CORROSION SCIENCE SECTION 105002-1 CORROSION—OCTOBER 2010 Submitted for publication April 13, 2010; in revised form, June 12, 2010. ‡ Corresponding author. E-mail: maalrodr@cnea.gov.ar. * Depto. Materiales – Comisión Nacional de Energía Atómica, Insti- tuto Sabato – Universidad Nacional de San Martín, Av. Gral. Paz 1499, San Martín, B1650KNA Buenos Aires, Argentina. ** GE Global Research, 1 Research Circle, CEB2505, Schenectady, NY 12309. (1) UNS numbers are listed in Metals and Alloys in the Unified Num- bering System, published by the Society of Automotive Engineers (SAE International) and cosponsored by ASTM International. Determination of the Crevice Corrosion Stabilization and Repassivation Potentials of a Corrosion-Resistant Alloy M. Rincón Ortíz,* M.A. Rodríguez, ‡, * R.M. Carranza,* and R.B. Rebak** ABSTRACT Ni-Cr-Mo alloys are highly resistant to general and localized corrosion, but they may suffer crevice corrosion in aggressive environmental conditions, such as high-chloride concentra- tions, applied potentials, and temperatures. It is assumed that localized corrosion will only occur when the corrosion poten- tial (E CORR ) is equal or higher than the crevice corrosion repas- sivation potential (E R,CREV ). The latter is measured by different electrochemical techniques using artificially creviced speci- mens. These techniques include cyclic potentiodynamic polar- ization (CPP) curves, the Tsujikawa-Hisamatsu electrochemical (THE) method, or other non-standard methods. Recently, as a variation of the THE method, the potentiodynamic-galvano- static-potentiodynamic (PD-GS-PD) technique was introduced. The goal of the present work was to determine reliable criti- cal or protection potentials for the crevice corrosion stabiliza- tion and repassivation of Alloy 22 (UNS N06022) in chloride solutions at 90°C. Conservative methodologies (which include extended potentiostatic steps) were applied for determining protection potentials below which crevice corrosion cannot sta- bilize and propagate. These values were compared to repas- sivation potentials obtained from the PD-GS-PD technique to assess its reliability. The CPP and THE methods were also considered for comparison. The repassivation potentials from the PD-GS-PD technique were conservative and reproducible, and they did not depend on the amount of previous crevice corrosion propagation in the studied conditions. KEY WORDS: ASTM G192, ASTM G61, chloride, corrosion, crevice corrosion, cyclic potentiodynamic polarization, repas- sivation potential, Tsujikawa-Hisamatsu electrochemical method, UNS N06022 INTRODUCTION Alloy 22 (UNS N06022) 1,(1) is a member of the Ni-Cr-Mo family designed to withstand the most aggressive industrial applications in both reducing and oxidizing acids. 2-3 It contains nominally 22% Cr, 13% Mo, and 3% W. 1 The base metal Ni contributes with its excel- lent resistance to hot alkalis. Cr enhances protection in oxidizing environments by promoting passive film formation. Mo and W enhance protection in reducing environments, in which generalized corrosion in the active state is observed, and they also improve the resistance to localized corrosion. 2,4-5 Alloy 22 has shown excellent resistance to pitting corrosion (PC), crevice corrosion (CC), and environmentally assisted cracking (EAC) in hot, concentrated chloride solu- tions. 2,4-5 Presently, Alloy 22 has a variety of industrial applications, including chemical processing, metal finishing, pollution control, nuclear waste treatment, pulp and paper industry, and flue-gas desulfurization plants. 2,4-5 Alloy 22 has been considered as a corro- sion-resistant barrier for high-level nuclear waste containers. 6 Alloy 22 is generally not susceptible to pitting corrosion in chloride-containing environments, but it might suffer crevice corrosion under certain aggres- ISSN 0010-9312 (print), 1938-159X (online) 10/000123/$5.00+$0.50/0 © 2010, NACE International