Engineering - Royal Australian Navy

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Naval <strong>Engineering</strong> Bulletin • June 2001 hold, can cause very rapid degradation and, ultimately, expensive catastrophic failure from ‘domestic object damage’ 3 . Corrosion can also accelerate the effects of other failure mechanisms, such as creep and thermal fatigue. In hot section components of gas turbines, the particular form of corrosion most prevalent and most destructive is known as hot corrosion. Hot Corrosion and its Consequences Chemical degradation of hot section components of gas turbines can be classed as either oxidation or hot corrosion. Oxidation in marine gas turbines is less predominant than in industrial or aircraft gas turbines because of their relatively lower operating temperatures, and will not be dealt with in this report. Hot Corrosion pProcess Because the combustor and HP/LP turbines are supplied with excess air for cooling, the hot combustion gases are constantly oxidising. Hot corrosion - sometimes referred to as turbine sulphidation - is an electrochemical process of accelerated degradation by oxidation of superalloys and coatings in combustion gases, which contain impurities such as sulphur, alkali metals, chloride and vanadium salts, as well as unburned carbon. It is more prevalent in lowflying aircraft engines (e.g. helicopters) and marine propulsion gas turbine engines, which are more susceptible to seawater ingestion. The basic chemical reaction is: 2NaCl (from sea salt) +S (from fuel) 4 + 2O 2 (from cooling air) → Na 2 SO 4 + Cl 2 • Type 2 corrosion (low-temperature), which typically occurs in the range 590 0 C to 820 0 C, with a maximum at about 700 0 C, characterised by uniform pitting corrosion attack with no denudation of alloying elements, and no sulphide formation observed in the microstructure. The LM2500 engine has a maximum HPT inlet temperature in excess of 900 0 C; hence it is potentially susceptible to both types of hot corrosion. Phases of Hot Corrosion Hot corrosion has been observed to occur in two distinct phases: • Initiation stage • Propagation stage In the initiation stage, the alloy is oxidised to form a protective barrier of Al 2 O 3 or Cr 2 O 3 , but sulphur penetrates this scale to form sulfides with the alloy. Over time, growth stresses develop in the oxide coating, and the oxides dissolve in the salt deposit. If allowed to continue, the initiation stage culminates in local salt penetration through the oxide scale to the alloy surface, causing rapid propagation. The propagation stage is associated with severe corrosive attack on the alloy by the Na 2 SO 4 . With today’s thin-walled, internally cooled turbine blades, catastrophic structural breakdown would not take long after the onset of rapid propagation - therefore, the alloy and/or coating of the blade must be selected so as to maximise the length of the initiation stage. Factors affecting the length of the initiation stage - which can vary from a matter of seconds to thousands of hours - include: Deposition of molten flux, based primarily on Na 2 SO 4 but also containing molten unreacted NaCl, dissolves the normally stable protective oxides on the turbine blades. Types of Hot Corrosion Two types of hot corrosion are generally recognised: • Type 1 corrosion (high-temperature), which typically occurs in the range 820 0 C to 920 0 C, with a maximum at about 870 0 C, characterised by the build-up of a non-protective oxide layer as oxidation and sulphidation destroy the metal substrate • alloy composition • fabrication methods • gas composition and velocity • salt composition • salt deposition rate • condition of salt • temperature • co-existence of erosion • specimen geometry in relation to gas path Consequences of Hot Corrosion Annex A contains photographs that illustrate the various effects of hot corrosion on HPT blades. 3 Turbine blade failure can cause up to $500,000 damage downstream. 4 Less predominant, but equally corrosive in its consequence, is the formation of vanadates by similar chemical reaction of vanadium in lower-grade fuels. 45
Naval <strong>Engineering</strong> Bulletin • June 2001 Prevention of Hot Corrosion To minimise the likelihood of hot corrosion, a number of ‘external’ drivers of the process can be managed in their design or implementation. Combustor and Turbine Design (gas flow dynamics) Good aerodynamic design of gas path components ensures that laminar flow is maintained, as the onset of turbulent flow patterns would be likely to accelerate the corrosion process. Air Filtration Only 30% of the intake air is used for combustion in the LM2500 engine: the remaining 70% is used for cooling. All intake air, nonetheless, needs to be filtered to remove foreign particles and airborne salt. This has the undesirable effect of reducing engine efficiency as excessively negative intake pressures are generated. The filters, then, are designed for a trade-off between effectiveness and air flow efficiency, and must be regularly and thoroughly washed down to remove salt deposits. This is not always achievable during extended ocean transits. Fuel Quality Marine gas turbines use marine diesel fuel, with specific requirements for low sulphur and vanadium content. Once embarked into ship’s bunkers, the fuel must be purified prior to consumption, to remove particulate matter and free water (caused by internal condensation of tanks). Water-Washing The LM2500, like most marine gas turbine engines, requires regular water-washing using a specialised detergent mixture to remove salt deposits from compressor and turbine gas-path components. The benefits of this are two-fold: potentially corrosive deposits are removed, and engine efficiency is improved by restoring the blade and vane surfaces to their designed laminar-flow profile. Inhibition of Hot Corrosion The inhibition of hot corrosion by the HPT blades themselves is achieved through the selection of appropriate alloying elements in the substrate metal. Progressive changes in alloy chemistry necessary to increase temperature capability and reliability have resulted in a very significant decrease in hot corrosion resistance in salt-contaminated environments: this trade-off in alloy properties has necessitated the introduction of corrosion-resistant protective coatings applied to the alloy substrate. Effect of Alloy Composition The most important alloying constituent to protect against hot corrosion is chromium; in particular, the protective layer of Cr2O3 is self-healing after breakdown, thus prolonging the initiation stage of the corrosion process. Cobalt-based superalloys are traditionally regarded as being superior to nickel-based alloys in corrosion resistance, but the latter has outstanding strength and oxidation resistance over a much wider temperature range. The use of chromium as the major alloying element in nickel-based superalloys, then, provides an optimal balance of properties. Table 1 has already shown the various constituents of hot-section alloys. Protective Coatings Increases in engine operating temperatures have dictated the path of development of hot-section superalloys towards maximising creep strength and minimising weight through internal cooling channels. Many of these alloys ultimately have inadequate hot-corrosion resistance, and must rely on coatings to prevent severe and life-shortening damage. By specifying low levels of contaminants (sulphur, vanadium) in fuels, simple diffused aluminide coatings are often sufficient for aero engines. However, the high and extremely transient temperature effects are still contributors to reduced service life - the situation is worse for marine propulsion gas turbines, where sea water ingestion accelerates hot corrosion processes. The reality of marine gas turbine operation also results in lower-grade fuels being used (there being little choice in many foreign ports of call). The principal selection criteria for coatings are: • high resistance to oxidation and/or corrosion • minimisation of solubility of molten salt • adequate ductility to withstand operational strains without cracking (i.e. must be matched well to mechanical properties of substrate alloy) • low rate of interdiffusion between coating and substrate • ease of application • low cost relative to LCC savings from improved service life Protective coatings in common use in hot-section applications can be grouped as follows (in ascending order of cost): • Aluminide coatings - most effective where metal temperature and/or environment are not extreme; easy to strip coatings off during refurbishment; able to form, and replenish, protective coatings of alumina; however, prone to brittle cracking at lower temperatures in high Al concentrations. 46
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Engineering - Royal Australian Navy

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