In pressurised water reactors (PWRs), hydrogen overpressure is used to keep the corrosion potentialbelow the threshold for onset of intergranular stress corrosion cracking (IGSCC) in type 304 SS. However, some regions may contain higher oxygen levels resulting in an increase in the potential. These "dead space" regions are difficult to access and during refuelling; oxygen may become trapped in these locations. The objective of this study was to investigate the influence of PGM additions on IGSCC susceptibility of type 304 stainless steels (SS) in the sensitised state within PWRs.The work presented herein investigates several aspects of the IGSCC problem. Virgin and platinum group metal (PGM)-modified (Ru and Pd) 304 SS have been studied. Material characterisation, including microstructural, tensile properties, hardness and grain size measurements, has been conducted. Crack initiation studies using U-bend samples in autoclaves simulating PWR environments have also been performed. In addition, crack propagation studies using circumferential cracked bar (CCB) specimens under constant extension in potassium tetrathionate solutions, a well-known medium to promote IGSCC on sensitised stainless steels, have been conducted in order to evaluate cracking resistance. Electrochemical studies using model solutions for PWR chemistry (containing boric acid and lithium hydroxide) and also potassium tetrathionate were carried out to look at the influence of the PGM on the kinetics of the main electrochemical reactions.The results revealed that PGM additions appeared to reduce crack initiation on sensitised type 304 SS under oxygenated conditions in high temperature water containing sulphate and chloride. PGM-doped and standard sensitised type 304 stainless steels revealed susceptibility to IGSCC propagation in 0.01 M K2S4O6, at pH=1.5 and 25°C. Electrochemical studies in potassium tetrathionate media showed smaller anodic dissolution peaks with PGM additions and metallography indicated less intergranular attack with PGM additions. In PWR model electrolytes, PGM additions, particularly 1 wt% Ru, were shown to catalyse the oxygen reduction reaction or hydrogen oxidation reaction, depending on the oxygen /hydrogen level. Overall findings showed that Ru additions can improve the IGSCC resistance of sensitised type 304 SS in PWR, while Pd additions are less effective.
|Date of Award||31 Dec 2010|
- The University of Manchester
|Supervisor||Stuart Lyon (Supervisor) & Andrew Sherry (Supervisor)|
Effect of Platinum Group Metal (PGM) additions on the stress corrosion cracking resistance of type 304 stainless steel in pressurised water reactors
Necib Ammour, O. (Author). 31 Dec 2010
Student thesis: Phd