generated. Sulphur, chlorine, sodium and oxygen were found at the crack tip, and an influence of loading on the corrosion mechanism’s preference to interact with either the γ or γʹ was observed. The microscopy analysis is in support of the corrosive mechanism being a combined stress and electrochemical corrosion linked with low temperature hot corrosion, where crack propagation occurs as a result of localised corrosion enhanced material degradation. High magnification EDX mapping identified W as segregating to the γʹ at room temperature.
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    A bi-objective decision support tool based on system dynamics and discrete event modelling for sustainable supply chain
    (DSRPT Consultancy, 2023-07-24) Yu, Jia; Hieu, Trung; Gray, Simon; Encinas-Oropesa, Adriana
    Based on the development trend of sustainable concepts and the implement ability of composites combined with Agave Bagasse Fibre (ABF) and Polyethylene Terephthalate (PET). The purpose of this paper is to design a bi-objective decision support tool for supply chain of composite material which combined with ABF and PET. Through the production, processing, recycling, and reprocessing of composite materials, the sustainable supply chain model of four different schemes is designed, and the data results of each scheme model are calculated and analysed. The tool can support supply chain modelling solutions that seek best practices for sustainable supply chains and optimize resource efficiency through cost and carbon dioxide emissions. The sustainable supply chain model was designed, created, and optimized in AnyLogic software using System Dynamics and Discrete Event Simulation modelling methods based on the supply chain model established by previous researchers. According to the analysis results of the model data, the reasonable design of the whole process can effectively reduce the cost and carbon dioxide emissions and achieve the effectiveness and implementation of the sustainable supply chain. The results of this study will provide reference for more sustainable supply chain models in the future. Further research on composite materials can be carried out by combining with practice.
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    Chlorine-induced stress corrosion cracking of single crystal superalloys at 550 °C
    (Springer, 2024-08-05) Duarte Martinez, Fabian; Dawson, Karl; Tatlock, Gordon; Leggett, J.; Gibson, G.; Mason-Flucke, J. C.; Nicholls, John; Syed, Adnan; Morar, N.; Gray, Simon
    This study has investigated the effect of NaCl and different gaseous environments on the stress corrosion cracking susceptibility of CMSX-4 at 550 °C. The presence of SOx leads to the rapid dissociation of NaCl into Na2SO4 and the release Cl2 and HCl, which then trigger an active oxidation mechanism and stress corrosion cracking. The incubation time for crack initiation at 690 MPa and in the presence of a sulphur containing environment is 10 min. A working hypothesis is that stress corrosion cracking occurs due to the hydrogen released at the oxide/alloy interface when metal chlorides are formed; however, this hypothesis needs to be further explored.
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    Combined hot corrosion and static stress or fatigue of single crystal superalloys.
    (2018-09) Brooking, Laurie Finian; Sumner, Joy; Gray, Simon; Simms, Nigel J.
    It has been observed that gas turbine (GT) blades operating in harsh environments can undergo material degradation in the form of crack initiation and propagation before that predicted by fatigue lifing. It is thought that this degradation can occur partly as a result of the growing demands for improved GT efficiencies. This is because the requirement for improved GT efficiencies is commonly achieved through increases in operational temperatures and pressures which the turbine stages operate at. These increases in the operational temperatures can consequently lead to the extended effect of hot corrosion in locations of the blade which would not normally impacted, such as the under platform region. Therefore, GT blades are subjected to continuous developments in terms of blade design and material properties and selection in order to achieve improvement in the GT efficiencies, reduce emissions and lower life cycle costs. However, at start of this research project it was postulated that the mechanism causing premature material degradation is a result of the extended effects of low temperature hot corrosion (LTHC), interacting with both cyclic and static loading conditions. In order to experimentally study these interactions statically and cyclically loaded specimens were tested in environments representative of the under platform region of both industrial and aviation gas turbine (GT) blades. A range of geometries were studied: C-rings, three point bend and cylindrical fatigue specimens. Using these specimens experimental studies were conducted investigating the impact of deposit flux, dwell time, multiaxial stress state and load application rate. Further investigations into the microscopic mechanism occurring at the crack tip have been conducted using high magnification transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Detailed stress state data has been produced using finite element analysis (FEA), this data has then been used to assess the impact of the stress state on crack initiation and propagation. It was found that turbine blade materials were susceptible to a form of high temperature stress corrosion cracking (SCC). Additionally, enhanced fatigue crack initiation and propagation was observed with test conditions consistent with low temperature hot corrosion (LTHC) conditions. There was shown to be a detrimental impact with increased rates of LTHC on the high temperature cracking mechanism. Detailed microscopy and analysis of specimens informed a proposed fundamental mechanism behind the enhanced cracking observed in LTHC environments. The five research papers presented within this thesis provide contributions to knowledge and developments in the understanding of crack initiation and propagation within superalloys exposed to simultaneous LTHC environments and loading.
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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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    Data for the paper titled Analysis of Combined Static Load and Low Temperature Hot Corrosion Induced Cracking in CMSX-4 at 550°C
    (Cranfield University, 2020-02-03 08:09) Gray, Simon; Brooking, Laurie; Nicholls, John; Sumner, Joy; Simms, Nigel; J. Tatlock, Gordon; Dawson, Karl
    Presentation containing figures within the paper
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    Data: The effect of trepanning speed of laser drilled acute angled cooling holes on the high temperature low cycle corrosion fatigue performance of CMSX-4 at 850 °C
    (Cranfield University, 2017-11-20 10:05) Gray, Simon
    The Excel file presents the raw fatigue data presented in the paper referenced below. It shows the effect of recast layer thickness of laser trepanned cooling holes on the corrosion-fatigue life of CMSX-4 at 850°C.
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    Effect of NaCl and SO2 on the stress corrosion cracking of CMSX-4 at 550°C
    (Taylor and Francis, 2023-05-01) Duarte Martinez, Fabian; Syed, Adnan; Dawson, Karl; Tatlock, G. J.; Morar, N. I.; Kothari, M.; Tang, C.; Leggett, J.; Mason-Flucke, J. C.; Gibson, G.; Nicholls, John R.; Gray, Simon; Castelluccio, Gustavo M.
    In the pursuit of more efficient gas turbine engines, components are required to operate for longer times at elevated temperatures. This increased time in service, together with a complex loading regime, can expose the material to environmental attack. This work has demonstrated that the interaction of stress, NaCl and a sulphur-containing environment is critical to cause crack initiation in the early stages of the exposure and accelerated corrosion rates in CMSX-4 at 550°C. The effect of having small concentrations of moisture in the gaseous environment or as water crystallisation in the salt is still to be investigated. A working hypothesis is that the interaction of alkali chlorides with a sulphur-containing atmosphere is the trigger to a self-sustaining cycle where metal chloride formation, vaporisation and oxidation lead to high amounts of hydrogen injection in a rapid manner and, therefore, hydrogen embrittlement.
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    Effect of stress state and simultaneous hot corrosion on the crack propagation and fatigue life of single crystal superalloy CMSX-4
    (Elsevier, 2018-05-03) Brooking, Laurie; Gray, Simon; Sumner, Joy; Nicholls, John R.; Marchant, G.; Simms, Nigel J.
    Operating conditions within industrial gas turbines are changing in response to pressures to reduce environmental impact and enable use of renewable sources. This is driving an increase in the operational temperatures and pressures of combustion in turbine systems. Additionally, diverse operating environments can result in higher sulphur and trace metal contaminant levels, exacerbating hot corrosion in GT systems. Low cycle fatigue (LCF) cycling can also be intensified as a result of increased start/stop shutdowns. The combined effects of hot corrosion and stress are experimentally studied on CMSX-4 single crystal (SC) γ/γ' system under both fatigue and static stress conditions, with either a multi-axial bending or uniaxial stress state. The associated stress intensity thresholds (KTH) under the various stress conditions were evaluated using finite element analysis (FEA). Cracking was observed both under static and fatigue stress conditions in a hot corrosion environment. Crack morphologies were analysed using SEM techniques. Bending stresses and fatigue cycles demonstrated increased crack propagation in the presence of hot corrosion with static uniaxial stresses showing the longest nucleation times and lowest propagation rates.
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    The effect of trepanning speed of laser drilled acute angled cooling holes on the high temperature low cycle corrosion fatigue performance of CMSX-4 at 850°C
    (Elsevier, 2017-05-04) Morar, N.; Roy, Rajkumar; Mehnen, Jorn; Nicholls, John R.; Gray, Simon
    The effect of laser trepanning speed and, as a result, recast layer thickness on the high temperature corrosion fatigue behaviour of CMSX-4 superalloy acute angled holes was investigated. The experimental test results show that an increasing laser drilling speed caused a reduction in corrosion fatigue life by 30-50% at 850°C, under low cycle fatigue regime. This reduction was found to correlate directly with the recast layer thickness and surface anomalies within the recast layer produced during the laser drilling process. Corrosion had a smaller effect on the overall life of the laser drilled specimens under the conditions tested. The results presented show that laser trepanning speedis influencial in limiting the life performance of laser drilled components in service.
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    The effect of volatilisation of refractory metal oxides on the cyclic oxidation of Ni-base superalloys
    (Springer Science Business Media, 2014-04-01T00:00:00Z) Brewster, Gyaneshwara; Edmonds, I. M.; Gray, Simon
    The role of refractory metals on the bare oxidation behaviour of a range of compositionally complex single crystal Ni-base superalloys has been investigated using a range of techniques. It has been shown that this series of fourth generation alloys were borderline alumina formers at 1,100 °C, and their cyclic oxidation performance was detrimentally affected by increased levels of Mo and Co. In addition, there was strong evidence that Re, Ru and Mo were lost through the process of cyclic oxidation, which supports the hypothesis that these elemental additions were lost through volatilisation/sublimation, contributing to their poor cyclic oxidation performance.
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    Effects of surface hardening by laser shock peening and shot peening on a nickel-based single-crystal superalloy CMSX-4
    (IOP Publishing, 2024-07-31) Hakeem, Aabid Husen; Morar, Nicolau I; Dawson, Karl; Tatlock, Gordon J; Gibson, Grant J; Gray, Simon
    Improving the expected life of nickel-based single-crystal superalloy turbine components by surface hardening treatments including laser shock peening (LSP) and mechanical shot peening (MSP) are of particular interest for mitigation of life limiting damage such as environmental assisted cracking in hot section components of gas turbines. In the present study the effects of LSP and MSP on the surface roughness, microhardness and work hardening of a nickel-based single crystal superalloy CMSX-4® have been assessed. Surface roughness was measured using laser profilometry. The degree of work hardening was measured using electron backscattered diffraction with local misorientation analysis. The analysis showed evidence for a work hardening layer in the MSP sample to a depth of approximately 70 μm. Sets of slip bands extending far into the bulk of the sample were observed in the LSP-treated sample, without any evidence of a work hardening layer. Microhardness measurements used to gauge the depth of residual stress showed that LSP produced a much deeper hardness profile than MSP, with compressive residual stress depths of 1000 μm and 200 μm in LSP and MSP respectively. The retention of hardness after a heat treatment of 50 h at 700 °C was more prominent in the LSP sample than in the MSP sample. LSP and MSP have therefore been shown to be at the opposite ends of the spectrum of surface hardening treatments of CMSX-4, with LSP giving milder hardening, but to a greater depth.
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    Environmentally assisted cracking of a single crystal nickel-based superalloy
    (Taylor and Francis, 2023-03-03) Dawson, Karl; Duarte Martinez, Fabian; Gray, Simon; Nicholls, John; Gibson, G.; Leggett, J.; Tatlock, G. J.
    Single crystal material, of CMSX-4® alloy composition, was cast and secondary orientation was controlled at the machining stage, to produce c-ring cross-section tubes with known crystallographic orientations. The c-ring tubes were coated with NaCl before being subject to loading up to 700MPa and heated for durations of up to 2 hrs at 550°C in flowing environments containing air and SO2. No cracking was observed in short term tests that were run in the absence of either NaCl, or SO2, indicating a symbiotic interaction is required to initiate cracking. Experiments confirm the presence of oxygen, chlorine and sulphur at the crack tips, formed along {001} crystallographic planes, however, they were distributed discretely, with several oxide and sulphide phases observed. In this work, we image, analyse and identify the phases formed during the cracking and corrosion of CMSX-4® superalloy and hypothesise on the complex chemical interactions that take place during crack initiation.
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    Impact of deposit recoat cycle length on hot corrosion of CMSX-4
    (Springer, 2017-04-05) Sumner, Joy; Aksoul, Quentin; Delgado, Joemar; Potter, Andrew; Gray, Simon
    Hot corrosion causes significant problems for both aerospace and power generation industries, where the combination of high temperature, corrosive gases, and contaminants severely limits component operating lifetimes in gas turbine hot gas streams. Multiple laboratory testing methodologies exist to study this hot corrosion, and these can be affected by a range of variables. This paper investigated the impact of varying deposit recoat cycle length when using the ‘deposit recoat’ testing method. CMSX-4 samples were exposed to simulated type II (pitting) hot corrosion conditions, with the same overall deposit load (averaged across the total exposure run), but different deposit recoat cycles. Post-exposure, samples underwent dimensional metrology analysis to compare metal loss resulting from different deposit recoat cycle lengths. Results for CMSX-4 suggest very small differences in corrosion losses, indicating CMSX-4 hot corrosion datasets obtained from deposit recoat experiments with different deposit recoat cycle lengths can be compared with confidence.
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    Influence of shot peening on high-temperature corrosion and corrosion-fatigue of nickel based superalloy 720Li
    (Taylor and Francis, 2016-04-08) Gibson, G. J.; Perkins, K. M.; Gray, Simon; Leggett, A. J.
    High-temperature corrosion fatigue, a combination of corrosion with a fatigue cycle, is an emerging generic issue affecting power generation and aero gas turbine engines and has the potential to limit component life. Historically, surface treatments, such as shot peening have been used to improve component life and have been optimised for fatigue response. Research into optimisation of shot peening techniques for hot corrosion and high-temperature corrosion fatigue has shown 6–8A 230H 200% coverage to provide overall optimum performance for nickel-based superalloy 720Li based on the limited data within this study. Utilisation of electron backscatter diffraction techniques, in combination with detailed assessment of corrosion products have been undertaken as part of this work. The resultant cold-work visualisation technique provides a novel method of determining the variation in material properties due to the shot peening process and the interaction with hot corrosion. Through this work it has been shown that all three shot peening outputs must be considered to minimise the effect of corrosion fatigue, the cold work, residual stress and surface roughness. Further opportunity for optimisation has also been identified based on this work.
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    Interaction of hot corrosion fatigue and load dwell periods on a nickel-base single crystal superalloy
    (Elsevier, 2018-07-25) Brooking, Laurie; Gray, Simon; Sumner, Joy; Nicholls, John R.; Simms, Nigel
    The effects of type II hot corrosion on the fatigue resistance of turbine blade superalloys is of growing interest as gas turbine (GT) original equipment manufacturers (OEMs) strive to optimise the operational efficiencies and versatilities of GT systems. Hot corrosion fatigue has been observed in the under platform regions of first stage GT blades, this location is subject to both relatively high principal stresses and stress gradients, combined with temperatures up to those associated with type II hot corrosion (500–700 °C). The effect of the deposition flux of corrosive salt species and the tensile stress dwell period on the fatigue performance and resultant crack morphologies of single crystal (SC) superalloy CMSX-4 has been studied at 550 °C. Deposit recoat methodologies were applied to specimens that were cyclically fatigued with a load-controlled trapezoidal waveform. It was observed that introducing a longer dwell period increased the number of {1 0 0} crack initiations and reduced the fatigue life (load cycles to failure). Optical and SEM microscopy and EDX techniques were used to examine specimen fractography, and mechanisms of crack advance and propagation discussed.
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    Interaction of stress corrosion cracks in single crystals Ni-Base superalloys
    (Elsevier, 2024-02-02) Elsherkisi, Mustafa; Martinez, Fabian Duarte; Mason-Flucke, Julian; Gray, Simon; Castelluccio, Gustavo M.
    Stress corrosion cracking (SCC) can be detrimental to nickel-based superalloy components exposed to harsh environments in aero-gas turbines. During flight, engines consume contaminants deposited on the surface of a blade, often leading to degradation. Cracking can initiate within minutes and rapidly propagate, depending on the temperature, contaminants, and applied stress. This study investigated the interaction between cracks in single-crystal turbine blades at intermediate temperatures by integrating experimental and computational methods. We performed C-Ring tests to quantify the time required for cracking, along with microscopic characterisation of the damage. In parallel, we developed a finite-element simulation for C-Ring tests using a phase field model calibrated to match the location of the cracks. The results demonstrated that the crack's characteristic spacing and length determine the likelihood of shielding or coalescing mechanisms.
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    Investigation into the effects of salt chemistry and SO2 on the crack initiation of CMSX-4 in static loading conditions
    (Springer, 2020-08-29) Martinez, Fabian Duarte; Morar, Nicolau I.; Kothari, Maadhav; Gibson, G.; Leggett, J.; Mason-Flucke, J. C.; Nicholls, John R.; Castelluccio, Gustavo M.; Gray, Simon
    Although evidence exists of the potential impact of stress, co-incident with corrosive environments at high temperature, for single crystal turbine blades, the mechanism responsible is not fully understood. This work explores the effect of CaSO4, Na2SO4 and sea salt on the scale formation and crack initiation of CMSX-4 at 550°C in 50 ppm of SO2 and synthetic air under a static stress of 800 MPa. The cross-sectional analysis showed that the CaSO4 and the Na2SO4 salted specimens did not undergo a significant degree of corrosion degradation and no cracks were detected after 400 hours of exposure. However, sea salt caused significant degradation to the scale and cracks were detected by X-ray CT scanning after 400 hours of exposure. The findings from this study suggests that the sulfation of chlorine containing species in sea salt led to the formation, vaporisation and re-oxidation of metal chlorides and this mechanism was found to play a key role in the formation of a non-protective scale. An active oxidation mechanism has been proposed to interpret the results. In conclusion, it is hypothesized that due to the synergistic effect of stress and the formation of a non-protective scale, fast diffusion paths for sulfur, oxygen and chlorine ingress were formed. Further work is currently being undertaken to understand the effect of these species on the local embrittlement of CMSX-4 that ultimately led to the initiation of cracks in the specimen.
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    Investigation into the environmental assisted crack initiation mechanism of CMSX-4 in simulated aero engine environments at 450 - 550°C.
    (Cranfield University, 2023-03) Duarte Martinez, Fabian; Nicholls, J. R.; Gray, Simon; Castelluccio, Gustavo M.
    The aviation industry has continued to increase the efficiency of gas turbine engines, which are now designed to operate on a wide variety of flight routes. In general, the efficiency drive has led to components spending longer times at temperatures, where accelerated corrosion can occur. This has led to a complex degradation mechanism being identified in the lower shank region under the platform of single-crystal turbine blades. This research aims to understand the mechanism of crack initiation due to the synergistic effect of stress and high temperature corrosion environments on CMSX-4 in the lower operating temperature range, 450°C - 550°C, of an aero gas turbine blade. The first part of the investigation consisted in comparing the effect of different salt deposits in a 50 ppm SO₂ - air environment at 550°C. A 50 ppm SO₂ – air concentration was considered because the air going through the lower shank is fed directly from the compressor, and not from the combustor (which is the main source of sulphur). Characterisation of the resulting scales were carried out using scanning electron microscopy, energy dispersive spectroscopy and X- ray diffraction. Results from thermodynamic modelling are also presented. The first part of the investigation showed that CMSX-4 sample under an applied stress and no applied salt did not experience accelerated corrosion attack or crack formation when exposed to 50 ppm SO₂ - air in a 400-hour period. The same observation was made for a CMSX-4 sample under an applied stress and salted with CaSO₄. Sea salt caused accelerated corrosion attack with cracks up to 1.3 mm through the substrate formed after 400 hours of exposure. Further tests using NaCl salt in 50 ppm SO₂ – air showed that cracks can initiate after just 10 minutes of exposure at 550°C. Crack growth rates are significantly reduced after two hours of exposure within a 50-hour salt cycle. Cracks with NaCl in 50 ppm SO₂ – air have also been observed at temperatures as low as 450°C. When NaCl salt was applied to CMSX-4 and exposed to air only for 50 hours, the corrosion attack was reduced considerably, and the initiation of cracks is either suppressed or significantly delayed beyond a 50-hour period. Although this PhD has only focused on a 50-hour period, longer exposure times should be carried out to determine if air exposures delay crack initiation time, or if crack initiation is completely supressed. This thesis has therefore shown that the interaction of stress, NaCl and a sulphur- containing environment are critical to cause early crack initiation in single crystal nickel-based superalloys in the temperature range 450 - 550°C. The effect of having small concentrations of moisture in the gaseous environment or as inclusions retained in the salt are still to be investigated. A working hypothesis is that that the interaction of alkali chlorides with a sulphur-containing atmosphere is the trigger to a self-sustaining cycle where metal chloride formation, vaporisation and oxidation leads to high amounts of H₂ formed at the scale/alloy interface. Potentially, the H₂ formed at the alloy/scale interface may dissociate into atomic hydrogen, and lead to hydrogen embrittlement. For further verification of this hypothesis, a set of tests have been suggested.