Repassivation Potential of Alloy 22 in Sodium and Calcium Chloride Brines
- PDF / 7,588,019 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 70 Downloads / 171 Views
Repassivation Potential of Alloy 22 in Sodium and Calcium Chloride Brines Raul B. Rebak,1 Gabriel O. Ilevbare2 and Ricardo M. Carranza3 1 Lawrence Livermore National Laboratory, Livermore, CA 94550, USA 2 Electric Power Research Institute, Palo Alto, CA, 94304, USA 3 Atomic Energy Commission of Argentina, 1650 San Martin, Argentina ABSTRACT A comprehensive matrix of 60 tests was designed to explore the effect of calcium chloride vs. sodium chloride and the ratio R of nitrate concentration over chloride concentration on the repassivation potential of Alloy 22. Tests were conducted using the cyclic potentiodynamic polarization (CPP) technique at 75°C and at 90°C. Results show that at a ratio R of 0.18 and higher nitrate was able to inhibit the crevice corrosion in Alloy 22 induced by chloride. Current results fail to show in a consistent way a different effect on the repassivation potential of Alloy 22 for calcium chloride solutions than for sodium chloride solutions. INTRODUCTION Alloy 22 (N06022) is a nickel base alloy especially 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 Because this Alloy 22 is Ni based, it does not suffer environmentally assisted cracking in hot chloride solutions. 3 The resistance of Alloy 22 to localized corrosion in chloride solutions is given by the combined presence of Cr, Mo and W. 7-12 However, Alloy 22 may suffer crevice corrosion when it is anodically polarized in chloridecontaining solutions. 8-10,13-15 The presence of nitrate (NO3-) in the solution minimizes or eliminates the susceptibility of Alloy 22 to crevice corrosion. 8-10,16-23 The value of the ratio R = [NO3-]/[Cl-] has a strong effect on the susceptibility of Alloy 22 to crevice corrosion. 16-25 The higher the nitrate to chloride ratio R, the stronger is the inhibition by nitrate. The minimum required R value for inhibition may depend on other experimental variables such as total concentration of chloride and temperature. Other anions in solution were also reported to inhibit crevice corrosion in Alloy 22. 19-20, 26-28 The objective of this work was to examine the susceptibility of Alloy 22 in several electrolyte solutions containing sodium chloride (NaCl), calcium chloride (CaCl2) and sodium nitrate (NaNO3) using the cyclic potentiodynamic polarization (CPP) technique. 29 Ratios R of nitrate over chloride from 0.0086 to 0.25 were investigated. EXPERIMENTAL TECHNIQUE Alloy 22 specimens were prepared from 1-inch thick plate. The specimens were creviced using a ceramic washer and PTFE tape. 15,30,31 The specimens were multiple crevice assemblies (MCA) 23 or lollipops. All the tested specimens had a finished grinding of abrasive paper number 600 and were degreased in acetone and treated ultrasonically for 5 minutes in de-ionized (D
Data Loading...
