Corrosion problems and solutions in waste-fired superheater tube materials are described and their corrosion mechanism is discussed in terms of molton salt corrosion, especially the difference in their corrosion behaviors among existing boilers.
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 waste incineration environment. Laboratory corrosion tests were conducted in a simulated incinerator gas on 4 alloys. A crucible method was compared with a coating method at 550°C with use of synthetic deposits composed of NaCl/KCl/Na2SO4=1/1/1 mole containing various amounts of alumina, which was aimed at controlling molten phase content in the deposit (MPC). The tests were conducted also 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. In the coating method, a bell-shaped change was observed only in limited cases, likely because the deposit layer is so thin that it does not hinder gas permeation at any MPC. The crucible method is thought to be more appropriate for such a low MPC environment as is seen in waste incinerator, because it is more aggressive and less disturbed by changes in the condition of deposit during test.
Laboratory corrosion tests were performed for investigating influence of ash composition on corrosion behavior of four candidate materials for the superheater of waste incineration plants. The synthetic ashes and deposits on actual boiler tubes were used in the tests. It was found that the corrosivity of the ash depended not only on the contents of metal oxides, like PbO and CuO, but also on the content of alkaline metal chlorides. Because Alloy 625 showed an excellent corrosion resistance in a molten chloride, alloying with Ni seemed to enhance corrosion reisistivity against the chloride melt. On the other hand, in a mixture of molten chloride and sulfate, the metal loss of Alloy 625 was almost equal with that of SUS 310 S and SUS 347 H. From this result, alloying with Cr seemed to relatively improve corrosion resistance in the mixture melt.
In order to study the effects of alloying elements on corrosion resistance of high alloy steels in a simulated environment of waste incinerator, laboratory corrosion tests were performed on high alloy steels with various contents of Cr and Mo. It was found that the rate of general corrosion decreased with the increase of the Cr+Mo content and the rate of maximum penetration, which was the sum of intergranular penetration rate and general corrosion rate, decreased with the increase of the Mo content in this condition. The type of corrosion shifted from the intergranular penetration to the general corrosion over 5 mass%Mo. It was assumed that Mo in the alloys acted to restrict the sulfidation of Ni and to accelerate the formation of the protective Cr2O3 layer.
Fracture dynamics of the molten chloride attack of SUS 304 steel with residual tensile stresses was studied using two types of displacement-sensitive heat resistant sensors. AE source simulation of dissipative elastic wave revealed that the Mode-I and -II brittle fractures were associated with the molten chloride attack at 873K. Emission rate of AE signals due to the Mode-I fracture reached the maximum after 15ks exposure and then gradually decreased, while the increase in the AE due to the Mode-II fracture slightly continued until the later period. The fall-off of grains, typical attack morphology by the residual surface tensile stresses, was estimated to be brought by the Mode-I fracture along the grain boundaries perpendicular to the free surface, followed by the mode-II fracture along the boundaries parallel to the surface.