Role of chromium in anomalous behaviour of the passive layer in Ni-Cr-Mo alloys in 1M HCl solution

This work seeks to understand the underlying mechanism involved in passivity of Ni-Cr-Mo alloys in a less concentrated HCl solution (1M) by systematically varying contents of Cr and Mo solutes in model Ni-Cr-Mo alloys. Corrosion behaviour was evaluated based on potentiodynamic polarisation tests carried out in conjunction with electrochemical impedance and x-ray photoelectron spectroscopies of passive films that formed on alloys during their exposure to the HCl solution. Results have shown that an increase in Mo alone is not sufficient to improve the corrosion resistance of the alloys at lower concentrations of HCl. Optimum concentrations of Cr and Mo solutes have been found to be in the vicinity of ~17 wt.% Cr and ~19 wt.% Mo for superior corrosion resistance of the alloys. This was attributed to the protection of the Cr2O3 layer as a consequence of the enrichment of Mo6+ ions in the passive film in 1M HCl solution.

Behaviors of Perfluorocarbon Emulsion during Dissolution of Oxide Layers

This study investigates the dissolution behavior of oxide layers containing radionuclides using perfluorocarbon (PFC) emulsion as a reusable medium. Chemicals such as PFC, anionic surfactant, and H2SO4 are used for preparing the PFC emulsion, and emulsified using an ultrasonication process. The FTIR results show O–H stretching that is formed by the interaction of the carboxyl group of the anionic surfactant with the hydroxyl group of water containing H2SO4, and find that the H2SO4 can be homogeneously dispersed in the PFC–anionic surfactant–H2SO4 emulsion. The dissolution test of the simulated Cr2O3 specimen is conducted using PFC emulsion containing KMnO4. Through the weight losses of specimens and Scanning Electron Microscope-Energy Dispersive X-ray Spectrometer (SEM-EDS) analysis, it is confirmed that the Cr2O3 layer on the SUS304 specimen is easily dissolved using PFC emulsion. During the dissolution of the Cr2O3, it is observed that the dispersed H2SO4–KMnO4 became unstable and separated from PFC emulsion. Based on these results, the behavior of the PFC emulsion during the dissolution of the oxide layer is explained.

Corrosion of Fe-(9~37) wt.%Cr Alloys at 700-800 oC in (N2, H2O, H2S)-Mixed Gas

Fe-(9, 19, 28, 37)wt.% Cr alloys were corroded at 700 and 800 oC for 70 h under 1 atm of N2, 1 atm of N2/3.2%H2O-mixed gas, and 1 atm of N2/3.1%H2O/2.42%H2S-mixed gas. The corrosion rate of Fe-9Cr alloy increased with the addition of H2O, and furthermore with the addition of H2S in N2 gas. Fe-9Cr alloy was always nonprotective. In contrast, Fe-(19, 28, 37) wt.%Cr alloys were protective in N2 and N2/H2O-mixed gas because of formation of the Cr2O3 layer. They were, however, nonprotective in N2/H2O/H2S-mixed gas because sulfidation dominated to form the outer FeS layer and the inner Cr2S3 layer containing some FeCr2S4.

Creep Behavior for Alloy 617 in Air at 950°C

Creep behavior for Alloy 617, which is considered as one of the major structural materials of a very high temperature reactor, was investigated in air at 950oC. Creep experimental data was obtained by a series of creep tests with different stress levels at 950oC. Alloy 617 revealed little primary creep strains and unclear secondary creep stages. A tertiary creep stage was initiated from a low strain level and was dominant in full creep curves. The creep constants of A, n, m, and C in Norton’s power law and Monkman-Grant relationships were determined. In microstructure observations of crept specimens, it was found that a Cr2O3 oxidation layer was formed on the surface, and just beneath the Cr2O3 layer, an internal Al-oxide sub layer was formed with rod shapes. Also, below the internal sub layer, a thick carbide-depleted zone was developed due to reaction of the chromia and carbide precipitates. The thickness of the outer Cr-oxide layer increased with increasing creep rupture times. The increasing tendency showed a smooth slope like a parabolic curve.