High-Temperature Corrosion Behavior of SUS310 Steels Embedded in Alkali Metal Chloride Salt Mixtures in Air
Authors: Hayashi Shigenari , Umehara Kenta , Furugaki Takashi , Tatsumi Keiji , Takahashi Hiromitsu , Shibata Kiyoshi 
Published: December 2020

Higher steam temperature is one of the crucial requirements to increase the efficiency of waste-to-energy (WTE) plants. However, current steam temperature in the WTE plants, up to 450°C, is much lower than that in fossil-fuel power plants which are commonly used up to 650°C. Superheater tubes used in waste incineration plants are exposed in extremely corrosive environments, which contains Cl2, SO2, O2, H2O, CO2 etc. The combustion ash containing not only oxides but also chloride salts and sulfate accelerates the corrosion of alloys and coatings. This is one of the main reasons hindering increase of steam temperature in WTE plants. High-temperature corrosion in such harsh environments are so complicated, and it strongly depends on the plants because waste components varies in different areas. In order to understand a fundamental aspect of the corrosion behavior of heat resistant steels, the corrosion behavior of commercial steels as well as austenitic Fe-20Ni-25Cr based model alloys under an embedding in the simulated ashes at different temperatures lower than the melting temperature of the ashes was investigated.
The commercial SUS310STB (base composition Fe-20Ni-25Cr-1Si), QSX5(Fe-20Ni-25Cr-1Mo-3Si), and model alloys of Fe-20Ni-25Cr with and without 2Mo or 2Si (in wt%) were used in this study. The specimens were embedded in the simulated ashes, which composed of CaO-4.5NaCl-4.5wt%KCl or CaO-3.1NaCl-3.96KCl-2.94CaCl2, in an Al2O3 crucible. Corrosion tests were conducted by a box furnace in air for up to 100 h at temperatures ranging from 400 to 580°C.
Fig. 1 shows the corrosion mass loss of steels and model alloys as a function of corrosion temperature. In this figure, the results in the ash obtained from the actual plant are also plotted for comparison. Both of simulated ashes are less corrosive comparing to the actual ash and the corrosion mass loss of SUS310STB was much higher than that of QSX5. Based on the corrosion behavior of model alloys, higher corrosion resistance of QSX5 can be attributed to higher Si content in the alloy.
The corrosion mass loss in the simulated ashes was low up to about 480°C and it stated to increase at 520°C in both of the simulated ashes, however, it was much lower in CaO-NaCl-KCl than in CaO-NaCl-KCl-CaCl2. The corrosion mass loss in CaO-NaCl-KCl was apparently increased at 580°C, which indicates that the presence of CaCl2 in ash accelerates corrosion. When steels were corroded at 580°C, the corrosion mass loss in CaO-NaCl-KCl-CaCl2 was significantly increased because the applied temperature is higher than the melting point of the ash.
Cross-sectional observation of the steels after corrosion test at 580°C in CaO-NaCl-KCl revealed that the alloys formed a sponge-like very porous Fe-oxide scale. Above the sponge-like Fe-oxide scale, a thick CaCrO4 layer was developed. Moreover, K2CrO4 was identified in the corrosion products from EPMA and XRD analysis, suggesting that KCl reacted with a Cr2O3 scale, which resulted in a breakdown of the Cr2O3 scale and generation of chlorine. Because CaCrO4 is considered to be formed by a reaction of Cr-chloride vapor and CaO in the ash, higher evaporation of Cr-chloride might increase the metal consumption at higher temperatures. In the CaCl2 containing ash, the chlorine potential in the atmosphere increases due to oxidation of CaCl2, which might result in further increase in corrosion mass loss even though steels were exposed at the temperatures below melting temperature of the salt.

Figure 1