Optimization of the double loop potentiokinetic reactivation method (DL-EPR) for detecting sensitization of UNS N06690

摘要

UNS N06690 is one of the current choices for nuclear power plant steam generator tubing. The alloy intensity to some submodes of stress corrosion cracking is a function of sensitization. Sensitization results from chromium carbide precipitation at grain boundaries when the alloy is exposed to temperatures of 500 to 800°C, causing local chromium depletion in the adjacent zone. The objective of this work was the optimization of the double loop electrochemical potentiokinetic reactivation (DL-EPR) method for UNS N06690. Alloy specimens were tested under different heat treatments, tailored to obtain different concentrations of chromium in solid solution at the grain boundary region. DL-EPR was performed in solutions with different concentrations of sulfuric acid and potassium thiocyanate, at 30 and 50°C. The optimal condition for detecting sensitization of UNS N06690 was 0.5 M H_2SO_4 + 0.001 M KSCN, at 30°C. When tested in the optimal condition, UNS N06690 showed an activation peak while a reactivation peak appeared for the aged but not for the fully solubilized specimen. Optical micrographs confirmed an intergranular type of attack under aged conditions. The ratio of the reactivation peak to the activation peak was correlated with weight loss in boiling nitric acid solution with Cr(VI) additions.