Burner rig trials with nickel- and cobalt-base alloys

Selected Ni- and Co-base alloys were hot corroded in a burner rig facility at temperatures cycled from ambient to 870 or 980°C respectively for times in excess of 3000h. A diesel fuel maintained at lw/oS and doped with sea salt to 476 ppm produced characteristic hot corrosion attack at 870°C and more classical oxidation attack at 980°C. This preliminary investigation shows that the products formed on the Ni-base alloys at 980°C were cracked and dominated by local surface protrusions (“warts”). Co-base alloys did not show these features nor were they observed following tests at the lower temperature. A small % of Y in a simple Nimonic alloy reduced the extent of scale damage but did not eliminate hot-corrosion/oxidation attack.     

Corrosion behaviour of the new gas turbine alloy 2100 GT in hot gases and combustion products

Not only excellent high temperature mechanical properties are needed to establish a new gas turbine alloy, but also a very good oxidation behaviour, together with good resistance to so‐called “hot corrosion”. This paper describes experimental studies on the corrosion behaviour in hot gases and combustion products of a new Ni‐Cr‐Ta alloy 2100 GT in comparison to the commercially established alloys 230, C‐263 and 617. Alloy 2100 GT is a newly developed cobalt, tungsten and molybdenum free Ni‐base superalloy of Krupp VDM. It contains as major alloying elements 25 wt.‐% chromium, 8 wt.‐% tantalum, 2.4–3 wt.‐% aluminium and 0.2–0.3 wt.‐% carbon. High temperature strength is achieved by the addition of tantalum, resulting in significantly increased solid solution strengthening, carbide hardening due to the formation of primary precipitated tantalum carbides, and γ′‐precipitation hardening by aluminium and tantalum. The isothermal oxidation tests showed that the parabolic rate constant of alloy 2100 GT is similar to that of alumina‐forming alloys. This is achieved by the remarkably high aluminium content for a wrought alloy. Additions of yttrium improve the spalling resistance under thermal cycling by the formation of very thin and tightly adherent oxide layers. No deleterious effect caused by the addition of tantalum could be found. In the cyclic oxidation tests performed at temperatures between 700°C and 1200°C alloy 2100 GT showed the lowest mass change of all the alloys investigated. Na₂SO₄ has been found to be a dominant component of alkali salt deposits on gas turbine components at elevated temperatures. Combustion gases contain SO₂ because of the impure nature of the fuel. To investigate the hot corrosion behaviour of alloy 2100 GT, tests were performed with salt deposits containing 0.1 mol Na₂SO₄ and a test gas comprising air and 0.1% SO₂. Test temperatures were 600°C, 700°C, 850°C and 950°C. Alloy 2100 GT exhibited the best performance at all test temperatures. It was the only alloy which did not suffer any fluxing of the oxide layer and only slight internal sulphidation was observed.     

钴对镍基高温合金抗热腐蚀性能影响的研究

用以Udimet500为基0～25%七种不同钴含量的合金在750℃、800℃、850℃含100ppm盐雾气氛中进行单管热腐蚀试验,试验时间为62.5小时。宏观观察及失重法结果表明,钴可以明显改善和提高合金的抗热腐蚀性能。 用光学显微镜、X-光、电子探针及辉光逐层分析等方法进行了微观分析。结果指出,增加合金钴含量有益于合金在热腐蚀孕育期形成连续的保护性氧化膜,提高氧化膜的粘附性。钴还可以有效地阻止硫在合金中的扩散。     

Optimisation of in‐service performance of boiler steels by modelling high‐temperature corrosion

The main objective of the EU OPTICORR project is the optimisation of in‐service performance of boiler steels by modelling high‐temperature corrosion, the development of a life‐cycle approach (LCA) for the materials in energy production, particularly for the steels used in waste incinerators and co‐fired boiler plants. The expected benefits of this approach for safe and cost effective energy production are: ‐ control and optimisation of in‐service performance of boiler materials, ‐ understanding of high‐temperature corrosion and oxidation mechanisms under service conditions, ‐ improvement of reliability to prevent the failure of components and plant accidents and ‐ expanding the limits of boiler plant materials by corrosion simulations for flexible plant operation conditions (steel, fuel, temperature etc.). The technical aim of the EU OPTICORR project is the development of modelling tools for high‐temperature oxidation and corrosion specifically in boiler conditions with HCl‐ and SO₂‐containing combustion gases and Cl‐containing salts. The work necessitates thermodynamic data collection and processing. For development and modelling, knowledge about the corrosion mechanisms and exact data are needed. The kinetics of high‐temperature oxidation and corrosion are determined from laboratory thermo‐gravimetric tests (TG) and multi‐sample exposure tests. The materials studied are typical boiler tubes and fin‐steels: ferritic alloys, the austenitic steel T347 and the Ni‐based alloy Inconel 625. The exposure gases are dry air, air with 15 vol‐% H₂O, and with 2000 ppm HCl and 200 ppm SO₂. The salt deposits used are based on KCl‐ZnCl₂ and Ca, Na, K, Pb, Zn‐sulfates. The test temperatures for exposures with deposits are 320 and 420°C and, for gas exposures, 500 to 600°C. At present the tools being developed are ChemSheet based programmes with a kinetic module and easy‐to‐use interface and a more sophisticated numerical finite‐difference‐based diffusion calculation programme, InCorr, developed for prediction of inward corrosion and internal corrosion. The development of modelling tools for oxidation and high‐temperature corrosion was started with thermodynamic data collection for relevant systems and thermodynamic mappings. Further, there are needs to develop the simulation model and tool for salt‐induced hot corrosion based on the ChemSheet approach. Also, the work on modelling and simulating with the InCorr kinetic modelling tool will be continued to demonstrate the use of the tool for various steels and alloys in defined combustion environments.     

Influence of alloying elements on the hot-corrosion resistance of Co-Cr alloys

The influence of additional elements such as Nb, Ta, Mo, W, Y and Al on the high temperature corrosion resistance Of Co-Cr alloys in air, hydrogen sulphide and in combustion gases was investigated. The reaction kinetics, the microstructures and the distribution of the elements in the corrosion scales were determined. In addition, the hot corrosion mechanism was studied by means of sulphidation-oxidation tests.     

The hot corrosion of Co-25Cr-10Ni-5Ta-3Al-0.5Y alloy (S-57)

A cobalt-base alloy, Co-25 Cr-10 Ni-5 Ta-3 Al-0.5 Y (S-57), was subjected to hot corrosion in Mach 0.3 burner rig combustion gases at maximum alloy temperatures of 900 and 1000°C. Various salt concentrations were injected into the burner; 0.5, 2, 5 and 10 parts per million synthetic sea salt and 4 parts per million sodium sulphate (Na₂SO₄). The extent of corrosion was determined by measuring the maximum depth of corrosion in the alloy and the corrosion process was studied by metallography, X-ray diffraction, scanning electron microscopy, and electron microprobe analysis. While S-57 was found to possess only moderate oxidation resistance at these temperatures, this alloy resisted significant hot corrosion attack under all but the most severe test conditions. The process of the hot corrosion attack under the most severe conditions of this study was primarily sulphidation.     

Gaseous corrosion of alloys and novel coatings in simulated environments for coal,waste and biomass boilers

The reduction of emissions from power generation plants is a key part of the Kyoto Protocol. Reduced emissions per unit of power produced can be achieved via increased thermal efficiency and this can be achieved by increasing steam parameters (i.e. temperature and pressure). Increased steam parameters in turn leads to accelerated corrosion of boiler components. Biomass and solid waste fuels introduce a number of aggressive species into process environments that result in enhanced rates of boiler degradation. This paper reports on studies, both theoretical and experimental, of the corrosion behaviour of high‐alloy steels and Ni‐base alloys as well as coatings for use in high efficiency coal and/or biomass‐ and waste‐fired power plants. Coatings produced within the SUNASPO project have been laboratory tested in gaseous atmospheres representative of coal combustion, biomass combustion and waste incineration. Laboratory tests were carried out mainly in the temperature range 500 °C to 800 °C. Initial results showed the poor performance of traditional uncoated low‐alloy boiler steels P91 (9% Cr) and HCM12A (12% Cr), as well as the higher alloy steel, 17Cr/13Ni. Results show the beneficial effects of coatings containing Al, Si, Al + Si, Al + Ti and Al + B in reducing the rate of corrosive attack. In a combustion product gas containing 100 ppm HCl and 1000 ppm SO₂, aluminizing affords corrosion resistance of low‐alloy steels such as HCM12A and P91 similar to that of Alloy 800 over 1000 h of test. The presence of Al inhibits internal, sometimes localized corrosion by promoting the formation of a protective surface oxide layer even at relatively low temperatures. The results of experiments in simulated coal; biomass and waste atmospheres are presented and discussed in terms of both corrosion kinetics and mechanisms of degradation.     

Effect of Experimental Procedures on the Cyclic,Hot-Corrosion Behavior of NiCoCrAlY-Type Bondcoat Alloys

A simplified test procedure was established to assess the hot-corrosion behavior of MCrAlY-type nickel-base alloys under the influence of molten sodium sulfate as well as sodium sulfate/potassium sulfate salt blends. Salt-coated specimens were exposed to 1-hr thermal cycles at 950°C in flowing oxygen for up to 500 cycles. Mass-change data of the specimens revealed a significant dependence of the corrosion attack not only on the average contaminant flux rate, as expected, but also on the initial amount of salt deposited during each recoating cycle. Furthermore, deposit removal before salt recoating markedly influenced the corrosion attack of the alloys. This was apparently related to changes in salt chemistry by the dissolution of elements such as Cr from the alloy, which can shift the basicity of the salt and thus affect the extent of attack. Substituting Na for K in sodium sulfate/potassium sulfate salt blends generally resulted in decreased attack. For K-containing salt deposits, increasingthe gross amount of alkali compared to sulfur resulted in increased sample weight losses due to scale spallation. In contrast, decreasing the amount of sulfur in such deposits which contained exclusively Na as the alkali resulted in a significantly reduced corrosion attack compared to stoichiometric sodium sulfate.     

Effect of ethanol as gasoline additive on vehicle fuel delivery system corrosion

In this research, the effect of ethanol as gasoline additive has been investigated into metal corrosion of a fuel delivery system. Corrosion tests have been performed in gasoline with different percentages of ethanol, using weight loss (immersion test) and electrochemical impedance stereoscopy (EIS) procedures. Surface of test specimens were checked by scanning electron microscopy (SEM) after 144 days of immersion in test solution. Also corroded components were analyzed using energy‐dispersive X‐ray analysis (EDAX) method. Test results, investigations, and analyses, altogether show more corrosion with the increase in ethanol percentage and water content in gasoline. Test results show that among different materials in fuel delivery system, aluminum alloys and hard soldering alloys have less corrosion than the others. Also chloride and sulfide were recognized as the main compound of corrosion products; therefore, the control of these two elements in fuel delivery system is a must in case of using ethanol as fuel additive in near future.     

Synopses of papers from other institute journals

Abstract

The corrosion behaviour of a range of readily available commercial copper based alloys was examined in three accelerated atmospheric corrosion tests: acidified salt spray, neutral salt spray at 80°C, and the GM (salt spray with thermal cycling) test. The samples exposed to the GM test evinced very little corrosion. During exposure, the weight of all the samples fluctuated in a random fashion, indicating that the corrosion products were only partially adherent and became detached from the surface at random times. The sequence of corrosion product formation on the specimen surfaces during exposure to the neutral salt spray test was very similar to that observed on coppers exposed to the atmosphere, indicating that this test can be used to give accelerated indications concerning atmospheric corrosion in humid marine regions. Four 24 h cycles of salt spray exposure are approximately equivalent to 1 year's exposure to the atmosphere. Alloying copper with phosphorus, tin, zinc, or silicon does not improve atmospheric corrosion resistance. The alloy containing 30%Zn exhibited dezincijication. The nickel silvers exbibited corrosion rates in the neutral salt spray at 80°C that were lower by a factor of at least 13 than those of the high purity coppers. For six of the alloys studied, the as received surface condition correlated with a slightly, but statistically significantly, higher corrosion rate, attributed to residual contaminants from rolling.     

Hot corrosion of four superalloys: HA-188, S-57, IN-617, and TD-NiCrAl

Four superalloys were subjected to cyclic oxidation and hot corrosion attack in a Mach 0.3 burner rig. Two of the alloys, HA-188 and S-57, were cobalt-base and the other two, IN-617 and TD-NiCrAl, were nickel-base. The alloys were exposed to maximum temperatures of 900 and 1000°C. For hot corrosion tests the burner rig flame was doped with various concentrations of sea salt and one concentration of sodium sulfate. Samples were evaluated based on maximum depth of attack. These data were subjected to a regression analysis for the development of model equations relating corrosion to temperature and for determining how significantly changes in salt concentration and composition affected the corrosion. The ranking of the alloys with respect to their corrosion resistance was found to vary with temperature, sea salt concentration, and salt composition. Some of the variation was attributed to changes in the mode of attack from straight oxidation to hot corrosion. Under those test conditions where the mode of attack was readily discernible S-57 was found to be the most hot corrosion-resistant alloy and TD-NiCrAl was the least resistant. This order was reversed when the attack was straight oxidation, TD-NiCrAl being the most resistant alloy and S-57 the least.     

Hot corrosion of TBC-coated components upon combustion of low-sulfur fuels

Gas turbine reliability is a crucial requirement for passenger safety in aviation and a secure energy supply. Hence, corrosive degradation of combustor parts, vanes, and blades in gas turbines must be prevented. One of the most severe forms of corrosion is alkali-sulfate-induced hot corrosion, which is associated with internal sulfidation of components and is usually anticipated to fade in importance in the absence of sulfur. However, the literature suggests that hot corrosion might still occur in low-sulfur combustion gases. In this study, established thermodynamic modeling methods are used to analyze the low-sulfur hot corrosion regime. Liquid sodium chromate is found to be stable in these conditions. A comparison of calculation results and engine findings suggests that high alkali levels can negatively impact thermal barrier coating life even if sulfur is absent in the fuel. Laboratory tests are carried out to validate the chromate formation on MCrAlY-coated specimens. It is shown that molten sodium chromate can alter the oxidation behavior of MCrAlY, promoting the formation of voluminous spinel. This represents a new and different form of hot corrosion compared to type I hot corrosion.     

铸造镍基合金K-3的热腐蚀及防护研究

用光学显微镜、X 光衍射、扫描电镜、能谱分析和化学分析等多种手段,对经热腐蚀后的K-3合金试样及渗铝的 K-3合金涡轮叶片进行了分折研究。结果表明叶片失效的原因是硫化及熔盐引起的热腐蚀;低铬、富难熔金属的 K-3合金的热腐蚀机制是硫化、碱性熔解和酸性熔解的综合作用。用 RFL 燃气腐蚀装置对不同的保护涂层进行抗热腐蚀性能的对比试验表明:渗铝的防护效果并不理想,而富硅的扩散型和融烧型料浆铝硅涂层则具有良好的抗热腐蚀性能。     

The effect of fuel-to-air ratio on burner rig hot corrosion

Samples of a cobalt-base alloy, Mar M-509, were subjected to hot corrosion in a Mach 0.3 burner rig. The corrodent was NaCl added as an aqueous solution to the combustion products of a sulphur-containing Jet-A fuel. The metal temperature was fixed at 900°C. The extent of hot corrosion increased by a factor of three as the fuel-to-air mass ratio was increased from 0.033 to 0.050. Because the depositing salt was always Na₂SO₄, the increased attack appeared to be related to the gas composition.     

Korrosionsverhindernde Additive in ölbeheizten Kesseln

Corrosion-inhibiting additives in oil-fired boilers A really effective inhibitor must have the following properties: It must be free from metallic particles so that no additional deposits occur; it must promote combustion so as to keep the sulphur trioxide content of the combustion gases low even in the presence of a great quantity of excess air; it must be capable of mixing freely with the fuel oil, and must prevent the formation of vanadium pentoxide. These conditions appear to be fulfilled by a pyridine-based additive which has been investigated at the laboratory and under working conditions. The prevention of sulphur trioxide formation is likely to be due to the fact that, during combustion, the additive reacts with the vanadium compounds of the fuel oil, forming lower vanadium oxides. An incidental result obtained during tests under working conditions was a considerable reduction in the iron sulphate deposits. The test results point to two possible applications of this additive, vis. (a) in large boilers while the additional air is reduced, and (b) in small boilers, irrespective of the quantity of excess air.     

Heißkorrosionsverhalten von SiC und Si3N4im Brennergas

High temperature corrosion behaviour of SiC and Si₃N₄ in burner gas The behaviour of the materials has been studied in the following media: hot air (1400 °C), gaseous combustion products of CH₄, molten salts (Na₂SO₄, Na₂CO₃, NaCl) and molten vanadium pentoxide, combustion gases into which solutions of the above salts are injected (the specimens were heated to 1200 °C and then after a short cooling period sprayed with salt spray), combustion gases with addition of H₂S and HCl. In the salt melts SiC is rapidly destroyed under alkaline conditions while it is highly resistant to NaCl and V₂O₅. Material obtained by pyrolytic deposition from the gas phase is considerably more resistant then sintered material. The resistance is about the same for Si₃N₄. Generally the resistance is due to the formation of a vitreous layer on the ceramic surface. In the hot gases containing salt spray the materials are also highly resistant (super alloys tested under identical conditions are destroyed within a few hours while the ceramic materials show no trace of corrosion even after 200 hours). The influence of H₂S and HCl is very negligable. Materials having low porosity are superior to porous (e.g. sintered materials).     

Evaluation of the Corrosion Resistance of Fe–Al–Cr Alloys in Simulated Low NO x Environments

Due to their excellent corrosion resistance, iron aluminum alloys are currently being considered for use as weld claddings in fossil fuel fired power plants. The susceptibility to hydrogen cracking of these alloys at higher aluminum concentrations has led researchers to examine the effect of chromium additions on the corrosion resistance of lower aluminum alloys. In this work, three iron aluminum alloys were exposed to simulated coal combustion environments at 500 and 700 °C for short (100 h) and long (5000 h) isothermal durations. Scanning electron microscopy was used to analyze the corrosion products. All alloys exhibited excellent corrosion resistance during short term exposures. For longer test times, increasing the aluminum concentration improved alloy corrosion resistance. The addition of chromium to the binary iron aluminum alloy prevented the formation iron sulfide and resulted in slower corrosion kinetics. A general classification of the scales developed on these alloys is presented.     

Hot corrosion behaviour of plasma sprayed alumina + YSZ particle composite coating

Hot corrosion is one of the damage mechanisms in thermal barrier coatings (TBCs) due to the molten salt effects as a result of combustion of low quality fuel. In this study, the hot corrosion behaviour of alumina–yttria stabilized zirconia particle composite coatings produced by thermal spraying for use as a thermal barriers on industrial gas turbines and in jet engines was evaluated. Plasma sprayed coatings with three different amounts of alumina- yttria stabilized zirconia particle composite have been exposed to 50 wt % Na₂SO₄ + 50 wt % V₂O₅ corrosive molten salt temperatures at 1050°C for 60 hours. Damages in the coatings surface and cross section after hot corrosion tests have been studied by using a scanning electron microscope to observe the microstructure and x-ray diffraction techniques to analyze the phase composition. The results have shown that the amount of YVO₄ crystals on the surface of YSZ coatings decrease while Al₂O₃ increases in YSZ + Al₂O₃ composition, therefore, the hot corrosion resistance of TBC improves with the addition of Al₂O₃.     

Influence of climatic factors in cyclic accelerated corrosion test towards the development of a reliable and repeatable accelerated corrosion test for the automotive industry

Two designs of experiments made of nine accelerated tests each were used in order to study the influence of a selection of important climatic parameters such as the concentration of NaCl, the drying level, the basic humidity cycle, the frequency of salt spray and the temperature. The accelerated corrosion tests were all performed using automatic chambers. Different automotive materials were selected in order to study the resistance to cosmetic corrosion, perforation corrosion and bi‐metallic corrosion. For cosmetic materials, coated panels of cold rolled steel (CRS), zinc‐coated steel and aluminium alloys were chosen. Perforation corrosion was investigated using crevice panels of CRS, zinc‐coated steel or different grades of aluminium alloys. From the results, the influence of testing conditions on the cosmetic and perforation corrosion of different automotive materials was obtained. As an example, it was shown that an elevation of the temperature from 35 to 45 °C in the cyclic corrosion test increased the scribe creep on painted CRS and aluminium alloys while it has no significant effects on painted hot dip galvanized (HDG) panels. The results were also compared to that obtained after 2 years of exposure at a marine exposure site and on busses driving in area using de‐icing salt. It was shown that one of the tests performed gives a good correlation to field results.     

Environmental factors that determine hot corrosion in marine gas turbine rigs and engines

There exists some confusion in the relationship between hot corrosion experienced by alloys used in gas turbine engines and the same alloys exposed to laboratory and combustion orientated experimental rigs. This report attempts to explain why differing results are obtained. The major areas creating the discrepancies are the salt injection sytems, the nature of the combustion device used and the aerodynamic character of the flow around the alloy specimen.