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Localized corrosion (LC) sometimes appears in boiler tubes exposed in severely corrosive combustion gases including Cl, S and O, but prediction and on-site maintenance of LC is actually difficult in many cases. Therefore understanding the cause of LC is considered to be important to maintain safety services and estimate lifetime in high temperature facilities. This LC occurs due to formation of partially different corrosive conditions due to fuels, condition of combustion, designing of boilers, and materials. In this paper, investigation results of macroscopic LC and influenced factors formed in actual boilers were described with the four cases of fossil fuel, biomass and WTE boilers, and also described against any coatings. Mechanisms of each LC were considered individually by using cut-off samples of damaged tubes, detailed analyses of corrosive environments and results of laboratory tests. From these considerations, protective oxide layers and not only static factors but also dynamic factors arising in operated conditions such as temperature and composition fluctuations were concluded to be important for the accurate lifetime evaluations of tube materials and coatings.

Corrosion-resistant materials (CRMs) and coatings are key technologies to increase power generation efficiency and reduce maintenance in waste-to-energy (WTE) plants. Corrosion environment became severe as steam temperatures have increased. The steam condition of more than 400 °C/3.9 MPa became possible in WTE boilers by using highly durable corrosion-resistant coatings, such as thermal spray of Al/80Ni20Cr alloy, HVOF-sprayed NiCrSiB alloy, Alloy 625 weld overlay for waterwall tubes and also superheater tubes. Also, the use of 310S type stainless steels and high Cr-high Mo-Ni base and high Si-Cr-Ni-Fe alloys have progressed because of a better understanding of corrosion mechanisms. Furthermore, high durability coatings using cermet and ceramic materials were applied to high temperature superheaters. This paper describes the major developments and the application of CRMs and coating technologies in the last 30 years in WTE plants, the corrosion mechanisms of alloys, the deterioration mechanisms of spray coating layers, and future subjects for the development of corrosion-resistant materials and coatings.

In order to improve the durability of high efficiency waste-to-energy boilers, it is essential to develop and apply high temperature corrosion-resistant materials having a long life time according to the intensity of the corrosion conditions. Two types of high Cr-high Si-Fe-Ni base and high Cr-high Si-Ni-Fe base alloy seamless tubes; MAC-N (26Cr-3.5Si-11Fe-Ni base) alloy and MAC-F (23Cr-3.8Si-38Ni-Fe base) alloy respectively which contain no expensive Mo and have better corrosion resistance than the existing alloys under severe corrosive environments of waste combustion gas, have developed. The optimum alloy composition range were defermined by confirming the effect of the alloying elements on the corrosion resistance in the laboratory corrosion tests. Furthermore, the seamless tubes were mounted on the actual superheater of the 500oC/9.8MPa high efficiency waste-to-energy boiler to examine the durability over a period of four years. As a result, it was confirmed that the MAC-N and MAC-F alloys have better corrosion resistance than the Alloy625 and 310HCbN. In addition, it has become clear that corrosion resistance of both alloys is displayed through the formation of SiO2 rich protective scale due to the combined addition of the principal elements, Si, Cr, Fe and Ni. Under conditions of high Cl content molten deposits and severe thermal cycle, the corrosion rate increases as the results of deterioration of protective oxidation scale.

高温腐食による材料の消耗, 廃棄は社会的な損失であり, エネルギー有効利用, 公害防止などの面でも腐食の阻止が重要と考えられる. さらに, 多くの機械に囲まれて生活している現代人にとって腐食の防止は安全性の確保と密接な係わりがある. 高温腐食はエネルギー設備, 化学反応装置, 公害防止機器, 輸送機械など生活を支える基盤設備において問題となり, 環境側と材料側の色々な要因が影響して発生する複雑な現象のため難解な面が多いが, 本紙では高温腐食へ入門する人達向けに高温腐食の単一反応, 複合腐食, 溶融塩腐食およびボイラ, 石炭ガス化プラントなどの実装置における高温腐食と腐食防止などについて出来るだけ簡潔に記述した.

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 PO2–high PCl2 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.

Field corrosion tests were conducted on eight single tube materials and two welded overlay materials in three typical Japanese waste incineration plants in an effort to develop new corrosi...

Fireside corrosion factors in high-temperature corrosion of waterwall tubes in waste incineration boilers were investigated. Factors accelerating corrosion were considered to be heavy metal (zinc and lead) chlorides, unburned carbon in deposits, and reduction of the gas atmosphere formed by the increase of unburned gas components and the decrease of oxygen (O2). Laboratory tests confirmed that increasing these factors increased the corrosion rate of carbon steel (CS). These tendencies were in good agreement with those in actual furnaces. A gas spray coating of two-layer Al/80% Ni-20% Cr, selected after laboratory corrosion tests, was applied to waterwall tubes at the test site. The Al/80% Ni-20% Cr spray coating maintained excellent durability for > 3 years, and its effectiveness in corrosion prevention was confirmed. This coating system has been applied at seven plants. Main deterioration factors affecting the coating layer were considered to be formation of corrosion scale and a decrease in bon...

Recently, the high-efficient thermal recycle technology of waste has become an important subject all over the world. In order to complete a highly-efficient waste burning boiler that produces the steam level of 500°C/100ata, the development and application of corrosion-resistant materials for use in a severely corrosive environment are key-technology, combining with corrosion prevention methods on designing and engineering of plants. The characteristics of high-temperature corrosion factors and corrosion behavior in waterwall and superheater tubes of waste-to-energy boiler are described first in this report. Then, the current development and application of corrosion-resistant boiler tubings and high-temperature coatings used for this kind of severe corrosive condition are summarized. Many kinds of alloys containing effective elements for corrosion tubings, and also applied by the coating processes of weld overlay and metal spraying. Furthermore, investigations of corrosion mechanism and laboratory evaluta...

The effect of the physical properties of boiler tube deposits on high-temperature corrosion of heat-resistant alloys was quantitatively examined to clarify the corrosion mechanism and to establish the most appropriate laboratory test method for the waste incineration environment. Laboratory corrosion tests were conducted in a simulated incinerator gas on four alloys. A crucible method was compared with a coating method at 550°C using synthetic deposits composed of NaCl/KCl/Na2SO4 = 1/1/1 mole containing various amounts of alumina, that was intended to control molten phase content in the deposits (MPC). The tests were conducted with deposits from two working plants. These deposits were found to be low in MPC. In the crucible method, a bell-shaped change of corrosion mass loss was observed for all the alloys. The change is thought to be due to a competitive effect of increase in MPC. The increase in MPC accelerates and depresses the corrosion because the molten phase is aggressive but hinders gas-permeation...

Nowadays, a promotion of high level recycling of waste and reduction of environmental load such as CO 2 and dioxins are required worldwide for waste treatment. The concepts of higher efficiency waste-to-energy (WTE) plants are positioned in the center of thermal recycling. A high total cost performance is required for those plants in the storm surrounding the society, such as the globalization of economy. The high-efficiency WTE boilers with steam conditions of more than 400 °C/3.9 MPa have been progressed recently. Also, the fluidized bed boilers burning industrial wastes such as many kinds of biomass fuels, RDF, RPF, etc., have been operated in 450 to 540 °C and 5.9 to 9.8 MPa.

Highly efficient electric power generation from biomass/waste fuels becomes an important worldwide issue to prevent global warming. In these plants, severe high-temperature corrosion and erosion-corrosion damage occur in boiler tubes influenced by HCl, SOx gases, and chlorides as contaminants in fuels. Coating technologies become important as a countermeasure for such damage, because of the easy maintenance, cost performance, and ease of application on various materials. In severe corrosive conditions of boilers, formation of dense, homogenous, and tough coating layers, as well as protective oxide layers of corrosion-resistant materials, are important. In the last 30 years, materials and coating processes applied in shop and on site have progressed based on many field observations and the consideration of deterioration mechanisms in order to maintain long lifetimes in the plants. Furthermore, new innovative coatings are now being developed by using advanced precise control, nanotechnologies, etc. This paper introduces recent trends of advanced coating developments and applications, such as weld-overlay, cladding, thermal spray coating, and slurry coating for biomass/waste boilers. Furthermore, the evaluation results of deterioration mechanisms and lifetime of coatings, and the future issue for innovative coatings, are presented.