STRESS CORROSION CRACK GROWTH RATE TESTING AND ANALYTICAL ELECTRON MICROSCOPY OF ALLOY 600 AS A FUNCTION OF POURBAIX SPACE AND MICROSTRUCTURE

摘要

Stress corrosion crack (SCC) growth rate tests and analytical electron microscopy (AEM) studies were performed over a broad range of environments and heat treatments of Alloy 600. This effort was conducted to correlate bulk environmental conditions such as pH and electrochemical potential (EcP) with the morphology of the SCC crack. Development of a 'library' of AEM morphologies formed by SCC in different environments is an important step in identifying the conditions that lead to SCC in components. Additionally, AEM examination of stress corrosion cracks formed in different environments and microstructures lends insight into the mechanism(s) of stress corrosion cracking. Testing was conducted on compact tension specimens in three environments: a mildly acidic oxidizing environment containing sulfate ions, a caustic environment containing 10% NaOH, and hydrogenated near-neutral buffered water. Additionally, stress corrosion cracking testing of a smooth specimen was conducted in hydrogenated steam. The following heat treatments of Alloy 600 were examined: mill annealed at 980℃ (near-neutral water), mill annealed at 1010℃ (steam), sensitized (acid and caustic), and mill annealed + healed to homogenize the grain boundary Cr concentration (caustic). Stress corrosion crack growth rate (CGR) testing showed that sensitized Alloy 600 tested in the mildly acidic, oxidizing environment containing sulfate ions produced the fastest cracking ( ～8.8μm/hr at 260℃), and AEM examination revealed evidence of sulfur segregation to the crack tip, consistent with expectations for an aerated environment. The caustic environment produced slower cracking ( ～0.4μm/hr at 307℃) in the mill annealed + healed heat treatment but no observed cracking in the sensitized condition. In the caustic environment, fully oxidized carbides were present in the crack wake but not ahead of the crack tip. In near-neutral buffered water at 338℃, the CGR was a function of dissolved hydrogen in the water and exhibited a maximum (0.17μm/hr) near the transition between Ni and NiO stability. The cracks in near-neutral hydrogenated water exhibited Cr-rich spinels and NiO-type oxides but no significant oxidation of grain boundary (GB) carbides. No clear effect of dissolved hydrogen on the crack wake morphology was apparent. In hydrogenated steam testing of a smooth specimen (CGR estimated as ～0.7μm/hr at 399℃), metallic nickel nodules were evident in both the crack wake and on the specimen surface. Oxide particles having a similar size and shape to the microstructural carbides were found in the crack wake, suggesting that these particles are carbides that were oxidized by contact with the steam. The present results show that different environments often produce unique crack tip morphologies that can be identified via AEM. The AEM results are also evaluated for consistency with three candidate SCC mechanisms: hydrogen-assisted cracking, film-rupture-oxidation (I.e., slip-dissolution), and internal oxidation.