Article By Joanne McIntyre
“The mission of JAEA is to conduct R&D in the field of nuclear energy, to open the way to the future of nuclear power, and to contribute to the welfare of human society,” explains Dr. Aoki. “I work within a research group for radiation materials engineering, which is part of the nuclear science and engineering centre at JAEA. We have been engaged in both experimental studies and simulation analyses to clarify degradation mechanisms of nuclear structural materials.”
Since he joined JAEA five years ago, Dr. Aoki has been involved in a research project for the stress corrosion cracking (SCC) mechanism of low carbon stainless steels used in light water reactor environments.
“As well as the research project for SCC mechanisms, I am also carrying out a study into the crevice corrosion mechanism of duplex stainless steels. I started this study theme when I was a master degree student and have carried on since then…it has become a lifetime study!”
Dr. Aoki went on to outline the objectives of his two main research projects.
Low carbon stainless SCC research
The objective of the research into the stress corrosion cracking (SCC) mechanism of low carbon stainless steels aims to clarify SCC initiation susceptibility and corrosion behaviour of the grain boundaries of long-term thermal aged low carbon austenitic stainless steels in simulated Boiling Water Reactor (BWR) conditions, explains Dr. Aoki. “The SCC initiation susceptibility of Type 316L is clearly increased by the combination of the cold work and the long-term thermal ageing, compared to Type 304L. Evaluation of the changes in microchemistry, microstructure and mechanical properties induced by the cold work and the long-term thermal ageing treatment is in progress to understand their correlation with the SCC initiation susceptibility.”
Crevice corrosion in duplex research
Dr. Aoki has also researched crevice corrosion mechanisms in duplex stainless steels. “The objective of this study is to elucidate crevice corrosion behaviour accompanied by the preferential dissolution and its mechanism on duplex stainless steel (DSS), based on the in-situ observation of crevice corrosion dissolution behaviour, and the analysis of dissolution behaviour of DSS, a and ? phases in a simulated crevice solution. Evaluation of the differences in crevice corrosion initiation and propagation behaviour between potentiostatic and galvanostatic conditions is in progress to examine and consider the suitability of accelerative measurements.”
In the nuclear industry, duplex stainless steels are most commonly used in pumps or valves in the primary loop recirculation (PLR) system and the primary coolant system.
The research carried out by Dr. Aoki and the researchers at JAEA focusing on austenitic stainless steels (such as grades 304L or 316L), duplex stainless steels (such as 2205), and lean duplex stainless steels are of relevance and interest far beyond the nuclear industry.
“My research results have been published as academic papers or proceedings of international conferences both in Japan and abroad,” explains Dr. Aoki. “The experiments are usually carried out in a sodium chloride aqueous solution simulating a seawater environment, so they are potentially interesting for the desalination industry, for seawater applications (i.e. a sea water pump, a chemical tanker), and the oil and gas industry.”
“We make efforts to explore collaborative framework with plant makers, heavy industry, government, and academia. Furthermore JAEA’s regularly participates in academic conferences and meeting both inside and outside Japan to present our research results there and learn about development trends (see box Stainless Steel World comes to Japan in 2020!).
Looking to the future
When asked what development he would most like to see for corrosion resistant alloys in the future, Dr. Aoki replies: “New applications in corrosive environments are emerging in pioneering technical fields. I would like to see the development of highly reliable ‘tailor-made’ stainless steel to manufacture components that these applications will need in future.”
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