Factors influencing stress corrosion cracking of the Incoloy-800 tubing of CANDU steam generator

Stress corrosion cracking is one of the major failure processes affecting the tubing of CANDU steam generator. This specific corrosion appeared as a result from the conjunction of strain stress and aggressive environment, which could affect the Incoloy 800 tubing on primary side (on the tube sheet in expansion transition region, at the support plates and in the U bent region) and on secondary side (in restrictive flow areas): - hydrogen charging in high pressure autoclave; - cathodic hydrogen charging test. The examination of materials tested has been performed by metallography. The results are presented as micrographs, on which one can distinguish the micro cracks initiation on tensioned cathodic hydrogen charged specimen. The results were: - Thermally treated samples presented intergranular dechromization which can affect the SCC behaviour of Incoloy 800 in hydrogenated water; - In SCC tests results suggest that hydrogen can enter the material only if a critical defect of film or a crack is previously initiated but after the crack initiation the hydrogen could play a major role in rapidly crack propagation; - Incoloy 800 was shown to be susceptible to cracking under cathodic hydrogen charging because under cathodic potential hydrogen absorption at the crack tip is higher than at the same potential obtained with a higher hydrogen overpressure; - The sensitized Incoloy 800 (700 deg. C/1 h) seems to be more susceptible at hydrogen induced SCC in primary water circuit conditions whereas the desensitized Incoloy 800 (700 deg. C/16 h) determined a better resistance at hydrogen induced SCC. The critical factor in stress corrosion cracking of nickel-based alloy seems to be the conjunction of hydrogen, stress and microstructure of material and not of each of them separately. (authors)