Development of Miniaturized Specimen Technique for Evaluating Stress Corrosion Cracking Suspeptibility

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The small specimen test technique (SSTT) combined with electrochemical measurements for irradiated miniaturized specimens has been developed to evaluate the resistance to stress corrosion cracking (SCC) and corrosion of fusion reactor structural steels in water-coolant environment under irradiation. Two newly developed apparatus for a small punch (SP) testing and a slow-strain-rate test (SSRT) using miniaturized specimens in high temperature and high pressure aqueous solutions under applied potential conditions were successfully demonstrated for Type 304 austenitic and HT-9 ferritic stainless steels. Anodic polarization curves were measured and SP fracture energies of stress corrosion were obtained as a function of potential and strain rate. SP-SCC tests using small specimens showed that sensitized Type 304 steel had the susceptibility zones to SCC at the regions of corrosion potential and transpassive potential, whereas the solution-annealed Type 304 and HT-9 steels had the potential independence of SP energies without lowering by the stress corrosion. Surface films were examined by scanning electron microscope, Auger electron spectroscopy and laser Raman spectroscopy. There was the relationship between nature of surface films and SCC susceptibility. The strain rates smaller than 1.5x10^<-6> s^<-1> were necessary for a significant reduction of the elongation required for intergranular stress corrosion of sensitized Type 304 steel in the SSRT using miniaturized tensile specimens. The strain rate value that brought about the highest SCC susceptibility in sensitized Type 304 steel was larger than that of the popular SSRT using the uniaxial tensile specimens because of the stronger bulge deformation in SP tests. The SP-SCC test may therefore develop as a rapid and useful stress-corrosion testing method using small specimens in order to evaluate the SCC resistance of irradiated materials in high temperature water environments.