Lead-Induced Stress Corrosion Cracking of Nickel-Based Alloy 690 (FULLY FUNDED)
Industrial Sponsor: Rolls-Royce plc
Nickel-base Alloy 690 and its weld metals have shown excellent resistance to stress corrosion cracking (SCC) in both laboratory experiments and civil nuclear reactor plants with over 20 years operating experience. This exceptional record has led to their consideration for application in future UK Pressurised Water Reactor (PWR) plant. However, despite this strong track record, work continues to quantify the longevity of these materials and the various composition, manufacturing and water chemistry factors influencing their susceptibility to SCC.
Lead-induced secondary side cracking of nickel-base PWR steam generator tubing alloys has been the focus of much attention because even part per trillion amount of lead in the high purity water, has been found to cause rapid transgranular cracking on Alloy 600 over a wide range of pH in civil nuclear reactor plants. Furthermore, the presence of lead has also been found to increase the SCC susceptibility of Alloy 690 at high pH in laboratory experiments, and may in fact be more susceptible than Alloy 600 in lead contaminated environments. At near-neutral and low pH, Alloy 690 may indeed be more resistant than Alloy 600.
Lead can react with other species to form an array of insoluble precipitates, thereby reducing the tendency to promote SCC. Critical species that react with lead to form insoluble compounds include for example, SO4, CO2, S, PO4, although the full range of species has not been fully identified. It is thought possible that these compounds may contribute to mitigating lead-induced SCC in real plant applications. Purification of the secondary system to eliminate impurities may therefore remove certain critical species that would normally have proved beneficial to reducing lead-induced SCC. However, the low solubility of Pb3(PO4)2, suggests that lead-induced SCC may not be a problem in plants operating with phosphate water chemistry, since this may eliminate the presence of soluble lead.
The aim of this project is to develop a more fundamental understanding of lead-induced SCC of Alloy 690 in plant relevant secondary chemistry environments. Initially a detailed literature review will be required on lead-induced SCC of Alloy 600 and Alloy 690 steam generator tubing. Experiments should then be performed to investigate the influence of lead and its compounds with respect to SCC susceptibility of Alloy 690, paying particular attention to possible combinations of pH and electrochemical potential which may arise in crevices under heat flux conditions for water chemistries potentially relevant to plant. In particular, the influence of phosphate water chemistry on lead-induced SCC should be examined. Various analytical techniques would be required to characterise crack tip oxides to develop mechanistic understanding.
Rolls-Royce is a global business providing power systems for use on land, at sea and in the air. The Group has a balanced business portfolio with leading positions in the civil and defence aerospace, marine and energy markets. We invest in technology and capabilities that can be applied to products and services in each of the global markets we serve. This project will be conducted with the Nuclear Materials, Chemistry & Corrosion department, Engineering Materials which supports the entire product lifecycle, from design and
manufacture to in-service support for both the Naval Nuclear Propulsion Plant and the Civil Nuclear business sectors.
Note: industrial placements at Rolls-Royce would only be available to UK nationals who were able to obtain security clearance.
This project is funded by EPSRC, the University of Manchester and our Industry partners. Funding is available to UK candidates. EU candidates are also eligible for certain projects. The successful candidates will have their fees paid in full and will receive an enhanced maintenance stipend.