Anodic Behavior of Alloy 22 in High Nitrate Brines at Temperatures Higher Than 100°C

Abstract

Alloy 22 (N06022) may be susceptible to crevice corrosion in chloride solutions. Nitrate acts as an inhibitor to crevice corrosion. Several papers have been published regarding the effect of nitrate on the corrosion resistance of Alloy 22 at temperatures 100°C and lower. However, very little is known about the behavior of this alloy in highly concentrated brines at temperatures above 100°C. In the current work, electrochemical tests have been carried out to explore the anodic behavior of Alloy 22 in high chloride high nitrate electrolytes at temperatures as high as 160°C at ambient atmospheres. Even though Alloy 22 may adopt corrosion potentials in the order of +0.5 V (in the saturated silver chloride scale), it does not suffer crevice corrosion if there is high nitrate in the solution. That is, the inhibitive effect of nitrate on crevice corrosion is active for temperatures higher than 100°C.

General Corrosion and Passive Film Stability

“…Felker et al (2006) did not test lower NO 3 :Cl ratios to determine critical values needed to inhibit localized corrosion in part because lower nitrate to chloride brines are not stable at the atmospheric testing conditions of their experiments. Note that defining temperature dependent NO 3 :Cl ratios necessary to inhibit the initiation of localized corrosion should be used with caution, because systematic studies of nitrate inhibition in the Ca-Cl-NO 3 system from 100 to 150˚C indicate that nitrate inhibition is dependent on the nitrate and chloride concentrations as well as the NO 3 :Cl ratio (Ilevbare et al, 2006) in agreement with the Yucca Mountain model at temperatures less than 120˚C (Section 5.2.1).…”

“…Additional electrochemical studies in Ca-Cl-NO 3 brines to 160˚C and K-Na-Cl-NO 3 brines to 150˚C also report that nitrate inhibits localized corrosion of Alloy 22 (Ilevbare et al, 2006;Felker et al, 2006). Localized corrosion observed in the passive 9-month experiments suggests that NO 3 :Cl ratios above 7.4 may be required to inhibit localized corrosion at 160 and 220˚C.…”

“…Localized corrosion is initiated within 30 minutes when the potential is held above the repassivation potential (Yilmaz et al, 2005). However, comparison of localized corrosion observed in the 9-month autoclave experiments at 160˚C in solutions with high nitrate to chloride ratios (Na-K brines, NO 3 :Cl = 7.4 and NO 3 = 18.5 molal) with no localized corrosion of Alloy 22 at the same temperature in accelerated electrochemical tests with much lower nitrate to chloride (Ca brines, NO 3 :Cl = 0.5 and NO 3 = 12 and 18 molal) (Ilevbare et al, 2006), indicate that electrochemical tests require longer exposure to capture localized corrosion in concentrated brines that form at higher temperature. Extension of the localized corrosion initiation model for Alloy 22 to high temperatures should be constrained by electrochemical data derived at multiple scan rates to ensure that transport of reactants does not mask electrochemical measurements.…”

“…The creviced specimens were included to test the hypothesis that nitrate inhibits localized corrosion of Alloy 22 in brines with NO 3 :Cl ≥ 0.5 at high temperature, as has been observed for Alloy 22 from 60 to 100ºC in Na-K-Cl-NO 3 brines and from 100 to 150ºC in Ca-Cl-NO 3 brines using electrochemical techniques (Ilevbare et al, 2005;Ilevbare et al, 2006). The limited number of tests conducted in FY05 were a follow-on study to Alloy 22 general corrosion experiments conducted from 120 to 220˚C as a function of time over a range of NO 3 :Cl , where solution compositions were based on knowledge of measured brine compositions at 120 o C .…”

Crevice Repassivation Potential of Alloy 22 in High-Nitrate Dust Deliquescence Type Environments

“…One study contained the behavior of Alloy 22 at temperatures up to 160°C in highly concentrated calcium based chloride and nitrate brines. 24 It was reported that the inhibiting effect of nitrate for crevice corrosion is also valid up to temperatures of 160°C. 24 Corrosion at the repository site may occur only in the presence of water.…”