Effect of Cr and Mo contents on the corrosion properties of Ni-Cr-Mo alloys was investigated in the supercritical water including extraction from the fly ash of incinerated municipal solid waste at 873 K/25 MPa. Among the alloys of which Cr content was 25 mass% or less, the more Mo content, the greater the mass gain became. On the contrary, the alloys of which Cr was 30 mass% showed a smaller mass gain as those of the Mo content increased. The alloys of which Cr content was 35 mass% or greater showed their mass gains 1 mg/cm 2 or less for 168-hour exposure. It was concluded that the Ni-Cr-Mo alloys, in order to exhibit preferable corrosion resistance, should contain Cr at least 30 mass % under this environment.
To decompose harmful compounds such as dioxin and PCB, the supercritical water oxidation process is gaining attention ~. The reasons why this process is favored are" (1) it uses water, un- harmful to environment; (2) it decomposes the harmful compounds completely in a short period of time; and (3) no secondary contamination occurs due to the enclosed process. Despite of these benefits, decomposing such compounds generates inorganic acids such as hydrochloric acid or sulfuric acid as well as halogenides such as chloride or fluoride, and those creates very severe corrosive environment to the device structural materials.
In the previous paper 2, corrosion behavior of various industrial alloys was studied in the extracted solution from the fly ash of the municipal solid waste between 473 K/1.6 MPa and 873 K/25 MPa. From the experiments, the following findings were obtained: (1) Although the $31603 (Type 316L) and $32900 (Type 329) exhibited a small degree of corrosion, they showed crevice-corrosion and stress corrosion cracking; (2) Ti grade 1 exhibited general corrosion at 873 K/25 MPa, which was smaller than what the N06625 (alloy 625) exhibited, and caused no stress corrosion cracking; (3) Some of the Ni-base alloys also showed general corrosion at 873 K/25 MPa and stress corrosion cracking at 673K/1.6 MPa and 723 K/17 MPa; (4) The Ni-base alloys that contain Cr more than 30 mass% showed preferable resistance to both the general corrosion and stress corrosion cracking; and (5) The effect of Mo content to the corrosion resistance of Ni-(16 to 22)mass% Cr alloys remained unclear.
