The Duplex steel 1.4462 and the Ni based alloy Centralloy ET45 micro were tested in an HC1 containing atmosphere with a low oxygen pressure at 680 °C for 72 h. The composition of the gas atmosphere was 3.8 vol.% HC1, 1.9 vol.% CO_2, 0.3 vol.% CO, 2.8 vol.% H_2,0.02 vol.% H_2S, bal. N_2. The calculated oxygen partial pressure is 10 bar and the chlorine partial pressure is 10"13 bar. Thermodynamic considerations show that the phases Cr_2O_3, FeCl_2 and metallic Niare stable in this gas atmosphere at 680 °C. The tests were performed in a horizontal quartz glass tube, which was heated with a tube furnace. Samples of the alloys were ground up to 1000 grit SiC paper to get a reproducible surface. After cleaning by ultrasonic the samples were placed alongside each other in the glass tube. The largest surfaces of the samples were parallel to the gas flow. A gas flow of 120 ml/min was adjusted for 72 h. The corroded samples were investigated by metallography, SEM/EDX and XRD. After the experiments the samples showed a surface layer of corrosion products consisting of Cr and Ni oxides and sulphides. Beneath this surface layer Cr chloride was found. The Fe concentration was low since Fe had formed a volatile Fe chloride, which was deposited in the colder parts of the quartz glass tube. Moreover, both materials showed a selective attack of the existent phases. In case of the Duplex steel especially the ferrite was attacked and the austenite showed only faint corrosion. In the Ni based alloy C and Cr had formed chromium carbide precipitates, which were attacked selectively, while the base material remained quite stable. Due to its higher Ni content the Ni based alloy showed a lower weight loss than the Duplexsteel during the high temperature corrosion experiments.
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