Yuuzou Kawahara

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 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.

Development and application of high-temperature corrosion-resistant materials and coatings for advanced waste-to-energy plants

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...

Corrosion Prevention of Waterwall Tube by Field Metal Spraying in Municipal Waste Incineration Plants

Abstract 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...

Evaluation of New Corrosion-Resistant Superheater Tubing in High-Efficiency Waste-to-Energy Plants

Abstract 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...

Role of molten phase content of deposits for high-temperature corrosion in waste incineration environment

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...

Development and Application of High Cr-High Si-Fe-Ni Alloys to High Efficiency Waste-To-Energy Boilers

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.

Advanced Coating Technologies for Aggressive Corrosion Environments in WTE and Power Plants

High durability, quality control and prediction of deterioration are key issues for the thermal spray coatings applied to fossil fuel and waste burning boilers. Small electrical resistance inspection (SERI) has been developed and applied for boilers to quantitatively evaluate the relationship between the deterioration mechanisms and the coating defects as well as life-times of coatings. Changes in the electrical resistance (ER) of coatings showed a good correlation with the deterioration causes and also with the bonding strength of the coatings. On the other hand, new processes such as thermal spraying and automatic fusing (SFU) process by high frequency induction heating have been developed to manufacture highly durable coatings. These SFU coatings, which are 0.5~3 mm in thickness and contain chemically bonded and dense NiCrSiB alloy layers have been found to have excellent durability which is almost equal to that of 625 weld overlays used in many WTE plants. Applications of these technologies are in progress to realize coatings with high cost performance, high efficiency and easy maintenance.