Chemistry and melting characteristics of fireside deposits taken from boiler tubes in waste incinerators

Twenty-three tube deposits taken from seven heat-recovery boilers of municipal solid waste incinerators were examined by chemical analyses and X-ray diffraction. These deposits were measured by Differential Scanning Calorimeter (DSC) in N₂ to investigate their melting characteristics. Sixteen deposits were used to evaluate their corrosiveness to carbon steel by high-temperature corrosion test conducted at 400 °C for 20 h in 1500 ppm HCl – 300 ppm SO₂ – 7.5%O₂ – 7.5%CO₂ – 20%H₂O – N₂. Total heat of endothermic reactions of the deposits taking place between 200 and 400 °C can be related to the corrosion rate of carbon steel at 400 °C. Corrosion initiated at temperatures when the deposits started to melt, became severe when fused salt constituents increased, and alleviated when the majority of the deposits became fused. The corrosion can be interpreted as fused salt corrosion caused by chloride and sulfate salts.     

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.     

Recent plant experience and research into fireside corrosion in Japan

The increasing use of heavy oil as a fuel in power stations has given rise to intensified research in the field of fireside corrosion in Japan. The problems which have to be tackled are described in terms of experiences gained in the operation of oil and coal fired boilers. The main types of damage encountered are nonuniform wastage of tubes in connection with carburization. In this context significant interdependences have been found in connection with coal deposition (especially sodium and sulphur contents). On the basis of this experiences and of the results of corrosion tests in the laboratory and in the field it has been found that the measure offering the best prospect for corrosion control might be the use of coextruded tubing which would allow a combination of high corrosion resistance and high creep strength. In addition to that some modified alloys (e.g. Alloy 800 H, CR 30 A or NF709) offer an interesting potential whereby the high chromium content (more than 20%) is the main criterium.     

Stress corrosion cracking of copper in swollen bentonite simulating nuclear waste disposal environment

Stress corrosion cracking (SCC) of pure copper in bentonite clay was examined using a slow strain rate test (SSRT). Bentonite was swollen with pure water or aqueous solutions containing NH₃ of 5 and 10 mM. Thick corrosion films and particulate deposits were formed on the copper surface after the SSRT. Typical tarnish rupture-type SCC occurred on pure copper in swollen bentonite with and without NH₃. The crack propagation rate was enhanced by NH₃. It is confirmed that a thick oxide layer was formed on copper during plastic deformation, resulting in tarnish crack-type SCC. Many particulate deposits observed on the surface were formed due to the rapid dissolution of Cu²⁺ ions to form porous CuO at local deformed sites, regardless of the SCC occurrence.     

Experience of superheater tubes in municipal waste incineration plant

Two years after the start of operations at the Tokachi Association for Municipal Waste Disposal and Treatment Services municipal waste incineration plant (name: Kuririn Center), which generates 400°C steam, an investigation of the plant's superheater tubes was conducted. The investigation consisted primarily of analyzing ash deposits, measuring tube wall thickness loss and observing tube sections after removal. The concentration of Cl in the deposited ash was found to increase as the temperature of the gas rose, and the amount of deposited ash tended to increase as the temperature of the gas rose. Tube damage consisted primarily of a uniform loss in thickness. Intergranular corrosion and other local corrosion was not observed. The results of thickness‐loss measurements showed that, after approximately two years in operation, tubes (SUS310) lost a maximum of 0.45 mm in thickness due to high‐temperature corrosion in locations where the steam temperature reached 400°C. Superheater tubes were also apparently damaged from the effects of a soot blower, resulting in an observed maximum loss of 0.9 mm thickness after approximately two years. Furthermore, the rate of thickness loss tended to increase after one year had passed.     

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.     

Field test of superheater corrosion in a CFB waste boiler: Part II – Scale formation characteristics

This study concerns the scales formed on the steels T22 , Alloy 310, Alloy 28 and the nickel‐based Alloy 65 in a superheater test coil at 460–540°C in a CFB waste boiler. The methods used for the characterisation of the scales included SEM, EDX, Auger spectroscopy and XRD. The deposits on the tubes consisted mainly of alkali chlorides and calcium sulphate. The scales formed consisted of Fe₂O₃ and Fe₃O₄ on the T22 steel, NiFe₂O₃ and Cr₂O₃ on Alloy 310 and Alloy 28, and Cr₂O₃ and NiO on Alloy 65. Rapid corrosion on the steel T22 was associated with the growth of an open columnar iron oxide below a thick porous chlorine‐containing scale. Pitting corrosion on Alloy 310 occurred and it may be associated with selective corrosion, first following the grain boundaries then uniformly attacking the metal. The only protective oxide was observed on Alloy 28 that formed an inner chromium oxide separating the chlorides from the metal. Dense thin chromium oxides were observed in the scale on Alloy 28, but no major cracks were found perpendicular to the tube. Alloy 65 suffered from grain boundary attack and was locally attacked under thick porous chromium oxide with nickel chlorides in the advancing front. Molybdenum was enriched at the interface to the metal on both Alloy 28 and Alloy 65.     

A thermodynamically consistent modelling of stress corrosion tests in elasto-viscoplastic materials

Slow strain rate (SSR) and constant load (CL) tests are the most important techniques used to rank the susceptibility of different materials to stress corrosion cracking in a specific environment. The present paper proposes a thermodynamically consistent phenomenological framework to model both SSR and CL tests of austenitic steels in acid solutions at room temperature. This global one-dimensional approach does not require the computation of local stresses or strains. The goal is to discuss the thermodynamic aspects that allow proposing a one-dimensional model that combine mathematical simplicity with the capability of describing a complex non-linear mechanical behaviour.     

An introduction to fireside corrosion experience in the central electricity generating board

This paper briefly reviews CEGB fireside corrosion experience. It first summarises the numbers and operating conditions of the plants, and indicates the range of materials used in the boilers. Experience of furnace wall and superheater/reheater corrosion in coal fired plant is summarised, emphasising the need to “control” fireside corrosion rather than being able to eliminate it, and the importance of more corrosion resistant materials, particularly as coextruded tube. Tube life prediction techniques in use by the CEGB are outlined, and three current areas of research are described.     

Laboratory scale tests on corrosion behavior of boiler materials in simulated combustion atmospheres (EU Project – OPTICORR)

Laboratory scale tests were made in the Plant Simulation Test Laboratory (PSTL) at JRC IE Petten and at VTT Industrial Systems in Finland. The multi‐sample exposure tests were carried out under isothermal conditions at temperatures of 500 and 600 °C in N₂‐8% O₂‐15% H₂O, N₂‐8% O₂‐15% H₂O‐2000 vppm HCl and N₂‐8% O₂‐15% H₂O‐200 vppm SO₂ atmospheres. The experiments were focused mainly on common ferritic and austenitic steels such as X10, X20, 2.25Cr1Mo, AC66, Sanicro28, Esshette 1250 etc. A Scanning Electron Microscope (SEM), with Energy Dispersive Spectrometer (EDS), and X‐ray diffraction (XRD) techniques were used to determine the chemical and phase composition of the corrosion products. The obtained results show that the presence of SO₂ generally suppresses the oxidation rate of ferritic materials. Suppression of the oxidation rate in an SO₂ containing atmosphere could be due to the presence of sulphides at metal/scale interfaces , which probably influences the ion transport through the oxide scale. When the oxidation reaction is surface controlled, absorbed sulphates interfere with the reaction of the oxygen on the surface. The presence of HCl in moist air at temperatures of 500 °C and 600 °C accelerates the oxidation rate of the studied materials, especially for the ferritic steels. The SEM/EDS studies suggest that in HCl containing atmospheres the corrosion mechanism is „active oxidation”︁.     

Study of accelerated corrosion test method for 16MnR steel waste containers in backfill materials

ABSTRACT

To research the long-term corrosion of 16MnR steel radioactive waste container in backfill materials that are lightweight aggregate concrete (LWAC) and Keerjian bentonite, accelerated corrosion test in the backfill environment was studied. The corrosive products during accelerated tests were compared to the ones during simulated tests through morphology observation, component analysis by XRD and grey correlation quantitative evaluation. An approach of the accelerated corrosion test combining enhancing medium with dry-wet alternation was designed. In either LWAC and bentonite enviroment, the corrosion rates of accelerated test and simulated test are quite different, but their corrosion products, corrosion laws and corrosion development trend are similar. For the zinc coating, the correlation coefficients of the accelerated test in LWAC and bentonite was 0.672 and 0.734, respectively. Moreover, the accelerated speedup of the designed accelerated corrosion test is greater than 337. Therefore, the designed accelerated corrosion test method has satisfactory correlation and acceleration effect.     

High temperature corrosion mechanisms and effect of alloying elements for materials used in waste incineration environment

Corrosion products on two typical materials, SA213-T12 steel and alloy 625 exposed to the actual combustion gas, were analyzed in addition to laboratory tests for penetration of corrosive matter. It has been clarified that corrosion products of oxides containing a little chlorides and sulphides show lamellar structures and that at the alloy-scale interface, chlorination, sulphidation, and oxidation occur under a low P_O2-high P_Cl2 condition. The formation of scale structures and the effect of corrosion-resistant alloying elements can be explained according to the stability tendencies of metals, chlorides, and oxides in the M-Cl-O equilibrium diagrams. The severity of corrosion environments at the interface is influenced by the penetration extent of corrosive matters through deposits and scales, and the protective effects of oxide films derived from alloying elements play an important role in preventing the corrosion. On the other hand, it has been shown that thermal fluctuation characterized in this kind of environment makes the lamellar scale structures and sometimes breaks and peels off the scale, and thus accelerates the corrosion. On the basis of the above mentioned knowledge, a new corrosion model is presented.     

High temperature failures of heat resistant steel castings in waste incinerators

In waste incinerators, heat resistant cast steel grates and side wall plates are exposed to uncontrollable temperatures up to approx. 1000°C and also to various corrosion attacks. Considerable temperature gradients may generate cracks, elevated Cl-contents or S-contents will produce high loss of wall thicknesses. Since failures may result from very different causes, all-round recommendations as to material selection cannot be given.     

A time-dependent corrosion wastage model for seawater ballast tank structures of ships

For assessment of time-dependent reliability or risk of aging ship structures, it is essential to have a mathematical model which provides the statistical characteristics (mean, variance, distribution) of corrosion wastage as a function of time (i.e., ship age). The aim of the present work is to develop such a model for low alloy carbon steel plates used for the structure of seawater ballast tanks in ships. Measurement data of structural wastage due to corrosion for such ship steel plates is collected and the statistical characteristics of corrosion loss and rate are quantified by statistical analysis in terms of ship age. The results and insights developed in the present study will be useful for predicting the wastage of corrosion in seawater ballast tank structures of ships. They will also be useful for designing corrosion tolerant structures subjected to the seawater environment.