High-Temperature Corrosion Behaviors of Structural Materials for Lead-Alloy-Cooled Fast Reactor Application

The corrosion of nuclear-grade steels in lead–bismuth eutectic (LBE) complicates the realization of high coolant temperatures. Corrosion tests of T91, HT9, and SS316L were performed in static cells at 600 °C for 2000 h at an oxygen level of 10−6 wt.%. The obtained corrosion surfaces of post-processed samples were characterized by several microscopy methods. Up to 1000 h, all the alloys exhibited an evolution of duplex oxide layers, which were spalled until 2000 h due to their increased thickness and decreased integrity. Following the spallation, a thin internal Cr-rich oxide layer was formed above the Cr-depleted zone for T91 and HT9. SS316L was penetrated by LBE down to 300 μm in severe cases. A comparison on the corrosion depths of the materials with regard to the parabolic oxidation law with abundant literature data suggests that it may lose its validity once the duplex layer is destroyed as it allows LBE to penetrate the metal substrate.

Effect of Atmosphere on Duplex Oxide Formed on 9Cr Steel at 550 °C

Commercial F91 steel was exposed to atmospheric pressure CO2 and laboratory air at 550 °C for exposure times up to 1000 h. In both atmospheres, a Fe-rich duplex oxide scale formed, but with different morphology, oxide phases and growth rates. In CO2, the duplex morphology was observed at the onset of oxidation and it was found that the cooling rate affect the oxide phases formed on the outer scale. In air, the alloy initially formed a protective chromium rich oxide layer, followed by the nucleation and growth of duplex iron-rich oxide nodules at random locations, leading to breakaway oxidation. Alloy carburization was also observed in CO2 but not in air environment.

Application of Raman Spectroscopy for Characterizing Synthetic Non-Metallic Inclusions Consisting of Calcium Sulphide and Oxides

The presence of non-metallic inclusions (NMI) such as sulphides and oxides may be detrimental to the control of the steel casting process and product quality. The need for their identification and characterization is, therefore, urgent. This study uses time-gated Raman spectroscopy for the characterization of synthetic duplex oxide-sulphide phases that contain CaS and the oxide phases of Al2O3, CA, C12A7, C3A, and MgO·Al2O3 (MA). Binary phase samples of CaS–MA, C3A–CaS, C12A7–CaS, Al2O3–CaS, and MA–CaS were prepared with varying phase contents. The relative intensities of the Raman peaks were used to estimate the samples’ phase content. For a quantitative estimation, linear regression calibration models were used to evaluate the change in phase content in the samples. The most suitable Raman peak ratios had mean absolute error (MAE) values ranging from 3 to 7 wt. % for the external validation error, and coefficients of determination (R2) values between 0.94 and 0.98. This study demonstrated the use of Raman spectroscopy for the characterization of the calcium sulphide, magnesium aluminate spinel, Al2O3, and calcium aluminate phases of CA, C3A, and C12A7 in a duplex oxide-sulphide system, and it offers potential for inclusion characterization in steel.

Kinetics of oxide scale growth on a (Ti, Mo)5Si3 based oxidation resistant Mo-Ti-Si alloy at 900-1300∘C

High-temperature oxidation behaviour of Mo-40Ti-30Si (at.%) alloy was investigated in the temperature regime of 900-1300∘C in air. Isothermal weight change data recorded up to 100 h of exposure revealed parabolic weight gain kinetics at all the tested temperatures. The protective oxide scale composed with SiO2 (silica) and TiO2 (titania) forming a duplex oxide microstructure consisting of TiO2 particles embedded in the continuous SiO2 matrix. The oxide scale showed parabolic growth kinetics, and the activation energies for the scale growth were found to be 72.2 kJ/mol in 900-1200∘C and 324.9 kJ/mol in 1200-1300∘C. The kinetics of the protective scale growth on the alloy surface was mainly controlled by the growth of the silica film and the inward diffusion of oxygen through the duplex oxide layer.

Porosity Developments within 9Cr-1Mo Steel Exposed to CO2

When 9Cr-1Mo steel is exposed to CO2-rich advance gas-cooled reactor (AGR) gases it forms a duplex oxide which consists of an outer Fe rich layer and an inner Cr rich spinel which provides oxidation resistance allowing the steel to resist the corrosive atmosphere of the plant. The oxide scale develops, growing both into the substrate and outwards from the initial metal/air interface. The spinel develops porosity through the coalescence of Fe vacancies which over time alters the properties of the oxide and potentially allows a transport network to form within the oxide. The porosity of the duplex oxide was measured using scanning electron microscopy of oxides on 9Cr steel samples oxidised in a CO2 atmosphere. Results show that samples which have suffered breakaway oxidation show larger oxide scales with alternating Fe/Cr bands whereas samples which have yet to suffer from breakaway show higher peak porosity values but thinner oxide scales. Furthermore the samples which are currently under protective oxidation show a high max porosity peak in comparison to those which have suffered breakaway.

Factors Affecting the Oxidation and Carburisation Behaviour of an Austenitic Stainless Steel Used in the UK Advanced Gas-Cooled Reactors

A significant number of stainless steel components within the boilers of the UK advanced gas-cooled reactor (AGR) plants are subjected to oxidation, carburisation and other changes in the underlying microstructure of the material during operation. This results from exposure to the pressurised CO2-based primary circuit coolant at temperatures from about 500 to 650°C. It is believed that there is a synergistic relationship between the pressurised CO2 coolant environment and creep-fatigue initiation and cracking. Devising and implementing an evaluation methodology to account for oxidation and carburisation to enable conservative lifetime assessments is essential to current and future plant safety. Therefore, the development of a new and fundamental understanding of environmentally assisted degradation and failure mechanisms is required. It has been postulated that the mechanism underlying the initiation of cracks is carburisation associated with the presence of a duplex oxide layer. In this study, the material-environment interaction for Type 316H stainless steel under simulated AGR conditions has been investigated to increase the understanding of the combined effects of stress, strain and surface preparation, for example, on oxidation and cracking behavior. Experimental data are presented which show that surface deformation promotes the formation of a thin, protective oxide scale, which does not protrude along the grain boundaries, whereas a deformation-free surface leads to the formation of thick duplex oxide layers as well as intergranular oxide penetration. Furthermore, an increased surface hardness due to carburisation has been observed for the undeformed surface only, suggesting that carburisation occurs at an early stage on a chemically treated surface. It is found that when the substrate is plastically deformed and under the effect of active stress, the thin oxide on the mechanically deformed surface can be disrupted, resulting in similar behaviour to a chemically treated surface with no deformation.