PhD opportunities

Intergranular stress corrosion cracking model of nickel-base alloys exposed to confined areas of the secondary circuit of pressurized water reactors

Thesis proposal

Area of expertiseMaterials science and engineering
Doctoral SchoolSystems Engineering, Materials, Mechanics, Energy
SupervisorMme Ccilie DUHAMEL
Research unitCentre of materials
ContactDUHAMEL Ccilie
http://www.mat.mines-paristech.fr/Accueil/Propositions-de-theses/
Starting dateOctober 1st 2020
KeywordsStress corrosion cracking, nickel-base alloys
AbstractStress corrosion cracking (SCC) of steam generator (SG) tubes on the secondary side is one of the major causes of ageing of pressurized water reactors (PWR). Stress corrosion cracking leads to the initiation and then the propagation of intergranular cracks under the synergetic effects of a corrosive environment and a tensile stress state. In 2012, the first case of cracking in the secondary side of a SG tube made of Alloy 600 (nickel-base alloy) was reported in France after 177000h of service. Observations of tubes show that secondary corrosion occurs in confined areas where impurities of the secondary water can concentrate and lead to local aggressive chemical environments.

Analyses of the deposits associated to the observed degradations suggest that the environments that may lead to such degradations are varied: magnetite (Fe3O4) and compounds containing Si, Al, Ca, Zn, PO4, SO4, Ca and Cu. In addition, lead and sulfur are found in the deposits close to areas affected by SCC. Thus, several model environments were defined:
- caustic environment: species like sodium decrease the stress threshold for crack initiation and increase the crack growth rate for basic pH.
- acid environment: SCC is all the easiest than the pH is acid
- complex environment: this environment consists of secondary water with additions of solid compounds like deposits or sludge that can accumulate in the SG. Up to date, this environment was less studied due to the multiplicity of parameters that may influence secondary SCC. It will be the main environment studied during the PhD.

In the past, SCC initiation and propagation empirical models were developed. However, these models dont rely on any physical mechanisms. The prediction they offer is quite limited (extrapolation to in-service components is difficult) and their representativeness is questionable. A local model was recently developed to predict SCC rate of Alloy 600 exposed to primary water. This model is chaining the three main steps of the cracking scenario:
- Incubation: time to reach a critical oxidation depth. Incubation is thus modeled as the kinetics of embrittlement of a grain boundary. In the case of Alloy 600, this kinetics mostly depends on temperature, grain boundary coverage by chromium carbides and electrochemical potential
- Initiation: probability to reach the critical stress to fail a sufficiently oxidized grain boundary.
- Propagation: this step is described by a low and a rapid crack growth rate using a sigmoid function of the stress intensity factor K.
ProfilePrerequisite (specific skills for this thesis):
Materials Science, Chemistry
Strong experimental skills
To be admitted to a doctorate, the candidate must hold a national master's degree or another diploma conferring the master's degree (engineering diploma, equivalent foreign diploma, etc.), following a course of training establishing his research aptitude.
Applicants will be selected after interview before a jury assessing their research aptitude and the adequacy of their training and knowledge with the proposed thesis subject. Strong scientific knowledge, a first research experience (internship 4 months minimum), a strong motivation for research, a good level in English will be required (minimum B2).
The application must include:
a motivation letter including the candidate's professional project
a detailed curriculum vitae mentioning studies, research internships carried out, diplomas with date obtained, professional experience, research axes envisaged
a transcript of marks from the previous course (engineer and / or master cycle)
one or more letters of recommendation ...
a certificate of English level.
for a non-French speaking student, an A2 / B1 level in French (European standard) is recommended when entering a doctorate
for students who do not hold a national research masters diploma, all documents attesting to the students qualifications in research training (research internship certificates with summary of the dissertation, recommendations, publications, etc.)
a copy of identity card or passport

The documents in pdf format must be sent to recruitment_these@mat.mines-paristech.fr

ONLY THE APPLICATIONS ADDRESSED BY E-MAIL WILL BE TAKEN INTO ACCOUNT
FundingConvention CIFRE