Localized Electrochemical Corrosion of Nickel-Based Alloys. Final Report

摘要

The technique of monitoring open-circuit potential over time to study pitting corrosion of Alloy 600 was demonstrated at 95 deg C. Chloride ion and oxygen levels were varied to determine the conditions required for pit initiation and propagation at 25 deg C and 95 deg C. Without applied potential pitting was not observed at 25 deg C in solutions of up to 2.6 M NaCl. At 95 sup 0 C pit initiation occurred above 0.22 M NaCl for a nitrogen sparged system, 0.042 M NaCl for a contaminated air sparged system and 0.059 M for an O sub 2 sparged system. At 95 sup C initiation followed by propagation was observed at 0.22 M, 0.12 M and 0.11 M NaCl for the N sub 2 , air and O sub 2 sparged systems, respectively. A theoretical model, using a hemispherical pit geometry and transport based on the Nernst-Einstein equation, was developed to predict changes in ion concentration, current and pit size. For a pit with an initial radius of 100 A, a fixed potential difference of 0.5 V and constant ionic diffusivities on the order of 10 sup -5 , cm sup 2 /sec, the model predicts that the solution within the pit will become saturated with metal chloride within 2 x 10 sup -7 seconds. The current density increases exponentially with time and reaches a maximum value of 7.2 x 10 sup 4 A/cm sup 2 at the point of saturation. (ERA citation 12:000737)