THE RELATIONSHIP BETWEEN THE CHARACTERISTICS OF OXIDE FILM AND STRESS-CORROSION SUSCEPTIBILITY OF NI-CR-FE ALLOY IN HIGH-TEMPERATURE WATER

The effect of environmental factors on stress corrosion cracking (SCC) for NiCrFe (Inconel alloy 600) alloy has been studied in deaerated water at 350°C by using the constant extension rate test (CERT) and U-bend test methods. SCC occurred in acidic and alkaline solutions. Especially, NiCrFe alloy suffered severe SCC, intergranular corrosion (IGC), and intergranular attack (IGA) in alkaline solution. On the other hand, SCC did not occur in neutral solution. The results due to Auger electron spectroscopy (AES) analysis showed that the oxide film of the NiCrFe alloy was Cr-enriched and was thin in acidic solution. As the pH of test solution increased, the Ni content of oxide film increased and the Cr content decreased. The oxide film of the NiCrFe alloy was thick and mainly composed of Ni in alkaline solution. The potential-pH diagrams for Ni-H2O and Cr-H2O systems and the results of corrosion potential measurement indicated that the oxide film of the NiCrFe alloy was stable in neutral water. On the other hand, the oxide films were unstable in acidic and alkaline solutions, respectively. According to these results, the relationship between the characteristics of oxide film and SCC susceptibility is discussed. © 1990.