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A Treatise on Corrosion Science, Engineering and TechnologyElectrochemical Impedance Studies on Corrosion of Stellite#6 in Permanganate-Based Decontamination Formulations

A Treatise on Corrosion Science, Engineering and Technology: Electrochemical Impedance Studies on... [Stellite is one of the hardest materials known and so it finds application in places where superior wear and corrosion resistance is required. It is used in components such as valves and pumps that are present in primary, moderator, and auxiliary circuits of many of the water-cooled nuclear reactors. These cobalt-rich alloys are the major contributors for particulate cobalt activity which promotes radioactive hotspots. In some of the low-temperature coolant circuits, metallic particles or colloids of eroded stellite material can cause increase in radiation dose. Extensive studies have shown that it is possible to dissolve these metallic particles by employing a redox process using a permanganate-based formulation as oxidizing agent and a mixture of EDTA, ascorbic acid, and citric acid (in the weight ratio 4:3:3) as reducing formulation. As stellite particles are mostly in metallic state, attempt was made to study its dissolution behavior using electrochemical techniques that measure the corrosion tendency of materials. The corrosion behavior of stellite alloys dictates their susceptibility for dissolution in any chemical reagent. Studies were done on one of the most commonly used stellite, namely stellite #6. Its corrosion behavior was studied in acidified permanganates such as permanganic acid (HMnO4), nitric acid permanganate (NP, a mixture of nitric acid and potassium permanganate) and alkaline permanganate (AP). Electrochemical impedance studies were carried out at 90 °C in NP, HMnO4, and AP. The redox potential of all the oxidizing agents favored transpassive dissolution of chromium from the alloy. In NP, only the chromium-depleted interphase boundary was attacked while most of the chromium-rich carbide phases were intact. On the contrary in HMnO4, uniform corrosion of the surface was observed. The nitrate ions in permanganate were found to promote the repassivation of the surface. In AP only, the carbide phases were attacked. As these carbides are lean in cobalt, AP was found to be least efficient in oxidizing Cr and dissolving cobalt from stellite. Among the three reagents, HMnO4 was found to be the best formulation for dissolving cobalt from the alloy and hence for reducing radioactivity due to stellite.] http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png

A Treatise on Corrosion Science, Engineering and TechnologyElectrochemical Impedance Studies on Corrosion of Stellite#6 in Permanganate-Based Decontamination Formulations

Part of the Indian Institute of Metals Series Book Series
Editors: Kamachi Mudali, U.; Subba Rao, Toleti; Ningshen, S.; G. Pillai, Radhakrishna; P. George, Rani; Sridhar, T. M.
Subramanian, Veena; Chandran, Sinu; Narasimhan, S. V.
A Treatise on Corrosion Science, Engineering and Technology — May 5, 2022
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Publisher
Springer Nature Singapore
Copyright
© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022
ISBN
978-981-16-9301-4
Pages
113 –124
DOI
10.1007/978-981-16-9302-1_7
Publisher site
See Chapter on Publisher Site

Abstract

[Stellite is one of the hardest materials known and so it finds application in places where superior wear and corrosion resistance is required. It is used in components such as valves and pumps that are present in primary, moderator, and auxiliary circuits of many of the water-cooled nuclear reactors. These cobalt-rich alloys are the major contributors for particulate cobalt activity which promotes radioactive hotspots. In some of the low-temperature coolant circuits, metallic particles or colloids of eroded stellite material can cause increase in radiation dose. Extensive studies have shown that it is possible to dissolve these metallic particles by employing a redox process using a permanganate-based formulation as oxidizing agent and a mixture of EDTA, ascorbic acid, and citric acid (in the weight ratio 4:3:3) as reducing formulation. As stellite particles are mostly in metallic state, attempt was made to study its dissolution behavior using electrochemical techniques that measure the corrosion tendency of materials. The corrosion behavior of stellite alloys dictates their susceptibility for dissolution in any chemical reagent. Studies were done on one of the most commonly used stellite, namely stellite #6. Its corrosion behavior was studied in acidified permanganates such as permanganic acid (HMnO4), nitric acid permanganate (NP, a mixture of nitric acid and potassium permanganate) and alkaline permanganate (AP). Electrochemical impedance studies were carried out at 90 °C in NP, HMnO4, and AP. The redox potential of all the oxidizing agents favored transpassive dissolution of chromium from the alloy. In NP, only the chromium-depleted interphase boundary was attacked while most of the chromium-rich carbide phases were intact. On the contrary in HMnO4, uniform corrosion of the surface was observed. The nitrate ions in permanganate were found to promote the repassivation of the surface. In AP only, the carbide phases were attacked. As these carbides are lean in cobalt, AP was found to be least efficient in oxidizing Cr and dissolving cobalt from stellite. Among the three reagents, HMnO4 was found to be the best formulation for dissolving cobalt from the alloy and hence for reducing radioactivity due to stellite.]

Published: May 5, 2022

Keywords: Stellite #6; Permanganic acid; EIS; Radioactive hotspots

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