Repassivation Potential of Alloy 22 in Chloride plus Nitrate Solutions using the Potentiodynamic-Galvanostatic-Potentios
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0985-NN03-13
Repassivation Potential of Alloy 22 in Chloride plus Nitrate Solutions using the Potentiodynamic-Galvanostatic-Potentiostatic Method Kenneth J. Evans1 and Raul B. Rebak2 1 Lawrence Livermore National Laboratory, Livermore, CA, 94550 2 Chemistry and Materials Science, Lawrence Livermore National Laboratory, 7000 East Ave, L- 631, Livermore, CA, 94550 ABSTRACT In general, the susceptibility of Alloy 22 to suffer crevice corrosion is measured using the Cyclic Potentiodynamic Polarization (CPP) technique. This is a fast technique that gives rather accurate and reproducible values of repassivation potential (ER1) in most cases. In the fringes of susceptibility, when the environment is not highly aggressive, the values of repassivation potential using the CPP technique may not be highly reproducible, especially because the technique is fast. To circumvent this, the repassivation potential of Alloy 22 was measured using a slower method that combines Potentiodynamic-Galvanostatic-Potentiostatic steps (called here the Tsujikawa-Hisamatsu Electrochemical or THE method). The THE method applies the charge to the specimen in a more controlled way, which may give more reproducible repassivation potential values, especially when the environment is not aggressive. The values of repassivation potential of Alloy 22 in sodium chloride plus potassium nitrate solutions were measured using the THE and CPP methods. Results show that both methods yield similar values of repassivation potential, especially under aggressive conditions.
INTRODUCTION Alloy 22 (N06022) is a nickel base alloy designed to be resistant to all forms of corrosion. Alloy 22 contains approximately 56% nickel (Ni), 22% chromium (Cr), 13% molybdenum (Mo), 3% tungsten (W) and 3% iron (Fe) (ASTM B 575) [1]. Because of its high level of Cr, Alloy 22 remains passive in most industrial environments and therefore has an exceptionally low general corrosion rate [2-6]. The combined presence of Cr, Mo and W imparts Alloy 22 with high resistance to localized corrosion such as pitting corrosion and stress corrosion cracking even in hot concentrated chloride (Cl-) solutions [7-12]. It has been reported that Alloy 22 may suffer localized corrosion such as crevice corrosion when it is anodically polarized in chloridecontaining solutions [8-10, 13-15]. It is also known that the presence of nitrate (NO3-) in the solution minimizes or eliminates the susceptibility of Alloy 22 to crevice corrosion [8-10, 1623]. The value of the ratio [NO3-]/[Cl-] has a strong effect of the susceptibility of Alloy 22 to crevice corrosion [16-25]. The higher the nitrate to chloride ratio, the stronger is the inhibition by nitrate. This ratio may depend on other experimental variables such as total concentration of chloride or temperature. Other anions in solution were also reported to inhibit crevice corrosion in Alloy 22 [19-20, 26-28]. The susceptibility of Alloy 22 to suffer crevice corrosion is generally measured using the cyclic potentiodynamic polarization (CPP) technique, which is des
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