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

Abstract

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. In the coating method, a bell-shaped change was observed only in limited cases, probably 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 a low MPC environment as seen in the waste incinerator, because it is more aggressive and less disturbed by changes in the condition of deposit during tests.