Niobium-stabilised austenitic stainless steel (20Cr-25Ni-Nb) has been immersed in sodium hydroxide, which is used as a corrosion inhibitor. The work shows how NbC precipitates may be degraded by use of pH 13 NaOH. Initial electrochemical measurements indicate that there is no benefit to this pretreatment as regards long-term corrosion inhibition, and post corrosion imaging shows the initiation of pitting corrosion at Nb-rich precipitates still present in the microstructure.
This article describes the radiation facilities and associated sample preparation, management, and analysis equipment currently in place at the Dalton Cumbrian Facility, a facility which opened in 2011 to support the UK’s nuclear industry. Examples of measurements performed using these facilities are presented to illustrate their versatility and the breadth of research they make possible. Results are presented from research which furthers our understanding of radiation damage to polymeric materials, radiolytic yield of gaseous products in situations relevant to nuclear materials, radiation chemistry in light water reactor cooling systems, material chemistry relevant to immobilization of nuclear waste, and radiation-induced corrosion of fuel cladding elements. Applications of radiation chemistry relevant to health care are also described. Research concerning the mechanisms of radioprotection by dietary carotenoids is reported. An ongoing open-labware project to develop a suite of modular sample handling components suited to radiation research is described, as is the development of a new neutron source able to provide directional beams of neutrons.
Advanced gas-cooled reactor (AGR) oxide fuels used in the UK are clad in bespoke grade 20%Cr-25%Ni-Nb austenitic stainless steel. Electrochemistry was first applied to correlate the breakdown potential with chloride ion concentration, temperature and pH for this alloy. At near-neutral pH the unsensitized material exhibited a linear E<sub>b</sub> = A + B log10[Cl<sup>-</sup>] relationship, where A = 0.7 V (vs. SCE), and B = 0.098 V/decade. Scanning Kelvin probe force microscopy revealed grain boundary regions in the heat-treated material up to 65 mV less noble to the matrix, whereas un-dissolved niobium carbide (NbC) precipitates were up to 55 mV more noble to the matrix. In-situ time-lapse microscopy and post-corrosion observations confirmed that sensitized grain boundaries were susceptible to pitting corrosion, further developing along intergranular corrosion pathways. It has however been shown that micro galvanic coupling between the Nb precipitates and matrix and / or sensitized grain boundary regions is not a factor in corrosion initiation as all experiments were performed under external potential control. Post corrosion observations showed the presence of pits at NbC precipitates promoting grain boundary corrosion. It is postulated that corrosion initiates at NbC precipitates as a pit, and when in close vicinity to Cr-depleted grain boundaries, then propagates along grain boundaries as intergranular corrosion.
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