Superheater fireside corrosion mechanisms in MSWI plants: Lab‐scale study and on‐site results

Combustion of municipal waste generates highly corrosive gases (HCl, SO₂, NaCl, KCl, and heavy metals chlorides) and ashes containing alkaline chlorides and sulfates. Currently, corrosion phenomena are particularly observed on superheater's tubes. Corrosion rates depend mainly on installation design, operating conditions i.e., gas and steam temperature and velocity of the flue gas containing ashes. This paper presents the results obtained using an innovative laboratory‐scale corrosion unit, which simulates MSWI boilers conditions characterized by a temperature gradient at the metal tube in the presence of corrosive gases and ashes. The presented corrosion tests were realized on carbon steel at fixed metal temperature (400 °C). The influence of the flue gas temperature, synthetic ashes composition, and flue gas flow pattern were investigated. After corrosion test, cross sections of tube samples were characterized to evaluate thickness loss and estimate corrosion rate while the elements present in corrosion layers were analyzed. Corrosion tests were carried out twice in order to validate the accuracy and reproducibility of results. First results highlight the key role of molten phase related to the ash composition and flue gas temperature as well as the deposit morphology, related to the flue gas flow pattern, on the mechanisms and corrosion rates.     

Effect of Zinc Chloride in Ash in Oxidation Kinetics of Ni-Based and Fe-Based Alloys

Corrosion by molten phases leads to severe corrosion of heat exchangers in waste-to-energy plants. In addition, the presence of heavy metal chlorides in ash deposit increases degradation at low temperature due to the formation of highly corrosive molten phases. In this study, two heat exchanger materials, a low alloy steel (16Mo3) and a nickel based alloy (Inconel 625) were exposed in air to three different synthetic ashes, with various chloride contents, including ZnCl₂ at isothermal temperatures of 450 and 650 °C in a muffle furnace. After the test, thickness and mass losses were evaluated on two separate samples, and metallographic cross sections of the specimens were characterized via SEM/EDX analyses. Both measurement results were in good agreement and showed that the corrosion observed on both materials was higher in the presence of zinc chloride in ash at 450 °C than in ashes without heavy metal chloride at 650 °C.     

Korrosion in Rauchgasen bei 1000 °C

Corrosion in flue gas at 1000 °C An improvement in efficiency of coal combustion power plants can be achieved by the combined cycle process with gas and steam turbines. The effective power is also optimized if the energy generated in the boiler is discharged by utilizing a liquid sodium heat exchanger, working at a temperature of 1000 °C. In this case the heat exchanger materials are exposed to severe conditions. Liquid sodium at a temperature ranging up to 1000 °C flows through the heat exchanger tubes and the outer surface of the tubes is exposed to the flue gas. The materials are stressed mechanically and are also subjected to high temperature corrosion. Therefore high temperature corrosion tests were carried out with Fe-Ni-Cr- and Ni-base alloys in a laboratory furnace in which the alloys are exposed to the flue gas at a temperature of 1000 °C for a total period of 3000 h. The experiments have shown that most of the materials have a sufficient resistance to weight loss corrosion but in some cases severe internal oxidation occurs. This kind of corrosion is mainly observed for materials with high concentrations of elements such as aluminium, silicon etc., i.e. by the elements with very high negative values of free energy of formation of oxides.     

Korrosionsverhalten von Gasturbinenwerkstoffen unter strömenden Heißgasbedingungen

Corrosion behaviour of gas turbine alloys under high velocity burnt fuels The aim of alloy development in the field of nickel based superalloys for flying and land based gas turbines is to enhance significantly the mechanical properties at high temperatures thus leading to a higher temperature capability. The higher temperature capability of the structural elements of gas turbines results in an increased efficiency, a lowered fuel consumption and less emissions. To achieve an increased high temperature capability, however, surface degradation of the material must be adjusted adequately, hence corrosion resistance has to be improved. Additional to the isothermal and cyclic oxidation tests which are performed in stagnant air the oxidation behaviour of alloy 2100 GT and alloy C‐263 was investigated by means of burner‐rig‐experiments under high velocity burnt fuels. In the burner rig test facility the sample is exposed to a hot gas stream of burned natural gas with gas velocities in the range of 60 m/s to 150 m/s. The metal temperature of the sample can be adjusted in the range of 900°C to 1200°C. In the tests described in this paper the gas velocities were chosen to be 60 m/s, 100 m/s and 140 m/s. The test duration was 1 h and 10 h. The test temperature was kept constant at 1000°C. After 1 h of testing both alloys showed mass gain which was significantly higher for alloy C‐263. After 10 h of testing the mass loss of alloy C‐263 was enhanced with increasing gas velocity. Alloy 2100 GT showed only at the highest gas velocity a mass loss. The examinations by means of SEM and light‐optical microscopy of the oxide scale and of the microstructure showed that alloy 2100 GT has a dense adherent alumina scale and suffers no internal oxidation even under burner‐rig‐test conditions. Alloy C‐263 forms a mixed chromia and Cr‐Ti‐mixed oxide scale. The chromia is evaporated with increasing gas velocity, leaving (Cr‐Ti)O₂‐needles on the surface. In the isothermal and cyclic oxidation tests alloy 2100 GT shows an excellent oxidation behaviour up to 1200°C with a corrosion rate of less than 0.1 mm/a. The aluminium content of app. 3 wt.‐% which is remarkably high for a wrought alloy leads to the formation of a thin dense and adherent alumina scale. Alloy C‐263 is a chromia former which is not suitable for temperatures higher than 1000°C.     

Field test corrosion experiments in Denmark with biomass fuels. Part 2: Co‐firing of straw and coal

In Denmark, straw is used for generating energy in power plants. However during straw combustion, potassium chloride and SO₂ are released in the flue gas and through condensation and deposition processes they will result in the formation of superheater deposits rich in potassium chloride and potassium sulphate. These components give rise to varying degrees of accelerated corrosion. This paper concerns co‐firing of straw with coal to reduce the corrosion rate from straw to an acceptable level. A field investigation at Midtkraft Studstrup suspension‐fired power plant in Denmark has been undertaken where coal has been co‐fired with 10% straw and 20% straw (% energy basis) for up to approx. 3000 hours. Two types of exposure were undertaken to investigate corrosion: a) the exposure of metal rings on water/air cooled probes, and b) the exposure of a range of materials built into the existing supertheaters. A range of austenitic and ferritic steels was exposed in the steam temperature range of 520–580°C. The flue gas temperature ranged from 925–1100°C. The rate of corrosion was assessed by precision measurement of material loss and measurement of oxide thickness. Corrosion rates are lower than for 100% straw‐firing. The corrosion products and course of corrosion for the various steel types were investigated using light optical and scanning electron microscopy. Catastrophic corrosion due to potassium chloride was not observed. Instead a more modest corrosion rate due to potassium sulphate rich deposits was observed. Corrosion mechanisms include sulphidation, oxidation and hot corrosion.     

Low temperature corrosion in domestic refuse boilers monitored with cooled corrosion probes and test heat exchangers

Corrosion properties of materials for utilizing of flue gas energy by cooling the gas below the acid dew point have been studied with cooled corrosion probes. The U-tube corrosion probe is found to be a reliable method where tests can be done in the actual plant environment. Results from one domestic refuse boiler plant with severe low temperature corrosion problems are reported and the results described indicating a minimum temperature of 75°C to avoid corrosion. Another set of results is reported with flue gas cleaning and cooling in an experimental heat exchanger. Unalloyed steel is found to show the same level of corrosion in both raw uncleaned and cleaned flue gas. AISI 316 and 254 SMO stainless steels have shown high corrosion rates in the raw gas but a satisfactory resistance in the cleaned gas.     

Steam oxidation and its potential effects on creep strength of power station materials

The oxidation behaviour of several commercial alloys in simulated steam was investigated and compared with that of model alloys with systematic variations of selected alloying elements in the temperature range of 550°C to 650°C. Also, creep tests of specimens pre‐oxidized in steam or in flue gas were carried out in order to study the interaction between creep and corrosion. The corrosion products were characterised by optical microscopy, XRD, SEM/EDX and Raman spectroscopy. It was found that the oxidation resistance of the materials increased with increasing chromium content. High chromium materials exhibited the best oxidation behaviour, whereas the low chromium materials formed thick, multilayered oxide scales, prone to spallation. Anomalous temperature dependences were found in materials with intermediate chromium contents. Creep tests showed a significant reduction in the creep strength of both alloys was caused by a 1000 h thermal exposure treatment at 650°C for P92 and at 800°C for Alloy 800. An additional, though much smaller reduction in strength was observed for the specimens that had been oxidized for 1000 h at 650 or 800°C prior to testing. Further testing is required for confirmation of this effect.     

Fireside Corrosion of Superheater Materials in Coal/Biomass Co-fired Advanced Power Plants

A series of laboratory-based fireside corrosion exposures were conducted to assess the effect of such conditions on superheater/reheater materials at higher than conventional metal temperatures. Controlled atmosphere furnaces combined with the “deposit recoat” test method were used to generate the exposure conditions; the gaseous environment simulated that anticipated from air-firing 20 wt% cereal co-product mixed with a UK coal. The exposures were carried out at 600, 650 and 700 °C with four candidate materials: T92, HR3C and 347HFG steels; nickel-based alloy 625. After the exposures, the samples were examined by SEM/EDX to characterize the damage. Pre- and post-exposure dimensional metrology were used to quantify the metal damage in terms of metal loss distributions. For the austenitic steels, the combined deposit/gas/temperature exposure conditions enabled quantification of the characteristic ‘bell-shaped’ curves (of damage as a function of temperature) for fireside corrosion.     

Korrosion von ölgefeuerten Zentralheizungskesseln

Corrosion of oil-fired domestic boilers Depending on the surface temperature of the flue gas side the corrosion of oil fired domestic boilers proceeds either mainly by acid corrosion or by oxygen corrosion:

• – At surface temperatures of 60°C and higher the corrosion mechanism of acid corrosion prevails and the corrosion rates amount to 0.1–0.3 mm/year (values referred to continuous burner operation). The corrosion products consist of soluble iron(II)- and iron(III)sulfates. Higher corrosion rates can be attributed to an appreciable catalytic formation of sulfur trioxide on the corrosion products formed on the convective heating surfaces.
• – At surface temperatures of 40°C the mechanism of oxygen corrosion already dominates and the corrosion rates are about ten times higher (1.5–3 mm/year, referred to continous burner operation). The high portion of ioron oxide hydrates, especially goethit (α-FeOOH), makes the corrosion products difficult to remove.
• – Distinctly reduced service lives are also expected for the so called reduced temperature boilers (“Niedertemperaturkessel”) and low temperature boiers (“Tieftemperaturkessel”): According to the manufacturers these boilers may be operated at boiler water temperatures well below 60°C, as they are equipped with constructive measures to enhance the surface temperature on the flue gas side. However, these measures are only fully effective under stationary conditions.

Some of the results were obtained from weight loss measurements on test specimen made from St 35.8 and gray cast iron, that were exposed to the flue gases of an oil fired experimental boiler. Other important results come from field measurements of the sulfuric acid content of about 30 boilers that are in practical use.     

High temperature oxidation of 9–12% Cr ferritic/martensitic steels under dual-environment conditions

ABSTRACT

In normal operations, the opposite surfaces of the power plant components are exposed to two different environments, i.e. air/flue gas on the one side and steam on the other side. Exposure under such dual-environment can lead to accelerated corrosion of the components on the air side. The oxidation behaviour of ferritic/martensitic steel T92 was investigated under dual-environment in a specially designed test equipment. The samples were exposed to dry oxyfuel flue gas (CO₂–27%N₂–2%O₂–1%SO₂) on one side and to steam on the other side up to 1000?h at 650°C. The formation of oxide scales was characterised by optical microscopy and scanning electron microscopy with attached energy-dispersive spectroscopy. Oxidation rate of specimens under dual-environment condition was almost three times higher than that in single-environment condition. This is explained based on hydrogen transport through the bulk alloy from the steam side to the flue gas side.

This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).     

Fireside corrosion of superheater materials in chlorine containing flue gas

Corrosion resistance of three types of candidate materials for superheater sections under simulated waste incineration conditions was evaluated. A 9Cr1Mo steel, an AISI 310SS, and the Ni-based alloy Sanicro 28 were tested on a laboratory and on a pilot scale with different flue gas compositions (up to 2500 mg/Nm³ of HCl and 1500 mg/Nm³ of fly ash). Laboratory tests were carried out in a furnace up to 200 h. Metal and gas temperature were kept constant at 500 °C. Pilot scale tests were carried out by using a 0.3 × 0.3 m cross-sectional combustor, with flue gas velocity of 5 m/s. Air-cooled probes, designed to operate at a metal temperature of 500 °C and facing gas temperatures as high as 600 °C, were used for 200 h as maximum test time. Qualitative correspondence was found between results obtained by the two sets of experimental tests, but quantitative values were not comparable. Metallographic evaluations, metal loss measurements, and weight loss analysis evidenced as the most suitable alloy Sanicro28. Maximum metal loss observed was 240, 182, and 107 μm, respectively, for 9Cr1Mo, AISI310SS, and Sanicro 28 under the most aggressive conditions. Intergranular corrosion attack was evidenced for AISI310SS, limiting the choice of materials to 9Cr1Mo and Sanicro 28, depending upon the lifetime expected at the design stage.     

Influence of sintering on the corrosion behavior of a Ti‐6Al‐4V alloy

This work studies the influence of the sintering conditions of a Ti‐6Al‐4V alloy on its corrosion performance. The alloy was vacuum sintered in different conditions of time and temperature. The density and microstructure (designating phase distribution) are evaluated. Corrosion resistance through electrochemical techniques (EIS) in 2 N and 6 N hydrochloric acid solutions, and oxidation resistance at 900, 1000 and 1100°C are appraised, and corrosion is studied by microstructural and X‐ray diffraction.     

Field test corrosion experiments in Denmark with biomass fuels. Part 1: Straw‐firing

In Denmark, straw and other types of biomass are used for generating energy in power plants. Straw has the advantage that it is a “carbon dioxide neutral fuel” and therefore environmentally acceptable. Straw combustion is associated with corrosion problems which are not encountered in coal‐fired plants. The type of corrosion attack can be directly ascribed to the composition of the deposit and the metal surface temperature. A series of field tests have been undertaken in the various straw‐fired power plants in Denmark, namely Masnedø, Rudkøbing and Ensted. Three types of exposure were undertaken to investigate corrosion: a) the exposure of metal rings on water/air cooled probes, b) the exposure of test tubes in a test superheater, and c) the exposure of test tubes in existing superheaters. Thus both austenitic steels and ferritic steels were exposed in the steam temperature range of 450–600°C. The corrosion rates were assessed by precision measurements of material loss and internal corrosion. The corrosion products and course of corrosion for the various steel types were investigated using light optical and scanning electron microscopy. Corrosion mechanisms are discussed in relation to temperature and deposit composition.     

Korrosionsreaktionen sauerstoffempfindlicher Metalle in flüssigem Natrium mit Oxidgehalten. I. Reaktionen von Zirkonium und Zircaloy-2

Corrosion reactions of metals sensitive to oxygen in liquid sodium containing oxide. I. Reactions of Zirconium and Zircaloy-2 The corrosion and oxidation behaviour of pure zirconium and of zircaloy-2 has been examined in a sodium circuit with forced sodium circulation. At temperatures ranging from 500 to 600°C, the pure metal as well as the alloy are oxidized, forming external oxide films and internal brittle zones. At a constant oxygen content of about 5 ppm, the reactions are governed by a parabolic law. The constants of the oxidation rate depend on temperature and correspond to an activation energy of 40 kcal/Mol in the case of zirconium. The zirconium alloys have a high resistance in pure sodium and lose virtually no material to the following sodium. In sodium with oxygen content, the corrosion rate up to 550°C is acceptably low. As far as the Getter effect of zirconium is concerned, the formation of external oxide films has an inhibiting effect. However, measurments of the decrease of the oxygen in the sodium showed that, due to the reaction with zirconium at 650°Ca technically useful Getter effect can be obtained.     

Field test of superheater corrosion in a CFB waste boiler: Part I – Metal loss characteristics

The corrosion was investigated on a superheater test coil in a CFB waste boiler. The alloys ranged from ferritic steel T22 to nickel‐based Alloy 65 and the metal temperatures were between 460 and 540°C. The thickness of the deposit was alloy and temperature dependent. The low‐alloyed steels developed thick deposits at all temperatures while the deposit thickness increased with the temperature on the high‐alloyed steels and the nickel‐based alloy. The corrosion attack was alloy dependent and related to the deposit crest. The nickel‐based Alloy 65 was preferentially attacked directly under the crest of the deposit while the other alloys were preferentially attacked at the edge. The corrosion rate increased with temperature for X20, Alloy 304L, Alloy 310 and Alloy 825; decreased on Alloy 65; and was bell shaped on T22 and Alloy 28. Alloy 310 suffered from severe pitting corrosion in a line following the edge of the deposit crest. The best overall corrosion resistant alloy was Alloy 28.     

High temperature corrosion studies in molten salt-FLiNaK

FLiNaK (LiF–NaF–KF: 46·5–11·5–42 mol.-%) is a promising candidate as high temperature coolant for next generation nuclear reactor coolants. Major technical challenge in implementation is the selection of structural material that can withstand corrosive nature of FLiNaK. Corrosion study of different alloys such as SS-316L, SS-317L, Inconel-625, Incoloy-800H, Hastelloy-B and Ni-201 was performed in FLiNaK salt. The tests were carried out in the temperature range of 650–850°C in graphite crucibles for 60 h under inert atmosphere. Weight loss of the alloy sample due to corrosion was measured, and corrosion rate was estimated. The salt composition after corrosion was analysed by inductively coupled plasma spectroscopy coupled with atomic emission spectroscopy. The surface morphology of the alloy samples was analysed by scanning electron microscopy. Ni-201 and Hastelloy-B showed better resistance to corrosion than other alloys. Corrosion rate was found to increase with increase in Cr content in the alloy.     

Sodium vanadate induced corrosion of MCrAlY coatings – Burner rig studies

The corrosion of several MCrAlY-coatings (M = Ni, Fe and Ni + Co) has been studied in a high velocity burner rig at 650, 800 and 950°C. The fuel used was diesel oil with additions of 3% sulphur, 200 ppm vanadium and 100 ppm sodium. The deposits formed on the specimens mainly consisted of sodium vanadates which were molten at the test temperature. Sodium sulphate was only found at and below 650°C. The corrosion mechanism involved was vanadate-induced hot corrosion. This corrosion mechanism is characterized by the formation of an oxide layer adjacent to the metal, the dissolution of oxide in the molten deposit, and precipitation of vanadates or oxides near the outer surface of the deposit. The continuous deposition of fresh vanadate on the surface served to maintain high corrosion rates for extended exposures.     

High-temperature corrosion behaviors of typical nickel alloy coatings in a simulated boiler coal ash/gas environment in the Zhundong region

The high-temperature corrosion behaviors of five nickel alloy coatings used in coal-fired boilers in the Zhundong region (Xinjiang province) were investigated in simulated coal ash and coal-combusted flue gas environment at 650°C for 250 and 500 hr. The samples were analyzed by weight gain experiment, X-ray diffraction test, and scanning electron microscopy technique with energy-dispersive spectroscopy. The results indicated that the corrosion level is in the order of NiCrMo13 ≈ Hastelloy C-276 (276) > NiCrBSi > Inconel 718 (718) > 45CT. The compositions of the corrosion scale in five nickel alloy coatings mainly consist of NiO, Ni₃S₂, and Cr₂O₃. The enrichment of Cr in the corrosion scale in 45CT, 718, and NiCrBSi coatings inhibits the formation of oxide and sulfide on the coating surface. The presence of W and Mo in nickel alloy coatings accelerates the formation of corrosion products, thus weakening the corrosion resistance of NiCrMo13 and 276 in simulated coal ash and coal-combusted flue gas environment.     

Effect of experimental conditions on the metal dusting phenomenon in several commercial nickel‐base alloys

The corrosion phenomenon, metal dusting, has been observed in many high‐temperature industrial plants, such as ammonia, syngas and steam‐reforming industries. An experimental research programme has been carried out into the degradation resistance of wrought and cast commercial high‐temperature nickel‐base alloys in H₂/CO gas mixtures at 650 °C. The results in this paper are focused on the effect of the experimental set‐up on the response of the alloy during exposure to the gas mixture; in particular, the specimen holders had an important role on the onset of carbon transfer from the gas to the alloy surfaces. Specimens of most alloys exposed to the gas mixture while suspended from quartz hangers were shown to have good resistance to the environment for periods up to 5000 h. On the other hand, specimens of these alloys, exposed while located in porous refractory ceramic holders, gained large amounts of coke and underwent severe damage. A series of short tests were carried out in order to ascertain the main factors influencing the alloy reactivities in the experimental rig. The presence of specimens of a less‐resistant alloy and slight variations in gas composition (with the addition of CO₂) were observed to have a marked impact on the alloy reactivities. Factors such as catalysts, contaminants and gas composition were considered and are discussed.     

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.