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Optimization of the DL-EPR Method for Detecting Sensitization in Alloy 690

UNS N06690 is one of the current choices for nuclear power plant steam generator tubing. The objective of this work was the optimization of the double loop electrochemical potentiokinetic reactivation (DL-EPR) method for that alloy. Specimens were tested under different heat treatments.

Product Number: 51317--9037-SG
ISBN: 9037 2017 CP
Author: Mariano Kappes
Publication Date: 2017
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Alloy 690 is one of the current choices for nuclear power plant steam generator tubing. However several submodes of stress corrosion cracking have been identified for this alloy in the laboratory which eventually could manifest as failures in service. The intensity of some stress corrosion cracking (SCC) submodes in particular SCC in the presence of reduced sulfur species increases with sensitization. Sensitization is produced in some alloys after exposure to temperatures in the order of 500 to 800°C when chromium carbides precipitate at grain boundaries causing a local depletion of chromium in the surrounding zone. Some process used in steam generator fabrication like welding and stress relief treatment can result in sensitization of grain boundaries. Sensitization can be detected electrochemically as has been accomplished for austenitic stainless steels with the double loop electrochemical potentiokinetic reactivation (DL-EPR) method.The objective of this work was the optimization of the DL-EPR method for alloy 690. Alloy 690 samples were evaluated 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 50ºC. Results were analyzed to conclude which solution can best correlate with the 690 alloy grain boundary condition.

Key words: UNS N06690, sensitization, intergranular corrosion, DL-EPR, steam generator

Alloy 690 is one of the current choices for nuclear power plant steam generator tubing. However several submodes of stress corrosion cracking have been identified for this alloy in the laboratory which eventually could manifest as failures in service. The intensity of some stress corrosion cracking (SCC) submodes in particular SCC in the presence of reduced sulfur species increases with sensitization. Sensitization is produced in some alloys after exposure to temperatures in the order of 500 to 800°C when chromium carbides precipitate at grain boundaries causing a local depletion of chromium in the surrounding zone. Some process used in steam generator fabrication like welding and stress relief treatment can result in sensitization of grain boundaries. Sensitization can be detected electrochemically as has been accomplished for austenitic stainless steels with the double loop electrochemical potentiokinetic reactivation (DL-EPR) method.The objective of this work was the optimization of the DL-EPR method for alloy 690. Alloy 690 samples were evaluated 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 50ºC. Results were analyzed to conclude which solution can best correlate with the 690 alloy grain boundary condition.

Key words: UNS N06690, sensitization, intergranular corrosion, DL-EPR, steam generator

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