The Open-Circuit Ennoblement of Alloy C-22 and Other Ni-Cr-Mo Alloys

摘要

The open-circuit corrosion and anodic oblation behavior of the C-series of Ni-Cr-Mo alloys (C-4, C-276, C-2000, and C-22) and alloy 625 have been studied ai 25 deg C and 75 deg C in 1.0 mol-L~(-1) NaCl + 1.0 mol-L~(-1) H_2SO_4. A combination of oven-circuit potential, potentiostatic polarization, and electrochemical impedance spectroscopy were employed in the study. The composition of the films formed was determined by x-ray photo-electron spectroscopy and time-of-flight secondary ion mass spectrometry. Passive oxide film resistances increase and defect oxide film concentrations decrease as films thicken and chromium and molybdenum segregate to the alloy/oxide and oxide/solution interfaces, respectively. The high-chromium alloys exhibit higher film resistances and lower film defect concentrations consistent with the more positive potentials observed on these alloys. The results show that the observed ennoblement in corrosion potentials with lime is coupled to the Cr/Mo segregation -process and the suppression of defect injection at the alloy/oxide interface. By all measures, C-22 exhibited the best passive properties.