Intergranular corrosion mechanisms of alloy 600: Final report

Some sections of alloy 600 steam generator tubes were cut in order to perform various analyses on samples that had undergone Intergranular Attack (IGA) or Intergranular Stress Corrosion Cracking (IGSCC). Micrographs of polished cross sections were taken showing in some cases intergranular penetrations of more than 200 micrometers (16% of wall thickness). Electron microprobe analyses on these samples indicated a decrease in chromium, nickel and iron concentration at the corroded grain boundaries as well as the presence of oxygen, sulfur and sodium in the same areas. Direct examinations and analyses of corroded grain surfaces were also carried out by scanning electron microscopy (SEM) and Auger electron spectroscopy (AES). The SEM examination of these intergranular surfaces showed the presence of a thin layer partially covering the IGA and IGSCC grains. This layer has not been fully characterized but AES analyses indicated the presence of oxygen, silicon and sulfur. Autoclave tests were also carried out on alloy 600 heats in pure NaOH and in NaOH + Na2SO4 solutions at 3500C with an overpressure of hydrogen. The aim of these experiments was to clarify the influence of impurities (in the alloy and in the solution) on IGA processes. These experiments were performed using tube sections and coupons of commercial grade alloy 600, both stressed and unstressed, and U-bend for the high purity specimen. Chromium sesquioxide was also added to the testing solution in some cases. The results of these tests showed that in pure caustic solution IGA had occured only on the stressed samples, and was enhanced by the presence of sodium sulfate and chromium sesquioxide