Determination of crevice corrosion susceptibility of Alloy 22 using different electrochemical techniques
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Determination of crevice corrosion susceptibility of Alloy 22 using different electrochemical techniques Mauricio Rincón Ortiz1, Martín A. Rodríguez1, Ricardo M. Carranza1 and Raul B. Rebak2 1 Depto. Materiales - Comisión Nacional de Energía Atómica, Instituto Sabato - Universidad Nacional de San Martín, Av. Gral. Paz 1499, San Martín, 1650, Buenos Aires, ARGENTINA. 2 GE Global Research, One Research Circle, CEB2505, Schenectady, NY 12309, USA. ABSTRACT Alloy 22 belongs to the Ni-Cr-Mo family and it is highly resistant to general and localized corrosion. It may suffer crevice corrosion in aggressive environmental conditions. This alloy has been considered as a corrosion-resistant barrier for high-level nuclear waste containers. It is assumed that localized corrosion may occurs when the corrosion potential (ECORR) is equal or higher than the crevice corrosion repassivation potential (ER,CREV). The latter is measured by means of different electrochemical techniques using artificially creviced specimens. These techniques include cyclic potentiodynamic polarization (CPP) curves, Tsujikawa-Hisamatsu electrochemical (THE) method or other non-standard methods, such as the PD-GS-PD technique. The aim of the present work was to determine reliable critical or protection potentials for crevice corrosion of Alloy 22 in pure chloride solutions at 90ºC. Conservative methodologies (which include extended potentiostatic steps) were applied for determining protection potentials below which crevice corrosion cannot initiate and propagate. Results from PD-GS-PD technique were compared with those from these methodologies in order to assess their reliability. Results from the CPP and the THE methods were also considered for comparison. The repassivation potential resulting from the PD-GS-PD technique was conservative and reproducible, and it did not depend on the amount of previous crevice corrosion propagation. INTRODUCTION Alloy 22 (UNS N06022) is a member of the Ni-Cr-Mo family designed to withstand the most aggressive industrial applications in both reducing and oxidizing acids.[1] It contains nominally 22% Chromium, 13% Molybdenum and 3% Tungsten. Alloy 22 has shown excellent resistance to pitting corrosion, crevice corrosion and environmentally assisted cracking in hot concentrated chloride solutions.[1,2] This alloy has been considered as a corrosion-resistant barrier for high-level nuclear waste containers.[3] It is assumed that localized corrosion will only occur when the corrosion potential (ECORR) is equal or higher than a critical potential (ECRIT).[3] The use of critical potentials for assessing the localized corrosion susceptibility is well documented.[4] Pitting corrosion (ER,PIT) and crevice corrosion repassivation potentials (ER,CREV) have been proposed as critical potentials. A variety of electrochemical techniques have been used for determining ER,CREV for Alloy 22 in different environmental and metallurgical conditions.[5-15] These techniques include cyclic potentiodynamic polarization [5-12] (CPP ASTM
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